Bias adjustment II - 'The Reckoning'
 

Apologies for the delay for this 'joint reply' - interrupted by a little
post-prandial 'digestif' by way of a spot of Delius and VW (Larka Sending
and Symphony No. 6 - still playing! :-)

*Massive thanks* to *all* who responded and were, between you, just about
right on the money, from what I can see of it!! (Fekkin' valvies - too damn
sad/skint to be out on the ****/tiles/razzle on a Saturday Night!! :-)

(I dunno!!)

OK. First up, the valves read as follows (Test voltage, amp on / Resistance,
amp off - the DVM wuz under me bed!!)

V1 = 0.317 V / 10.2 ohms (10R2 ??)

V2 = 0.303 V / 10.0

V3 = 0.322 V / 10.2

V4 = 0.318 V / 10.5

(I'll check tomorrow, but I thought the voltage readings were all "+" ???)

Next, the valve labels are all as follows:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

Now, call me a bluff old cove, but this looks to me a bit like we want
is -37 VDC on the grid (am I right?) - ie. the 'test' voltage?? If so the
valves are all(consistently) 'underbiased?

What do each of these figures mean? (I understand about the valves being
'matched' but I thought was mA / V or summat??)

Many thanks, once again.

Bias adjustment II - 'The Reckoning'
 

I presume the test point is the cathode of the output valve, which has a 10 ohm
resistor. If the voltage is .32 then the current across it should be 32mA.
That's not very much - I wonder what the B+ could be - over 400v if you want
any power I imagine. Andy

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Bias adjustment II - 'The Reckoning'
 

"Andy Evans" wrote in message
...
I presume the test point is the cathode of the output valve, which has a 10
ohm
resistor. If the voltage is .32 then the current across it should be 32mA.
That's not very much - I wonder what the B+ could be - over 400v if you
want
any power I imagine. Andy


OK, what about the label?

Ua: 430V

Vg2: 440V

Vg1: -36V


What are these telling us exactly?

(Apologies for my Ignore Ants.... ;-)

Bias adjustment II - 'The Reckoning'
 

Ua: 430V this is the voltage on the anode of the output valves

Vg2: 440V this is the voltage on the screen grids (g2) -

Vg1: -36V this is the fixed bias on the grid, or g1 (grid one, the control
grid, where the signal goes in)

32mA at 430v would only give you about 13 watts, which is pretty conservative,
but OK I guess.

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Bias adjustment II - 'The Reckoning'
 

"Andy Evans" wrote in message
...
Ua: 430V this is the voltage on the anode of the output valves

Vg2: 440V this is the voltage on the screen grids (g2) -

Vg1: -36V this is the fixed bias on the grid, or g1 (grid one, the
control
grid, where the signal goes in)

32mA at 430v would only give you about 13 watts, which is pretty
conservative,
but OK I guess.


No, summat's not right here - this amp'll trounce my KiT88 which pushes out
32wpc. (Volume, not*quality* mind....;-)

It's running into a pair of Fluffy Bunny DM2As (84dB according to that nice
Mr P) and it stoke's 'em like a wild thing! (Runs out of puff at about the 2
o' clock mark, tho'....???)

Bias adjustment II - 'The Reckoning'
 

No, summat's not right here

Well, according to Ohms law .32 volts across 10 ohms is 32mA. I'm assuming this
is the cathode....................... but I don't know the circuit. You'd
expect about double that, really.

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Bias adjustment II - 'The Reckoning'
 

"Keith G" wrote in message
...
Apologies for the delay for this 'joint reply' - interrupted by a little
post-prandial 'digestif' by way of a spot of Delius and VW (Larka Sending
and Symphony No. 6 - still playing! :-)

*Massive thanks* to *all* who responded and were, between you, just about
right on the money, from what I can see of it!! (Fekkin' valvies - too
damn sad/skint to be out on the ****/tiles/razzle on a Saturday Night!!
:-)

(I dunno!!)

OK. First up, the valves read as follows (Test voltage, amp on /
Resistance, amp off - the DVM wuz under me bed!!)

V1 = 0.317 V / 10.2 ohms (10R2 ??)

V2 = 0.303 V / 10.0

V3 = 0.322 V / 10.2

V4 = 0.318 V / 10.5

(I'll check tomorrow, but I thought the voltage readings were all "+"
???)

Next, the valve labels are all as follows:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

Now, call me a bluff old cove, but this looks to me a bit like we want
is -37 VDC on the grid (am I right?) - ie. the 'test' voltage?? If so the
valves are all(consistently) 'underbiased?

What do each of these figures mean? (I understand about the valves being
'matched' but I thought was mA / V or summat??)

Many thanks, once again.





Hi Keith,

I'm just wondering if they supply the amplifier with brand new but tested
valves i.e. not burnt in for say 10 hours to allow anode current to
stabilise. If this is the case they may supply the amplifier underbiased
with instructions (if you had them!!) to run it for a while then increase
the bias point to the correct anode current when all the output tube anode
currents are stable (i.e. the bias reading at test points are finally static
and settled down) Just a thought - I'd still get back to the supplier.

Cheers

Mike

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:

V1 = 0.317 V / 10.2 ohms (10R2 ??)
V2 = 0.303 V / 10.0
V3 = 0.322 V / 10.2
V4 = 0.318 V / 10.5

If you adjust the tweakers so that the voltages measure as follows...

V1 = 0.3162V
V2 = 0.31V
V3 = 0.3162V
V4 = 0.3255V

....that should give each valve an Ia of 31mA. I'm assuming that the 31mA on
the valve label indicates what the anode current should be for all valves.
(Do they all have the sticker, and do they all have the same value for Ia?)


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:

No, summat's not right here - this amp'll trounce my KiT88 which
pushes out 32wpc. (Volume, not*quality* mind....;-)

It's running into a pair of Fluffy Bunny DM2As (84dB according to
that nice Mr P) and it stoke's 'em like a wild thing! (Runs out of
puff at about the 2 o' clock mark, tho'....???)

That sounds like a sensitive input, as if it's maxing out at a relatively
low volume setting due to the size of the signal it's being fed. After that,
it would just get progressively dirtier. (On mine, o/p from the DAC straight
into the power amp reaches max volume at about 1/3rd of the way up.)

My amp's anode current is about 60mA, and the HT is, I think, 450V (hence
the spec 27W absolute max: 0.06 * 450 = 27). Yours has about half the
current for a similar HT, so Andy's 13W sounds about right.

Does it go audibly louder than the KiT88 (both amps at max, use a preamp to
control volume)? What's the rated output of the KiT88? Could one amp find
the speakers easier to drive than the other?


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment II - 'The Reckoning'
 

"Andy Evans" wrote in message
...
No, summat's not right here

Well, according to Ohms law .32 volts across 10 ohms is 32mA. I'm assuming
this
is the cathode....................... but I don't know the circuit. You'd
expect about double that, really.


OK. Thanks for that.

I gotta go do a movie now - Swim's getting the arse!! :-(

(I know, I know - slap that bitch's face!!! )

;-)

Bias adjustment II - 'The Reckoning'
 

"Mike Gilmour" wrote in message
...

"Keith G" wrote in message
...
Apologies for the delay for this 'joint reply' - interrupted by a little
post-prandial 'digestif' by way of a spot of Delius and VW (Larka Sending
and Symphony No. 6 - still playing! :-)

*Massive thanks* to *all* who responded and were, between you, just about
right on the money, from what I can see of it!! (Fekkin' valvies - too
damn sad/skint to be out on the ****/tiles/razzle on a Saturday Night!!
:-)

(I dunno!!)

OK. First up, the valves read as follows (Test voltage, amp on /
Resistance, amp off - the DVM wuz under me bed!!)

V1 = 0.317 V / 10.2 ohms (10R2 ??)

V2 = 0.303 V / 10.0

V3 = 0.322 V / 10.2

V4 = 0.318 V / 10.5

(I'll check tomorrow, but I thought the voltage readings were all "+"
???)

Next, the valve labels are all as follows:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

Now, call me a bluff old cove, but this looks to me a bit like we want
is -37 VDC on the grid (am I right?) - ie. the 'test' voltage?? If so
the valves are all(consistently) 'underbiased?

What do each of these figures mean? (I understand about the valves being
'matched' but I thought was mA / V or summat??)

Many thanks, once again.





Hi Keith,

I'm just wondering if they supply the amplifier with brand new but tested
valves i.e. not burnt in for say 10 hours to allow anode current to
stabilise. If this is the case they may supply the amplifier underbiased
with instructions (if you had them!!) to run it for a while then increase
the bias point to the correct anode current when all the output tube anode
currents are stable (i.e. the bias reading at test points are finally
static and settled down) Just a thought - I'd still get back to the
supplier.

Cheers

Mike
Another thought, if the cathode resistor is 10 ohms then the output valves
are under running by about half (I guess...not gospel) so you should be able
to get an idea by the envelope temperature of the valves i.e. are they
running less hot than you'd nornally expect?

I know VTL amp output valves generally run cool cos de man sez keeps dem
electrons under restraint :-) i.e. instead of 60 or 70mA they run at 45mA
or so and a lot cooler and last longer - he says they sound better...I don't
agree but hey it's a free world (but they sure last looooooonger)

Cool,

Mike

Bias adjustment II - 'The Reckoning'
 

"Mike Gilmour" wrote

I'm just wondering if they supply the amplifier with brand new but tested
valves i.e. not burnt in for say 10 hours to allow anode current to
stabilise. If this is the case they may supply the amplifier underbiased
with instructions (if you had them!!) to run it for a while then increase
the bias point to the correct anode current when all the output tube anode
currents are stable (i.e. the bias reading at test points are finally
static and settled down) Just a thought - I'd still get back to the
supplier.


Not a bad idea - I can struggle through a bit of 'written' German (it's been
a while) but I'm wishin' and hopin' that the seller (Proelektra.de) will be
able to put me onto the designer. I don't think the seller will know and his
English is not too good (better than my German by a long chalk....).

I'll wait for the second amp to turn up (Mon/Tue with any luck) and give the
single sheet of instructions another go (this one's up in the loft, in the
box).

Bias adjustment II - 'The Reckoning'
 

"Wally" wrote in message
...
Keith G wrote:

V1 = 0.317 V / 10.2 ohms (10R2 ??)
V2 = 0.303 V / 10.0
V3 = 0.322 V / 10.2
V4 = 0.318 V / 10.5

If you adjust the tweakers so that the voltages measure as follows...

V1 = 0.3162V
V2 = 0.31V
V3 = 0.3162V
V4 = 0.3255V

...that should give each valve an Ia of 31mA. I'm assuming that the 31mA
on
the valve label indicates what the anode current should be for all valves.
(Do they all have the sticker, and do they all have the same value for
Ia?)


This getting very interesting. Yes, all 4 valves have the same sticker with
the same values, but I have seen diffent values (33, not 31) on the labels
on the eBay pic. I have another identical amp coming in a couple of days
(hopefully) and it will be interesting to see what the values are on that
one.

It almost looks like they measure every valve and match them - is this
likely or is it ********???

OK, I see one or two more posts coming in - I gotta go (or I'll get a
slappin') I'll get back on this tomorrow - many thanks to all, it's
interesting and extremely useful. (It's either *me* now or Phil North in a
few days when he get his 'identical' amp!! ;-)

Rock on....

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:

This getting very interesting. Yes, all 4 valves have the same
sticker with the same values, but I have seen diffent values (33, not
31) on the labels on the eBay pic. I have another identical amp
coming in a couple of days (hopefully) and it will be interesting to
see what the values are on that one.

It almost looks like they measure every valve and match them - is this
likely or is it ********???

I would say that that's entirely possible. (Take a look at the Groove Tubes
web site to see how far they take this in guitar land.) It sounds like
they're matching the valves they use in a particular amp chassis, but that
they have a tolerance for the actual Ia (31-33mA, possibly wider).

I agree with Mike that running them cooler will make them last longer - the
electrons that flow (flow, pure electron *flow* - ye canny beat the magic of
Plasma Power! Who gives two ****s if the technology's 100 years old?) - to
the anode come out of the cathode, and there's a finite amount to be had.
The opposite is to run them hot enough to get the anodes to glow red (you'll
have seen what an arc welder can do with current, right?), but they don't
last.


Rock on....

....d0000dz!


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:

Apologies for the delay for this 'joint reply' - interrupted by a little
post-prandial 'digestif' by way of a spot of Delius and VW (Larka Sending
and Symphony No. 6 - still playing! :-)

*Massive thanks* to *all* who responded and were, between you, just about
right on the money, from what I can see of it!! (Fekkin' valvies - too damn
sad/skint to be out on the ****/tiles/razzle on a Saturday Night!! :-)

(I dunno!!)

OK. First up, the valves read as follows (Test voltage, amp on / Resistance,
amp off - the DVM wuz under me bed!!)

V1 = 0.317 V / 10.2 ohms (10R2 ??)

V2 = 0.303 V / 10.0

V3 = 0.322 V / 10.2

V4 = 0.318 V / 10.5

(I'll check tomorrow, but I thought the voltage readings were all "+" ???)

Next, the valve labels are all as follows:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

Now, call me a bluff old cove, but this looks to me a bit like we want
is -37 VDC on the grid (am I right?) - ie. the 'test' voltage?? If so the
valves are all(consistently) 'underbiased?

What do each of these figures mean? (I understand about the valves being
'matched' but I thought was mA / V or summat??)

Simple.

There's a tiny amount of cathode bias generated by the 10Rs in the cathose
circuit. 31mA * 10R does indeed give the 0.3 odd volts you mention.

The bulk of the g1 negative bias is doubtless set by those trimpots.

-36V sounds about right to me for EL34s.

If you were really fussy you could adjust the trimpots so that V1, 2, 3 ,4 were
all identical - although that would only be sensible if the resistors were
accurately matched too !

Graham

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:
"Wally" wrote in message
...

Keith G wrote:


V1 = 0.317 V / 10.2 ohms (10R2 ??)
V2 = 0.303 V / 10.0
V3 = 0.322 V / 10.2
V4 = 0.318 V / 10.5

If you adjust the tweakers so that the voltages measure as follows...

V1 = 0.3162V
V2 = 0.31V
V3 = 0.3162V
V4 = 0.3255V

...that should give each valve an Ia of 31mA. I'm assuming that the 31mA
on
the valve label indicates what the anode current should be for all valves.
(Do they all have the sticker, and do they all have the same value for
Ia?)



This getting very interesting. Yes, all 4 valves have the same sticker with
the same values, but I have seen diffent values (33, not 31) on the labels
on the eBay pic. I have another identical amp coming in a couple of days
(hopefully) and it will be interesting to see what the values are on that
one.

It almost looks like they measure every valve and match them - is this
likely or is it ********???


Very likly, I think the numbers on the valves are just that, for
matching. They set up the valve tester at that anode, g1, and g2
voltages, plug in the valve, and write down what current flows through
it. Then when they send a amp out, they pick 4 valves with the same
number. Thats why you will have seen pictures with different numbers on,
they have matched another set about another current.

Doesn't help you to know what the design current is though. I would try
and find out from the makers. Without knowing what the B+ is you can't
know what dissapation the valves are running at, and even if the valves
would work fine at a higher current, you don't know what the power
supply can give. If it is running at 34ma, thats as Andy said quite low,
but remember the anode dissapation is not directly related to the output
power, it depends on when the move from A to B happens. if the valve is
dissapating 13w (for example) thats power thats not being passed to the
speakers, what you are interested in (in terms of output) is the power
the valve "dissapates" in the load, the output transformer primary, and
so to the speakers

--
Nick

Bias adjustment II - 'The Reckoning'
 

"Keith G" wrote in message
...

"Wally" wrote in message
...
The snip!

This getting very interesting. Yes, all 4 valves have the same sticker
with the same values, but I have seen diffent values (33, not 31) on the
labels on the eBay pic. I have another identical amp coming in a couple of
days (hopefully) and it will be interesting to see what the values are on
that one.

It almost looks like they measure every valve and match them - is this
likely or is it ********???


No ********, with computerised testers it's a very quick operation.(Valves
using solid state for their own advantage - ironic eh?) If you want the
best out of your amp and to know it's nicely balanced in regard to gm etc
then matched valves are the only way to go. With your amount of valve
equipment growing it might be worth picking up a valve characteristic
meter/tester on ebay and doing your own testing/checking.

Cheers,

Mike

Bias adjustment II - 'The Reckoning'
 

In message , Mike Gilmour
writes

"Keith G" wrote in message
...

"Wally" wrote in message
...
The snip!

This getting very interesting. Yes, all 4 valves have the same sticker
with the same values, but I have seen diffent values (33, not 31) on the
labels on the eBay pic. I have another identical amp coming in a couple of
days (hopefully) and it will be interesting to see what the values are on
that one.

It almost looks like they measure every valve and match them - is this
likely or is it ********???


No ********, with computerised testers it's a very quick operation.(Valves
using solid state for their own advantage - ironic eh?) If you want the
best out of your amp and to know it's nicely balanced in regard to gm etc
then matched valves are the only way to go. With your amount of valve
equipment growing it might be worth picking up a valve characteristic
meter/tester on ebay and doing your own testing/checking.

Cheers,

Mike



Matching valves is easier said than done. OK, it's a simple matter to
match for a particular anode current, at a particular anode voltage, for
a given grid voltage, and this is all that is normally possible by
measuring the cathode current.

But this is only measuring one parameter at one setting. Valves vary so
much from one to another (compared with SS) that to find the best match
over the operating range that the two (in a P-P amp) have to cover is
non-trivial without a full curve tracer.
--
Chris Morriss

Bias adjustment II - 'The Reckoning'
 

"Chris Morriss" wrote in message
...
In message , Mike Gilmour
writes

"Keith G" wrote in message
...

"Wally" wrote in message
...
The snip!

This getting very interesting. Yes, all 4 valves have the same sticker
with the same values, but I have seen diffent values (33, not 31) on the
labels on the eBay pic. I have another identical amp coming in a couple
of
days (hopefully) and it will be interesting to see what the values are
on
that one.

It almost looks like they measure every valve and match them - is this
likely or is it ********???


No ********, with computerised testers it's a very quick operation.(Valves
using solid state for their own advantage - ironic eh?) If you want the
best out of your amp and to know it's nicely balanced in regard to gm etc
then matched valves are the only way to go. With your amount of valve
equipment growing it might be worth picking up a valve characteristic
meter/tester on ebay and doing your own testing/checking.

Cheers,

Mike



Matching valves is easier said than done. OK, it's a simple matter to
match for a particular anode current, at a particular anode voltage, for a
given grid voltage, and this is all that is normally possible by measuring
the cathode current.

But this is only measuring one parameter at one setting. Valves vary so
much from one to another (compared with SS) that to find the best match
over the operating range that the two (in a P-P amp) have to cover is
non-trivial without a full curve tracer.
--
Chris Morriss

I understood that a number of the major valve suppliers have computerised
characteristic testers that plot and map various parameters rapidly. I know
a manual plot is very time consuming (got the T shirt ;-) but it's a very
different ball game with their equipment.

Mike

Bias adjustment II - 'The Reckoning'
 

I understood that a number of the major valve suppliers have computerised
characteristic testers that plot and map various parameters rapidly. I know
a manual plot is very time consuming (got the T shirt ;-) but it's a very
different ball game with their equipment.Mike

I use an AVO3 - what am I missing out on here? Andy

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Bias adjustment II - 'The Reckoning'
 

Andy Evans wrote:
I understood that a number of the major valve suppliers have
computerised characteristic testers that plot and map various
parameters rapidly. I know a manual plot is very time consuming (got
the T shirt ;-) but it's a very different ball game with their
equipment.Mike

I use an AVO3 - what am I missing out on here? Andy

Rapidiy of execution? ATEs were invented for a reason, you know... :-)


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment II - 'The Reckoning'
 

"Wally" wrote in message
...
Keith G wrote:

No, summat's not right here - this amp'll trounce my KiT88 which
pushes out 32wpc. (Volume, not*quality* mind....;-)

It's running into a pair of Fluffy Bunny DM2As (84dB according to
that nice Mr P) and it stoke's 'em like a wild thing! (Runs out of
puff at about the 2 o' clock mark, tho'....???)

That sounds like a sensitive input, as if it's maxing out at a relatively
low volume setting due to the size of the signal it's being fed. After
that,
it would just get progressively dirtier. (On mine, o/p from the DAC
straight
into the power amp reaches max volume at about 1/3rd of the way up.)

My amp's anode current is about 60mA, and the HT is, I think, 450V (hence
the spec 27W absolute max: 0.06 * 450 = 27). Yours has about half the
current for a similar HT, so Andy's 13W sounds about right.


OK, it's crunch time. I'm going to have to get into this quite a bit deeper,
I can see...



Does it go audibly louder than the KiT88 (both amps at max, use a preamp
to
control volume)? What's the rated output of the KiT88? Could one amp find
the speakers easier to drive than the other?


I haven't had the chance to compare the amps directly, but I would say the
EL34 goes quite a bit louder than the Kit88 which is 'supposed' to be rated
at 32 wpc with the cathode resistor mod that I carried out on it....???

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:

I haven't had the chance to compare the amps directly, but I would
say the EL34 goes quite a bit louder than the Kit88 which is
'supposed' to be rated at 32 wpc with the cathode resistor mod that I
carried out on it....???

What value are the cathode resistors on the KiT88, what's the voltage across
them, and what's the anode voltage?


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment II - 'The Reckoning'
 

"Nick Gorham" wrote


It almost looks like they measure every valve and match them - is this
likely or is it ********???


Very likly, I think the numbers on the valves are just that, for matching.
They set up the valve tester at that anode, g1, and g2 voltages, plug in
the valve, and write down what current flows through it. Then when they
send a amp out, they pick 4 valves with the same number. Thats why you
will have seen pictures with different numbers on, they have matched
another set about another current.


Yep, much as I suspected.


Doesn't help you to know what the design current is though. I would try
and find out from the makers.


I'm going to see if I can get the circuit from the designer.


Without knowing what the B+ is you can't know what dissapation the valves
are running at, and even if the valves would work fine at a higher
current, you don't know what the power supply can give. If it is running
at 34ma, thats as Andy said quite low, but remember the anode dissapation
is not directly related to the output power, it depends on when the move
from A to B happens. if the valve is dissapating 13w (for example) thats
power thats not being passed to the speakers, what you are interested in
(in terms of output) is the power the valve "dissapates" in the load, the
output transformer primary, and so to the speakers


OK. (There'll be more questions.... ;-)

Bias adjustment II - 'The Reckoning'
 

"Mike Gilmour" wrote


No ********, with computerised testers it's a very quick operation.(Valves
using solid state for their own advantage - ironic eh?) If you want the
best out of your amp and to know it's nicely balanced in regard to gm etc
then matched valves are the only way to go. With your amount of valve
equipment growing it might be worth picking up a valve characteristic
meter/tester on ebay and doing your own testing/checking.


Yes, I've been thinking of getting summat simple and learning how to use it.

It's about time I got a bit more 'into' it!! The idea of getting these cheap
Chinese amps is to do exactly that and so far it's getting to be very
interesting!!

Thanks to all, once again.

Bias adjustment II - 'The Reckoning'
 

"Pooh Bear" wrote


What do each of these figures mean? (I understand about the valves being
'matched' but I thought was mA / V or summat??)

Simple.

There's a tiny amount of cathode bias generated by the 10Rs in the cathose
circuit. 31mA * 10R does indeed give the 0.3 odd volts you mention.


(I can see Ohms Law poking out here and there!! ;-)


The bulk of the g1 negative bias is doubtless set by those trimpots.

-36V sounds about right to me for EL34s.

If you were really fussy you could adjust the trimpots so that V1, 2, 3 ,4
were
all identical - although that would only be sensible if the resistors were
accurately matched too !


Yes, Wally worked out the exact values. I have to say , even with the
variances I measured the amp ssounds fine and looks perfectly OK. I will
'square it up' but not until I've got the next one (due, hopefully tomorrow
or Tuesday) and made some direct comparisons first.

Many thanks.

Bias adjustment II - 'The Reckoning'
 

"Keith G" wrote in message
...

"Nick Gorham" wrote


It almost looks like they measure every valve and match them - is this
likely or is it ********???


Very likly, I think the numbers on the valves are just that, for
matching. They set up the valve tester at that anode, g1, and g2
voltages, plug in the valve, and write down what current flows through
it. Then when they send a amp out, they pick 4 valves with the same
number. Thats why you will have seen pictures with different numbers on,
they have matched another set about another current.


Yep, much as I suspected.


Doesn't help you to know what the design current is though. I would try
and find out from the makers.


I'm going to see if I can get the circuit from the designer.


I'll agree with that as well. My matched quads of KT88's & 6550C's came with
similar test labels & identical numbers. It's easy with a tester to do a of
quick 'n dirty static test, & the've always been within okay limits. You
know yourself if you get an untested batch lot then they can vary quite a
bit sample to sample but I've always found the Svetlana output valves with
not too much unit variation (or have I been lucky?). Anyone else found
this? But if you want true accuracy then dynamic matching is the only
answer.

Bias adjustment II - 'The Reckoning'
 

"Wally" wrote in message
...
Keith G wrote:

I haven't had the chance to compare the amps directly, but I would
say the EL34 goes quite a bit louder than the Kit88 which is
'supposed' to be rated at 32 wpc with the cathode resistor mod that I
carried out on it....???

What value are the cathode resistors on the KiT88, what's the voltage
across
them, and what's the anode voltage?


The original design of the KiT88 produced 36wpc with 150R cathode resistors.
It ate a couple of valves (common on this amp I found out) and I swapped
them for 220Rs on the advice of Nick Lucas at WAD. He said that would reduce
the output to 32wpc. Since then it has been fine. (They didn't carry this
kit for long as I gather it was a bit of a bugger - Nick G might know more
about this.) Having said that, it's a fine amp IMO.....

I'm sorry, but I can't lay hands on the paperwork to see what the voltages
measured when I built it and I'm not easily in a postion to measure them
atm.....

Bias adjustment II - 'The Reckoning'
 

"Mike Gilmour" wrote


Doesn't help you to know what the design current is though. I would try
and find out from the makers.


I'm going to see if I can get the circuit from the designer.


I'll agree with that as well. My matched quads of KT88's & 6550C's came
with similar test labels & identical numbers. It's easy with a tester to
do a of quick 'n dirty static test, & the've always been within okay
limits. You know yourself if you get an untested batch lot then they can
vary quite a bit sample to sample but I've always found the Svetlana
output valves with not too much unit variation (or have I been lucky?).
Anyone else found this? But if you want true accuracy then dynamic
matching is the only answer.


TBH, I've always trusted that the valves I've bought have been tested (as
ordered) and have taken the view that unless you have a big stock of them,
you can't do much to match them yourself. Having said that, I've got a small
clutch of 'not too sure' valves that I would like to be able to test.

The daft thing is that one could probably afford to chuck a lot of 'unknown
quantities' before getting anywhere near the price of a tester...???

Bias adjustment II - 'The Reckoning'
 

"Keith G" wrote


I'm sorry, but I can't lay hands on the paperwork to see what the voltages
measured when I built it and I'm not easily in a postion to measure them
atm.....


There's a .pdf of the circuit here if you are curious/interested:

http://www.apah69.dsl.pipex.com/keit...t88circuit.pdf

Bias adjustment II - 'The Reckoning'
 

It's about time I got a bit more 'into' it!! The idea of getting these cheap
Chinese amps is to do exactly that and so far it's getting to be very
interesting!!

The problem for tweaking these cheepo chinese amps is that
a) everything is on pcbs, I think, so a pain to fit larger componants in the
same slots
b) there may not me be much room inside the chassis for adding stuff, e.g.
chokes.

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:

What value are the cathode resistors on the KiT88, what's the voltage
across them, and what's the anode voltage?

The original design of the KiT88 produced 36wpc with 150R cathode
resistors.

According to the voltages specced on the diagram, it has 10V at the 150R,
giving 67mA for Ia (V/R=I). HT is 510V, so It should be producing 510 *
0.067 = 34W. I can't imagine that changng to 220R would drop the power
substantially - should still be much more than the 13W we reckon the EL34
jobbie is producing.


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment II - 'The Reckoning'
 

Wally wrote:
Keith G wrote:


What value are the cathode resistors on the KiT88, what's the voltage
across them, and what's the anode voltage?


The original design of the KiT88 produced 36wpc with 150R cathode
resistors.


According to the voltages specced on the diagram, it has 10V at the 150R,
giving 67mA for Ia (V/R=I). HT is 510V, so It should be producing 510 *
0.067 = 34W. I can't imagine that changng to 220R would drop the power
substantially - should still be much more than the 13W we reckon the EL34
jobbie is producing.



Not quite, Ht is 510, cathode is at 10v so at 67ma, it would be
500*.067=33.5 the spec for kt88's states the max anode dis is 40w, so
quite a way from that. The change of cathode resistor will make more of
a change than you are suggesting. increasing the value, will increase
the voltage on the cathode, and in that way slightly reduce the voltage
across the valve, BUT it will also reduce the current, so having more of
a effect of anode dis than you may be expecting.

Also, the powers being quoted here 34W and 13W are the power being
dissapated by the valve at idle, not directly related to the power
output of the amp. To find that, you need to draw a set of composite
lines for the valves at the paticulat DC bias point, and then plot the
loadline of the reflected load the speaker presents via the output
transformer.

This is a good starting site for such composite load lines.

http://members.aol.com/sbench102/composit.html

Actually its a good starting site for many valve related things.

The fact that the El34 is running at 13w, just points that its a AB1
amp, not a A1 amp, no big deal, and only sensible for El34's IMHO given
the market I suspect they are trying to get into.

--
Nick

Bias adjustment II - 'The Reckoning'
 

"Wally" wrote in message
...
Keith G wrote:

What value are the cathode resistors on the KiT88, what's the voltage
across them, and what's the anode voltage?

The original design of the KiT88 produced 36wpc with 150R cathode
resistors.

According to the voltages specced on the diagram, it has 10V at the 150R,
giving 67mA for Ia (V/R=I). HT is 510V, so It should be producing 510 *
0.067 = 34W. I can't imagine that changng to 220R would drop the power
substantially - should still be much more than the 13W we reckon the EL34
jobbie is producing.


At the time Nick Lucas said I wouldn't notice any difference and I
haven't....

There's *no way* the EL34 amp is only producing 13W though.....???

Bias adjustment II - 'The Reckoning'
 

"Nick Gorham" wrote in message
...
Wally wrote:
Keith G wrote:


What value are the cathode resistors on the KiT88, what's the voltage
across them, and what's the anode voltage?


The original design of the KiT88 produced 36wpc with 150R cathode
resistors.


According to the voltages specced on the diagram, it has 10V at the 150R,
giving 67mA for Ia (V/R=I). HT is 510V, so It should be producing 510 *
0.067 = 34W. I can't imagine that changng to 220R would drop the power
substantially - should still be much more than the 13W we reckon the EL34
jobbie is producing.



Not quite, Ht is 510, cathode is at 10v so at 67ma, it would be
500*.067=33.5 the spec for kt88's states the max anode dis is 40w, so
quite a way from that. The change of cathode resistor will make more of a
change than you are suggesting. increasing the value, will increase the
voltage on the cathode, and in that way slightly reduce the voltage across
the valve, BUT it will also reduce the current, so having more of a effect
of anode dis than you may be expecting.

Also, the powers being quoted here 34W and 13W are the power being
dissapated by the valve at idle, not directly related to the power output
of the amp. To find that, you need to draw a set of composite lines for
the valves at the paticulat DC bias point, and then plot the loadline of
the reflected load the speaker presents via the output transformer.

This is a good starting site for such composite load lines.

http://members.aol.com/sbench102/composit.html

Actually its a good starting site for many valve related things.

The fact that the El34 is running at 13w, just points that its a AB1 amp,
not a A1 amp, no big deal, and only sensible for El34's IMHO given the
market I suspect they are trying to get into.


OK, this is all very interesting. All I can say is that the EL34 amp is most
definitely louder into 84 dB speakers than the KT88 amp is into 86/87 db
speakers. In real terms the amp is *plenty* loud enough where it is and the
valves look *decidely* unruffled....???

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:



OK, this is all very interesting. All I can say is that the EL34 amp is most
definitely louder into 84 dB speakers than the KT88 amp is into 86/87 db
speakers. In real terms the amp is *plenty* loud enough where it is and the
valves look *decidely* unruffled....???



Not wanting to sound like I have gone over to the "other side" :-), but
connect your sig gen to the input and your scope to the output, and see
what voltage out causes them to clip.

Would be better for this if you had a 50w 8R load, but its only messing
about.

--
Nick.

Bias adjustment II - 'The Reckoning'
 

"Nick Gorham" wrote in message
...
Keith G wrote:



OK, this is all very interesting. All I can say is that the EL34 amp is
most definitely louder into 84 dB speakers than the KT88 amp is into
86/87 db speakers. In real terms the amp is *plenty* loud enough where it
is and the valves look *decidely* unruffled....???



Not wanting to sound like I have gone over to the "other side" :-), but
connect your sig gen to the input and your scope to the output, and see
what voltage out causes them to clip.

Would be better for this if you had a 50w 8R load, but its only messing
about.


OK, I'll wait until the second one turns up (tomorrow FFS ??????) - it's a
bugger to get to it where it is and I'll not disturb it.....

(I'll give you a shout and we can get it rigged up right. I've got some
'resisitor loads'....)

Bias adjustment II - 'The Reckoning'
 

Nick Gorham wrote:

Not quite, Ht is 510, cathode is at 10v so at 67ma, it would be
500*.067=33.5 ...

Ah, yes, it's a 10V section and a 500V section, each with the same current.
:-)


Also, the powers being quoted here 34W and 13W are the power being
dissapated by the valve at idle, not directly related to the power
output of the amp. To find that, you need to draw a set of composite
lines for the valves at the paticulat DC bias point, and then plot the
loadline of the reflected load the speaker presents via the output
transformer.

Could the volume difference be down to how each amp handles the speakers?


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment II - 'The Reckoning'
 

Keith G wrote:

... All I can say is that the EL34 amp
is most definitely louder into 84 dB speakers than the KT88 amp is
into 86/87 db speakers. In real terms the amp is *plenty* loud enough
where it is and the valves look *decidely* unruffled....???

Maybe you could compare each amp into one pair of speakers, and then into
the other pair.


--
Wally
www.artbywally.com
www.wally.myby.co.uk

Bias adjustment II - 'The Reckoning'
 

Wally wrote:
Nick Gorham wrote:


Not quite, Ht is 510, cathode is at 10v so at 67ma, it would be
500*.067=33.5 ...


Ah, yes, it's a 10V section and a 500V section, each with the same current.
:-)



Also, the powers being quoted here 34W and 13W are the power being
dissapated by the valve at idle, not directly related to the power
output of the amp. To find that, you need to draw a set of composite
lines for the valves at the paticulat DC bias point, and then plot the
loadline of the reflected load the speaker presents via the output
transformer.


Could the volume difference be down to how each amp handles the speakers?




Yes, one amp could have a OPTX wound for 8R and the other 4R for
example. I think its also the case that the Kit88 is a pure class A
design, whereas the el34 may well shift into B quite early on, the low
bias would certainly make me think thats the case.

--
Nick