Ping Don..
 

(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday), so perhaps you could rummage your 'valve
savvy' and assist me with a problem?

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in 90
degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad distortion
kicks in I'm told. Any immediate ideas where to look as to cause? I have
both amps here and can make comparison measurements - I'm told all the
passives measure OK...??

Here they are *again*

http://www.apah69.dsl.pipex.com/show...yAmpsAgain.JPG


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!

:-)

Ping Don..
 

"Keith G" wrote in message
...

(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday), so perhaps you could rummage your 'valve
savvy' and assist me with a problem?

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in
90 degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad
distortion kicks in I'm told. Any immediate ideas where to look as to
cause? I have both amps here and can make comparison measurements -
I'm told all the passives measure OK...??

Here they are *again*

http://www.apah69.dsl.pipex.com/show...yAmpsAgain.JPG


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!

:-)


Actually, it's even worse than that - I would like to rip all the 'bias
meter' palaver out of 'em and change them to cathode bias.

(No idea how to do that, needless to say!! :-)

Ping Don..
 

On Thu, 12 Jul 2007 14:42:35 +0100, "Keith G"
wrote:


(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday), so perhaps you could rummage your 'valve
savvy' and assist me with a problem?

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in 90
degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad distortion
kicks in I'm told. Any immediate ideas where to look as to cause? I have
both amps here and can make comparison measurements - I'm told all the
passives measure OK...??

Here they are *again*

http://www.apah69.dsl.pipex.com/show...yAmpsAgain.JPG


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!

:-)




Swapping those valves could easily have damaged something, because
they would have both been going flat out with nothing to limit the
anode current (neither grid would have been connected). Best I can
suggest is swapping valves between the two amps to see which one
carries the fault with it, then get a new one of those.

I haven't looked at the pinouts of the two to see what connects to
what when you swap them, but this is my best guess.

Passives damaged by too much current tend to show the evidence pretty
dramatically. But if it is a passive, measure the value of the cathode
resistor and make sure it hasn't gone low. Also measure the resistance
from grid to ground to check it isn't open circuit.

d

--
Pearce Consulting
http://www.pearce.uk.com

Ping Don..
 

Keith G wrote:
(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday),

Chance would be a fine thing. I never got your email Keith. Anyway....

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in 90
degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad distortion
kicks in I'm told. Any immediate ideas where to look as to cause? I have
both amps here and can make comparison measurements - I'm told all the
passives measure OK...??

Here they are *again*

http://www.apah69.dsl.pipex.com/show...yAmpsAgain.JPG


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!


Without knowing the circuit its a bit of a guess, but my first thought
would be to check if the coupling caps have become leaky. (assuming
there are any).

Check the voltage between the 300b grid and ground. I would expect
something like -70v. If the coupling cap is leaking it will be higher
than that.

Have you considered painting the valve bases and sockets so he may be
able to work out what goes where? Or faining that, strike hime on the
head with the nearest outout transformer and tell him to get a clue?

--
Nick

Ping Don..
 

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

(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday), so perhaps you could rummage your 'valve
savvy' and assist me with a problem?

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in
90 degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad
distortion kicks in I'm told. Any immediate ideas where to look as to
cause? I have both amps here and can make comparison measurements -
I'm told all the passives measure OK...??

Here they are *again*

http://www.apah69.dsl.pipex.com/show...yAmpsAgain.JPG


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!

:-)



Actually, it's even worse than that - I would like to rip all the 'bias
meter' palaver out of 'em and change them to cathode bias.

(No idea how to do that, needless to say!! :-)


Again, depends on the circuit, you may not have enough B+ to do that.

--
Nick

Ping Don..
 

"Nick Gorham" wrote

Again, depends on the circuit, you may not have enough B+ to do that.


OK - that's a *secondary objective* atm!

(The meters have been swapped and both seem to be OK in the 'good' amp!)

Ping Don..
 

"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 14:42:35 +0100, "Keith G"
wrote:

a sorry tale


Swapping those valves could easily have damaged something, because
they would have both been going flat out with nothing to limit the
anode current (neither grid would have been connected). Best I can
suggest is swapping valves between the two amps to see which one
carries the fault with it, then get a new one of those.


No, all that's been tried - all/any of the valves (and the meter) can be
swapped to the good amp willy nilly and it continues to work perfectly.



I haven't looked at the pinouts of the two to see what connects to
what when you swap them, but this is my best guess.

Passives damaged by too much current tend to show the evidence pretty
dramatically. But if it is a passive, measure the value of the cathode
resistor


Seems OK/same on both amps.


and make sure it hasn't gone low. Also measure the resistance
from grid to ground to check it isn't open circuit.


OK, with no power on (?):

Good amp = 100 kOhm

'Bad' amp = 40 kOHm

??

Ping Don..
 

"Nick Gorham" wrote in message
. uk...
Keith G wrote:
(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday),

Chance would be a fine thing. I never got your email Keith. Anyway....


Direct response received and replied to!



If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop
for the odious techie *Andrew* to sort them out and I don't think I
could stand the 'loss of face'...!!


Without knowing the circuit its a bit of a guess, but my first thought
would be to check if the coupling caps have become leaky. (assuming
there are any).


He has replaced the two 100 uF caps which I deem to be the coupling caps
with a single 220 uF cap on the 'bad' amp and it has been fine for a
while...



Check the voltage between the 300b grid and ground. I would expect
something like -70v. If the coupling cap is leaking it will be higher
than that.


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?



Have you considered painting the valve bases and sockets so he may be
able to work out what goes where? Or faining that, strike hime on the
head with the nearest outout transformer and tell him to get a clue?


Oh, he's learning!! - It doesn't help the bloody amps are *handed*!! :-)

They're both still on - are we doing further damage??

Ping Don..
 

"Keith G" wrote in message
...

"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 14:42:35 +0100, "Keith G"
wrote:

a sorry tale


Swapping those valves could easily have damaged something, because
they would have both been going flat out with nothing to limit the
anode current (neither grid would have been connected). Best I can
suggest is swapping valves between the two amps to see which one
carries the fault with it, then get a new one of those.


No, all that's been tried - all/any of the valves (and the meter) can
be swapped to the good amp willy nilly and it continues to work
perfectly.



I haven't looked at the pinouts of the two to see what connects to
what when you swap them, but this is my best guess.

Passives damaged by too much current tend to show the evidence pretty
dramatically. But if it is a passive, measure the value of the
cathode
resistor


Seems OK/same on both amps.


Additional:

*VDC* across the cathode resistors:

Good amp = 59VDC

Bad amp = 154 VDC

Does this add anything - certainly don't look right???

Ping Don..
 

"Keith G" wrote

Additional:

*VDC* across the cathode resistors:

Good amp = 59VDC

Bad amp = 154 VDC

Does this add anything - certainly don't look right???


Additional additional:

Anode voltages (amps are warm now)

Good boy = 438 VDC

Bad boy = 312 VDC

Both steady....

??

Ping Don..
 

"Keith G" wrote in message
...

"Keith G" wrote

Additional:

*VDC* across the cathode resistors:

Good amp = 59VDC

Bad amp = 154 VDC

Does this add anything - certainly don't look right???


Additional additional:

Anode voltages (amps are warm now)

Good boy = 438 VDC

Bad boy = 312 VDC

Both steady....

??


Another additional additional:

The 'bad' amp *stinks* - Swim says like a 'Chinese amp' but much worse!!
:-)

(Summat's not happy there, that's for sure!!)

Ping Don..
 

On Thu, 12 Jul 2007 15:44:49 +0100, "Keith G"
wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?

If you swap the output valves between amps, does the fault go with
them? That grid voltage should be pretty much tied to ground. This one
uses an interstage transformer, yes? Check the resistance of the
secondaries, and make sure it hasn't gone open circuit. That grid
voltage should only move with signal.

d

--
Pearce Consulting
http://www.pearce.uk.com

Ping Don..
 

"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 15:44:49 +0100, "Keith G"
wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?

If you swap the output valves between amps, does the fault go with
them?


No - all or any of the valves/rectifiers can be freely swapped to and
fro with no adverse effect, I'm told. Even the meters were swapped (at
my suggestion) and the bad amp stayed bad and the good amp continues to
work OK. (I will check this for myself a little later.)


That grid voltage should be pretty much tied to ground. This one
uses an interstage transformer, yes?


No. This is the circuit the amp is *supposed* to be based on:

http://www.apah69.dsl.pipex.com/show/300Bcircuit.gif


Check the resistance of the
secondaries, and make sure it hasn't gone open circuit. That grid
voltage should only move with signal.


Hmm....

What bothers me is that on my 2A3 amp there is only a single wire to
each of the heater pins (from the filament trannies, naturally) - on
this amp with the meters, there's a whole bunch of crap connected to
them and I'm lost to see how any adjustment can work which is ultimately
connected to the cathode/heater ???

These are the 300B connections:

http://www.apah69.dsl.pipex.com/show/GoodAmp.JPG

http://www.apah69.dsl.pipex.com/show/BadAmp.JPG

Ping Don..
 

On Thu, 12 Jul 2007 16:58:51 +0100, "Keith G"
wrote:


"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 15:44:49 +0100, "Keith G"
wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?

If you swap the output valves between amps, does the fault go with
them?


No - all or any of the valves/rectifiers can be freely swapped to and
fro with no adverse effect, I'm told. Even the meters were swapped (at
my suggestion) and the bad amp stayed bad and the good amp continues to
work OK. (I will check this for myself a little later.)


That grid voltage should be pretty much tied to ground. This one
uses an interstage transformer, yes?


No. This is the circuit the amp is *supposed* to be based on:

http://www.apah69.dsl.pipex.com/show/300Bcircuit.gif


OK, so the grid of the output valve is grounded through a pair of
resistors in series - total 101k. The bad amp is 40 odd k, so what
that says to me is that the coupling cap between the valves has gone
short. That means that the hundred and one k is now in parallel with
all those resistors in the power supply - a quick look at values says
they come out to about what you measured. So you need a new 0.33uF
from the anode of the 6SN7 to the grid of the 300B.

Turning it off in the meantime would be a really good idea.

d
--
Pearce Consulting
http://www.pearce.uk.com

Ping Don..
 

Keith G wrote:


OK - powered up with dummy loads.

Grid to Ground:

Good Amp - a *continually* changing 0.04 to 0.06 VDC

Bad Amp - steady 202 VDC (???)

Are we on the money so soon?



Looks likely.

220uf seems to be a odd value coupling cap, I would have expected 0.22uf
or so. Looking at the picture, I would have thoulf what looks like a big
PIO would be the chap.

BTW, if its 0v, and you have a cathode resistor, then it will be cahode
bias (I would expect)

Track down the coupling cap and remove/replace it. Any old 0.22uf cap
will do to try, as long as its got a voltage rating of 500v or more.

--
Nick

Ping Don..
 

"Nick Gorham" wrote

Looks likely.


OK, I have transferred Don's response here to cover both bases in one
reply.


220uf seems to be a odd value coupling cap, I would have expected
0.22uf or so. Looking at the picture, I would have thoulf what looks
like a big PIO would be the chap.


OK (see pix).



BTW, if its 0v, and you have a cathode resistor, then it will be
cahode bias (I would expect)


Except there are the trimpots for adjusting the bias...??



Track down the coupling cap and remove/replace it. Any old 0.22uf cap
will do to try, as long as its got a voltage rating of 500v or more.


From Don:

OK, so the grid of the output valve is grounded through a pair of
resistors in series - total 101k. The bad amp is 40 odd k, so what
that says to me is that the coupling cap between the valves has gone
short. That means that the hundred and one k is now in parallel with
all those resistors in the power supply - a quick look at values says
they come out to about what you measured. So you need a new 0.33uF
from the anode of the 6SN7 to the grid of the 300B.

Turning it off in the meantime would be a really good idea.


Did that! :-)

Right. I'm having a bit of a problem identifying the values you both
state as none of the caps seem to quite fit the bill valuewise. Are we
talking about the big Copper Foil PIO bugger he

http://www.apah69.dsl.pipex.com/show/L1000575.JPG

http://www.apah69.dsl.pipex.com/show/L1000576.JPG

http://www.apah69.dsl.pipex.com/show/L1000577.JPG


You both seem to be zoned in on it??

If so, it seems to be 1.0 mF/630V - nothing like 0.33uF as depicted in
the circuit (which I think can almost be disregarded) or what you
mention - unless that it is a valid substitute??

If that's the likely culprit, I would be tempted to rip the one out of
the good amp and try it - good idea or not??

(Thanks to both of you in the meantime - I really would like *not* to
see this amp go off to the shop!! :-)

Ping Don..
 

On 2007-07-12, Don Pearce wrote:
On Thu, 12 Jul 2007 16:58:51 +0100, "Keith G"
wrote:
... This is the circuit the amp is *supposed* to be based on:

http://www.apah69.dsl.pipex.com/show/300Bcircuit.gif

OK, so the grid of the output valve is grounded through a pair of
resistors in series - total 101k. The bad amp is 40 odd k, so what
that says to me is that the coupling cap between the valves has gone
short. That means that the hundred and one k is now in parallel with
all those resistors in the power supply - a quick look at values says
they come out to about what you measured. So you need a new 0.33uF
from the anode of the 6SN7 to the grid of the 300B.

Turning it off in the meantime would be a really good idea.

Seconded. The anode current in the bad amp looks like it may be
*significantly* more than the 100 mA maximum (if I read correctly
a 300B datasheet I found online).

154V / 825 Ohms = 187 mA (if the cathode resistors are actually as
in the circuit diagram at the link).

--
John Phillips

Ping Don..
 

On Thu, 12 Jul 2007 17:47:02 +0100, "Keith G"
wrote:


Right. I'm having a bit of a problem identifying the values you both
state as none of the caps seem to quite fit the bill valuewise. Are we
talking about the big Copper Foil PIO bugger he

http://www.apah69.dsl.pipex.com/show/L1000575.JPG

http://www.apah69.dsl.pipex.com/show/L1000576.JPG

http://www.apah69.dsl.pipex.com/show/L1000577.JPG


You both seem to be zoned in on it??

If so, it seems to be 1.0 mF/630V - nothing like 0.33uF as depicted in
the circuit (which I think can almost be disregarded) or what you
mention - unless that it is a valid substitute??

If that's the likely culprit, I would be tempted to rip the one out of
the good amp and try it - good idea or not??

Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.

You won't get any signal through, of course.

d

--
Pearce Consulting
http://www.pearce.uk.com

Ping Don..
 

"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 17:47:02 +0100, "Keith G"
wrote:


Right. I'm having a bit of a problem identifying the values you both
state as none of the caps seem to quite fit the bill valuewise. Are we
talking about the big Copper Foil PIO bugger he

http://www.apah69.dsl.pipex.com/show/L1000575.JPG

http://www.apah69.dsl.pipex.com/show/L1000576.JPG

http://www.apah69.dsl.pipex.com/show/L1000577.JPG


You both seem to be zoned in on it??

If so, it seems to be 1.0 mF/630V - nothing like 0.33uF as depicted in
the circuit (which I think can almost be disregarded) or what you
mention - unless that it is a valid substitute??

If that's the likely culprit, I would be tempted to rip the one out of
the good amp and try it - good idea or not??

Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.


OK. Sounds like a good ploy. I've had to clear the decks for tea and
have got some errands later, so it might be a while 'til I get back on
it! (The trouble with working on the *kitchen table*!! :-)



You won't get any signal through, of course.


Don't matter - my dummy loads are **** transducers anyway!! :-)

Mora anon....

Ping Don..
 

On Thu, 12 Jul 2007 18:10:52 +0100, "Keith G"
wrote:


"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 17:47:02 +0100, "Keith G"
wrote:


Right. I'm having a bit of a problem identifying the values you both
state as none of the caps seem to quite fit the bill valuewise. Are we
talking about the big Copper Foil PIO bugger he

http://www.apah69.dsl.pipex.com/show/L1000575.JPG

http://www.apah69.dsl.pipex.com/show/L1000576.JPG

http://www.apah69.dsl.pipex.com/show/L1000577.JPG


You both seem to be zoned in on it??

If so, it seems to be 1.0 mF/630V - nothing like 0.33uF as depicted in
the circuit (which I think can almost be disregarded) or what you
mention - unless that it is a valid substitute??

If that's the likely culprit, I would be tempted to rip the one out of
the good amp and try it - good idea or not??

Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.


OK. Sounds like a good ploy. I've had to clear the decks for tea and
have got some errands later, so it might be a while 'til I get back on
it! (The trouble with working on the *kitchen table*!! :-)



You won't get any signal through, of course.


Don't matter - my dummy loads are **** transducers anyway!! :-)

Mora anon....


No idea why that cap is so big. In combination with the 100k grid bias
resistor it comes out with a lowest operating frequency of about 1Hz.
A cap of about 0.1uF would be plenty big enough.

d

--
Pearce Consulting
http://www.pearce.uk.com

Ping Don..
 

"Don Pearce" wrote


No idea why that cap is so big. In combination with the 100k grid bias
resistor it comes out with a lowest operating frequency of about 1Hz.
A cap of about 0.1uF would be plenty big enough.


Well, I've got the owner's blessing to butcher the amps as I (and
you/Nick) see fit, so tomorrow will be interesting.

(I fully expect him to come round anyway - he works Saturdays at the
local 'Bike shop and has Fridays off!!)

Ping Don..
 

Keith G wrote:
"Don Pearce" wrote



No idea why that cap is so big. In combination with the 100k grid bias
resistor it comes out with a lowest operating frequency of about 1Hz.
A cap of about 0.1uF would be plenty big enough.



Well, I've got the owner's blessing to butcher the amps as I (and
you/Nick) see fit, so tomorrow will be interesting.

(I fully expect him to come round anyway - he works Saturdays at the
local 'Bike shop and has Fridays off!!)





Well, the cynic in me would wonder:

Audio Note 1uf 600v (copper) PIO = $69.60
Audio Note 0.1uf 600v (copper) PIO = $41.69

Though without starting a flame war, I would have suggested

Mundorf M-CAP® Supreme Silver & Oil 0.1uf 1200v = $23.45

But you could try

BC Components 2222 368 Series 100nf 400v = 13p

--
Nick

Ping Don..
 

"Don Pearce" wrote


Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.

You won't get any signal through, of course.


OK, did that and the bias meter behaved exactly the same - climbed to
the endstop after a few moments warm-up??

(I lifted the capacitor connection from the grid pin - presumably
correct?)

Ping Don..
 

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



No idea why that cap is so big. In combination with the 100k grid
bias
resistor it comes out with a lowest operating frequency of about 1Hz.
A cap of about 0.1uF would be plenty big enough.


Well, the cynic in me would wonder:

Audio Note 1uf 600v (copper) PIO = $69.60
Audio Note 0.1uf 600v (copper) PIO = $41.69

Though without starting a flame war, I would have suggested

Mundorf M-CAP® Supreme Silver & Oil 0.1uf 1200v = $23.45

But you could try

BC Components 2222 368 Series 100nf 400v = 13p


Thanks for the suggestions but the cap hasn't been proved to be faulty
yey - from what I can see of it?

(My suspicions still hover round all the extra crap between the 300B and
the meter itself..??)

Ping Don..
 

Keith G wrote:
"Don Pearce" wrote



Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.

You won't get any signal through, of course.



OK, did that and the bias meter behaved exactly the same - climbed to
the endstop after a few moments warm-up??

(I lifted the capacitor connection from the grid pin - presumably
correct?)






Yes, assuming there is still the other connections to the grid pin.

Ok, so is the voltage on the grid still high?

It would help having a clue how the amp was wired, I am not sure just
how having adjustable bias fits with having a cathode resistor (ok it
can be a mix of both). And if it soes have adjustable bias I would not
have expected the grid to be a 0v.

Maybe a close up picture of the amp would allow us to see how the thing
is wired.

--
Nick

Ping Don..
 

"Nick Gorham" wrote in message
. uk...
Keith G wrote:
"Don Pearce" wrote



Don't do that yet. First just disconnect one end of that cap and see
if the bias currents return to normal. If they do, then the cap is
certainly bad.

You won't get any signal through, of course.



OK, did that and the bias meter behaved exactly the same - climbed to
the endstop after a few moments warm-up??

(I lifted the capacitor connection from the grid pin - presumably
correct?)






Yes, assuming there is still the other connections to the grid pin.


No, this is what's mystifying me - there's only the cap connected to the
grid pin. I don't see how all the crap connected to the heater pins can
alter/affect the grid voltage? (See pix below...)


Ok, so is the voltage on the grid still high?


No, only 1.8V or thereabouts (not steady).



It would help having a clue how the amp was wired, I am not sure just
how having adjustable bias fits with having a cathode resistor (ok it
can be a mix of both). And if it soes have adjustable bias I would not
have expected the grid to be a 0v.

Maybe a close up picture of the amp would allow us to see how the
thing is wired.


Do these help:

http://www.apah69.dsl.pipex.com/show/L1000575.JPG

http://www.apah69.dsl.pipex.com/show/L1000576.JPG

http://www.apah69.dsl.pipex.com/show/L1000577.JPG

??

Ping Don..
 

Keith G wrote:


Yes, assuming there is still the other connections to the grid pin.



No, this is what's mystifying me - there's only the cap connected to the
grid pin. I don't see how all the crap connected to the heater pins can
alter/affect the grid voltage? (See pix below...)


Ok, so is the voltage on the grid still high?



No, only 1.8V or thereabouts (not steady).



Ok, the grid will be floating, so the voltage you are reading will be
from grid current.


Do these help:


Yes, I guess you have disconnected the 100k resistor thats connected to
the cap. Either connect this resistor directly to the grid, or reconnect
the cap, and disconnect the other end of the cap, so the grid resistor
is still there to pull the grid down to ground.

Looking at that, it is using cathode bias, its just got a meter across
the cathode resistor.

That resistor looks like a vishay bulk film, seems a tad excessive for a
grid resistor. Ho, well, keeps in with using a big cap when a smaller
one would do as well.

--
Nick

Ping Don..
 

"Nick Gorham" wrote


Yes, I guess you have disconnected the 100k resistor thats connected
to the cap.

No, I hadn't...


Either connect this resistor directly to the grid, or reconnect
the cap, and disconnect the other end of the cap, so the grid resistor
is still there to pull the grid down to ground.


But disconnecting the *other* end of the cap has put the bias meter
*exactly right*!!!

:-)

Is that it then - the cap's fuct?



Looking at that, it is using cathode bias, its just got a meter across
the cathode resistor.


I would still like the meters out of the way, but not if they are going
to behave - that's a decision for someone else at another time!!


That resistor looks like a vishay bulk film, seems a tad excessive for
a grid resistor. Ho, well, keeps in with using a big cap when a
smaller one would do as well.


It's suppose to be 'PR's (you know who) best work...??

Ping Don..
 

Keith G wrote:
"Nick Gorham" wrote



Yes, I guess you have disconnected the 100k resistor thats connected
to the cap.


No, I hadn't...


Either connect this resistor directly to the grid, or reconnect

the cap, and disconnect the other end of the cap, so the grid resistor
is still there to pull the grid down to ground.



But disconnecting the *other* end of the cap has put the bias meter
*exactly right*!!!

:-)

Is that it then - the cap's fuct?

My money is on that.

--
Nick

Ping Don..
 

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


Is that it then - the cap's fuct?

My money is on that.


Well, your money is safe - I swapped the cap out of the Good Amp and put
it in the Bad Amp and the Bad Boy is singing like a good 'un as I type
and...

.....rushes to check again....

the meter is (dare I say it?) - *rock steady*!!

:-)

Well done Nick and many thanks (you too, Don) - with your brain and my
soldering iron and we could conquer the world!!

Of course, the Good Amp's pretty silent now - what we want now is a
*pair of caps*. What do you suggest - same rating? (Forget *bling* -
just gimme the specs and I should think Tony'll get summat from
Maplins!! :-)


Checks again - yep - still right on the same mark on the meter!!

Ping Don..
 

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

Keith G wrote:



Is that it then - the cap's fuct?

My money is on that.



Well, your money is safe - I swapped the cap out of the Good Amp and put
it in the Bad Amp and the Bad Boy is singing like a good 'un as I type
and...

....rushes to check again....

the meter is (dare I say it?) - *rock steady*!!

:-)

Well done Nick and many thanks (you too, Don) - with your brain and my
soldering iron and we could conquer the world!!

Of course, the Good Amp's pretty silent now - what we want now is a
*pair of caps*. What do you suggest - same rating? (Forget *bling* -
just gimme the specs and I should think Tony'll get summat from
Maplins!! :-)


Checks again - yep - still right on the same mark on the meter!!






Well, from Maplins, I would think VM88V would do. 220nf 630v. (its
marked as web only on their site).

Any poly cap of .22uf or bigger, or lower, down to 0.1u should be fine
as Don said. Check the B+ and select a voltage rateing more than that.

Or maybe a

SONIQS SAX 0.22uF Ref: SAX0u22

From the WD site, they deliver quickly.

--
Nick

Ping Don..
 

"Nick Gorham" wrote

Well, from Maplins, I would think VM88V would do. 220nf 630v. (its
marked as web only on their site).

Any poly cap of .22uf or bigger, or lower, down to 0.1u should be fine
as Don said. Check the B+ and select a voltage rateing more than that.


OK, the amp's still good and steady. I have spoken to Tony and told him
to try and get a pair of '1mF' (is that the same as 1uF?) minimum 500V -
the Anode Voltage on the Good Amp was about 438, IIRC..??



Or maybe a

SONIQS SAX 0.22uF Ref: SAX0u22

From the WD site, they deliver quickly.


OK.

Ping Don..
 

Keith G wrote:
"Nick Gorham" wrote


Well, from Maplins, I would think VM88V would do. 220nf 630v. (its
marked as web only on their site).

Any poly cap of .22uf or bigger, or lower, down to 0.1u should be fine
as Don said. Check the B+ and select a voltage rateing more than that.



OK, the amp's still good and steady. I have spoken to Tony and told him
to try and get a pair of '1mF' (is that the same as 1uF?) minimum 500V -
the Anode Voltage on the Good Amp was about 438, IIRC..??


Err, not mF is 10^-3F, uF is 10^-6F, so 1mf = 1000uf. In any case 1uf
will work but is far bigger than it needs to be, but whatever.

It will be one hell of a output transformer that will allow the
difference between a 1uf and 0.22uf cap to be heard in that situation.


--
Nick

Ping Don..
 

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


Well, from Maplins, I would think VM88V would do. 220nf 630v. (its
marked as web only on their site).

Any poly cap of .22uf or bigger, or lower, down to 0.1u should be
fine as Don said. Check the B+ and select a voltage rateing more than
that.



OK, the amp's still good and steady. I have spoken to Tony and told
him to try and get a pair of '1mF' (is that the same as 1uF?) minimum
500V - the Anode Voltage on the Good Amp was about 438, IIRC..??


Err, not mF is 10^-3F, uF is 10^-6F, so 1mf = 1000uf. In any case 1uf
will work but is far bigger than it needs to be, but whatever.


Here's a close-up of the actual:

http://www.apah69.dsl.pipex.com/show/L1000577.JPG


It will be one hell of a output transformer that will allow the
difference between a 1uf and 0.22uf cap to be heard in that situation.


OK.

Ray's been here and has gone home to see what he's got in his bin - I'd
love 'em out the way now!!

Ping Don..
 

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

Keith G wrote:

"Nick Gorham" wrote



Well, from Maplins, I would think VM88V would do. 220nf 630v. (its
marked as web only on their site).

Any poly cap of .22uf or bigger, or lower, down to 0.1u should be
fine as Don said. Check the B+ and select a voltage rateing more than
that.



OK, the amp's still good and steady. I have spoken to Tony and told
him to try and get a pair of '1mF' (is that the same as 1uF?) minimum
500V - the Anode Voltage on the Good Amp was about 438, IIRC..??


Err, not mF is 10^-3F, uF is 10^-6F, so 1mf = 1000uf. In any case 1uf
will work but is far bigger than it needs to be, but whatever.



Here's a close-up of the actual:

http://www.apah69.dsl.pipex.com/show/L1000577.JPG



sarcasmWell, there you go then, it must be the same, I can't imagine
Audio Note making such a mistake /sarcasm

Well, last time I looked

m = milli = 10^-3
u = micro = 10^-6
n = nano = 10^-9

Maybe they didn't have a greek font, so used the spare m off the from t
of micro,

--
Nick

Ping Don..
 

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

Keith G wrote:

"Nick Gorham" wrote



Well, from Maplins, I would think VM88V would do. 220nf 630v. (its
marked as web only on their site).

Any poly cap of .22uf or bigger, or lower, down to 0.1u should be
fine as Don said. Check the B+ and select a voltage rateing more
than that.



OK, the amp's still good and steady. I have spoken to Tony and told
him to try and get a pair of '1mF' (is that the same as 1uF?)
minimum 500V - the Anode Voltage on the Good Amp was about 438,
IIRC..??


Err, not mF is 10^-3F, uF is 10^-6F, so 1mf = 1000uf. In any case 1uf
will work but is far bigger than it needs to be, but whatever.



Here's a close-up of the actual:

http://www.apah69.dsl.pipex.com/show/L1000577.JPG



sarcasmWell, there you go then, it must be the same, I can't imagine
Audio Note making such a mistake /sarcasm

Well, last time I looked

m = milli = 10^-3
u = micro = 10^-6
n = nano = 10^-9

Maybe they didn't have a greek font, so used the spare m off the from
t of micro,


I hafta say the m = u confusion didn't help, but we (Ray) have evolved
that any cap from 0.47 - 1.5 uF/min 450V should be OK - foil, poly or
whatever?