Dynavox VR70-E
 

"Mike Gilmour" wrote in message
...

Iain, I would add here to ensure all bias pots are fully down before
applying B+ !!!
I also find adding a bit of anode current helps the warmup time prior to
setting the bias.

Hi Mike,

Yes. Good point. But how does one know which way round the bias
pots are connected? Fully anticlockwise may not be "fully down"
Sometimes they are wired as attenuators. But usually there is a series
resistor either side of the pot to prevent one from going to zero bias.

Also, with maximum bias, and the valves drawing no current,
the psu voltage could rise beyond the workimg voltage of the
reservoir capacitor, which could well be 500V wkg, in a budget
amp.

So perhaps a "quick and dirty centre setting" might be a better place to
start, and then quickly set each pot to its approximate setting. Then
after an hour or so, go back for tighter trimming.

I have a pal who reckons he can hear the sweet-spot and set the bias by
ear. His amp sounds good, but get gets through a lot of valves:-)
Not recommended:-)


Guitarist seem to like doing this..but hey they can generally afford the
replacements :-)

And claim them as expenses against tax too:-))

Iain

Dynavox VR70-E
 

"Mike Gilmour" wrote in message
...

"Mike Gilmour" wrote in message
...


Iain, I would add here to ensure all bias pots are fully down before
applying B+ !!!
I also find adding a bit of anode current helps the warmup time prior to
setting the bias.

I mentioned this because a friend of mine (not too clued up about valves)
actually put a new set of valves in with the bias set to provide him with
glowing anodes!!
its good to have some current through the valves during warm up as it
provides some load for B+


Yes, the cherry (cheery) anode brings a new dimension to the listening
experience. At least for a while:-)


:-)

Iain
---If it don't glow, it don't go---

Dynavox VR70-E
 

"Iain M Churches" wrote in message
...

"Mike Gilmour" wrote in message
...

Iain, I would add here to ensure all bias pots are fully down before
applying B+ !!!
I also find adding a bit of anode current helps the warmup time prior to
setting the bias.

Hi Mike,

Yes. Good point. But how does one know which way round the bias
pots are connected? Fully anticlockwise may not be "fully down"
Sometimes they are wired as attenuators. But usually there is a series
resistor either side of the pot to prevent one from going to zero bias.


Thats exactly why it didn't mention clockwise or anticlockwise... points to
trip-up the unwary or ppl lacking knowledge. Mind you that said, the
thought of the uniniated routing around valve amps fills me with dread -
there really should be a bright red government health warning label with
skull and crossbones on every one! It's not only valves that die!!



Also, with maximum bias, and the valves drawing no current,
the psu voltage could rise beyond the workimg voltage of the
reservoir capacitor, which could well be 500V wkg, in a budget
amp.


Yes, thats why I said to run some anode current through the valves prior to
setting bias as it helps the warm up and puts some load onto B+ as you say
with budget or poorly designed valve power amplifiers the excessive rise in
B+ under no load conditions.



So perhaps a "quick and dirty centre setting" might be a better place to
start, and then quickly set each pot to its approximate setting. Then
after an hour or so, go back for tighter trimming.


A guesstimate could be a bit risky, it depends a lot on the range/value of
the bias pots - some amps have a wide bias range to encompass changes of
output valve types so that a small rotational change could result in a large
bias change - that is the case on my Lumley's.


I have a pal who reckons he can hear the sweet-spot and set the bias
by
ear. His amp sounds good, but get gets through a lot of valves:-)
Not recommended:-)


Guitarist seem to like doing this..but hey they can generally afford the
replacements :-)

And claim them as expenses against tax too:-))

:-))


Iain

Dynavox VR70-E
 

"Nick Gorham" wrote

And draw a line up from the x axis (Anode voltage, and see where it
crosses the grid curve at the grid voltage, that will give the expected
current (look at the pentode mode one, as g2 is in use).


Just an acknowledgement to thank all for the responses and to say I'll be
scruting them closely later on....

Dynavox VR70-E
 

"Iain M Churches" wrote



Va
Plate (anode) voltage - measured anode to cathode.
which, in your case it is purely academic as the cathode is less than a
volt above ground.


OK


Vg2
Screen grid voltage


OK, but mystified as to why this appears to be 10V higher than the Anode
Voltage??




Vg1
Input grid DC voltage, i.e. the bias. At -36V this is mighty close to
the -35V I thought you might find:-)


OK, but see below....



There are actually three grids, but g3 (pin 1) is usually tied to
the cathode (pin 8) Take a peep:-)


Yes I gather that, therefore 0V...???




Ia
Anode current. i.e. the idling current flowing through the valve with
no audio present. This is the figure we are trying to achieve when we
set the bias.


OK, but see below



There are many sets of operating conditions for valves in push pull,
depending on the circuitry, fixed bias, cathode bias, triode connected,
UL etc etc,


Understood (but not understood yet).... ;-)



My guess would be that the bias can be set at 36mA (?), but what does
that
handwritten figure mean? It's obviously taken from measuring the valves.
All four valves have the same figure and are presumably 'matched'???

Yes. Manufacturers/builders usually match power valves in pairs, but one
does not always see a label attached:-)
That's good attention to detail.

On the other amp the figure on each of the valves is 27mA. Does this mean
there is a different bias figure for each amp?

Yes presumably.


OK, but see below....


Also, what is the best way to set the bias - mA or Volts?

Do you have a set of test points to measure the voltage across the 10 Ohms
cathode resistor, cathode to ground? If you have, then that is the way to
do it.
So set the bias to give you 310mV on your meter then you will have 31mA of
current flowing.


I ask as my meter can (and does) give me readings either way - V or mA - is
one way any 'better' than the other, using the test points and trimpots??


OK, thanks for all that - the questions remaining are as follows:

From what you say above the Vg1 figure on the label (labels printed the
same - ie both show Vg1: -36V) appears the bias value, yet you appear also
to say the Ia figure (Anode Current, which is different on each of the amps)
is also the bias figure or have I misunderstood?

So which is it -36V or the value on the labels? And if it's the value on the
labels will the two amps have different output levels?? :-)

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


Is the correct bias a 'valve thing' or an amplifier thing'? - ie designed
into the amp irrespective of the valves it is likely to be fitted with in
the future and which will almost certainly vary (within certain limits) ???

Apologies, once again, if these questions sound a bit thick!!

:-)

Dynavox VR70-E
 

"Keith G" wrote in message
...

"Iain M Churches" wrote


(huge snip)

There are actually three grids, but g3 (pin 1) is usually tied to
the cathode (pin 8) Take a peep:-)


Yes I gather that, therefore 0V...???

No. no quite. From the information you gave, your cathode has
a 10 Ohm resistor to ground, and a potential difference of 310mV

(another huge snip - info covered)

OK, thanks for all that - the questions remaining are as follows:

From what you say above the Vg1 figure on the label (labels printed the
same - ie both show Vg1: -36V) appears the bias value, yet you appear also
to say the Ia figure (Anode Current, which is different on each of the
amps) is also the bias figure or have I misunderstood?

Slightly misunderstood, or perhaps I did not word it clearly enough.
The bias is the negative voltage (-36VDC) which is applied to the grid, Vg1.
Ia (the anode current) is the current which flows in the valve when the
correct bias voltage has been applied.


So which is it -36V or the value on the labels? And if it's the value on
the labels will the two amps have different output levels?? :-)

When you set the bias by reading the voltage specified across
the cathode resistor, (310mV you stated IIRC ) you will then have
-36V at the grid and the correct and the correct idling current in
the valve.

Is the correct bias a 'valve thing' or an amplifier thing'? - ie designed
into the amp irrespective of the valves it is likely to be fitted with in
the future and which will almost certainly vary (within certain limits)

It's the operating condition for the valve running in that particular
circuit. When replacing valves in a push pull amp it is
wise to order matched pairs of valves for each channel,
or better still a matched quad for a stereo amp.

Cheers
Iain

Dynavox VR70-E
 

"Iain M Churches" wrote

Yes I gather that, therefore 0V...???

No. no quite. From the information you gave, your cathode has
a 10 Ohm resistor to ground, and a potential difference of 310mV


OK


Slightly misunderstood, or perhaps I did not word it clearly enough.
The bias is the negative voltage (-36VDC) which is applied to the grid,
Vg1.
Ia (the anode current) is the current which flows in the valve when the
correct bias voltage has been applied.


OK. Would I be right if I said the Ia is not measurable from the bias test
points then?



So which is it -36V or the value on the labels? And if it's the value on
the labels will the two amps have different output levels?? :-)

When you set the bias by reading the voltage specified across
the cathode resistor, (310mV you stated IIRC ) you will then have
-36V at the grid and the correct and the correct idling current in
the valve.


Is there any merit in setting the bias a lower or higher setting - valve
life/power output trade-off kinda thing?? If so, which way - higher or
lower?


Is the correct bias a 'valve thing' or an amplifier thing'? - ie designed
into the amp irrespective of the valves it is likely to be fitted with in
the future and which will almost certainly vary (within certain limits)

It's the operating condition for the valve running in that particular
circuit. When replacing valves in a push pull amp it is
wise to order matched pairs of valves for each channel,
or better still a matched quad for a stereo amp.


Understood. Many thanks for all that.

Dynavox VR70-E
 

"Keith G" wrote in message
...

"Iain M Churches" wrote

(snip for points clarified)

OK. Would I be right if I said the Ia is not measurable from the bias test
points then?

Well, yes and no. There are three parameters here, all inter-related. The
object of the -36V negative bias on the grid, is to set the operating
conditions
to give, in your case 31mA idling current.

The way we do this, is to measure the DC voltage across the cathode resistor
of 10 Ohms, and then adjust the bias pot until we read 310 mVDC across
the cathode resistor. When you see 310mVDC on your meter you know that
by Ohm's Law there is 31mA flowing throught the resistor.

V= I*R.
0.310= 0.031 * 10

Is there any merit in setting the bias a lower or higher setting - valve
life/power output trade-off kinda thing?? If so, which way - higher or
lower?

I wonder when we would come to that one:-)
Usually the manufacturer has done his homework on the operating
conditions under which the valves will run in a particular design.
I can see litle point in running the amplifier "lean" i.e. overbias with
lower idling current, and running it "rich", or "hot" will shorten the life
of the valves.


Iain

Dynavox VR70-E
 

"Keith G" wrote in message
...

"Iain M Churches" wrote

Keith:
Is the correct bias a 'valve thing' or an amplifier thing'? - ie
designed
into the amp irrespective of the valves it is likely to be fitted with
in
the future and which will almost certainly vary (within certain limits)


Iain:
It's the operating condition for the valve running in that particular
circuit. When replacing valves in a push pull amp it is
wise to order matched pairs of valves for each channel,
or better still a matched quad for a stereo amp.




Hi Keith.

A picture is worth a thousand words. So please download the
data sheet for the EL34, which you can find at:

http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf


You may find it worth while to print this out, and keep it for future
reference.
Now turn to page C1. Move along the bottom horizontal axis of the chart
to the point which represents Vg1 at -36V (that's the bias) and two squares
to the right of -40V Now draw a vertical line with a pencil and ruler from
the -36V point upwards until it intersects with the Va = 400V curve
(which IIRC is close to the operating conditions of your amp)
Then draw a horizontal line across from that point and read off the
Ia (mA) which is the idling current.

What figure do you get? just above 30mA?
We can calculate the power dissipated while the valve is idling.
W= V*I. = 400 * 0.030 =12W

Now imagine you set the bias to only -20V.
Draw a vertical line from that point
and mark where it intersects the Va=400V line.
Draw a horizontal line from that point across to
the right hand margin and read off the I(mA) current.

What figure do you get? 180mA ?
Now we can calculate the power dissipated for
this set of operating conditions
Now we are really cooking:-) Let's make the
calculations quickly before the valve melts:-)
W= V*I. =400 * 0.180 = 72W

Can you see how the parameters are connected?
Is it starting to make sense?


Iain

Dynavox VR70-E
 

"Iain M Churches" wrote


A picture is worth a thousand words. So please download the
data sheet for the EL34, which you can find at:

http://www.mif.pg.gda.pl/homepages/f...129/e/EL34.pdf


You may find it worth while to print this out, and keep it for future
reference.

Print it out??? It's 19 sodding pages - I haven't read that much writing
since halfway through school, when I gave it up as a bad job!!

:-)

(I've saved it for later for the odd, brief perusal...!!)


Now turn to page C1. Move along the bottom horizontal axis of the chart
to the point which represents Vg1 at -36V (that's the bias) and two
squares
to the right of -40V Now draw a vertical line with a pencil and ruler from
the -36V point upwards until it intersects with the Va = 400V curve
(which IIRC is close to the operating conditions of your amp)
Then draw a horizontal line across from that point and read off the
Ia (mA) which is the idling current.

What figure do you get? just above 30mA?
We can calculate the power dissipated while the valve is idling.
W= V*I. = 400 * 0.030 =12W

Now imagine you set the bias to only -20V.
Draw a vertical line from that point
and mark where it intersects the Va=400V line.
Draw a horizontal line from that point across to
the right hand margin and read off the I(mA) current.

What figure do you get? 180mA ?
Now we can calculate the power dissipated for
this set of operating conditions
Now we are really cooking:-) Let's make the
calculations quickly before the valve melts:-)
W= V*I. =400 * 0.180 = 72W

Can you see how the parameters are connected?
Is it starting to make sense?


Oh yeah, crystal clear now that you mention it......

cough, cough...

OK, I do see (roughly) what you are on about (or will, when the time comes
and it's needed - that's the way I work), but don't forget I'm only a
Fettler Grade II - which is only *slightly* higher than an 'analogue audio
design engineer'...... ;-)