Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83
valve complement) which seems to be misbehaving...
OK so I'm using it as a guitar amp as well but that shouldn't be much of a
bother.
I've noticed that when playing music through it, the anode voltage seems to
drop faster than I'd expect. For guitar, where I'd be deliberatly trying to
get the amp to distort nicely (there is such a thing,,,) as soon as the
anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd
phasing noises and this is in the early stages of break-up.
To give you an idea, I can get an in-between clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory.
I know this isn't the "right" newsgroup for guitar amp stuff but I must say
that my recent experiences here really do make it a much better place to
post this than alt.guitar.amps.
Thanks guys.
cb

"Chris Berry" wrote in message
...
Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83
valve complement) which seems to be misbehaving...
OK so I'm using it as a guitar amp as well but that shouldn't be much of a
bother.
I've noticed that when playing music through it, the anode voltage seems
to
drop faster than I'd expect. For guitar, where I'd be deliberatly trying
to
get the amp to distort nicely (there is such a thing,,,) as soon as the
anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd
phasing noises and this is in the early stages of break-up.
To give you an idea, I can get an in-between clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory.
I know this isn't the "right" newsgroup for guitar amp stuff but I must
say
that my recent experiences here really do make it a much better place to
post this than alt.guitar.amps.
Thanks guys.
cb

Did I miss something?
cb

Chris,

I was reading your post when my 2 year old started wailing on the keyboard.
:-) Sent an empty post.

I suspect maybe a PS issue, caps maybe need attention.

Vin


"Chris Berry" wrote in message
...
Did I miss something?
cb

try another GZ34 ,but before that-try with different electrolytic cap(s)
after GZ34.

--
--
--
.................................................. ........................
Choky
Prodanovic Aleksandar
YU

"don't use force, "don't use force,
use a larger hammer" use a larger tube
- Choky and IST"
- ZM
.................................................. ...........................
"Chris Berry" wrote in message
...
Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83
valve complement) which seems to be misbehaving...
OK so I'm using it as a guitar amp as well but that shouldn't be much of a
bother.
I've noticed that when playing music through it, the anode voltage seems
to
drop faster than I'd expect. For guitar, where I'd be deliberatly trying
to
get the amp to distort nicely (there is such a thing,,,) as soon as the
anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd
phasing noises and this is in the early stages of break-up.
To give you an idea, I can get an in-between clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory.
I know this isn't the "right" newsgroup for guitar amp stuff but I must
say
that my recent experiences here really do make it a much better place to
post this than alt.guitar.amps.
Thanks guys.
cb

"JVC" wrote in message
...
Chris,

I was reading your post when my 2 year old started wailing on the
keyboard.
:-) Sent an empty post.

I suspect maybe a PS issue, caps maybe need attention.

Vin

That's put a smile on my face.
Could well be. I'm investigating further tomorrow.
Thanks.
cb

"Choky" wrote in message ...
try another GZ34 ,but before that-try with different electrolytic cap(s)
after GZ34.

Just tried a JJ GZ34 - same thing.
It's a buzzing sound, a bit like when you switch off an amp and still have a
signal going in.
You all reckon it's power supply related then as I suspect?
cb

Chris Berry wrote:
Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83
valve complement) which seems to be misbehaving...
OK so I'm using it as a guitar amp as well but that shouldn't be much of a
bother.
I've noticed that when playing music through it, the anode voltage seems to
drop faster than I'd expect. For guitar, where I'd be deliberatly trying to
get the amp to distort nicely (there is such a thing,,,) as soon as the
anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd
phasing noises and this is in the early stages of break-up.
To give you an idea, I can get an in-between clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory.
I know this isn't the "right" newsgroup for guitar amp stuff but I must say
that my recent experiences here really do make it a much better place to
post this than alt.guitar.amps.
Thanks guys.
cb

My first thought is that this amp may have never been "meant" to go into
signal excursions beyond the clipping point. It very possibly has a
negative feedback loop to help keep distortion low during normal
operation in the linear range.

Difficulty with such NFB loops, when they do break up they can do so
quite unpredictibly. Try to find a schematic of your amp, and see if
there is a resistor and/or capacitor connected from the speaker output
back to an earlier point in the circuit (often the cathode of a preamp
tube). Disconnect this feedback network, and see what it does to your
sound. It should also significantly increase the amplifier's overall
gain, which might be a bonus also.

If you can't decide which you like better, add a switch and label it
something funky like "cozmik overdrive". ;-)

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

Chris,

Blocking distortion, most likely, that amp probably isn't quite comfortable
running WOT.

Vin


"Chris Berry" wrote in message
...

"Choky" wrote in message
...
try another GZ34 ,but before that-try with different electrolytic cap(s)
after GZ34.

Just tried a JJ GZ34 - same thing.
It's a buzzing sound, a bit like when you switch off an amp and still have
a
signal going in.
You all reckon it's power supply related then as I suspect?
cb

Chris Berry wrote:

Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83
valve complement) which seems to be misbehaving...
OK so I'm using it as a guitar amp as well but that shouldn't be much of a
bother.
I've noticed that when playing music through it, the anode voltage seems to
drop faster than I'd expect. For guitar, where I'd be deliberatly trying to
get the amp to distort nicely (there is such a thing,,,) as soon as the
anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd
phasing noises and this is in the early stages of break-up.
To give you an idea, I can get an in-between clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory.
I know this isn't the "right" newsgroup for guitar amp stuff but I must say
that my recent experiences here really do make it a much better place to
post this than alt.guitar.amps.
Thanks guys.
cb

I would have no idea what "To give you an idea, I can get an in-between
clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory." means.

However, when the amp is grossly over driven, it isn't unusual to get a drop of
B+
of 10%.
Most amps when used without any overdrive or clipping will maintain their
operating voltages.

If the amp isn't a true guitar amp, and is a hi-fi amp, and has negative
feedback,
and high value coupling caps, then perhaps what you hear is paralysis.
This is caused by the coupling caps chraging up due to the grid current flow,
and the direct voltage across the caps is like over biasing of the output
tubes,
and they make the amp act like a class C amp.
The only way to prevent this in an RC coupled amp is to minimise the size of
the
coupling caps.
In a hi-fi amp, the output coupling caps are ususally at least 0.1 uF,
with grid bias resistors of 220k or higher.
Guitar amps often use 0.022uF and 100k which is a faster time constant, so that

discharge of charge build up in the caps is much faster, and follows the fading
note better.
The is much less feedback in a guitar amp. Feedback makes paralysis occur much
more severly,
so reduce the amount of fedback for musical instrument use.

Once the hi-fi amp has its feedback reduced, and the coupling caps reduced,
the amp is then the equal of most guitar amps.
The F response may be slightly reduced, but still be quite adequate for
guitar.

The changes of coupling caps *must* be accompanied by feedback
reduction because the amp may become quite unstable, ie, oscillate
at some LF tone if the feedback is left as is.

Many hi-fi amps have 20 dB of feedback.
Only a few guitar amps have no feedback at all, such as a Vox AC30,
but quite a few have 10 dB, just enough to reduce the
effective output resistance a bit.

When making changes to feedback and coupling caps, the amp should still be
stable with no load connected; if it is, then it will be stable with a speaker
connected.

Patrick Turner.

One thing to try is to (temporarily) add add'l filter capacitance. There is
a limit to the capacitance connected directly to the anode of a GZ34 but you
can add add'l storage downstream if the amp has a filter choke or a pi
network.

Or, to continue a prior discussion, you can add a Weber Copper Cap and you
shouldn't get much sag plus you can add a fair amount of capacitance right
after the rectifier.

It may be that your old caps have dried out a bit.

I had an Eico St-70 that did wild wired stuff under load (B+ dropped a lot
& there was a lot of distortion). Adding 30 MFD solved the problem.
Permanently.

Jon

From: "JVC"
Newsgroups: rec.audio.tubes
Date: Thu, 15 Apr 2004 00:20:25 GMT
Subject: Anode voltage sag.

Chris,

Blocking distortion, most likely, that amp probably isn't quite comfortable
running WOT.

Vin


"Chris Berry" wrote in message
...

"Choky" wrote in message
...
try another GZ34 ,but before that-try with different electrolytic cap(s)
after GZ34.

Just tried a JJ GZ34 - same thing.
It's a buzzing sound, a bit like when you switch off an amp and still have
a
signal going in.
You all reckon it's power supply related then as I suspect?
cb

The tone you want may not be available on that amp--guitar amps create the
'signature' sound through distortion laden circuits. Your Dyna amp is too
clean. As for the voltage drop, you are probably just driving it hard and
the power supply can't take the abuse. You could add a constant voltage or
current source circuit (http://www.bottlehead.com/et/parts/parts.htm) to
solve your problem, or beef up the power supply. Either one will make the
sound cleaner and work against the blues sound a little, but you could solve
that problem with a little distorted NFB a'la Fender design..


"Chris Berry" wrote in message
...
Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83
valve complement) which seems to be misbehaving...
OK so I'm using it as a guitar amp as well but that shouldn't be much of a
bother.
I've noticed that when playing music through it, the anode voltage seems
to
drop faster than I'd expect. For guitar, where I'd be deliberatly trying
to
get the amp to distort nicely (there is such a thing,,,) as soon as the
anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd
phasing noises and this is in the early stages of break-up.
To give you an idea, I can get an in-between clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory.
I know this isn't the "right" newsgroup for guitar amp stuff but I must
say
that my recent experiences here really do make it a much better place to
post this than alt.guitar.amps.
Thanks guys.
cb

"Jon Yaeger" wrote in message
...
One thing to try is to (temporarily) add add'l filter capacitance. There
is
a limit to the capacitance connected directly to the anode of a GZ34 but
you
can add add'l storage downstream if the amp has a filter choke or a pi
network.

Or, to continue a prior discussion, you can add a Weber Copper Cap and you
shouldn't get much sag plus you can add a fair amount of capacitance right
after the rectifier.

It may be that your old caps have dried out a bit.

I had an Eico St-70 that did wild wired stuff under load (B+ dropped a
lot
& there was a lot of distortion). Adding 30 MFD solved the problem.
Permanently.

It's more likely to be the capacitors being a bit dry than them needing a
bigger value. There's already 47+47 in there.
I think I'll also test it with diodes to see if with a better supply this is
greatly improved.
The webber would simply take too long to get here for my liking although it
is a possibility in the long run.
Thanks.
cb

"Fred Nachbaur" wrote in message
news:Kckfc.18357$mn3.17840@clgrps13...
My first thought is that this amp may have never been "meant" to go into
signal excursions beyond the clipping point. It very possibly has a
negative feedback loop to help keep distortion low during normal
operation in the linear range.

Here's the preamp circuit:

http://www.albertkreuzer.com/pics/el...nt_pre_sch.gif

and the power amp:
http://www.albertkreuzer.com/pics/el...nt_pwr_sch.gif

I don't know why but I didn't think of that... You're right!. I think I'll
clip the NFB resistor and put a 1Meg log pot in series with it and see how
that helps.


Difficulty with such NFB loops, when they do break up they can do so
quite unpredictibly. Try to find a schematic of your amp, and see if
there is a resistor and/or capacitor connected from the speaker output
back to an earlier point in the circuit (often the cathode of a preamp
tube). Disconnect this feedback network, and see what it does to your
sound. It should also significantly increase the amplifier's overall
gain, which might be a bonus also.

Thanks Fred. I'll try it out and tell you how it fares.


If you can't decide which you like better, add a switch and label it
something funky like "cozmik overdrive". ;-)

Or with a pot just label it Tight/loose with "Good Girl" in the middle....

cb

After looking at the poweramp scheme, I'd say, since your using this for a
guitar amp delete the 1meg resistor on the inst input and connect it to the
top of the 500k log pot or even straight to the tube input. I'd put a 1k -
5k resistor right at the tubes input or as close as possible either way.

- Dyslexics of America Untie! keithw...


"Chris Berry" wrote in message
...

"Fred Nachbaur" wrote in message
news:Kckfc.18357$mn3.17840@clgrps13...
My first thought is that this amp may have never been "meant" to go into
signal excursions beyond the clipping point. It very possibly has a
negative feedback loop to help keep distortion low during normal
operation in the linear range.

Here's the preamp circuit:

http://www.albertkreuzer.com/pics/el...nt_pre_sch.gif

and the power amp:
http://www.albertkreuzer.com/pics/el...nt_pwr_sch.gif

I don't know why but I didn't think of that... You're right!. I think I'll
clip the NFB resistor and put a 1Meg log pot in series with it and see how
that helps.


Difficulty with such NFB loops, when they do break up they can do so
quite unpredictibly. Try to find a schematic of your amp, and see if
there is a resistor and/or capacitor connected from the speaker output
back to an earlier point in the circuit (often the cathode of a preamp
tube). Disconnect this feedback network, and see what it does to your
sound. It should also significantly increase the amplifier's overall
gain, which might be a bonus also.

Thanks Fred. I'll try it out and tell you how it fares.


If you can't decide which you like better, add a switch and label it
something funky like "cozmik overdrive". ;-)

Or with a pot just label it Tight/loose with "Good Girl" in the middle....

cb

"dcoff" wrote in message
...
The tone you want may not be available on that amp--guitar amps create the
'signature' sound through distortion laden circuits. Your Dyna amp is too
clean.

Supposedly clean... There's masses of gain there - it starts overdriving on
1 - 1.5....
Just removed the NFB and it's the same phasing sound - most probably power
supply related.
Will take a closer look this afternoon.


As for the voltage drop, you are probably just driving it hard and
the power supply can't take the abuse. You could add a constant voltage
or
current source circuit (http://www.bottlehead.com/et/parts/parts.htm) to
solve your problem, or beef up the power supply. Either one will make the
sound cleaner and work against the blues sound a little, but you could
solve
that problem with a little distorted NFB a'la Fender design..

I'm guessing but those EL34's are being pushed quite hard indeed - certainly
more than the 40W the manufacturer states for this amp but that's the clean
rating.
Luckily, I'm using a hot plate (output attenuator) to get this moving though
and won't have dead ears at these volumes.
With no NFB it's sounding a little better. I think the problem might not be
leaky capacitors after all but that the capacitor is too large!
This would take longer to charge and hence cause my phasing effect.
cb

"Keithw" wrote in message
...
After looking at the poweramp scheme, I'd say, since your using this for a
guitar amp delete the 1meg resistor on the inst input and connect it to
the
top of the 500k log pot or even straight to the tube input. I'd put a 1k -
5k resistor right at the tubes input or as close as possible either way.

- Dyslexics of America Untie! keithw...

Thanks Keith.
That won't affect the phasing problem so when I get round to improving the
preamp, I'll take that into consideration.
cb

"Chris Berry" wrote in message
...

"Fred Nachbaur" wrote in message
news:Kckfc.18357$mn3.17840@clgrps13...
My first thought is that this amp may have never been "meant" to go into
signal excursions beyond the clipping point. It very possibly has a
negative feedback loop to help keep distortion low during normal
operation in the linear range.

Here's the preamp circuit:

http://www.albertkreuzer.com/pics/el...nt_pre_sch.gif

and the power amp:
http://www.albertkreuzer.com/pics/el...nt_pwr_sch.gif


Marvelously simple design !
If you use 'INST' input, just try a 1nf cap in lieu of the 10nf at the
input.
Of course, the tone stack is not what is usual for guitar.


I don't know why but I didn't think of that... You're right!. I think I'll
clip the NFB resistor and put a 1Meg log pot in series with it and see how
that helps.

Difficulty with such NFB loops, when they do break up they can do so
quite unpredictibly. Try to find a schematic of your amp, and see if
there is a resistor and/or capacitor connected from the speaker output
back to an earlier point in the circuit (often the cathode of a preamp
tube). Disconnect this feedback network, and see what it does to your
sound. It should also significantly increase the amplifier's overall
gain, which might be a bonus also.

Thanks Fred. I'll try it out and tell you how it fares.


If you can't decide which you like better, add a switch and label it
something funky like "cozmik overdrive". ;-)

Or with a pot just label it Tight/loose with "Good Girl" in the middle....

cb

Hi Chris,
By looking at the schematic I would think that your amp would
work pretty well for a guitar and should not respond a whole lot different
than your average 2 X EL34 push pull AB design.

The 40 v sag is normal - under pretty severe clipping anyhow. If you are
sagging your supply down that much at low power though, then something is
drawing too much current. For sure check or replace the filter caps but from
what you are describing I think maybe you have a bad tube. You might try
lifting that level indicator tube off the 4 ohm output tap just to be sure
something there isn't leaking and screwing up the OT's load.

What are the DC voltages at your power tubes? How old are those tubes
anyways?
Todd

"Patrick Turner" wrote in message
...


Chris Berry wrote:

Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83
valve complement) which seems to be misbehaving...
OK so I'm using it as a guitar amp as well but that shouldn't be much of
a
bother.
I've noticed that when playing music through it, the anode voltage seems
to
drop faster than I'd expect. For guitar, where I'd be deliberatly trying
to
get the amp to distort nicely (there is such a thing,,,) as soon as the
anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd
phasing noises and this is in the early stages of break-up.
To give you an idea, I can get an in-between clean-overdriven bluesy
tone
but not get anywhere near a blues/lead territory.
I know this isn't the "right" newsgroup for guitar amp stuff but I must
say
that my recent experiences here really do make it a much better place to
post this than alt.guitar.amps.
Thanks guys.
cb

I would have no idea what "To give you an idea, I can get an in-between
clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory." means.

It means that there's a little break-up that's usable when much more would
normally be expected.


However, when the amp is grossly over driven, it isn't unusual to get a
drop of
B+
of 10%.
Most amps when used without any overdrive or clipping will maintain their
operating voltages.

When overdriving at about what I expect to be the levels I want it to, B+
sag is 30% - 290V


If the amp isn't a true guitar amp, and is a hi-fi amp, and has negative
feedback,

NFB's been removed and a little improvement has resulted

and high value coupling caps, then perhaps what you hear is paralysis.
This is caused by the coupling caps chraging up due to the grid current
flow,
and the direct voltage across the caps is like over biasing of the output
tubes,
and they make the amp act like a class C amp.
The only way to prevent this in an RC coupled amp is to minimise the size
of
the
coupling caps.
In a hi-fi amp, the output coupling caps are ususally at least 0.1 uF,
with grid bias resistors of 220k or higher.
Guitar amps often use 0.022uF and 100k which is a faster time constant, so
that

discharge of charge build up in the caps is much faster, and follows the
fading
note better.
The is much less feedback in a guitar amp. Feedback makes paralysis occur
much
more severly,
so reduce the amount of fedback for musical instrument use.

Once the hi-fi amp has its feedback reduced, and the coupling caps
reduced,
the amp is then the equal of most guitar amps.
The F response may be slightly reduced, but still be quite adequate for
guitar.

I'll check that later.


The changes of coupling caps *must* be accompanied by feedback
reduction because the amp may become quite unstable, ie, oscillate
at some LF tone if the feedback is left as is.

Like I said, with NFB removed completely, there's a slight improvement.


Many hi-fi amps have 20 dB of feedback.
Only a few guitar amps have no feedback at all, such as a Vox AC30,
but quite a few have 10 dB, just enough to reduce the
effective output resistance a bit.

When making changes to feedback and coupling caps, the amp should still be
stable with no load connected; if it is, then it will be stable with a
speaker
connected.

Here, the hot plate is my friend. I must be getting 1/2 a watt out with the
amp at full powergiving the amp a 16 ohm load.

Thanks for the tips.
cb

wrote in message
...
Hi Chris,
By looking at the schematic I would think that your amp
would
work pretty well for a guitar and should not respond a whole lot different
than your average 2 X EL34 push pull AB design.

The 40 v sag is normal - under pretty severe clipping anyhow. If you are
sagging your supply down that much at low power though, then something is
drawing too much current. For sure check or replace the filter caps but
from
what you are describing I think maybe you have a bad tube. You might try
lifting that level indicator tube off the 4 ohm output tap just to be sure
something there isn't leaking and screwing up the OT's load.

That's it 40V at mild clipping is what I'm getting.


What are the DC voltages at your power tubes? How old are those tubes
anyways?
Todd

-40V bias 400V B+, Vg2 330V
Valves are new and tested with avo valve tester.
Thanks.

cb

400V is on the low side. My Eminent runs at 440V (quiescent) with 50mA
through each EL34. I haven't checked what happens to the anode voltage
under full load or overdrive. Maybe your GZ34 isn't up to spec anymore,
or the filter caps need to be replaced.

The GZ34 and EL34s in my Eminent are Philips (made in Holland) tubes.

Corné



Chris Berry wrote:

Hi all,
I've a question concerning my Dynacord Eminent amp (2xEL34+GZ34+ 5xECC83
valve complement) which seems to be misbehaving...
OK so I'm using it as a guitar amp as well but that shouldn't be much of a
bother.
I've noticed that when playing music through it, the anode voltage seems to
drop faster than I'd expect. For guitar, where I'd be deliberatly trying to
get the amp to distort nicely (there is such a thing,,,) as soon as the
anode voltage drops from 400V (quiescent) to 360V, I start to hear wierd
phasing noises and this is in the early stages of break-up.
To give you an idea, I can get an in-between clean-overdriven bluesy tone
but not get anywhere near a blues/lead territory.
I know this isn't the "right" newsgroup for guitar amp stuff but I must say
that my recent experiences here really do make it a much better place to
post this than alt.guitar.amps.
Thanks guys.
cb

"Corné Janssen" wrote in message
...

400V is on the low side. My Eminent runs at 440V (quiescent) with 50mA
through each EL34. I haven't checked what happens to the anode voltage
under full load or overdrive. Maybe your GZ34 isn't up to spec anymore,
or the filter caps need to be replaced.

The GZ34 and EL34s in my Eminent are Philips (made in Holland) tubes.

Corné


Which one do you have? Mine's the 225-102 but I have a few of their circuits
if you need them (supplied by Dynacord themselves)
I'm using a JJ GZ34 and EL34's made by Halltron (well Branded at least).
cb

The one I have is a 125-102. A few years ago I scanned the circuit
diagram and Duncan Munro put it on his site at:
http://tdsl.duncanamps.com/pdf/eminent.gif

If you didn't know the TDSL site has a lot of tube data and amplifier
circuit diagrams. TDSL can be found at: http://tdsl.duncanamps.com/

I'll check anode voltages at full load and overdrive conditions. If I
can find the time I'll do it this weekend.


Corné

Chris Berry wrote:

"Corné Janssen" wrote in message
...

400V is on the low side. My Eminent runs at 440V (quiescent) with 50mA
through each EL34. I haven't checked what happens to the anode voltage
under full load or overdrive. Maybe your GZ34 isn't up to spec anymore,
or the filter caps need to be replaced.

The GZ34 and EL34s in my Eminent are Philips (made in Holland) tubes.

Corné


Which one do you have? Mine's the 225-102 but I have a few of their circuits
if you need them (supplied by Dynacord themselves)
I'm using a JJ GZ34 and EL34's made by Halltron (well Branded at least).
cb

Here are the measurements

These measurements are taken at 1kHz
Quiescence anode voltage : 435V
anode voltage @ 40W clean : 382V
anode voltage @ 50W over driven : 365V

When th amp is overdriven hum from the powersupply gets into the output
signal this is clearly visable on an oscilloscoop. This may explain the
phasing noise you hear.


Corné

"Corné Janssen" wrote in message
...

Here are the measurements

These measurements are taken at 1kHz
Quiescence anode voltage : 435V
anode voltage @ 40W clean : 382V
anode voltage @ 50W over driven : 365V

When th amp is overdriven hum from the powersupply gets into the output
signal this is clearly visable on an oscilloscoop. This may explain the
phasing noise you hear.

well that problem is now sorted out and I reckon you're very right about the
ripple getting through.
Maybe the supply is still weak but with less current draw through the valves
it's stable and in the region you're getting.
Now I've a different problem with the bias on one valve dropping from -40V
to -48V while the other rises 1 or 2 volts max...
cb

unmatched tubes or grid cap leak?

--
--
--
.................................................. ........................
Choky
Prodanovic Aleksandar
YU

"don't use force, "don't use force,
use a larger hammer" use a larger tube
- Choky and IST"
- ZM
.................................................. ...........................
"Chris Berry" wrote in message
...

well that problem is now sorted out and I reckon you're very right about
the
ripple getting through.
Maybe the supply is still weak but with less current draw through the
valves
it's stable and in the region you're getting.
Now I've a different problem with the bias on one valve dropping from -40V
to -48V while the other rises 1 or 2 volts max...
cb

Hey Chris-
From a few posts ago you said your voltages at your EL34's we
-40V bias 400V B+, Vg2 330V

I assume you are taking these measurements at idle with zero signal input?

These voltages are the same at both tubes?

By the looks of your schematic - there is something wrong with your Vg2
level. It should be about the same as your plate voltage (at idle). I think
you should replace your filter caps before going any further with any of
this - they are likely the main cause of all your troubles.

Once you have new filters -including the bias filters, go ahead and change
the screen grid resistors (100 ohm) and make sure the 1k grid series
resistors are good - or just replace those too. Mount both the Rg2 and the
Rg1 as close to the tube socket as possible.

By what you describe in your last post - your schematic shows a 25 k trim
pot for each output tube- are you saying that one of these voltages is
drifting after they have been set for equal currents?
Do you have reasonably equal amounts of bias voltage at each tube when you
initially set for matched (push/pull) current? Or - are you taking this
measurement with signal applied?
Todd



"Chris Berry" wrote in message
...

"Corné Janssen" wrote in message
...

Here are the measurements

These measurements are taken at 1kHz
Quiescence anode voltage : 435V
anode voltage @ 40W clean : 382V
anode voltage @ 50W over driven : 365V

When th amp is overdriven hum from the powersupply gets into the output
signal this is clearly visable on an oscilloscoop. This may explain the
phasing noise you hear.

well that problem is now sorted out and I reckon you're very right about
the
ripple getting through.
Maybe the supply is still weak but with less current draw through the
valves
it's stable and in the region you're getting.
Now I've a different problem with the bias on one valve dropping from -40V
to -48V while the other rises 1 or 2 volts max...
cb

wrote in message
...
Hey Chris-
From a few posts ago you said your voltages at your EL34's we
-40V bias 400V B+, Vg2 330V

It's risen with grid resistor changes in the output stage to 430V ish and
the bias supply has increased (but not to schematic values - close though)
with the changing of the bias supply diode.


I assume you are taking these measurements at idle with zero signal input?

No. The measurements above are but the sag is with a strong signal input.


These voltages are the same at both tubes?

At idle yes - not under load.


By the looks of your schematic - there is something wrong with your Vg2
level. It should be about the same as your plate voltage (at idle). I
think
you should replace your filter caps before going any further with any of
this - they are likely the main cause of all your troubles.

Replaced most of the capacitors thus far - still need a 22uF+22uF for the
main supply though.


Once you have new filters -including the bias filters, go ahead and
change
the screen grid resistors (100 ohm) and make sure the 1k grid series
resistors are good - or just replace those too. Mount both the Rg2 and the
Rg1 as close to the tube socket as possible.

Already done that. Improved the supply but the bias does it's funnies.


By what you describe in your last post - your schematic shows a 25 k trim
pot for each output tube- are you saying that one of these voltages is
drifting after they have been set for equal currents?

Set for 40V as per the schematic. They drift under load - Left one - rises
to -39V, the right one drops to 48V

Do you have reasonably equal amounts of bias voltage at each tube when you
initially set for matched (push/pull) current?

Well I'm setting for voltage - not current but with 2 sets of tubes (matched
at one point) it's always the right hand socket that produces this
behaviour.

Or - are you taking this
measurement with signal applied?

Setting it is with no signal drift is measured under load.

Thanks for taking a poke...

cb

I would suggest fixing one thing at a time. Let's clarify: What are the DC
voltage measurements at your output tube sockets with no signal applied?

wrote in message
...
I would suggest fixing one thing at a time. Let's clarify: What are the DC
voltage measurements at your output tube sockets with no signal applied?

Todd, I'll be able to tell you tomorrow or the day after when I'm back with
the amp and have a moment.
Thanks though..
cb