bias feed resistors? Master Volume mod...

[diddymix]

Hello Folks,

I was hoping someone with expertise on output stages, with regard to fixed bias might be able to help me with something I want to accomplish.. Basically on my amp shown, I have a post PI master volume. I have made changes to my preamp stage and done a direct clean channel with daughtercard and it all works great, and whilst I love the tone, I have a feeling that now that I'm going direct to PI from V1.. the signal is hitting the PI is very hot, and probably distorting the PI before the output stage, contributing to an almost 'too fat' tone with even hardly any gain turned up.
To cut to the chase I would like to try moving my master volume from just before the EL84's... to just before the PI input cap (22nf)... making it a pre PI MV. I feel that this could give me more of a clean dynamic range from the fingertips..
So basically, at the moment the bias for the output tubes (roughly 550mv) goes directly through the actual master volume pot. So my question is, if I was to move the master volume pot to before the PI... would I need to include 'bias feed' resistors from the bias source, and have them link up to the grids as before?? I would imagine the MV pot is currently doing the same thing but variable.. Am I correct that this could work?? Could this be a problem considering it would be static and on hitting the grids all the time?? I see it like this on other designs... (such as the VOX AC30, and Peavey classic 30). If so what value of bias feed resistor would I choose... 220k?
Thanks for reading everyone and relaly appreciate any input and experience with regard to my situation.

[Stevem]

First off surly you do not mean that the bias voltage feeding those 220k resistors is a positive voltage no less only 550 millivolts?
That voltage should be a negative 50 volts!
And by the way that - 50 volts is too high a setting , but your posted 550mv is nonexistent, lol!

[diddymix]

First off surly you do not mean that the bias voltage feeding those 220k resistors is a positive voltage no less only 550 millivolts?
That voltage should be a negative 50 volts!

Ahh! I think yes its correct.. I've attached the bias instructions for the amp here.. and as you can see it says recommended voltage 1.3V.. (my later DSL401 revision I found suggests 0.55V). I just thought of this though.. perhaps this is the reason... if you look at the notes at the bottom, it says you must turn the master volume pot to OFF. Perhaps as the bias voltage goes through the volume pot... could this maybe explain why the suggested voltage for bias is so small? (as the off would be max/nearly resistance..)?? I guess I could try taking measuring the bias feed somewhere before the volume pot to find out...

Great and thanks for spotting this, appreciate your advice!

[sluckey]

You are confusing two different things...

1. The bias voltage (VBIAS) goes to the output tubes grids via the MV. It's probably about -15V for EL84s.

2. The bias instructions have you monitoring cathode current by measuring the small voltage drop across a 10Ω resistor (R33). The monitor point is CON5.

When you turn the bias pot, you are actually changing the VBIAS voltage that goes to the grids via the MV. This in turn changes the current flowing through the tubes which changes the voltage at your bias monitor point.

As for moving the MV, you will simply replace the dual pot with two 220K half watt resistors. If you want to be precise use two 200K resistors since the MV pot is 200K.

[diddymix]

You are confusing two different things...

1. The bias voltage (VBIAS) goes to the output tubes grids via the MV. It's probably about -15V for EL84s.

2. The bias instructions have you monitoring cathode current by measuring the small voltage drop across a 10Ω resistor (R33). The monitor point is CON5.

When you turn the bias pot, you are actually changing the VBIAS voltage that goes to the grids via the MV. This in turn changes the current flowing through the tubes which changes the voltage at your bias monitor point.

As for moving the MV, you will simply replace the dual pot with two 220K half watt resistors. If you want to be precise use two 200K resistors since the MV pot is 200K.

Thanks Sluckey perfect That makes sense now, I was confusing the two.. I'm gonna try this out i have a feeling the pre PI MV should sound better, hopefully open up a bit.. There's a question I'd love to get your opinion on if I may.. hope its alright but I've been scratching my head about this one. Just to the left of the master volume on the schematic there is a link between the channels.. with a 2M2 resistor and a 47pf cap.. What do you think this is doing?? Is it an anti oscillation thing perhaps..? I find it wierd but I guess it is a PCB amp with alot going on on the board.. Thanks again mate

[pdf64]

C110 and R132 are across the balanced signal paths feeding the power tube grids. Not related to the amp’s ‘channels’ per se.
They form a mild shelving filter, bringing high frequencies down a notch. May be intended for tonal purposes, phase correction, improving the margin of stability etc.

[diddymix]

C110 and R132 are across the balanced signal paths feeding the power tube grids. Not related to the amp’s ‘channels’ per se.
They form a mild shelving filter, bringing high frequencies down a notch. May be intended for tonal purposes, phase correction, improving the margin of stability etc.

Great thanks! I thought it might be something like that.. Id like to remove them and see if theres any noticicable tonal change.. I dont have a scope though so cant check for instability... maybe best just leave em.??

[diddymix]

You are confusing two different things...

1. The bias voltage (VBIAS) goes to the output tubes grids via the MV. It's probably about -15V for EL84s.

2. The bias instructions have you monitoring cathode current by measuring the small voltage drop across a 10Ω resistor (R33). The monitor point is CON5.

When you turn the bias pot, you are actually changing the VBIAS voltage that goes to the grids via the MV. This in turn changes the current flowing through the tubes which changes the voltage at your bias monitor point.

As for moving the MV, you will simply replace the dual pot with two 220K half watt resistors. If you want to be precise use two 200K resistors since the MV pot is 200K.

Hey Sluckey, thanks for the info on this. I was hoping you could give me some advice on my findings after trying the mod.. First up the amp sounds alot better! More punch and more open, also the clipping has become lezz buzzy and compressed somehow, and is a cleaner overall tone and I want this to stay! The only problem now is, there is a noticable 'buzz' which sounds like the output tubes humming.. it changes with the pre PI MV up and down.. I used the 220k bias resistors, and measured the bias voltage after those resistors as -19.5V. When I put back the old Post PI MV like before the mod, the buzz is not there... so I think it is in the modification.. DO you think this could be because of incorrect bias value?? Also, i grounded the MV output (not the wiper), to where the PI tail resistor is grounded.. I dont know if this location matters..
Anyways thanks for your time and appreciate any advice or knowledge on this

[pdf64]

...The only problem now is, there is a noticable 'buzz' which sounds like the output tubes humming.. it changes with the pre PI MV up and down.. I used the 220k bias resistors, and measured the bias voltage after those resistors as -19.5V. When I put back the old Post PI MV like before the mod, the buzz is not there... so I think it is in the modification.. DO you think this could be because of incorrect bias value?...

The stock master must be turned up to max in order to compare its sound and noise levels etc to a mod that takes it out of circuit.
The appropriateness of the applied bias voltage is determined by the voltage across the cathode resistor.

[diddymix]

...The only problem now is, there is a noticable 'buzz' which sounds like the output tubes humming.. it changes with the pre PI MV up and down.. I used the 220k bias resistors, and measured the bias voltage after those resistors as -19.5V. When I put back the old Post PI MV like before the mod, the buzz is not there... so I think it is in the modification.. DO you think this could be because of incorrect bias value?...

The stock master must be turned up to max in order to compare its sound and noise levels etc to a mod that takes it out of circuit.
The appropriateness of the applied bias voltage is determined by the voltage across the cathode resistor.

OK!! Thats kinda what I thought... in that before the bias would only be 200Kish with the pot on max.. So, do you think that there are other changes necessary to the bias circuit or ground path in order to calm the buzz down?? Is it maybe that Im somehow now 'stressing' the output tubes??

[diddymix]

...The only problem now is, there is a noticable 'buzz' which sounds like the output tubes humming.. it changes with the pre PI MV up and down.. I used the 220k bias resistors, and measured the bias voltage after those resistors as -19.5V. When I put back the old Post PI MV like before the mod, the buzz is not there... so I think it is in the modification.. DO you think this could be because of incorrect bias value?...

The stock master must be turned up to max in order to compare its sound and noise levels etc to a mod that takes it out of circuit.
The appropriateness of the applied bias voltage is determined by the voltage across the cathode resistor.

OK!! Thats kinda what I thought... in that before the bias would only be 200Kish with the pot on max.. So, do you think that there are other changes necessary to the bias circuit or ground path in order to calm the buzz down?? Is it maybe that Im somehow now 'stressing' the output tubes?? Do you mean that an incorrect bias value into the grids here could cause this buzzing effect?? Thanks so much !

[pdf64]

Dunno, just suggesting that if in doubt, as it's so easy, check the cathode current to verify that bias is suitable.
Are you familiar with the noise level from the amp with the master at max?

[diddymix]

Hey guys!

I'm just back regarding the hum on the output.. as its been a mystery to me.. but now have thought of this, was hoping anyone here could tell me what they think??

Basically the hum on the output, which is quite loud when I move the master volume pot to before the PI. I completely overlooked this but have been using Tunsgram El84 tubes... only I bought two pairs of these separately. So each pair is matched, but both pairs may well spec differently. Could these mismatched output tubes cause a considerable output hum (50Hz UK)... (regardless of pre PI master volume setting)?

This would be great to know, thanks so much people

[martin manning]

I completely overlooked this but have been using Tunsgram El84 tubes... only I bought two pairs of these separately. So each pair is matched, but both pairs may well spec differently. Could these mismatched output tubes cause a considerable output hum (50Hz UK)... (regardless of pre PI master volume setting)?

Yes. There is considerable ripple remaining on the plate voltage, and if the current on each half of the OT is not matched then the ripple isn't cancel, with the difference being carried through to the secondary. The miss-match required to produce an audible hum is more than a few mA, though.

[diddymix]

I completely overlooked this but have been using Tunsgram El84 tubes... only I bought two pairs of these separately. So each pair is matched, but both pairs may well spec differently. Could these mismatched output tubes cause a considerable output hum (50Hz UK)... (regardless of pre PI master volume setting)?

Yes. There is considerable ripple remaining on the plate voltage, and if the current on each half of the OT is not matched then the ripple isn't cancel, with the difference being carried through to the secondary. The miss-match required to produce an audible hum is more than a few mA, though.

Thanks Martin! Trying this out today got some new matched tubes arriving.. I remember you also mentioned the 2 x 100R resistors for the heater supply that could be off balance.. If it still hums with the new tubes do you think its worth testing these?? COuld that explain the hum too? (Again the hum does not change with pre PI master v, its just there moderately loud, oh and another symptom is when I switch off the main power switch, I get quite a loud blap/pop...)