Bias Problem

bolomule · Post by **bolomule** » Tue May 19, 2015 4:58 pm

Hello. I have two individual bias pots wired according to the Brown Note layout. There is -44 to -25 sweep on both pots but when a pair of EL34 tubes are installed I cannot get lower than approx. 80 ma. I have tried 6L6 tubes that I know are good and they too bias too hot. Is it just a matter of changing the bias range resistor? I currently have 1k2 in there now. The pots are 10k with 15k tails. Thanks for any help.

Stevem · Post by **Stevem** » Tue May 19, 2015 6:00 pm

What has the bias voltage range that you can adj changed to with the tubes installed?
You need to cool the tubes down by having more bias voltage and you can do that by lowering the value of the resistor that feeds the ac voltage into the rectifier, or increase the value of the resistor that goes to ground to keep more voltage that you already have, or you my need to do a bit of both!

bolomule · Post by **bolomule** » Tue May 19, 2015 7:04 pm

SteveM thanks for the reply. I swapped the resistor feeding the rec to 470R. With tubes installed, I can now get a range of -52 to -30. This translates to about 1.5 mv to way too hot. The dual bias pots are pretty touchy. It seems I need to keep idle bias at about 12.5 mv on both tubes. While playing the bias jumps to about 30 to 40 mv depending how hard its being driven. If I go above that low initial idle bias, it will eventually jump to around 80 mv during playing and make a motorboating sound. I then hit the standby and bring the bias back down. I might ditch this two pot setup. It doesn't seem vary stable unless I have a bad pot or something else I'm missing.

Phil_S · Post by **Phil_S** » Tue May 19, 2015 7:09 pm

Show us a drawing of the complete bias circuit from the PT supply to the tube grids. None of this sounds right to me.

bolomule · Post by **bolomule** » Tue May 19, 2015 7:22 pm

Phil_S. Here is the Brown Note layout. I wired it the same except for 10k pots and 15k tails. My PT's bias tap puts out 47 VAC. Thanks.

tubelectron · Post by **tubelectron** » Tue May 19, 2015 7:46 pm

double posting while writing

, sorry

tubelectron · Post by **tubelectron** » Tue May 19, 2015 7:48 pm

My PT's bias tap puts out 47 VAC.

That's a correct value, found on most Fenders, Boogies and Marshalls.

A layout doesn't speaks too much to me, I prefer a schematic, sorry... But I am not Phil_S !

Like him, I would supsect a problem not necessarily due to the bias circuit.

0 - Did your amp worked fine as is before that problem ? Or is it a kit you have just finished and powered ?

1 - If there is a feedback loop, are you sure it is connected in the right sense ? The motoboating effect you speak about drives me to say that, and in addition I suspect more or less an ultrasonic oscillation of your phase splitter and power stage, which gives you those hi reading idle currents... This oscillation can be tied indeed to a wrong sense FB connection, or a parisitic coupling somewhere : do you hear abnormal hiss in the speaker ? Try to disconnect the FB line and see if at no signal or low signal the problem still continues.

2 - The motorboating can also be due to a filtering or a coulping capacitor leak problem...

A+!

sluckey · Post by **sluckey** » Tue May 19, 2015 7:55 pm

Your bias circuit could benefit by having a filter cap between ground and the wiper of each bias pot. Like this...

http://home.comcast.net/~seluckey/amps/ ... e_1971.pdf

ToneMerc · Post by **ToneMerc** » Tue May 19, 2015 8:49 pm

bolomule wrote:SteveM thanks for the reply. I swapped the resistor feeding the rec to 470R. With tubes installed, I can now get a range of -52 to -30. This translates to about 1.5 mv to way too hot. The dual bias pots are pretty touchy. It seems I need to keep idle bias at about 12.5 mv on both tubes. While playing the bias jumps to about 30 to 40 mv depending how hard its being driven. If I go above that low initial idle bias, it will eventually jump to around 80 mv during playing and make a motorboating sound. I then hit the standby and bring the bias back down. I might ditch this two pot setup. It doesn't seem vary stable unless I have a bad pot or something else I'm missing.

It would help if you would clarify the exact circuit layout. You do have the 220K grid resistors split right? Also, why are you biasing at only at 12.5mV at idle?

TM

Phil_S · Post by **Phil_S** » Tue May 19, 2015 9:46 pm

You'll need to give me a disclaimer for any errors in the attached, but this is what I make of that awful busy layout. Drat! I've got the polarity of the cap labeled incorrectly.

bolomule · Post by **bolomule** » Wed May 20, 2015 1:21 am

So my EL34 tubes are bad. I tried the 6L6 tubes again and the bias is stable at 42 ma. I have about 460 on the plates and I'm happy with the tone. Sorry for the trouble and thanks for the advice all.

roberto · Post by **roberto** » Wed May 20, 2015 6:34 am

Hi Phil,

it would be safer for the tubes to receive the raw bias directly through a resistor, and then andjust through a trimmer going to ground (raw bias - resistor - bias tap - trimmer - resistor - ground).

This way if the trimmer has some troubles the tube is biased to raw bias (so cold) instead of very hot (or unknown) using that schematic.

Phil_S · Post by **Phil_S** » Wed May 20, 2015 1:45 pm

No argument from me, Roberto! I just tried to draw what I see in the layout diagram. We're trying to get a handle on the bias supply circuit. I do it the way you describe.

This split bias arrangement seems a bit overdone to me. Even if you dial in the bias on unmatched tubes, I'm not sure what it gets you. It's a 4-holer with 2 bias adjustments. At least it has a tail resistor to ground.

roberto · Post by **roberto** » Wed May 20, 2015 8:59 pm

Yeah, I'm working on something inbetween the ginger idea of sharing 30% of the bias between each pair with an unbypassed common cathode resistor, plus fixed bias plus a self adjusting bias for each tube.

By now, using that configuration, with a tube that should draw two times the current of the other (when using one common cathode resistor), I've obtained 1.5 mA maximum difference between the two tubes (35.5 and 38 mA).

More sims will be done shortly, trials will take some more time.

romberg · Post by **romberg** » Wed May 20, 2015 11:20 pm

bolomule wrote: I can now get a range of -52 to -30. This translates to about 1.5 mv to way too hot.

This sounds ok. What is your plate voltage?

. While playing the bias jumps to about 30 to 40 mv depending how hard its being driven.

I'm assuming you are measuring this voltage across a 1ohm resistor mounted on the power tube cathodes? This measurement is only meaningful while the amp is idle. When the amp is idle the current flowing through these resistors is DC (and the voltage measured across the resistor is too). When you are playing the amp then there is also an AC signal. So, your meter is not telling you anything meaningful.

Just bias your amp while idle (no signal entering the amp). Play it a bit and then check the bias again (for drift) while the amp is idle.

Mike

The Amp Garage

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