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Adjustable Bias?

bassmodder

bassmodder

Dec 9, 2006
1,084
10
Sioux Falls, SD
#1
This amp needs an adjustable bias really bad. Where should I put the pot? What value of pot and resistor should I use with said pot?

I am thinking that I should put it where the 100K resistors are after the bias voltage is inserted...

Opinions? Schematic is posted below.
Untitled-1.gif
 
#2
Replace the 68KΩ bias shunt resistor with a 50K pot in series with a 33K to ground. This will enable you to increase or decrease the bias as you wish.

Paul

P.S. I would also remove the single resistor feeding the screens of the '34s and replace it with one 1KΩ 5W resistor for each tube screen.
 
#4
bassmodder said:
What effect would changing the screen resistor have on the tubes? Right now, my screen voltage is kissably close to the plate voltage.
Click to expand...

It will be. Separating the resistors will allow the tubes to draw the screen current it needs. With one resistor one tube can hog the current available. Not perhaps the most scientific explanation but it will have to do. A couple of glasses of wine limits my verbosity! :)

Paul
 
#5
Heheheh! Don't I know that! Before you know it 3/4 of the bottle is gone! I will add those two resistors so each tube will have its own resistor.

When you are sober, why would the adjustable bias not be installed where the 100k's are right after the -36 volts gets injected?

Pardon my newbeness here, I just rebuilt this amp from the ground up (every component replaced) and am getting my fingers around how they work.
 
#6
The negative voltage is developed at the junction of the 10KΩ resistor and the 68KΩ to ground. This is connected to the two 100KΩ grid leak resistors. By changing the ratio of the 10K to 68K you change the voltage present at that point. Higher than 68K more negative voltage. Less than 68K - lower voltage. Thus pot at minimum = 33K to ground lower negative voltage: tubes run hotter. Conversely pot at maximum = 50K + 33K or 83K to ground - more negative voltage and tubes run cooler. As a matter of interest what is the current bias voltage you are measuring? -36V should be running '34s slightly cool. I usually set my designs at -34V and they always seem happy at that level.

Paul
 
#7
I have -32 volts on the bias measured directly on the tube socket.

Plate voltage is 419 volts
Screen is 417 volts
Input voltage is 117 volts.

The plate on one tube REALLY glows when you lay the volume to the floor. The other tube could care less.
 
#8
OOOOOH! I see what you are sayen here... You would change the "master" bias voltage and not go into the circuit to change the voltage. I will give it a whirl and see what happens. If I do this, a matched set of tubes is very important right?

This is a PA amp and I am looking for the most clean headroom.
 
#9
Matched tubes are a fairly recent phenomenon. When these amps were built they just stuck the next couple of tubes out of the box into the amp. Ideally both tube should be running the same current but you won't hear any difference if they are not.

32V is low and your tubes will be running hot. See how the amp performs with -36V and new screen resistors.

Paul
 
#12
:) Just purchased the supplies today! Was hard to make it to the store to get the stuff due to work and all.

I had to use a 75K to get the bias up high enough. However, I now have two choices in the bias voltage: 32.2 or 36.1.

I can not hear any difference between the two, so I chose the 36. The plate stopped glowing aswell. (Sorta like poking a lazy person... poke too much and it moves and moves but takes lots of tuning in the opp. direction to move it back...)

One last question, when I dime the piss out this amp, my bias goes to about 50 volts and the plate begins to glow again on that one tube.

I reduced the bias down to high 20's and the other tube's plate will not glow, but the other one will really glow. It follows the tube when you switch sockets.
 
#13
Have to order the screen resistors though... I will hardly ever drive this amp into distortion, but it is nice to know that I can probably do that safely after I change those screen resistors.

Since cooling the tubes down, I can no longer fry eggs on the power transformer! The whole thing really sounds exactly the same as it did before, just runs much cooler.

Thank you BassmanPaul for your help and suggestions toward improving the performance of this little amp!
 
#14
Put them 1K's in today. Did not really change my screen voltage by any large amount. However I can drive wide open without having a nuclear meltdown in my overactive tube. Just some slight glowing, but when driven to that level, you can not even make out the words any more, so that kind of drive seems useless in a real world application.
 

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