I need more negative voltage for biasing

[pbman1953]

For a Traynor YBA3 Custom, many have suggested to remove the 47 ohm screen resistors. In its place, to install a 1k resistor to each tube on pin 4. When this is done the bias voltage is decreased. There are one or 2 resisitors to swap out to get more. Here is the schematic-

http://www.freeinfosociety.com/electronics/schematics/audio/traynor_customspecial_yba3c.pdf

There are 3 resisitor inviolved. A 150k, 47k and 15k. Please let me know which one changes and to what value to start at

Thanks

[sluckey]

remove the 47 ohm screen resistors. In its place, to install a 1k resistor to each tube on pin 4. When this is done the bias voltage is decreased.

That should not happen. Are you sure you're doing this right?

There are 3 resisitor inviolved. A 150k, 47k and 15k. Please let me know which one changes and to what value to start at

Decreasing the 150K will increase the bias voltage. Try 100K.

Increasing the 47K will increase the bias voltage. Try 68K.

[pbman1953]

thanks

Are you saying to do both or either one?

[sluckey]

Do the 47K first. If you can't get enough, try the 150K also.

[Willabe]

pbman1953,

Doug has great info on biasing an amp in his "tube amp library". Go to > Tube amp library > servicing Fender amps > biasing the amp.  This should be of great help, IMHO, I find it very helpful, again IMHO.

         Brad             

[chocopower]

For a Traynor YBA3 Custom, many have suggested to remove the 47 ohm screen resistors. In its place, to install a 1k resistor to each tube on pin 4. 

Why do that? you have a 1K 20w resistor just before protecting the screens. It have been there for a lot of years doing his work properly. The only thing you add with those new 1k resistors is more compresion or sag....

[pbman1953]

that makes a lot of sense,
I'll leave it stock and replace those old brown resistors instead

Thanks

[pbman1953]

On that 1k 20 watt, and others like it. Besides a meter measurement , are there visible signs that these should be changed?

On the botton side there is discoloration, showing kind of a whitish residue at the ends and bottom. They measure fine

There are 2 others but smaller values

[sluckey]

remove the 47 ohm screen resistors. In its place, to install a 1k resistor to each tube on pin 4. When this is done the bias voltage is decreased.

That should not happen. Are you sure you're doing this right?

After reading your other thread about this amp, I'm uncertain if you are actually talking about bias voltage. I think you are talking about the cathode current that you measure with an adapter. Cathode current WILL DECREASE when you install those 1K screen resistors. The negative bias voltage is the voltage you would measure on the control grid (pin 5). Pin 5 controls the cathode current, but the negative voltage on pin 5 will not change when you replace those screen resistors.

Now, if what I just said is correct, and you can't get enough cathode current after replacing the screen resistors, you will have to DECREASE the negative bias voltage to get the higher cathode current. Just do the opposite from what I said earlier with the bias circuit resistors.

Having said all that, what are you trying to accomplish? I agree with choco that this amp is fine like it is.

I would not replace those power resistors just because of some discoloration.

[pbman1953]

Great news and thanks for the help to all!


Happy New Year

[sluckey]

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