Traynors, screen voltage....tube choice?

[GerryJ]

Thanks for all the helpful info. Switching topic a bit from screen grids to bias- Most of you know the YBA-1 Traynor bassmaster has a fixed bias resistor, & the YBA1A MkII has a wiper (adjustable) resistor. With the amps stock. I'm able to get 27 mA in the YBA-1 & 34 mA current in the YBA-1A, using Tesla EL34s. If I switched to JJ KT77s, does anyone know if I'd have to modify the bias supply hardware? Thanks.

[Stevem]

To keep the 77s in check I would say you would want a extra 10 more volts of negative bias on hand over the 34s if the plate voltage is between 475 to 500.

[GerryJ]

To keep the 77s in check I would say you would want a extra 10 more volts of negative bias on hand over the 34s if the plate voltage is between 475 to 500.

Thanks- On the YBA-1A I'm measuring cathode current of 34 mA, using a bias probe device (for safety), & the schematic says the amp puts 525 V on the screen grid, and puts 537 volts on the plate. For current flow of 34 mA, I've got the bias wiper resistor turned (iirc) all the way counter-clockwise, ie, if I start turning it more toward the center, the cathode current increases. So if I install KT77s and I need to turn the bias wiper clockwise to allow more current (because it's under-biased), I've got plenty of room to move. If on the other hand, the KT77 gives me too high a current with the bias wiper full counterclockwise, I guess I'd have to modify the circuit?
Sorry for not knowing too much, I'm endeavoring to self-educate myself more on the fundamentals - thanks again.

[Stevem]

When a output tube is under biased and in turn running too hot it is due to not having a high enough negative voltage on its input grid.

Over biased is just the opposite.

There is then too high a negative voltage on the input grid.

In this case the fix can be done in one of two ways.

You can increase the resistor value feeding the bias diode so the circuit then has less negative voltage, or you can lower the value of the bleed resistor that goes to ground.

This will sink more negative voltage into the chassis.

If your bias circuit has a bias trim pot then you may need to reset the value of both of these two resistors so that the bias voltage your looking for can be had with the trim pot near the middle of its sweep range.

If you increase the value of the resistor that feeds the diode ( the resistor with ac voltage on one end) then you should increase its wattage rating by 1/2 watt in the least just to play it safe.

If you do not have enough negative bias voltage as in the tubes are idling too hot, then just the opposite of what I posted about above in regards to the resistors needs to be done, ( IE) lower the value of the bias diode feed resistor and or increase the value of the bias bleed resistor.

98% of the time I have found that once I get the bias voltage into the ball park for getting the idle current that I want, then if the bias trim pot gives me 1.5 volts of sweep on either side of center then I am golden!