Traynor Yba-1A Bias

[tdlunsfo]

Finishing up a Traynor Mark 2 repair with some new GT 6CA7's. This amp came to me with a dead 6CA7 where the filaments worked but zero emissions. Interestingly, the other tube survived..but I digress. Someone has added a bias pot but with it all the way in I am at 39 and 46ma's. Obviously these are not a matched set but my concern is the bias may still be too hot. I can change some values in the bias circuit to fix this if needed, I am just unsure if I need to. I have never biased these tubes and I have read to bias them hotter, others say the same as a regular EL34. Then there is the fan, can you go to 100% with the fan? My plate voltage is around 580v.

[g1]

That is way hot. What kind of bias voltage are you getting at the grids? One schem. shows -48V, another shows -56V.
Do you know which is the correct schematic for the amp you have? Where did they put the bias pot?

[tdlunsfo]

I've been using the -48v schematic and I originally set it for that voltage but the tubes started to red plate. Upon checking the bias it was off the charts probably due to either the wrong schematic (which I didn't realize another existed) or the higher line voltage. The pot is wired as a variable resistor and in between the 15k (r32) and the 47k. Someone before me changed the 15k to a 22k. Swapping it back to a 15k brought the bias down to 36 and 42ma's. I am thinking about placing a 10k in there to get a better range.

[g1]

The problem with having the pot in series with R32 like that is that if the pot gets dirty or goes bad, you lose all bias voltage and have meltdown.
Preferable would be replacing R31 (47K) with a 25K pot in series with a 25K resistor. That way if the pot fails, you go full cold bias. Having the 25K resistor in series with the pot limits the full hot setting.

[tdlunsfo]

Great point and thanks for the advice. I will try it out. Where would you feel comfortable running these tubes with the fan blowing on them?

[g1]

I would say 18W max per tube, as I don't believe anything about them being more than a 25W tube.
As you suggested, you can still decrease R32 to 10K for more range.

[loudthud]

Check the 150K 2W resistor that connects between the bias supply and the HV winding. It usually drifts up in value over the years. Replace with a 2W metal oxide type.

[tdlunsfo]

I actually changed that resistor during my initial troubleshooting. Just as you said, it had drifted up. Someone had completely recapped this thing prior to me except for the bias capacitors, which I also changed.

[Enzo]

Fan has nothing to do with the operating conditions inside the tube.

[tdlunsfo]

So I guess I incorrectly assumed the fan setup was like a heat sink and you could push the tubes if you could remove the heat. Seems the fan is more about increasing tube longevity rather than operating set point.

[Enzo]

No, the fan is about improving the longevity of everything ELSE but the tubes. The tubes are already sealed in a vacuum, natures own best insulator. They do radiate heat all over, and the chassis and surrounding parts get heated up. A fan can help remove heat from them, but it will have zero affect on red plating and such inside the tube.

[J M Fahey]

Just imagine this: put red hot tubes inside an expanded polystyrene cooler.

Fill all available space with fiberglass, cotton, wool or vermiculite.

Wrap the whole cooler in a couple thick blankets.

Now point a small puny fan towards the outside of the blankets.

Imagine how much you are cooling the tubes

Ok, very high vacuum inside the tube glass bottle is a better insulator than what I just suggested

[tdlunsfo]

Understood and thanks for assisting. I ended up with 29 and 33 ma's. Amp sounds amazing!