Traynor Bassmaster (YBA-1) fixed (nonadjustable) bias- problematic?

[LOUDthud]

In my YBA-1A Mk II, which does have an adjustable bias trim pot, my trim pot is almost (but not completely) counterclockwise (i.e., allowing lowest cathode>plate current flow, 'cold') and it shows a plate current of 38 mA, which is 81% of assumed maximum dissipation of 47 mA (25 watts/537 V). Playing this quite loud (over 100 dB) but only with a touch of nice warm harmonic distortion, in a dark room the plates stay completely dark, no hint of any redplating whatsoever.

Look closely at the EL34s. On my YBA-1A you could see that the screen grids were glowing a dull orange. That's the way it came when it was new. When I replaced the power tubes, I didn't have a chance to play the amp, even at low volume. I just set the bias colder so the screens didn't glow and packed the amp across town to the rehearsal space. When I played it through my two 2x15 cabinets, it sounded horrible. After I adjusted the bias hotter, (I had installed a pot that was accessible without opening the amp) it sounded normal again.

Check what I said about the 470K 2W resistor in reply 10. I'll bet yours will measure high and that will cause you to turn the bias pot almost as cold as it will go. Replace with a 2 or 3 Watt MOX resistor.

[GerryJ]

Thanks! To the best of my ability, I think the only thing glowing red inside (and it's bright red) are the filaments. There's also a little blue glow nearby but not much.
On my YBA-1A schematic - came with the amp - it's a 1972, and the resistor coming off the PT to feed the bias (R33 if I'm correct) on this model is 150k, 2 W. I'll have to go inside and measure it when I get a chance. The line then goes thru a few more resistors, including R7A, a 10k pot, then on to the tubes. I've enclosed the schematic.
Some people on other sites have posted that some earlier YBA-1As didn't even have adjustable bias.
Of interest about my amp, there's a note about using the R7A bias pot (actually a trim tab in the amp), note #7 - "adjust R7A to allow 4 V DC across R25 (470 ohm 10 W) screen resistor." Not biasing by cathode to anode voltage or idle current. Whatever thought process is behind that is way over my head, lol.

Traynor YBA-1A Mk II your amp ('72).jpg

[LOUDthud]

Yes, R33 is the one.

[GlideOn]

Yes, there's a 70% sweet spot for Class AB amps and 90% sweet spot for Class A amps, though keep in mind there's no policemen enforcing these rules and you can perfectly well adjust by ear to a setting the you find suitable.

Some high voltage Silverface era Fenders run 55%-60% bias but they debatably sound better than evenly biased. They also have larger coupling caps in the preamp which gives more signal bandwidth (bass, treble) so in some ways a cold bias may allow an amp to sound clearer than not.

These are not HiFi amps designed to sound clinical - these are designed to be distorted, watts and all so don't be afraid to run colder, warmer or whatever you want. At the end of the day they are a wear and tear item that will need to be replaced sooner or later. If you want to save money on tubes, stick to Shuguang EL34Bs which can take pretty much whatever abuse you can throw at them, sound great and are less than $20 a piece around holiday sales.

[GerryJ]

Thanks. Since the discussion extended to the YBA-1A Mk II (the one with the fan), I was perplexed how they say on the schematic to adjust the bias trim pot (R7A) " to allow 4 V DC across the Screen resistor R25 (470 ohm, 10 watt)"
I understand how the bias pot adjusts negative voltage for the control grid, but I don't see how it relates to achieving a 4 V drop across the screen resistor. Currently I just use a Eurotube bias pro (measures plate current and voltage).
Can anyone explain the electronics here?- thanks!

[LOUDthud]

As long as there is no significant change in Plate Voltage, Screen current will change in proportion to Plate current.

[pdf64]

Yes, there's a 70% sweet spot for Class AB amps ... Some high voltage Silverface era Fenders run 55%-60% bias ... Shuguang EL34Bs ...

6L6GC 30W anode limit is under the design maximum rating system, the EL34 25W limit is under the design centre system.

The 6L6GC anode does not have 20% more dissipation capability than the EL34 anode.

A design centre limit for the 6L6GC might be about 26W.

I suggest that a fixed bias AB idle limit guideline for either type is around 17-18W.

60% of a design maximum limit seems roughly equivalent to 70% of a design centre limit.

[GerryJ]

As long as there is no significant change in Plate Voltage, Screen current will change in proportion to Plate current.

...so by lowering the voltage across the screen grid resistor (by 4 V DC as suggested on the schematic) you're lowering the screen current, and this implies (looking at the schematic) that you're increasing the negative voltage (bias) applied to the control grid and therefore idle plate current is lowered. Ok. But it just seems like a very odd way to adjust bias- instead of saying on the schematic, for example, "adjust R7A bias resistor for idle plate current of 25-32 mA".