Tube subs (12AY7, 12AU7) in old Traynor bassmasters?

[GerryJ]

Hello, I have some old bassmaster amps, all 3 preamp tubes are 12AX7. The volume pots on these amps go very quickly from silent (at '0', or say 7 o'clock) to LOUD (at '1', 8 o'clock) to VERY loud at 3 (9 o'clock). Beyond 3, it just thickens up more and starts overdriving. I'm not really interested in changing out the volume pots to higher ohms at this time. I have spare 12AY7s and 12AU7s, and wonder if I sub these in at V1 (and/or V2 or V3?) if it would make the volume taper up a little more gradual.
Some people have written that going from 12AX7 ('gain factor' of 100) to 12AU7 ('gain factor' of 20) may be harmful in some amps because 'the 12AU7 will draw much more current.' I've also seen this 'gain factor' defined as 'mu' or 'voltage gain.'
I've done some web searching, but haven't seen any sources that might educate me on the real differences between the 12A_7 family of tubes, and how one or the other tube type might draw too much voltage or current in simple amps (like these) which use 3 12AX7s and 2 power tubes. Any short explanation or links would be appreciated!

[ViperDoc]

If your question is about when you turn the pot, you notice an immediate increase in gain and an unappreciable difference as you continue to turn the knob, this is an effect of the pot taper itself. The only way to alter that is to install a pot whose taper is more gradual, such as a 10% vs a 30% taper. Alpha and even modern production PEC pots have a resistor track that ends prematurely on a longer 3-lug ground connector than you would expect, essentially initiating the ground connection "earlier than zero". It makes the low end more abrupt when engaged. I don't like that either.

Swapping the tube for a lower amplification factor will lower the overall gain offered by the tube itself. I expect you would encounter less gain in the beginning of your pot rotation if you changed the tube, but whether that's enough for you requires an experiment. Try swapping a 12AY7 (mu of 44) in for V1 and see if you like the results. You can also try a 5751, about mu 70.

[GerryJ]

Thanks! As you probably know, those 60s-70s Traynor tube amps are pretty rugged both physically and electrically. I did some more reading and research, and the caveat about preamp tube subs for some amps- best I can tell - might be relevant for switching tubes to a higher amplification factor (gain factor, mu) rather than a lower one, based on heater/filament voltage and current draw. In other words, if you're starting with a 12AX7 (gain factor of 100), going down to lower gain tubes of 12AT7 (60), 12AY7 (45) or 12AU7 (20) should be safe. Whereas, in some amps, going up on tube substitution (i.e., putting a 12AX7 in a spot designed for a 12AU7 or 12AY7), might pose a problem for the circuit due to the increased current draw of the heater. ( Probably more of an issue in delicate home stereo tube amps, or very old smaller guitar amps...)

[sluckey]

might pose a problem for the circuit due to the increased current draw of the heater.

All tubes mentioned in this thread draw the same heater current.

[dorrisant]

Just a thought... If you want to reduce gain, you could change the Volume pots out to a lower value, not a higher one. This bleeds more of the gain of through the pot's connection to ground. A lower value pot presents a better path to ground. In a lot (not all) of the YBAs, the Volume pots are 4MA. Most people find that swapping them for 1MA pots is just what is needed... Go lower if you need it. With these vales stock, the problem is less about the taper and more about less bleed to ground.

Flog me if I'm wrong.

[_Steve]

You'll be fine. Play with V1 & V2. V2 is a little more preferable from signal/noise ratio perspective, but just find what you like. 5751s are a good candidate.

[GerryJ]

Thanks guys! Just spent a few minutes subbing at V1 to start with- both tubes did make the volume pot more linear than the 12AX7, without any adjustment needed in tone controls. I'm not currently gigging, but using these tubes would probably be very helpful in dialing in the right volume more easily, if I was playing a place where the soundman or proprietor want 'background music' levels!