6aq5 "18 Watt" TMB

[Matt D]

So my next project is going to be a 6aq5 version of Ritchie's 18 Watt TMB circuit. I'm going to use a Hammond AO44 power and output transformer.

I'm keeping the EZ81 from the Hammond but running it off of a separate filament transformer to free up enough heater current for three 12ax7s. Not the most elegant solution but I'm working with what is cheap and/or free.

The Hammond PT will give me 310 volts on the plates, 300 on the screens, and I should be able to get the voltages for the 12ax7s down from there by experimenting with the dropping string resistors from the HT.

I've been able to find a bunch of stuff about using 6v6's in this circuit but little about the 6aq5s. Will the same changes to the PI and tail resistor be appropriate or do I need to do something else considering the lower voltage?

I'm thinking 250R for the power resistor, as that is what the Gibson Scout uses.

Anyone build this one with the 6aq5's? Also, if anyone has any suggestions about what values to start of with for the HT dropping resistors that would be great.

Thanks.

[camerongrieve]

Check out a datasheet for the 6AQ5 - you'll want ~250V DC on the plates, so you'd have to find a way to drop your B+ by 60V or so

[topbrent]

If it were me,

I would ditch the 6AQ5 idea and run 6V6's.
I would also axe the EZ-81, as it is a heater current hog.

If you go this route, you will have plenty of heater current to run 2-6V6's and 3 12AX7's.

I am running an AO44 as an 18 watt Lite IIB right now with a SS diode rectifier, 2-EL84's and 2-12ax7's and the PT doesn't even get warm.
- Sounds great too.

[Phil_S]

I own a Gibson Scout. I think I understand your approach. There is no problem running the 6AQ5's at 310, but you do need to watch the screens. If I recall, my Scout runs at about 310 plate 305 screen.

Opinions may differ on this point. The 6AQ5 is electronically equivalent to 6V6. The difference and the problem is the bottle size or probably more properly the miniaturized structure inside the bottle. It won't stand up to the same high plate voltage as the 6V6. It is fairly well accepted that 6V6 will run at 340-350 plate volts, even higher sometimes, without concern. The 6V6 spec sheet is identical to the 6AQ5 and also discusses 250V which appears to have been regularly ignored in the good/bad old days.

If you are building a new amp, unless you happen to have a good stock of 6AQ5 like I do (about 50 of them), what's the point? I think that Topbrent is pointing you in the right direction. Build it with 6V6's, use a solid state rectifier, and include a sag resistor if you like to give the feel of a tube rectifier.

Whichever tube you choose, I'm thinking 250 Ohms is going to run hot. You'll have to see where you end up with the build. I'd start with 270 Ohms, and maybe look at 300 Ohms as an alternate. Also, the exact same PI is appropriate to both tubes equally.

As for the HT supply ladder, I'd start with the standard 18W dropping resistors and go from there if the result isn't to your liking.