Whole-Watt 6AU6 amp

[PRR]

Stereo Headphone amp. Mod for guitar.

https://pdf.yt/d/8Qrati6FKe_KWtG3

[DummyLoad]

kewl... 


--pete

[Backwoods Joe]

So the OT for one 6AU6 would have a 25K primary. What primary impedance would you need if you ran a quad of 6AU6's in push /pull at around 300V? Add a hot preamp and a LTP phase inverter. Sounds like fun!

[DummyLoad]

50K Ra-a for 2 tubes in P-P class A1


half that for 4 tubes in P-P class A1


--pete

[PRR]

> the OT for one 6AU6 would have a 25K primary.

*Depending on the Volts and Amps* that you run the tube at!!

For a 160V supply I figured that 6AU6 could still work near 10mA but then likes 15K (happens to be a Fender Reverb transformer).

> a quad of 6AU6's

The "advantage" of 6AU6 is that it needs very small drive, which may save a half-tube in the total path.

If you are peppering the chassis with sockets, you gotta ask: "would it be simpler to add a small-tube driver so I can use fewer, less sensitive, but more robust power tubes?"

Also 300V leads to high OT impedance which is either a more expensive winding, or gives lame bass (see response curves in article). A pair of 25L6/6Y6 at 160V will zone-in around 5Kpp which is a much easier winding (and more common part) for the same 4 Watts.

[DummyLoad]

> the OT for one 6AU6 would have a 25K primary.

*Depending on the Volts and Amps* that you run the tube at!!

For a 160V supply I figured that 6AU6 could still work near 10mA but then likes 15K (happens to be a Fender Reverb transformer).

> a quad of 6AU6's

The "advantage" of 6AU6 is that it needs very small drive, which may save a half-tube in the total path.

If you are peppering the chassis with sockets, you gotta ask: "would it be simpler to add a small-tube driver so I can use fewer, less sensitive, but more robust power tubes?"

Also 300V leads to high OT impedance which is either a more expensive winding, or gives lame bass (see response curves in article). A pair of 25L6/6Y6 at 160V will zone-in around 5Kpp which is a much easier winding (and more common part) for the same 4 Watts.

understood. what i posted assumed the same conditions only expanded. perhaps i should have clarified.


--pete

[drgonzonm]

kewl... 


--pete

DL,
whatever our differences, you have a nice website, and from what I can tell you produce decent and unusual products.

Drgonzonm

[drgonzonm]

Stereo Headphone amp. Mod for guitar.

https://pdf.yt/d/8Qrati6FKe_KWtG3

From I understand in your posting, the values found in the attachment use one channel.

Thanks for info and the new application. 

[PRR]

> assumed the same conditions

Joe actually asked about a higher B+ than the published plan. Since it is already near Pdiss limit, current must be lower, so V/I Impedance must be even higher.

However 25K is already annoyingly high, choices become very limited, and the "25K" in the article is probably just a best-fit to what was in his catalog. And yes, class A P-P will want twice that, 50Kct, maybe 60Kct at the higher voltage. Which is just absurd for full-range audio without micro-managed winding techniques ($$$). Going for four is a step in a better direct, 25Kct-30Kct. Still high when you can get 4 Watts with just two fatter bottles at somewhat lower B+ and much more favorable OT impedance. Or maybe work the same 250V-300V in even bigger bottles and happy impedance to get much more than 4 Watts, if you can stand the power.

"High"-power audio has always been a compromise between what the tubes like (high impedance) and what a happy transformer gives (medium/low impedance). Getting plate resistance down was identified back in the day that a 1 Watt amp was "high power" for home use. Type '10 yielded to types 45 and 50, then to 2A3. Pentodes had to tread the same path; here it is dynamic large-signal resistance (knee height) instead of incremental plate resistance.

That said, I've worked small tuner-triodes at 280V in 30Kct (military surplus) to make a part-watt. Burned up the tubes fast, but they were going out of style and were cheap in the day.

Eight pair (sixteen 6AU6) comes near 8K loading and 16 Watts. That is a lot of hole-punching, and almost 5 Amps of 6V; two 6V6 do about the same at 1/5th the heater power.

[JB]

I quite like the idea of actually trying it as a headphone amp, but with lower B+ and lower Z output.  I bought a couple of 6AU6 back when you did your 1/3W amp, might dig them out and have a go. A Hammond 125ASE would do it.  I've got a 125CSE I could try as a single channel lash up.

I reckon it could be a candidate for building into a little box and powering from a reverse driven 12V mains transformer off an AC wall wart.  Over here, UK, there are some EU/US transformers with a split 115/115 primary which you can series or parallel as appropriate.  Might end up with around 150V after losses.  I guess at this reduced HT the screens could live at similar voltage, just a small dropper from B+?  Could even go fancy and have separate left and right HT supplies from the two windings!

Headphones these days seem to be significantly higher than 8 ohm - more like 32, 64, 200 etc.  Would it be sensible to parallel an 8 ohm resistor across the transformer secondary to keep the reflected load happy?  Shouldn't matter too much about throwing some signal away, unless using high impedance cans.

 

[HotBluePlates]

I reckon it could be a candidate for building into a little box and powering from a reverse driven 12V mains transformer off an AC wall wart.  Over here, UK, there are some EU/US transformers with a split 115/115 primary which you can series or parallel as appropriate.  Might end up with around 150V after losses.

There are no 240v:240v isolation transformers? Those might be cheap-ish, and all you need is a filament transformer to build the whole thing according to plan.

[jojokeo]

That said, I've worked small tuner-triodes at 280V in 30Kct (military surplus) to make a part-watt. Burned up the tubes fast, but they were going out of style and were cheap in the day.

...going out IN style I say...or you could say that at least they're "dying a hero"!