12AT7 Single ended

[walkman]

Hi All,

Found this design for a 2 tube receiver using a 12AT7 (or 12AU7) that may be able to be used in a mini amp design.
Likely not much power maybe 50mw and not much headroom.

https://www.industrial-electronics.com/elec_exp_1959_9.html



T1 is 4000, and b+ would be roughly 165v.

Best bet  for a guitar amp may be to have a pentode preamp into the first half of the 12at7.
For safety a power transformer or wallwart with voltage multiplier would be the way to go.

I have a 3300:16 OT and suitable power supply that will give 40ma at 120v after full wave rectification.

Anyone prepared to ‘sketch’ this out or comment on design considerations?

[Latole]

With no isolation Power Transformer, these amp are call widowmaker amp.
Don't built it as is, use isolation PT


https://www.tropicalfishvintage.com/blog/widowmaker-amplifiers-explained

[walkman]

With no isolation Power Transformer, these amp are call widowmaker amp.
Don't built it as is, use isolation PT


https://www.tropicalfishvintage.com/blog/widowmaker-amplifiers-explained

Certainly and thanks for restating the risks. As pictured it is unsafe design.

I should have said I have a suitable (240v) power transformer to be clear.

[PRR]

I guesstimate 125mW at 20% THD, so 50mW "not stinky" is probably in the ballpark. (About 5mA max in the 'power' stage, and obviously less in the volt-amp stage.)

If you can spring for more tube in front, the same 12AT7 parallel at 300-400V makes a large part of a Watt, as seen in thousands of Fender Reverbs' reverb drivers. But that mean more supply voltage and preferably a 15K OT, and more risk of lease-breaking.

[walkman]

I guesstimate 125mW at 20% THD, so 50mW "not stinky" is probably in the ballpark. (About 5mA max in the 'power' stage, and obviously less in the volt-amp stage.)

Do you think 4000 is a reasonable load at 150v - 165v?

Would this also work with the volt stage?

In one point in a different build with a low b+ round 100v I had an triode stage equivalent to 12at7 running with 3300 plate load at about 2.3ma but was not really sure that the plate load should be less than the internal resistance something like 11k.

I’ve read the optimal load would be 1.5 - 2 times internal plate resistance.

I’m still trying to understand what is ‘more important’ plate current, voltage swing or gain. Obviously will vary from design to design and its a balancing act.

[Latole]

Low power amp.

Use the amp schematics we know it work

[Latole]

Or Pepco ;

[PRR]

Do you think 4000 is a reasonable load at 150v - 165v?
...not really sure that the plate load should be less than the internal resistance something like 11k.
...understand what is ‘more important’ plate current, voltage swing or gain.

You already decided you don't want the MOST power or the LEAST distortion. In most real situations, the ear is tolerant. So don't over-think. Build it and see.