Any examples of a 12AU7 LTP phase inverter?

[Lauri]

Example of direct coupled 12AU7 LTP.

[Reeltarded]

I did exactly that in my 4-4-0. 12ax7 drives those little triodes too hard. I was too lazy to modify the circuit, so I swapped that in. Much better.

Lazy coward!

Antisolderist!

[johnnyreece]

I did exactly that in my 4-4-0. 12ax7 drives those little triodes too hard. I was too lazy to modify the circuit, so I swapped that in. Much better.

Lazy coward!

Antisolderist!

You know, I should get in there and do it "right". Someone (I think romberg?) came up with some good values for a voltage divider at the PI. Maybe that's something I should do while I'm between projects...You know what they say: If it ain't broke, fix it 'til it is!

[B Ingram]

Example of direct coupled 12AU7 LTP.

Raezzor will have to be careful here: direct-coupling ahead of this long-tail means the DC conditions of the inverter, as well as the stage prior, is critical, all to save the phase-shift of a coupling cap.

Meanwhile, a 100mV guitar signal only needs a gain of 35x to smack the output stage silly. Why are we working so hard here? "Anything" will work here (even a very poorly conceived & executed plan), except in the case that the circuit oscillates or the output tubes arc due to over-voltage.

[Raezzor]

I feel like I've been officially labeled a disturber of the peace!

But I'm glad it stirred up some good discussion. lol

I certainly wouldn't directly couple the PI to the power tubes. I planned on using screen and grid stopper resistors anyway so oscillation shouldn't be an issue. Reading that data sheet on the 6EH5 having issues with lead dress and oscillation further cemented that decision for me.

Thanks for the help all, it is much appreciated! I certainly have a lot to think about now!

[martin manning]

What B+ are you planning? The 6CA5 data sheet lists 130V max.

[didit]

What B+ are you planning? The 6CA5 data sheet lists 130V max.

A thread with comments that hold a high opinion of how much higher a plate voltage these will tolerate ::

https://www.diyaudio.com/community/thre ... me.355113/

My only direct experience showing similar results was with some 6005s (mil-spec 6AQ5) pushed well above 350V. However TubeLab does have reputation for sharing consistently wise observations and results.

..

[martin manning]

You know, I should get in there and do it "right". Someone (I think romberg?) came up with some good values for a voltage divider at the PI. Maybe that's something I should do while I'm between projects...You know what they say: If it ain't broke, fix it 'til it is!

R Aiken has tutorial on AC coupled LTP design here: https://www.aikenamps.com/index.php/des ... e-approach
Assuming a 12AU7 and using his rules of thumb, I get plate loads 16k5 and 18k, bias resistor 1k, and tail resistance of 19k (including any tail resistance for GNFB.
It has open loop gain of ~12.5 dB, where a 12AX7 LTP might have ~18.

What B+ are you planning? The 6CA5 data sheet lists 130V max.

A thread with comments that hold a high opinion of how much higher a plate voltage these will tolerate ::

https://www.diyaudio.com/community/thre ... me.355113/

My only direct experience showing similar results was with some 6005s (mil-spec 6AQ5) pushed well above 350V. However TubeLab does have reputation for sharing consistently wise observations and results.

Good to know! Sticking with the data sheet max would have the preamp tubes running at very low voltages.

[Helmholtz]

I like this LTPI calculator: https://www.ampbooks.com/mobile/amplifi ... alculator/

[Raezzor]

I like this LTPI calculator: https://www.ampbooks.com/mobile/amplifi ... alculator/

I did try using some of the calculators on that site, but I think the low voltages I plan on using send them into a tizzy so I wasn't too sure how accurate Mr. Kuehnel's calculators were.

You know, I should get in there and do it "right". Someone (I think romberg?) came up with some good values for a voltage divider at the PI. Maybe that's something I should do while I'm between projects...You know what they say: If it ain't broke, fix it 'til it is!

R Aiken has tutorial on AC coupled LTP design here: https://www.aikenamps.com/index.php/des ... e-approach
Assuming a 12AU7 and using his rules of thumb, I get plate loads 16k5 and 18k, bias resistor 1k, and tail resistance of 19k (including any tail resistance for GNFB.
It has open loop gain of ~12.5 dB, where a 12AX7 LTP might have ~18.

Those numbers are almost exactly what my back-of-the-envelope calcs were at. Like others have mentioned my concern now is voltage swing, but from everything I'm seeing it should be plenty at 120ish B+ to the PI to swing the ~9.4 VPP or 10.5ish dBv needed to drive a 6EH5 push-pull pair to full power.

I plan on rigging up a prototype rig here shortly with just the PI and power amp sections to see what my output looks like using these numbers.

What B+ are you planning? The 6CA5 data sheet lists 130V max.

The 6EH5 sheets list 140v as max on the plate. I plan to use something close to that, but I'm still figuring out what exactly my power rectification and filtering will look like since I was hoping to find a way to run the 12AU7 at a higher voltage than the 6EH5 screens. I've never seen the screens voltage taken after the PI but if there isn't any reason it couldn't be done I may just do that.

[cdemike]

A little late, but I stumbled across this looking for something else. Just in case you haven't started building yet.



Original thread is here: https://ampgarage.com/forum/viewtopic.php?t=34492