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alexg 14th June 2012 07:01 AM

12at7 dc coupled to 12at7 cathodyne to drive EL34
I just acquired a pair of Dynaco Mk 3 chassis with power transformer and OPT, without the driver pcb.

Am thinking of using EL34 for power tube and 12at7 for preamp (half of the 12at7 for voltage amplifier dc coupled to the other half acting as cathodyne phase inverter).

Would the 12at7 work? ( I have a few of these tubes that is why I wanted to use it).

Or I can have a 5842 as voltage amplifier dc coupled to a 12at7 long tailed pair.

Any insights? Thanks.

SY 14th June 2012 09:39 AM

You could do that, but the swing requirement would be pushing it. You certainly won't have enough gain to wrap some feedback around it. Your first stage gain is likely to be somewhere around 40-50, second stage gain will be 0.8-0.9, which means that it will take nearly a volt of input to drive the output tubes- you have no excess gain to use for feedback reduction of output impedance and distortion.

You could either use a higher gain input tube (like a good pentode) or an input voltage amp driving a long tail pair.

TonyTecson 14th June 2012 10:16 AM


5842 has a mu of 40, the 12at7 as LTP gives you a gain to about x30 so you have lots of open loop gain to play around with....

in my latest EL34 pp build, i used a Russian 6J9 (6688/EF180) pentode as input and 6CG7 ltp drivers in a mullard 5-20 topology....i liked the result....

the Russian 6J9 was a revelation to me, it was very quiet.....

Osvaldo de Banfield 14th June 2012 11:58 AM

If well design in the stage, in terms of voltage gain, impedances, etc, the stage will work. 12AT7is a mu 70, so some 12AX7 design will work approximate, and some re-work may be necessary.

pauldune 14th June 2012 01:49 PM

Menno van der Veen did it all the time; Ecc81 as voltage amp, dc coupled to cathodyne, to drive 2x el34. I build one of his schematics some 20 years ago; it sounded good. Later I thought it strange to bias it in the worst part of its characteristics, but after some studying, it was actually a lot more linear than it looked at first. If you look up his name in google, I'm sure you'll find some schematics.


pauldune 14th June 2012 02:00 PM

Hey I even found my own thread:

In the first post is the original schematic.

After long experiments with ccs's and so on, I reverted to the original; with some good 6201's it sounded the best.

rongon 14th June 2012 02:08 PM

Why not a 12AT7 (or 6DJ8) LTP into a 12BH7 push-pull driver? (6CG7 would also work for the push-pull driver, but would be harder to use DC-coupled from the first stage, as both 12AT7 and 6CG7 require fairly high plate voltage.) All balanced to the output tubes, but you'll get enough gain (if you do it right). Definitely more complex than the voltage amp to LTP (Mullard 5-20) style, but sounds better to me.


TonyTecson 14th June 2012 11:02 PM

why not restore the Mark 3 to original specs? the input tube can be a 6GH8, 6BL8, etc...not so hard to do...

the space for the pcb can be replaced with a blank piece of aluminum and p2p wiring can be done...

alexg 15th June 2012 12:41 AM

Tony, I was planning to replace the pcb with blank aluminum, but would really love to only have one tube, 12at7.

If this will not work, I also have some 5842, 5687, 6688 and 6cg7.

alexg 15th June 2012 12:44 AM

Paul, I really like 12at7 so I might try what you have suggested. Thanks.

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