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12at7 dc coupled to 12at7 cathodyne to drive EL34

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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.
 
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.
 
alexg,

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.....
 
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.

Paul
 
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.

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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...

mark3.gif
 
alexg,

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.....

Hi Tony,

Can you post your schema? I have a pair of 6688 waiting for a good use.

Alex
 
the scheme follows that of the mullard 5-20 except that instead of ef86, i used the 6688, i do not have the amp with anymore, but let us see...

the 6688 likes higher plate current at low volts, different from an ef86.....

run your ltp on 400volts B+, plate currents at 2mA each so a 4mA CCS at the tail and 56k/2w plate resistors...

the 6688, i run on about 5mA at B+ of about 250volts, screen current is 1/5th of plate current, you can size your resistors accordingly.....i came up with 39k plate load resistor, 180k g2 resistor and cathode resistor of about 390 ohms....plate voltage is around 75volts

going back to the LTP, with tail current of 4mA, tail resistor can be 18k, 3watts in case you do not want to use the CCS...
 
the scheme follows that of the mullard 5-20 except that instead of ef86, i used the 6688, i do not have the amp with anymore, but let us see...

the 6688 likes higher plate current at low volts, different from an ef86.....

run your ltp on 400volts B+, plate currents at 2mA each so a 4mA CCS at the tail and 56k/2w plate resistors...

the 6688, i run on about 5mA at B+ of about 250volts, screen current is 1/5th of plate current, you can size your resistors accordingly.....i came up with 39k plate load resistor, 180k g2 resistor and cathode resistor of about 390 ohms....plate voltage is around 75volts

going back to the LTP, with tail current of 4mA, tail resistor can be 18k, 3watts in case you do not want to use the CCS...

6688 is dc coupled to the 6cg7?
 
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