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Old 15th June 2012, 12:57 AM   #11
alexg is offline alexg  Philippines
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Quote:
Originally Posted by Tony View Post
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
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Old 15th June 2012, 02:37 AM   #12
AJT is offline AJT  Philippines
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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...
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Old 15th June 2012, 08:15 AM   #13
alexg is offline alexg  Philippines
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Quote:
Originally Posted by Tony View Post
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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Old 15th June 2012, 11:50 AM   #14
AJT is offline AJT  Philippines
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^yes...
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