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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL84 Triode PP Design

sorenj07

Member
2006-05-04 9:15 pm
Berlin
[IMGDEAD]http://img83.imageshack.us/img83/2083/el84triodeppsignalschemog2.th.jpg[/IMGDEAD]

I have the EL84's, the OPT's, and the 6N23P's are on the way.

Power supply:
Hammond 273BX (350-0-350 175mA) rectified by two UF4007 per leg bypassed by .1uF 1KV, into 193M choke (10H 300mA 63 ohms) and 440uF 450V cap, bypassed with 1uF 630V, feeding B+. B++ taps off a 20H 20mA 1.67K and 100uF 400V bypassed with another 1uF 630V, for the two 6mA regulators at 275V. The C- supply is a voltage doubler running off the unused 5A tap.
 

Salas

diyAudio Chief Moderator
Paid Member
2002-10-08 11:31 am
Athens-Greece
But how it works in Baby Huey? Is it partial?
 

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Is there any way to get 5mA+ through the 6DJ8s other than to use a negative voltage supply? I'm looking at doubling off a Hammond 166G80 (80V .5A) for a -210V supply, which, with a 21K 5W tail resistor, should spit out 10mA for the cathodes to share right? My plate resistors would be 24K 2W.

I assume that a LM334 won't like putting out 10mA very much...

About the bias adjustment setup: would a 10K pot and 5K resistors to ground work better?
 
Audio,

Perhaps neither Stuart or I have explained completely (Stu, if I have you completely wrong, scold at will). The problem here is a regulator (voltage or current) that will reliably operate at a drop of 2v or less. Under conditions of dynamic cathode swing, maybe a lot less. What if your tail is going in and out of regulation with signal?

With the right CCS chip, maybe you could even take a bridge and supply off the heater winding (the heaters, with their several amps draw would probably never even notice), and get your chip over the problem; but the bias supply looks like the best bet to me, maybe because I'm planning to do the exact same thing.

To me, trying to regulate current over a volt or two drop looks pretty shaky.

Aloha,

Poinz
 
All right. I've finalized design with a negative voltage supply feeding a simple tail resistor as in the Musical Machine. I'm open to experimentation once the amp is done, however, and a CCS might be worth it. If it's legit to tap off the heater voltage to feed a CCS, the 9V or so that would result seem ideal. Anybody got any chips/implementations that can do 10mA? Kind of a broad question, I know..

Also. this one is for Poindexter about his Musical Machine gain. What is the reason for the ~4-5% difference between the top and bottom plate resistors? I echo this setup in my latest iteration, 51K and 47K on the top, 2x51K on the bottom. I read somewhere that this corrects phase imbalance between the top and bottom triodes or something? I'm using snooty quality PRP 1-watters from PartsConnexion, for peace of mind :)
 
One reason to buy Morgan Jones "Valve AMplifiers" is the current sink designs he has there. They work very well - something like page 134 b) or c) - they also use a -15v supply.

I'm listening to exactly such an amp as I write. The front end in in this case is an ECC40 with one of the CCS above under it. Sounds very detailed. I have the EL84s in triode and no global feedback.

I prefer the ECC40 by some way - it has glorious treble - but it is a rimlock socket and so not eerybody's cup of tea. Socket and tube easily available in Europe and on ebay, though.