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Old 25th August 2010, 11:00 AM   #1
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Default "Universal" octal amplifier?


I am thinking of building a tube amp (actually a pair of monoblocks) and I was hoping I could make it universal so I can try the different output tubes.

I have a pair of 4K P-P OPT's with UL tap. (35% I think).

I was hoping to be able to use a design that allows for the different kinds of octal power tubes available with some minor modifications. I have some 6L6GC's, but I may want to put in KT88's or 6550's 5881's, EL34's EL156 maybe etc...

Is this possible? What would need to be changed for each tube (bias obviously... what else?) Is there such a design already?

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Old 25th August 2010, 12:31 PM   #2
chrish is offline chrish  Australia
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I have just completed this pair of monoblocks 6L6GC AB2 Amp

have tried 6P3S-E, 6L6, EL34, KT88, 6BG6GA (with adaptor). Just requires an adjustment for bias with the tube change.


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Old 25th August 2010, 12:55 PM   #3
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Thanks! Interesting read, I'm skimming the thread now, lots of useful info.

Do you think I can use my 4K OPT's for this purpose?
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Old 25th August 2010, 01:52 PM   #4
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You might like to have a choice of B+ voltages too - a Tektronix 'scope power transformer could give you a number of possibilities...
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Old 25th August 2010, 01:58 PM   #5
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That's a good idea actually. I do already have two xformers, 300-275-250-0-250-275-300 @250mA and with one 3A 6.3v winding. Pretty basic, but they'll probably do.

Thinking about using 4 6D22S diodes I have in bridge (ah the joys of single tube diodes).

If the xformers are too weak I'll use them for my EL84 project, which is next after this project.
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Old 25th August 2010, 02:04 PM   #6
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FWIW, Pete Millett sells a 6SN7 based universal driver PCB for just such an amp design........although his schema shows a 6600 ohm output transformer.

With the 6L6GC, KT88, & EL34, they can be plug-n-play since they have the same pinout, just use a selector switch to change the cathode current (assuming cathode bias).

Push-pull KT88 class A amp with "universal" driver PCB
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Old 25th August 2010, 02:16 PM   #7
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I'm sure I'll be able to modify the circuit I end up using once I get my head around that calculating loadlines stuff. 4k does not seem unreasonable for a 6L6,KT88 or EL34.
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