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Serow 13th January 2003 04:36 AM

modifications to compact EL84 amp
The following schematic is found at triode electronics :

Serow 13th January 2003 04:42 AM

I really like the simplicity of this circuit and the EL84/6BQ5 output tubes. However, I would like help with the following modifications:

1) Increasing power to greater than 10 watts.

2) Possibly changing the driver tube. I haven't heard much about the 6C4, but I know it is out of production. How about an ECC99 or 6SN7? I read in the circuit description that it only needs to swing 11 volts.

I'll need some detailed assistance, as I'm just starting out.


planet10 13th January 2003 08:06 AM

And it wouldn't hurt to lose the tone control R3 & C1 I believe.


fdegrove 13th January 2003 11:52 PM


Planet10 is correct,loose the tone control...

Up C4 to 470 mF,up C2 to 0.22mF and your going to have a little powerhouse.

The 6C4 should still be around at reasonable prices,check Antique Electronic supply.

All of this won't yield a a more powerful amp but the upgrade will give quite a bit more headroom and better bass performance.

Heck,I may build one for the bedroom system...:cool:


Colt45 14th January 2003 12:03 AM

6C4 = 1/2 of ECC82

personally id drop the resistor splitter thing, use a 6SN7/6CG7, use 1/2 for gain, and 1/2 as a cathodyne.

bump B+ to 300v, leave Rk the same..
should give you ~17w, IIRC.

fdegrove 14th January 2003 12:11 AM

6C4 = 1/2 of ECC82


personally id drop the resistor splitter thing, use a 6SN7/6CG7, use 1/2 for gain, and 1/2 as a cathodyne.
And loose the elegance in the process for what?
A dB or two?

If you need the power look elsewhere I'd say??:(

I dubbed this the "Tiny Penthode"

BTW,a nice compact circuit can be made using a pair of ECL86s this it the "Pocket Amp"


Colt45 14th January 2003 12:45 AM

what elegance?

planet10 14th January 2003 01:01 AM


Originally posted by Colt45
what elegance?
The passive phase splitter -- that is the whole point of this amp.


Colt45 14th January 2003 02:26 AM

i dont see the point though.

12AU7 is the same size as 6C4, same tube, just 2 in 1, and you could use a cathodyne instead of the funky phase splitter.

but whatever works..

Tom Bavis 14th January 2003 03:36 AM

Would a ONE resistor mod retain the elegance? Remove the ground at the low end of the cathode resistor and add another 100-200 Ohms from that point to ground. The balance of the output stage will be improved. Calculation of the wattage is left as an exercise for the reader (translation...Where did that calculator go?). Of course you'll need more drive voltage - nothing is free. But it isn't hard to find a tube with more gain than a 6C4.

For more power, you'll need more than 250V, especially with the extra drop of the added resistor.

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