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Old 28th Jul 2006, 9:29 pm   #1
adibrook
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Default power amp design

I'm still working on this power amp (formally the KT61 amp).

I finally found some power valves that i like. I have a pair of Mullard EL37's. I like them because they have a nice classic shape (unlike the el34) and they glow nicely.

I think they are suitable for use in this amp, and they look well on the shassis.

I'v ripped out all the old circuitry anyway, so i'll have to rebuild it to suite el37's.

The rectifier is a 5Z4G, which was originally in the amp. I also have a GZ32 and some other suitable replacements. The original 5Z4G is pretty well used.

The phase splitter is a 6SN7 in the usual confuguration (when one triode is grounded grid).

The only thing i'm not sure about is the 1st stage. I have an octal sockets fitted, and at the moment it's got a 6SJ7. But it doesnt look too good. It hardly glows.

I'v kind of changed my design philosophy abit. Instead of having lots of gain in one unit, i think that a better solutions is to have several units of moderate gain. This keeps feedback lower.

Does anyone know what i can use? Maybe even a single triode will be allright?

This amp will get use on stage at at least several shows, so i want it to look cool.
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Old 28th Jul 2006, 10:10 pm   #2
jjl
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Default Re: power amp design

Hello Adi

Don't know whether this has been mentioned before but the EL37 is not a common valve & is sought after in valve collector and "audiophool" circles, so it might not be such a good idea to push your EL37 pair too hard.
I believe that the EL37 was the Philips / Mullard response to the RCA 6L6 range & the GEC / Marconi / Osram KT66 & that the EL34 replaced the EL37 in the Philips / Mullard range in the early '50s .
My understanding is that the KT61 is a close relative of the 6L6. The 6L6 must have been made in huge numbers in the past & variants are in current manufacture so I'd suggest that it would be a good idea to try out some 6L6 variants in your amp e.g. the 6L6GC.

Hope this helps


John
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Old 29th Jul 2006, 6:23 pm   #3
adibrook
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Default Re: power amp design

oh...ok.

Well, my el37's are slightly used (but not by me yet....so should be alive). If they are really more sought after than other power valves than if anyone wants to swap give me a pm. I'll consider anythgin of approximatle simular power output, and glass (preferably classic shape) construction. I dont think glowability is an issue with power valves of that size...they all glow pretty well.
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Old 29th Jul 2006, 11:03 pm   #4
boiss
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Default Re: power amp design

Hi Adibrook,
I don't know the EL37, but I am collecting the bits for a stereo amp using 813s and they really light up, I reckon that visually they're one of the best at over 7ins high , I picked them up on ebay in the states for $12 US. In triode mode they put out 25W SE and as tetrodes 45W SE (100W in PP). Only real problem is the heater supply 10V at 5A which requires DC for SE use. Valve sockets and top caps are available for around 3$ each, do a search on "my 813" for some circuits etc and suppliers of relevant bits.
good luck,
Mike.
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