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I'm testing a EL84 SEP (that's SE Pentode) design. It started with a biamped Motorola comsole chassis with two 6BQ5 HF amps and a mono 6V6PP bass amp.What amazes me is I am measuring 17 watts at 1khz to 15khz with only 2% distortion on the EL84's. Now the tube manual does state this power level but not the distortion which is amazingly low for an SE at this power level. That's using the stock OPT's which are tiny. With a cheap Hammond 125CSE, I get down 1.5% thd. Now of course below 1khz, things go bad prety fast but for an HF amp crossed at 1khzwe don't care. The 6BQ5 needs 8v of drive to put out 17w so a good driver would be a SRPP. Presently I'm driving with my signal generator, a Heath IG82.
I am thinking of replacing my 6V6 PP feedback hf amps with this design. I have yet to listen to it. I don't consider triode operation feasible as you get about 1.6 watts max. There must be something else evil here as it can't be this simple to make a good HF amp - or is it?
Follow Ups:
You are not going to get 17 watts from a single EL84 even in pentode mode. The manual gives 5.7 watts, which is very close to the maximum possible 50% efficiency at the maximum plate dissipation of 12 watts.17 watts is available in push-pull AB1, again according to the specs.
I measured 17v @ 1khz. Perhaps my test jig was being fooled somehow so I'll run it again. I was using a high enmd scope with built in RMS math functions. I think I'll use the good old DMM as well this time.One tube manual, I think it was GE did claim 17w but it didn't say it was PP or SE. It was one of the later charts. Perhaps it was class C?
ANyway I have to agree, this is too good to be true so I must be getting fooled somehow.
With an 8 ohm load works out to 4.5W.
No it was RMS??? Well I'll have to double check because something isn't right here.
I checked it again and founf the scope set to measure PP versus RMS. So the tube manual is correct as it should be!
It wasn't just a wild guess. Sure does bug you when things don't make sense!
not bad for a like amp.
I use a triode strapped class A1 PP 6V6 amp with no feedback. My B+ is about 300VDC and the tubes aare cathode biased. It gets almost 3 watts. I run it full range. Given some of the speakers I use it has to be clipping half way often but I'll be darned if I notice it. In short I don't think the low wattge will be a problem for a HF amp and would encourage you to give it a try. I don't think the distortion will be a problem either. I feel it is a very clean sounding amp. It started life as a Magnavox too and I have found their OPT's to sound a lot better than they look. While I am no longer using the OPT's I still use the PT and choke.
I looked on the Marchand site but I didn't see any schematics. Can you point me in the right direction? Amps are easy but I need help with building a crossover:(
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