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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 05 Dec 2010, 12:58 
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Joined: 05 Jan 2010, 21:16
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Location: Las Vegas Nevada USA
I couldn't resist putting in a ridiculously low bid on a Peavey Classic 120/120, 120 watts RMS per channel (presumably class B) all tube amp. Much to my surprise I won the auction and am now the proud owner of the amp.

It came with Chinese 6L6GCs (there are EIGHT, 4 per channel), but I swapped them out with Russian 6P3S-E power tetrodes ($5 ea.for $40 total) and the amp sounds much better. For a multi-purpose amp with so much power it sounds really good IMHO. There is no practical limitation on bandwidth in the range of human hearing, and the bass is tight, POWERFUL, and well defined using the 4 ohm output tap on 8 ohm speakers. The response is very flat. These will provide full, unattenuated bass through 3-way passive crossover speakers with a flat response when using the the lower voltage, lower impedance taps. There is no audible clipping prior to developing the fear that your speakers are about to go up in smoke. It's that loud. And I'm using 125 WRMS per channel rated JBL4412 3-way studio monitors that aren't that efficient (90dB).

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The rack case with input and output connectors, the two humongous power trannys and the two huge output trannys alone were worth more than the $237 I paid for it, It's in great condition.

Now the project becomes to rebias the tubes into a class A amp. I realize I'll take a big hit on power (35 watts perhaps instead of 120), but the distortion should go down. I'm also considering some changes to the preamp section, but first I need to lay my hands on a schematic. The power amp I can trace out. Same with the preamp but schematics would be nice.

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PostPosted: 05 Dec 2010, 23:03 
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I should have called this "my first listen to a class AB tube amp".

Although it sounds punchy, the detail is not quite what I want in a hi-fi amp. The Oddwatt sounds a lot more "transparent". This amp colors the sound. It sounds good, just not up to the standards of a true hi-fi amp.

I can see the traces from the top of the circuit board so tracing the circuit schematic should be easy. I'll modify the circuit to run class A and to run the anodes at 25 or 30 watts idle dissipation and we'll see...

If that doesn't work I could always simply use the power transformers and the chassis for a rackmount KT120 Oddwatt and buy new Edcor OT iron for it. I REALLY like the appeal of a fully enclosed hi-fi amp with a kid on the way. It's a lot safer than an exposed amp, and I MIGHT get away with this in the living room if I can claim it's a normal amp. This chasss would make it simple.

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PostPosted: 06 Dec 2010, 13:27 
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Is that big amp marketed as a pro-audio amp?

Why is a tube amp not normal? ;)
Cheers

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PostPosted: 06 Dec 2010, 13:46 
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It seems to be marketed as a general purpose amp, either for PA use, or for bass or guitar use along with full range speakers and a guitar preamp that has a guitar cabinet emulator, or with a guitar preamp and guitar or bass cabinets that have no tweeters.

It's not very old. This would be perfect for a subwooder amp. The two channels can be bridged for 240 watts RMS of tube power. As a stereo guitar amp head it would rock.

It has presence and resonance controls for shaping the tone of a bass or guitar, but when these are turned all the way down they have no effect and the amp is flat. The specs say 120 Watts RMS per channel into 4, 8, or 16 ohms (selectable) at 5% THD, or 240 watts mono.

The price was right. It weighs over 60 lbs and I'm thinking I may not modify it but use it for a subwoofer amp instead.

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