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Old 31st March 2005, 10:08 PM   #21
crtubes is offline crtubes  United States
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From performance perspective, OP's design is much like class B even if no cut-off.
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Old 31st March 2005, 10:10 PM   #22
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OP's?
Really am tired now, been up since 7
Steve
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Old 31st March 2005, 10:49 PM   #23
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Hello,
Is it ok to use normal low voltage transformers for the rectifier heaters, as they're floated up to b+ level...
Thanks,
Steve
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Old 31st March 2005, 11:40 PM   #24
MelB is offline MelB  Canada
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Oh Boy! A triode strapped KT88 amp. I have been enjoying mine for about 5 years now. Just retubed the KT88's with the same SED ones. The silver stuff was almost all gone inside the old ones. They were on about 10 hours a day on average for that long. I did a triode connected EF86 to a 12au7 split-load phase splitter => KT88's. I'll do up a crappy drawing (I can't draw!)and post it tonight. My B+ is about 420v and I get about 20w/ch out. I'm pushing a Hammond 272JX to supply both channels...we get a bit-o-sag on the B+ when both channels are pushed to 20w. I don't think that happens too often under regular listening.. MelB
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Old 31st March 2005, 11:44 PM   #25
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Cheers!!
Yeh mine sags loads at the moment, the blue glow on the glass dissapears on peaks n the sound goes flat Ive noticed the getter disappearing fast on my valvearts. my amps on like your ten hours a dayish, but it's only been going a year. Already had one die too, jus stopped conducting. Anyways, lookin forward to your schematic!!
Steve
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Old 1st April 2005, 03:10 AM   #26
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Default schematic!!

Hello,
I'm getting there I think...
Steve
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Old 1st April 2005, 03:54 AM   #27
MelB is offline MelB  Canada
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Here it is Steve. Like I said...it's a mess. Forgot I directed coupled the EF86 to the 12AU7...doh! It's a damn fine sounding amp though. No feedback! MelB
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Old 1st April 2005, 05:38 AM   #28
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The new schematic seems just fine. But there are a few of those "i'd do it differently but it doesn't really matter" things. I would have bumped up the filter caps in the bias supply, and used a resistor since there is no need for a choke. I also typically put a small value capacitor from the wiper of the bias pot to ground, but it isn't needed. I also do not understand the filament winding fo the KT88's. I see a "virtual center tap" going to a capacitor, as if you were planning to put a DC bias on them to reduce hum, but I see no place where this voltage will come from. I'd just eliminate that capacitor and connect the "virtual center taps" to ground, output stages won't benefit from an elevated DC voltage on the heaters. Your driver tube has the center tap grounded, while filament induced hum will be essentially negligible here, you could put a DC bias on it from either one of the cathodes of that tube, or by added some extra rectification and stuff on the bias winding.

Looks like an awsome amplifier! be sure to take pictures
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Old 1st April 2005, 10:13 AM   #29
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Hello,
Cheers Mel for going to all that trouble! very appreciated! Do you want to swap transformers? hehe, I want a center tap... Anyway, does it sound any better if you bias it a bit further towards class A? I've given my bias pots the ability to put voltage out from -30 to -45V in an effort to get more accuracy from the pots, maybe I should extend that to -55V for times when I'm not so worried about the quality....or my rooms getting too hot. Also do you know what primary impedance your outputs are?

Dr Photon cheers! Yes I'd like to have a bit more capacitance, I'll have to see how the funds go and if I've got anything left I can double up on the 75uF ones, space permitting as well. What does the small value cap on the bias pot wipers do? Just take noise to ground? Might be a good idea. For the heaters, I saw that 'virtual center tap' in a thread round here somewhere for getting rid of heater hum. Has to be done on the tube socket though I think or it doesn't work so well....I think, not sure of the reasons. I'll look into that a bit more I think as the simpler the better for me.
Cheers,
Steve
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Old 1st April 2005, 02:37 PM   #30
MelB is offline MelB  Canada
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Hi Steve:

I've never tried getting it up to class A. I'm limited by my power transformer here as it's rated 300-0-300 @250mA and it's driving both channels. I could try lowering the bias a tad I guess and run them at 50mA each. I never got around to that...I was just so happy with how it sounded at the time I left it. I do have an o-scope and can't "see" any cross-over distortion at 40mA. I don't think true class A is within my reach. If anyone could suggest an upgrade for a power transformer here. Something I've always wanted to do put never got around to. Looking for 320-300-0-300-320 maybe at >400mA that can also supply 6A @ 6.3v and will fit here: http://www.diyaudio.com/forums/attac...amp=1107823338

The output trany's are 4.3K rated 60watts Hammonds. I must say they do a much better job on the finish of their output transformers than they do on their power transformers.

Missed a resistor on my drawing in the bias supply. Between the first cap and the 63v Zener is a 4.7K.
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