Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: CascoSieg on September 24, 2021, 08:16:26 pm
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No, not James, Harry! :icon_biggrin:
This is just amp #3 so, you know, I still get excited easily ;)
I got interested in Hiwatts and was thinking of building something along the lines of DR504, but don't need another big amp, so decided on a push-pull EL84 power section. I love Mark Huss's site and was inspired by his DR504 layout, which it appears is fairly true to the 70's model he was emulating. I drew from Frank57's "Custom 20 Tube", the Trinity design, the other Hiwatt designs in Doug's schematics page, and various and sundry other bits around here, and came up with this. I didn't like the sound of it at first, but then after revisiting my work a few times and addressing some things (like unsoldered connections!), it's got a solid clean-on-the-verge-of-almost-dirty sweet spot, and behaves pretty well now. The saturating OD will take some getting used to, but I can imagine using that in some applications. It's not a Hiwatt, but it's unique among my other amps, so I'm satisfied. Still waiting on tolex and corners...
There's still a squealy oscillation problem (around 2kHz) when running near to wide open (preamp and OD), that I was able to improve but not fix with some wire route changes. If anyone sees anything glaringly wrong with my layout or dress that might cause such an oscillation, I welcome suggestions.
The B+ is relatively high, but I think not unheard of for EL84s adn 12AX7's. As long as I bias the power tubes appropriately, is there any reason to be concerned about, or to lower the HT voltage (see schem)?
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and a few more pics...
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Nice, well done!
I suggest to take the bias feed direct from the winding, before the fuse.
Consider if that fuse blows spuriously, the output valves will run unbiased for while, until the other fuse blows.
The transformer should be protected of course, but (especially with modern valves), it can be damaging to run a high cathode current.
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Thanks pdf64! Will do that.