I finally got around to trying the zener trick I mentioned a few months ago. Bodie's idle bias voltage was around 21V, and that surged up to 31V under heavy overdrive. A pair of 12V 5W zener diodes in series now clamp the cathode voltage at 24V. Maximum clean output power has risen from 19W to 23W. On the 'scope, crossover distortion is now only barely present during overdrive and the overdriven tone has increased rather dramatically. I think next time I'm going to have to try just straight fixed bias; 7591s lose so much in cathode bias. Not that I really need another 10-15W of output, but the tone is much improved. I can probably lower the plate voltages to something more sane than, say, 480V to keep the output level reasonable.
I'm going to be tweaking this amp for years, I just feel it.
For now, I still need to come up with a decent bass control. While I really like the "independent tone controls scattered throughout the preamp" approach, there aren't many very good one-knob bass controls. Sure, you can take the baxandall stack apart and just use the bass control from that, but it didn't work too well. It may be time to try a flat tilt control in place of just a dedicated bass control. That would give the user the option of cutting treble early and/or late, which would open up the preamp distortion characteristics a bit.
I'm tempted to start looking at FFTs of the frequency response to see if the NFB loop is causing any high-frequency strangeness. Given the (deliberately) limited bandwidth of the amp, square wave analysis is somewhat problematic, but it seems to indicate there's some unwanted phase shifting at high frequencies resulting in excess treble.
Then again, I might not notice that if I weren't using these vintage EV SRO speakers. It might even be good with a particularly dark speaker, but I feel a well-designed amp should work well with any speaker, though that may be something of an impossible goal.
Anyway.
It seems I lucked out picking a voltage for the zeners to latch the cathodes to; the plates are happy at the dissipation they're subjected to under heavy overdrive. I wonder if I could go a little colder though, get a little more squish out of the output section.
And I'm still thinking about the power supply. It's working fine, but could it be better...?
Yep, years of tweaking ahead.
Do you have a diagram of the circuit or a link to one? I think that I know how you hooked the zeners up, but I'm not certain.
ReplyDeleteHere's a good one: PP Output Stage with Zener Clamp
Deleteand here's the page: TubeCAD
Alternatively, some people use a zener in series with a small cathode resistor and a large bypass capacitor in parallel with both, which is probably a more reliable way of getting mixed fixed/cathode bias. I think Fender ruined the idea for a lot of people during the Silverface era, just like they did with ultralinear outputs.
Zeners apparently have even chances of failing open or short, when they do fail, so either fusing the cathodes or the fusing the B+ is a good idea. Then again, fusing the B+ is always a good idea...
I used a pair of 1N5349B's in series, both to get the 24V and to keep the potential dissipation high.
I'll post an updated schematic; I've done a few more tweaks that I haven't posted about.