Since the Crate's OT is fine I'm just going to use it. I have to double-check the turns ratio but if it's a 4k:4/8/16 I'll have a really hard time justifying the cost of buying a new Edcor or ClassicTone.
For the power transformer, I've decided on the Antek AS-1T275 partially for the cost, partially for the insane performance boost you get from using a toroidal transformer. Completely unloaded, it'll put out ~400VDC through a bridge rectifier. If I somehow manage to completely load it (e.g. power tubes completely overdriven to hell... and I manage to throw away another 50mA somehow) the plate voltage will only sag to ~370VDC. And, of course, it can handle the heater current without breaking a sweat.
The only downside is I'll have to use a separate bias transformer, though people have been known to find bias taps on these that aren't specified on the data sheet. In that case I'll have to use the more traditional full-wave rectifier instead of the full-wave bridge, but it's got two identical secondary windings with the phases indicated so that shouldn't be a problem.
Actually building this thing will be a little tricky. It would be great if Crate hadn't put the tubes right smack dab in the center of the chassis. This means my options are limited:
- I could drill all new holes in the chassis for the tube sockets so that I have a nice big area in which to mount a turret board, but without buying chassis punches I'm thinking that would come out poorly.
- I could leave the sockets where they are mount a turretboard above the sockets. I'm not sure if I have the vertical clearance to be able to do this, and since the tubes hang down they're going to be pouring heat right into the turret board. Not ideal.
- Or lastly I can get some skinny turret boards or even single-row turret strips, ignore the factory layout, and do something a little closer to point-to-point.
This will also force me (allow me?) to disregard the original layout. That's fine; I don't particularly feel like buying some expensive cap cans anyway, and I can also design a quieter ground scheme.
For the bias supply: I'm going to try a Triad VPL36-140 which should be more than sufficient. Way more current than I need for a bias supply, but they're cheap. I've already purchased one of these for Bodie - running those 7591s in cathode bias has been a fun experiment but it kinda kills me to throw away ~15W of output power. Plus I have to bias them rather warm at idle to keep the rising cathode voltage from pushing them so far into cutoff, and even then the distortion tone suffers.
Of course, I could just follow the original schematics (in both cases) and build capacitor-coupled bias supplies. I've already had one bad run-in with those when I first started rebuilding Bodie - if they're not sufficiently loaded (say, you fire it up without the tubes in) the bias supply instead becomes a charge pump and all your filter caps see a rapidly-rising voltage. Props to whatever capacitors I was using back then for failing gracefully (instead of exploding) when subjected to 1000VDC. There have also been some rants against capacitor-coupled bias supplies over at MEF regarding their long-term reliability, so I think I'll just go with the $10 bias transformer and a half-dozen components, thanks.
Aah yes, and the filter caps. I'm keeping the plate voltages low (at least for a plexi) so standard 450V caps should be fine. Keeping the stock 450 and up plate voltages would mean needing to totem-pole some caps which takes up a whole bunch of room. Also I'm building this with the idea of using a quad of 6V6s (which is what was originally in the amp) and I know modern 6V6GTs can take that kind of plate voltage but why push it? So the amp'll break up a little earlier, oh no, not in a Plexi!
I still have to decide on whether or not to use a choke. I think most people would expect one in this sort of build, though they'd probably assume the bias transformer was one if I didn't tell them otherwise. And of course, this amp is only going to resemble a Plexi, not be a sincere attempt to clone one. The power supply is already going to be rather stiff, even if I go with the 68/69 filter cap values. Headroom would be improved, and the amp would be a touch louder.
I think I probably will use a choke, if only because I have to consider using two different types of output tubes. While the 6V6s won't pull much screen current, the EL34s definitely will. The question is will a quad of 6V6s pull as much screen current as a pair of EL34s? I'm thinking no. So with a dropping resistor for the screen supply, the screen and preamp voltages will drop more with the EL34s than with the 6V6s. Noticeably? I'll have to do the math. That may be a good thing though, really enhance the difference in the output tube choice. Pair of EL34s, it'll break up earlier. Quad of 6V6s, a little more headroom.
But with a choke, the EL34s will probably have similar headroom to the 6V6s. While arguments can be made about the plate curves of beam tetrodes (diode line) vs true pentodes (no knee) and their effects on distortion, the whole power section is wrapped in NFB anyway. There have been amps made that run a pair of pentodes alongside a pair of beam tetrodes with a pot to allow the player to pick one, the other, or some intermediate setting (IIRC a pair of 6L6GCs and a pair of EL34s) and most people report subtle differences at best.
So, I think that using an RC filter for the screen supply instead of an LC filter would enhance the "difference" in the two tube types. Artificially, it could be argued, sure. But the tone, man, the tone! Of course people will complain. Jim used chokes because Leo used chokes and you're not using a choke so this can't be any good. Bah. Bah!
I've been thinking that I'd like to try to sell this one but maybe this would be too much for the average guitarist. Well, no, I can put external bias points on the back of the amp. And the bias might not have to shift much. And hey, if the the primary impedance of the OT is what I suspect, they can even run a pair of 6V6GTs in there if they use a different speaker tap.
Man, I haven't even gotten to my preamp tweaks yet.
So yeah, a whole lot of work goes into these firebottles.
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