Just a SWAG; it’ll either be OK or possibly run a little ‘cool’. But I’d get a simple little bias probe type tool and check, and be safe now rather than sorry later. I believe you can run a little more heat through a 6550/KT88, so it should be OK. It’ll have a lot more headroom and probably more bottom end, so let’s hope that’s what you’re after.

It shouldn’t hurt a tube to run it cold, but it can sound like crap. That huge crossover notch sounds pretty raspy, and just not good. With a bunch of fuzz you’d never know, but if you play clean at all you won’t like it. But that’s a worst case scenario.
I believe you can put any tube in any amplifier, or close to it. Even if you put a 6V6 in a Fender Twin Reverb, you won’t necessarily ruin the tubes or the amp. Say there was -52VDC for bias on the Twin, or even -56VDC just for argument. A Deluxe Reverb may have -35VDC for bias with +415VDC for Plate voltage. I believe most Twin schematics show +460VDC, so that bias voltage may be high enough to not kill 6V6’s! Please note I would never do it without first putting the bias as high as possible (say -60VDC or -65VDC) and measuring the idle Plate current. But the point is that anything can be done, and it’s not complete crazy to not worry too much about blowing up an amplifier.
Try it. You may like it, you may not. You may need to adjust the bias to add some heat through KT88’s, or you may not. But I still say the clean headroom goes up a lot, as does the bottom end. Have some fun, and learn some useful stuff as you go. But you should be using a bias probe, and that will come in handy for future experimenting with output tubes.

I said ‘close to it’, and there’s no reason for me to back down. It can be done. I’ve done the 6V6 in a Twin trick, and it works. Tubes have a different impedance, but most of the tubes we use today are close to being neighbors that it doesn’t enter into my mind. And you can load 6L6’s at almost any impedance you want. The old stuff was up to 10K and Fender went down to 4200-ohms. I’ve bought 3800-ohm OPT’s and they work just fine. The power output goes up, and so does the bandwidth and distortion. I’ve measured 6V6 OPT’s anywhere from 6600-ohms down to 5K. I built a 4 x 6V6 amp and used a Twin OPT (which is where I got the 6V6 in a real Twin idea) and it worked. That’s 2100-ohms. Marshall used 1900-ohms. That is too low, even for a fool like me. But anything else? Fair game if you crank the Plate current down to nothing and check everything first before adding heat.
No, this is where I have more test equipment than God, less common sense than a soap dish, and the experimental curiosity to be dangerous. If you have sense, maybe you aren’t as foolhardy as me, but what you can learn in one evening is worth more than spending a year in front of a computer reading Forum posts. The GEC KT88 data shows 4K for a pair, 460VDC, and generating 100-watts. So maybe, and that’s a big maybe, the power transformer can’t handle the extra filament current, or the Plate current. Biased up properly (no crossover notch), maybe the output transformer can’t……..
Ah, just measure everything, and maybe you can’t do it. But I wouldn’t say ‘NO!’ until I measured everything. That’s what I do anyway, just to kill a lazy Sunday. Remember many Marshalls came without any Screen Grid resistor at all, and they’ve lived this long. It’s up to tube quality, and how hot you run them. If you measure the voltage drop across a Screen grid resistor, you’ll know if it’s up to the task. Remember, the smaller the value, the less of an IR drop, so lower wattage can be fine.
Without knowing what Mr. Grim knows, I shouldn’t kept my mouth shut, or at least tempered the answer. But still, if I put the bug in his ear on learning more, then my work here is done.

Hi guys,

I have a Studiomaster VLM (50W MesaBoogie Mk2 clone) that has gone terminal! My amp tech advises that the torroidal transformer is u/s and that, unless I find a replacement, the amp is as much toast as the transformer!

The problem is, we have no clue as to the spec of the transformer.

Studiomaster have a few guys from the 80s who remember the amp but the tech-specs have long gone and only hazy half-guessed information remains.

I wonder whether you could help us out, please? My expertise is limited to “man with screwdriver and meter” level and thus I’d need to be spoon-fed to some degree.

I believe we need to know:

1. what are the primary and secondary winding specs?
2. can a substitute torroidal transformer be obtained (or are we into bespoke re-wind territory)?
3. is there a suitable external transformer that could be used to sit in the case or hang off the underside of the amp?

Your thoughts and expertise would be much appreciated. Thanks!

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