What to do with Super Twin Reverb?

[martin manning]

Put some AC on the secondary and measure the plate and screen voltages.

[Roe]

I have another idea. What about converting the power section to four KT88's?

Do you think the OT would be close enough to use safely?

should work, but the PT could be stressed somewhat due to 1a extra heater current draw (4x1.6a vs 6x0.9a). Plate voltage with 6l6s seems to be around 500-520v

[66Fuzz]

Performed the cap job, replaced the tubes, balanced. It sounds pretty good, but all the pots need cleaning...I will clean later tonight and report again...

[solderhead]

Yes, balancing isn't a problem when the tubes don't vary all that much. IME it becomes a real problem though, when the tubes get out of balance and the balance isn't monitored or controlled in some way. When one tube out of 3 ends up drawing excessive current the problem can rapidly get worse while escaping detection until the tube reaches end of life and it's too late to deal with the problem. When things go too far the problem can result in a catastrophic/destructive failure mode that could have been avoided or at least controlled if a functional protection circuit was in place.

Anecdote:
I run a pair of SVT 2-Pro rack units for bass, one as the main, the other as a backup. I had a tube go dead-short and the protection circuit shut the amp down before any further damage occurred. I switched to the backup amp and played on. When it was time to service the amp all that was required was a tube change. Without the protection circuit a dead short would have resulted in dumping B+ at full current across the OT, heating it to the point of melting the insulation on the wires and shorting out the OT. That would stress the PT. If the PT insulation failed before the main fuse blew then the amp would have self-destructed and taken out both the OT and PT, which would cost somewhere in the range of $1k to replace on an SVT, or about $750 for the STR.

My point in telling this story isn't to argue that it's unsafe to run a sextet and manage the balance problem by tube-juggling. It's to say that if/when you eventually suffer a catastrophic failure with a sextet then you're better off having some kind of protection circuit in place than not having one. High power and Protection were two of the main features that Ampeg brought forth with their "Super Vacuum Tube" or "Superior Valve Technology" marketing. The SVT was designed to protect itself, initially with fusible resistors and later with cathode current comparators. The Fenders didn't bother to do anything about protection. The result was the difference between easy repairs vs. catastrophic destruction. IMO the push-pull sextet is an inherently risky topology, and that if you want to use that topology then it makes sense to employ some sort of fault monitoring / circuit protection to protect the amp from a catastrophic failure mode.

Going beyond the balancing issue, there's another reason that nobody designs 6x or 8x 6L6 amps today -- cost. In addition to the load balancing problem, the tubes are just so much more expensive today compared to the 70s that it makes sense to use a smaller number of higher powered tubes. The design is a lot simpler, the build cost is cheaper and reliability is less of a problem.

When I was doing my STR restoration I had thought that converting to a 4x6550 amp would be a nice option, as managing a sextet of 6L6 just seemed silly to me, in addition to the cost of 6x6L6 being higher than 4x6550 at the time. But as Roe said, the heater rating for the PT would likely be under the necessary current rating by a full 1A, which kills that idea. To go with KT88 you'd have to add an extra low V transformer in what is an already very cramped chassis. I gave up on that idea.

I think the STR is a good example of how Fender optimized the design of their big amps in the 70s in order to minimize production cost. Back in the 70s I think it was more expensive to add iron and copper windings to a PT to increase it's current output than it was to add a couple more low current tubes. 6L6 tubes were dirt cheap in the 70s but they're a lot more expensive now. The balance of cost at the time favored a sextet of low cost 6L6 and a lower current PT. Things have changed a bit today, as vacuum tubes in general are not as common today and costs are higher. Specifically, 6L6 are a lot more expensive than they used to be. Today it's very uncommon to see any new design that uses a sextet or an octet of 6L6 -- everyone prefers 6550 or KT88 when high power is required.

[66Fuzz]

I finished servicing the amp. It is quiet, very powerful, and does sound great. I'll leave it as is for now. I cleaned all the pots and touched up the most critical solder joints.

I will add a muffin fan to ventilate and I may opt for some Neo speakers in the future, as this amp is very heavy as is...

Changing it to four KT88'S seems like a fool's errand at this point....

[solderhead]

Good to hear that you've got it done. Be sure to wear earplugs. That's one of the loudest amps I've ever heard.

[66Fuzz]

Here is a clip I made:

https://youtu.be/5jZdO79Eefc?si=hpVXii99y8UYQN-o

[solderhead]

Sounds like mine. Those Pyle drivers are punchy speakers. I've used that amp for bass a few times.