What to do with Super Twin Reverb?

[66Fuzz]

I was gifted a Super Twin Reverb and it is going to need a cap job and new power tubes. But I'm wondering if I should modify it or just leave it as is? It is very heavy. Perhaps put Neo speakers in it? Or?

Would love some suggestions, schematics, etc...

[Stevem]

I would first recap it and stuff in new power tubes and of course set the bias balance.

Then judge what you want to be different about the amp.

[solderhead]

Warning: Long Post. Here's what I did with mine:

The STR is essentially the same amp as the Fender Studio Bass from the same era. It makes a nice punchy amp for keyboards or 4-string bass, especially if you've got the original Pyle speakers. A common mod is to move the electronics into a head cabinet and use EVM-15L in a TL-606D cabinet. That's like having a mini-SVT. But the STR tends to be a maintenance headache (for reasons discussed below) and they're sort of an odd bird because nobody really really wants or needs a super-clean 180W Fender amp for guitar -- it's so damned loud, as in Ted-Nugent-ruin-your-hearing-loud, that I don't use mine for guitar. For bass it's a nice amp but it's no SVT. I went through mine and did a full restoration but after spending a lot of time fussing over the project I find that I don't use the amp and it's rarely brought into rotation. As a guitar amp it's just too loud, and as a bass amp the SVT does everything it does but much better.

Here are some ideas for resto-mods. They address deficiencies in the amp's design that IMO need to be addressed before considering mods:

1. Power Tubes. Re-tubing is a bit of a PITA as it is not easy to find a matched sextet of 6L6 that you can buy with a click -- you'll have to custom order them. As the tubes age you'll find that maintaining bias is a bit of a pain.

2. Biasing 3 tubes in parallel is a pain and the circuit's simple balance control is worthless. You'll find that even if you work with matched tubes they won't stay matched for long. Eventually one tube will end up pulling more current than the others and it will fail. To keep the power amp performing properly you're going to need to spend a lot of effort making sure that the parallel tubes share the load evenly. Fender paid no attention to this in designing the amp. Ampeg did. Ampeg built special comparator boards to monitor the cathode current of each tube and to adjust bias accordingly. The Ampeg monitoring circuit will even shut the amp down if there's a critical failure. Fender has no equivalent circuit so if something goes very wrong the amp will just go up in flames. You really need to install complete transformer protection circuits to avoid spending $300 on a new OT and $400 on a new PT.

3. Custom Bias Circuit. At the absolute minimum I would recommend redesigning the bias circuit to have 6 individually adjustable bias pots. Better yet, build an Ampeg-like circuit to monitor cathode current, control bias and protect the output transfromer. R.G. and I corresponded way back when about designing a microcontroller-based bias module for the sextet.

4. Selectable Power and Speaker Switching. Most of the time you won't need to have a full sextet of 6L6 running at 500V. You might consider switching off pairs depending on how much power you need, to have a 60/120/180 watt amp. With this I'd add speaker switching to disconnect one driver or to change the wiring between series-parallel so that you're working with a matched load.

5. Re-Cap. Even if your caps are recent they're likely to be bad. This amp is the hottest of the Twin designs and it bakes your caps like a Kenner Easy-Bake Oven bakes cupcakes.

6. Get Rid of the 6C10. Compactrons are ridiculously expensive when you can find them. Most people either drill the chassis to add another tube and run a pair of 12AX7 instead of the 6C10. Some will create daughter boards to allow a pair of tubes to be plugged into a board that plugs into the Compactron socket.

7. Install MOV on the PT primary supply to protect it from voltage spikes.

8. Install MOV on the OT primary, each plate to CT.

9. Install a high-Z load resistor in parallel with the speaker jack to protect the OT.

10. Fuses: AC Mains, B+, heaters.

11. Reverb. It's weak. V1B drives both the Reverb driver V2B and the EQ driver V2A in parallel and the reverb driver gain doesn't seem to be all that great. Some people suggest that rewiring these circuits so that the output of the EQ is used to drive the reverb circuit. I haven't addressed this in my amp, though I think you should be able to get plenty of gain out of the V2B pentode stage without the need to change the amp's topology.

11. Fan Cooling. With 6x6L6 it gets HOT in there. Passive cooling doesn't work all that well. Any kind of air circulation will help prolong the life of your parts.

12. Remove Distortion Circuit. The distortion circuit on this amp is pretty worthless and that front panel pot takes up valuable real estate. I'd look at using the switch for variable NFB mods.

13. Isolate Circuit Ground from Chassis Ground. This is a bigger project. Removing the death cap and re-wiring the amp's grounding system to raise the circuit ground above chassis ground will help with noise -- but it's going to require isolation jacks and complete redesign of the ground system.

14. Dropping the B+ rail to the preamp. That 30k 20W resistor creates a sawtooth waveform that rides on top of the B+ rail. The variable/sawtooth B+ will intermodulate with signal and produce nasty sonic artifacts. There are several ways to reduce B+ that can avoid the sawtooth problem.

IMO these are the basic problems with the design of the amp that I thought needed to be addressed for reliability/safety before I would consider spending time modding it. This list isn't complete, but it's getting long enough already. Doing all of this to an STR is going to be a huge project -- which might explain why your friend gave you the amp.

That said, this amp takes pedals like a Fender and if you need something that's loud and clean then you've got the right chassis for your project. When properly tuned, I don't like this amp all that much for guitar or bass, but it can be a killer setup with a Rhodes or a pedal steel.

Have fun!

[66Fuzz]

Oh darn...

I thought about the individual pots for the bias, but its too much work for me at this point, oof!

I was toying with the idea of making it a bass amp similar to the Mesa D180. But, I might just get it up and running and pass it on to some one else...

[Roe]

I've been thinking about a 180w head but the issue here is the lack of secondaries. just a 4 ohms output is not flexible enough. Perhaps more secondary taps can be added though

[Stevem]

It's very easy to get even a matched octete of output tubes from Euro tubes on the west coast.

[Roe]

It's very easy to get even a matched octete of output tubes from Euro tubes on the west coast.

TAD also offers matched sextettes https://www.tubeampdoctor.com/en/6l6gcm ... RT106&c=76

[66Fuzz]

I was able to order caps and a matched sextet from Antique Electronic Supply. I'll just get the amp functional and see what it sounds like for now...

[martin manning]

With six output tubes you have more options to swap them around to balance the current flow on the OT primary.

[solderhead]

With six output tubes you have more options to swap them around to balance the current flow on the OT primary.

When I received my STR the previous owner must have been doing exactly that. The amp had 6 different brands of 6L6 in it -- no two tubes were of the same brand. I can only surmise that the PO was a really smart guy who was practicing your method of manual current balance by frequently juggling the tubes and taking current measurements ... or that he was just a dufus/cheepskate who refused to buy a matched set of tubes and just stuck any available 6L6 into the sockets when he needed to do so. Based on his destruction of the reverb pan by manhandling the amp, I'm inclined to think he was a dufus/cheepskate.

The problem I've had with using the average current balancing method is that it's good for the current flow in the OT primary, but it doesn't guarantee that the 3 tubes on one side will always share the load evenly. IME with tubes that aren't even close to being matched (like the dufus example), or with tubes that are given enough time to wear away their matching, there is eventually going to be one tube that ends up doing the heavy lifting and that ultimately leads to the tube failing. To protect against that kind of failure I feel better if there's some form of protection circuit built into the amp. that can vary from simple fusible resistors (like Ampeg used in the original SVT) or more advanced comparator circuits (like Ampeg uses now). Using sextets and octets in parallel push-pull configurations is kind of risky because of the load balancing problem. Most designers avoid that topology and don't go beyond a quartet unless they can take steps to protect the amp. I think this is pretty important as I've been burned by this before.

For you guys who are buying matched sextets online -- are the sellers offering matched sextets now in the pull-down menus or do you have to call them to order a matched sextet? As I recall I never saw matched sextets in the pull-down website menus and I had to place phone orders to get them. But I have to admit that I haven't looked for a sextet in a very long time as I just don't use the amp.

[solderhead]

I've been thinking about a 180w head but the issue here is the lack of secondaries. just a 4 ohms output is not flexible enough. Perhaps more secondary taps can be added though

Hammond 1650WA

[martin manning]

Tube swapping is good for dealing with the couple of mA differences the you see in matched sets or that develops as the tubes age.

[66Fuzz]

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?

[martin manning]

Do you know what the primary impedance and screen tap voltage ratios are?

[66Fuzz]

Do you know what the primary impedance and screen tap voltage ratios are?

1K7 Primary. Not sure of the UL ratio.