Super Twin Reverb... UL?

[martin manning]

So the voltage ratio you measure putting signal into the secondary is quite different from what you got when the signal was applied to the primary. You could try applying the signal to the screen taps and measure the secondary voltage, get that ratio, and divide it by the 20.16 you got applying the signal across the plate leads to see if there is a difference.

If the DL fraction is 12.6%, that just means less negative feedback. Mullard included 20% DL as well as 43% on their EL84 data sheet, with different output power and distortion results.

[h4x0r]

iirc (and it is a long time since I have owned, much less worked on a super twin) the OT's are driven as parallel triodes. The actual output transformers are non-existent. You will not find a replacement off the shelf and need one to be custom wound if yours does the doodoo.
Also ST mandates a fully matched set of tubes... they are quite the problem for mismatched singles. I learned the hard way a few times. If you want to use a super twin, you have to "pay to play" as they say. Matched sextet of OT's is going to hurt.
I have run my ST and one customer's ST with 7581's and found them to be somewhat a more stable / sweeter. It's a personal preference. I think they appreciated not blowing OT's all the time.
Also if I recall the transformers tap was closer to 18-20%, which should come as no surprise, since the amps were designed for maximum power and not High Fidelity, (contradictory really, Fidelity is what ultralinear designs are really targeting).

I'm not sure about the whole 43% tap thing. In all honesty, the times where I have had to actually measure windings to a higher degree of accuracy outside of obvious mismatching taps, I can faithfully say I've not found a 43% tap. In fact on guitar amps, quite the opposite. mostly less than 15%.

Is there are particular reason for the exercise other than a theoretical one? Do you have a problem with one of the taps/windings? I hope there is no problem. If there is, you will need to have a transformer custom wound. I am not aware of any off-the-shelf solutions for an ST or STR. I do applaud your deep-dive into the STR, because they are not the easiest thing to work on.

Unfortunate that you have the compactron. the ST's did not have it if I recall, and I have never seen an STR in this neck of the woods. I doubt any ever came here tbh. Plenty of ST's, but never seen one with reverb or a compactron. Having said that, I think you will find that the hundred bucks for the 6C10 is cheaper than punching holes in the chassis (or finding space) for three 12ax7's...

[dorrisant]

Having said that, I think you will find that the hundred bucks for the 6C10 is cheaper than punching holes in the chassis (or finding space) for three 12ax7's...

Hi, h4x0r

Welcome,

No disrespect, but you must be joking... I have been waiting for the time to completely rebuild it into some sort of Dumble circuit... SSS seems most likely. I have no real affection for most spring reverb and this one is nothing worth a $$$ compactron for. This amp circuit works fine as is but there too many things that I would need to be completely rebuilt to make it worth keeping it. Not to mention the distortion circuit that sounds like ass. Maybe the Super Twins were better... I have never played one.

Also, I will rebuild almost any amp that I get my hands on... I polish turds. Lot's of beaks and feet builds that end up turning out nice, and worth more than the parts I started with. I built a Small Special with 10 watt Hammond iron into a 30's tool box. Maybe I didn't post that... it is killer! I may post it in the morning if I can't find it here... Or now: https://ampgarage.com/forum/viewtopic.php?f=4&t=34451

With respect, I do appreciate your input. The numbers you are claiming and Martin's comments reveal a deficiency in my last testing efforts. I will have to get that heavy thing back out of the cab so I can take better measurements. I may have to unsolder a few leads this time. I think the line out portion of the circuit may be affecting my results. I'll unsolder and remove doubt.

[h4x0r]

Ahh I misinterpreted...
I thought you were having an issue with the OT and were rebuilding the unit (resto to standard), hence my 6c10 vs 3x12ax7 replacement comment.