Spec for '65 Supro TBolt T3778 output transformer?

[tubeswell]

I am looking at the 5U4 rectified version (see attachment) (And yep I know that the voltage chart looks a bit wrong, but I cant take credit for it)

As with other TBolt schemes I've seen, the circuit states 'T3778' for the OT, but I can't seem to locate any definitive spec sheet for that.

I would guess its somewhere between 3k2 and 5k. Anyone got any more specific clue than that (that you would be willing to divulge)? Or even an opinion?



TIA

[EFK]

I've had and still have a lot of Supros. It's always fun trying to figure out what the heck Valco was doing. They used undersized transformers across the board and even used 6V6 transformers on some of the late 50s 6L6 models. I don't currently have a thunderbolt but I have had one, and IIRC the transformer looked identical to what is in my 88T (2 x 6L6GC and a 5U4). This transformer is 4K into 4 ohm (2 8 ohm speakers in parallel).

[tubeswell]

4k - Thanks EFK - So you think maybe in that schematic use a 30W OT (or 20W?)

[EFK]

I doubt any Supro transformer, ever, was rated more than about 20W. Between the unbypassed cathode resistor and all the signal padding, I have yet to find any 2X6L6 Supro that can even hold its own - volume wise - with a blackface deluxe. Of course the tone on all of them is great, very thick and greasy on the normal inputs, but they're definitely not ear busters. Keep in mind for the thunderbolts, the OT had to be small as they shoved it inside the chassis (at least on those I've seen/had). The schematic you show has a 8 ohm speaker listed - unfortunately, I don't remember whether mine was 4 or 8, but if it was 8 it probably had a higher impedance reflected as Valco seemed to reuse the same small number of transformers regardless of speaker load, so w/ a 1000 to 1 impedance ratio it might have been reflecting back 8k. I can tell you, having converted two 2x10 1690T amps to 1X12, those sound best with a 4K impedance (they are 2X6L6 as well), and sound the loudest too, as least as far as I've been able to determine. I have a bit of an addiction to these weird old amps and can't seem to stop accumulating them!

What do you think of the 5U4 with no standby switch?

[tubeswell]

What do you think of the 5U4 with no standby switch?

Being a rectifier tube, it should have slow start, but I have noticed that with some 5U4Gs, they seem to slam on instantly (although I also should note that since I built in rectifier anode diode protection in all my amps these days, it could be that those 5U4Gs were bad anyhow)

[EFK]

I was always under the assumption that the 5U4 tubes were direct heated = instant on. I know the debate rages as to whether or not that's an issue. I was after more volume w/ my 88T anyway, so I use a 5AR4 and just scaled the main dropping resistor to the preamp as well as the cathode bypass resistor to maintain the same approximate preamp voltages and power tube bias. Being cathode bias and pretty compressed, the slight added punch is nice.

[loudthud]

I have a Thunderbolt with a 5U4. It needs a cap job but I measured the ratio real close to 625 at 100Hz and 200Hz so that's 5000:8 . I thought I remembered this thing having one of those filter caps that plugs into an octal socket. But this one has a skinny Mallory 20 10 10 at 450V.


Edit: OOOPS the voltage ratio was about 25. Squared the impedance ratio is 625.

[EFK]

Those mallory tri-cap cans are usually the only filtering any of the Supros have. Sometimes there is one extra on the screens in parallel with one in the can, for some reason which I can't explain. I've never seen any up in the preamps of these amps; usually the last 10uf serves for the entire preamp through PI, but it can help to stick an extra 8 uf or 10 uf up on the power feed for V1. Doesn't seem to affect anything other than to cut a bit more hum I also found it helps with bleed through pulsing/hum off the trem circuit (recently had that problem with the 88T).

I just picked up an Airline amp which is the 1624T circuit but it has the 12" Jensen stock, and this thing is as silent as the grave WITH the original 1964 Mallory can!!!! I can't believe it - I would never believe caps that old would still be functional, but apparently they are.

[tubeswell]

I was always under the assumption that the 5U4 tubes were direct heated = instant on.

That would explain it. I thought there was some reason* that I hadn't clicked 'bout why a 5AR4 lit things up more slowly than the 5U4Gs I've measured. Now looking askance at the tube schematics again I see it clearly (and note that 5Y3G and 5R4 is the same type of directly-heated cathode). And in that regard, using a 5U4G is certainly no worse than using SS rectification, which is what the other supros are like. It just means that the filter caps ought to be rated to contain the inevitable surge, which if the B+ voltage is around 400ish, a 450 cap oughta be able to handle.

[tubeswell]

I have a Thunderbolt with a 5U4. It needs a cap job but I measured the ratio real close to 625 at 100Hz and 200Hz so that's 5000:8 .

many thanks loudthud. 5k it is then

[Sweetfinger]

I doubt any Supro transformer, ever, was rated more than about 20W. Between the unbypassed cathode resistor and all the signal padding, I have yet to find any 2X6L6 Supro that can even hold its own - volume wise - with a blackface deluxe.

All the Valco transformers I've seen have been paper insulated which is physically smaller than a non interleaved plastic bobbin transformer like you see on BF/SF Fenders. I would caution against size comparisons between disparate manufacturers and construction methods because it is not at all an accurate gauge of "power rating". I've heard a few Valco amps that will hold their own with a Pro-Reverb or Bandmaster, CERTAINLY a Deluxe Reverb. My Gretsch 6162 has EL-84s and will hold with a DR on volume.
Another reason you might assume the transformers were wimpy is that Valco never seemed interested in really driving their amps hard. Take the Harmony 525. It has a fairly large output transformer(Valco-wise), larger than most 30 watt outputs. For some reason, Valco designed the amp to be really clean. Probably because the speaker was a C15P, which wouldn't handle much signal before blowing. I've seen the same(or similar) outputs in the Valco made Montgomery Ward heads. Those are fairly loud amps- definitely over 30 watts. I've got one here. The iron in those measures about 3" X 2.5" X 1.5" with 2.5" mounting. Plenty big, IMO when you consider it is paper wound.

[soundmasterg]

Here are the original T-bolt transformers being made again by the same manufacturer who made them originally. They say it is a 35 watt, 8 ohm transformer, 5k CT. This is the same one I have for my as yet not started on Tbolt clone.

Greg

http://www.classictone.net/40-18061.html

[EFK]

Want to see something REALLY interesting? They also make the power transformers for those as well, and they were the OEM manufacturers so you'd assume they have the original spec sheets. All of the thunderbolts I've seen used 5U4 rectifiers, but take a look at the 5V rating on the spec sheet for the power transformer....

[tubeswell]

Want to see something REALLY interesting? ...take a look at the 5V rating on the spec sheet for the power transformer....

http://www.classictone.net/40-18060.pdf

Interesting, could be a typing error. But the High-Tension winding rating looks lowish for 6L6s as well

[soundmasterg]

Maybe that is why those power transformers tend to run hot and fail over time?

I went with the Heyboer listed on the schematic myself. The guy I fixed the T-Bolts for had to replace one years ago for one of his and he had ordered 5 at the time and I bought one from him for my project. It looked beefier than the stock PT's and gave the same voltages out as the stock ones...though it was the one meant for the solid state rectifier so it would not work for the tube one. One of his amps had the tube rectifier and the other two had the solid state rectifier, and the SS ones sounded better.

Greg