1965 Kay 703C - OT replacement questions

[Carlsoti]

I've been gifted this vintage tube amp on two separate occasions, in two different states, by three different friends. It's a weird story, but that's for another time. The first time it was given to me, it was Non-Op. No speaker or it's associated output tranny. The second time, it was fitted with a 4 ohm Jensen C8R speaker and a seemingly odd choice for an output transformer, a 3W Hammond 125A. NOT the 125ASE one would expect. The center tap is not used. The speaker side is wired to pins 1 and 5, for a primary impedance of 2,800 ohms, per the spec sheet, which roughly falls in line with the 60FX5 spec sheet's load resistance of 3K ohms.

Question 1: PP v SE aside, with this transformer being over-spec'd by almost/more than double, does this seem like a "reliable" implementation?

I know SE transformers are typically much physically larger than an equivalent wattage PP tranny, because they're air-gapped to prevent core saturation, minimizing distortion. FWIW, I'm not at all concerned about clean headroom on this amp, but I'd like it to be somewhat representative of the original tone. A little more grit wouldn't bother me, though.

Question 2: How would I CALCULATE the DC I'd expect to see on the input of this transformer, if possible, or is it a matter of having to measure the circuit in operation?

Question 3: If the above scenario is untennable, would using an 8ohm speaker wired to pins 1 and 6, giving a primary of 3K ohms, be a better option? If so, why?

Tube spec sheets are typically only used as a "guide" in relation to guitar amps. Voltages seen in MANY guitar amps are WAY over what the tube manufacturers call out for Max ratings. With that in mind..

Question 4: How much audio power does a 60FX5 typically produce when run "full-tilt-boogie" in a circuit such as the Kay 703C? Tube sheet says they do 1.3 Watts.

Additionally, I've been neglecting this amp for a few decades, due to an irrational and uniformed fear of the "Widomaker" moniker applied to them. The filter-caps were replaced with a CE Dist C-ER33-47-47 "firecracker" when the transformer and speaker were put in it, and I've since put a dead-ground-leg 3-prong cord in it to help ensure correct polarity when plugged into a properly wired outlet. It's hacky and it does work, but I'm strongly considering installing an isolation transformer using Pete's Safety Mod schematic drawn up in 2015, so that I can sleep well, knowing it's properly and safely wired.

Any direction you could provide would be greatly appreciated.

Thanks,
Tim C
Valley of the Sunburn, AZ

[sluckey]

That amp was a bottom of the barrel entry level amp. There were no high quality parts used in it. Fix it as is. Don't try to turn a sow's ear into a silk purse. Buy new at Western Auto for under $50.

[Carlsoti]

That's why I asked the question(s), Sluckey. It was already "repaired", and it works, but on the surface, this PP OT shouldn't be used in this amp. I'd like to be able to "get away" with keeping it, and only having to put in the isolation transformer, then play the ever-lovin' crap out of it.

[PRR]

The original OT was cheaper. Don't sweat it. DO FIX THE HOT CHASSIS!!!!!!

> Voltages seen in MANY guitar amps are WAY over what the tube manufacturers call out for Max ratings.

Why would voltages be "wrong"? It (the amp and also the tube-set) was made for anything near 117VAC direct. You will feed it near 123V through a small transformer, which will be so close to 117VAC that you can't slip a pick between. Is it 1.1W, 1.9W, 0.9W? If you actually cared, you'd probably want 5 or 50 watts. Anyway even 0.1 Watts is often "Turn That Down!"

[Carlsoti]

I guess I'll just put in an appropriate isolation transformer, run it hard, and hope the oddly-spec'd replacement OT doesn't give up. I suppose I could check the temp on it after a good long while. Thanks.

[sluckey]

That's a good plan. You can always add a proper OT later if the improper one fails.

[PRR]

The OT is not going to die, working SE instead of PP. It will bass-distort early, and you would have better bass per buck if the same amount of iron were designed for SE. But Hammond actually sold this series for lo-fi SE before they developed the 125xSE series.