Looking for an Electar Tube 10 Power Transformer

[SlidePicker]

My little Electar Tube 10, one of my favorite amps, is blowing fuses when I turn on the power switch with the standby switch off. All four diodes check out OK. Has to be a short in the power transformer. The number on the bottom of it is SKT-17622. Might y'all have one lying around in some dusty corner? Or have a suggestion for a workable substitute? Thanks!

[Prairie Dawg]

Does it do this with the transformer disconnected from the chassis? It should be a pretty simple thing to disconnect the high voltage and the filament where they plug into or are soldered to the board. Also what sort of footprint does the transformer have?

[SlidePicker]

I haven't checked it disconnected from the chassis. I assumed the standby switch did that past the diodes. I guess I'll have to sacrifice another fuse (maybe) to the cause. I'd love it if it turned out not to be the PT! I'll have to pull the board and desolder the secondary leads to do that, after work today or over the weekend. It's a little horizontal mounting thing about the size and shape of a tweed Champ PT, mounted through the (vertical) chassis, with 120 and 240V primaries (switchable in the amp) and 600VCT & 6.3V secondaries. Need to measure the holes and spacing while I'm at it. Thanks!

[52 Bill]

The last thing that I suspect is a bad power transformer. Power tube, power supply diodes and filter caps would all be the first things that I'd check.

Does the fuse blow with the tubes pulled?

Build yourself a light bulb limiter to save your fuses.

[SlidePicker]

If you look at the schematic, when the standby switch is off the HV circuit is broken between the rectifier diodes and first filter cap. The only things in the circuit at that time are the power transformer, the diodes and the 6.3V heater circuit. The diodes aren't shorted. Filter caps and power tube B+ aren't involved. Pulling the tubes would interrupt the heater circuit, and I suppose I should do that. A heater short could cause my problem, and I hadn't considered that.

[52 Bill]

Sorry, I don't have a copy of the schematic, my advice was based on my general experience with tube amps.

[SlidePicker]

Thank you, Bill. That was excellent advice. I had the advantage of the schematic. But you did make me remember the heater circuit could be the problem, too, which I can't wait to get home and investigate! If I just need to replace the 6L6, I'll be whooping it up!

[Enzo]

Even odder things happen. I had to work to convince someone his standby switch was at fault once, it was arcing inside to the switch frame.

The test is to disconnect ALL the secondary wires and apply mains voltage to the primary only. NOW if it still blows fuses, the tranny is bad.


And more fuses? Look up "light bulb limiter", make one and use it.

[SlidePicker]

I'm still measuring things with the PT in the chassis. The fuse blew in an instant with the tubes out and the standby still off. The primary has continuity at 7.0 ohms, with infinite resistance to ground. The B+ secondary is 132 ohms to ground on one side, 150 on the other, and 282 across both ends, as it should be. The four diodes test as they should. The 6.3V secondary measures 0.6 ohms end to end, and 0.5 ohms from either end to the center tap at ground. That the two halves don't add up may reflect the sensitivity limits of my multimeter on the low end, but does 0.6 ohms seem low for a 6.3V winding?

(BTW, I am scratching my head wondering about things like arcing inside a switch. But in case I end up having to replace the PT, or someone else wonders what PT will fit a Tube 10, Weber's W022772EU "Power Transformer for Tweed Champ, Champ, Vibro Champ, and Princeton. 120, 220, 240 inputs, 600V and 660V HV taps" is a perfect fit mechanically and electrically. The one with just the 115V primary would work fine if you don't mind sacrificing the voltage selector switch, which I'll likely never use.)

[SlidePicker]

I DO need to build a light bulb current limiter!

[SlidePicker]

WHOA! I think I've narrowed the problem down! I turned the voltage selector switch to 230V and plugged it into my 125V wall outlet. The fuse didn't blow. The pilot light glowed dimly and flickered, and I could hear a faint sizzling arcing sound coming from the PT. In seconds, a wisp of smoke started coming out of the end bell with the secondary leads!

[Steve Conner]

It's new PT time!

[SlidePicker]

Yep! Looking around to see if I can find a match for the international-voltage Champ PT without the unneeded 5V secondary, but I'll probably end up with it and just tape off the leads. I've noticed that the big electronic parts wholesalers have mighty limited selections of power transformers in tube amp voltage ranges these days. They've become a "boutique" specialty item.

[Steve Conner]

Funny you should say that. In the second edition of The Art Of Electronics, published in 1980-something, Horowitz and Hill complain about "old transformers for tube equipment cluttering up the catalogs".

Surely Hammond will have a suitable part.

[SlidePicker]

HA! Yeah, Hammond seems to be the go-to source.