Problems Installing an Isolation Transformer in a Silvertone 1430

[64deluxe]

I am working on a Silvertone 1430 trying to make it safe with an N-68X isolation transformer. Before I started work it powered up fine with a loud hum but it did pass signal and it did sound "OK", for one of these. They are an acquired taste.

I have attached the schematic created by Clark Huckaby which matches the circuit on this amp better than the other 1430 schematic on the web.

I have never tried this before, but this was my strategy. Wire the primary side of the N-68X to the fuse tip and then through the switch and back to the transformer, with the ground wire going to the chassis. Wire the secondaries to the two points in the circuit where the original AC went. Then remove the death cap.

I wired in the transformer and replaced the filter caps. The original caps had their negative ends tied to one side of the on/off switch. The 40uf and the 20uf [in this one they had already been replaced with a modern CE red cylindrical twin cap with 47uf and 33uf] were tied to the switch through an NC pin on the rectifier socket by a wire back to the switch and the other cap, a 10uf, was connected directly to the same terminal on the switch. That same terminal also had a green wire from the junction of the 560K and 150R resistors.

I connected the negative ends of the replacement caps to a new terminal strip, along with the green wire, and connected them to one side of the secondary from the isolation transformer.

The other side of the secondary goes to another new tag strip where the wire from the plate of the rectifier and the 100 ohm, now 150 ohm, resistor are now connected.

I changed the 5w 100 ohm resistor on the filament supply to 150 ohms based on a suggestion in the Ampage archive.

Here, I think is where I may have made my error. I removed the 68k resistor and .05uf cap as I had read they form the "death cap" combination in this amp and should be removed.

When I power up the amp the heaters come on and I get a fairly loud hum that is unaffected by the volume pot but no sound.

I have obviously made a rookie wiring error, being a rookie, and I suspect someone out there can spot it for me, if not from my description I can post photos as well.

[Prairie Dawg]

The loud hum when you first powered it up was undoubtedly from deceased electrolytic capacitors and you probably should have tested everything before you started another bit of surgery. It's never a good idea to tackle more than one modification at a time without testing.

Your secondaries on the isolation transformer should have gone to the power cord of the amp and the other side of the transformer to line voltage. What you might want to do is back out of the isolation transformer modification for now and solve your hum issue before you go farther. That'll serve to validate and localize the problem.

Those series string three tube screamers are nothing but fun. I've got a recently acquired Custom Kraft which administered too many jolts to its owner. I, on the other hand, do not play in bare feet on a cement floor.

[64deluxe]

I did replace the electrolytics and the primary is connected to the mains and the amp circuit is isolated completely. The main problem was where I thought it was. I jumpered the points where the .05uf/68k combination were removed and finally got signal but a strong hum still. With the jumper in place there was no significant voltage, AC or DC, on the chassis so I guess in this case there is no concern if that cap goes open. So I went ahead and put back the .05uf/68k and replaced all remaining capacitors. It sounds much better now but one problem remains.

The pot sounds very scratchy. It's fine when it isn't in motion but if you turn it it is scratchy. I noticed this earlier so when I was replacing the capacitors tied to the pot I also replaced the pot but still get considerable scratchiness when I turn the pot.

Aside from that it does sound very cool. Reasonably hum free and clean with the volume below half, and gnarly and beautiful when dimed. I can live with the scratchiness but want to know if it is normal or a problem that should be fixed.

[loudthud]

Here, I think is where I may have made my error. I removed the 68k resistor and .05uf cap as I had read they form the "death cap" combination in this amp and should be removed.

When I power up the amp the heaters come on and I get a fairly loud hum that is unaffected by the volume pot but no sound.

Did you notice (in the Huckaby article) how after the transformers were installed that the negative side of the filter caps and the cathode circuit of the 50C5 are now connected to the chassis? Basically it's a short across where the .05uF/68K was.

Did you replace the .01 disc cap from pin 1 of the 12AU6 to the Volume pot?

[g1]

Also of note, he used the 50VA isolation transformer for the HV only, and a separate transformer for the heaters.
Apparently, half-wave rectifiers will cause more heat build up (than full wave rectifiers) in the isolation transformer. It is possible the isolation transformer may overheat even though the VA rating is sufficient.
Not sure if this is scientifically sound, but better safe than sorry. Watch for excessive heat at the isolation transformer.

[JHow]

If you don't mind my asking, how did you handle the heaters? It looks to me like Hucakaby made a power supply to run the heaters in parallel off of 50V or so. He used a second transformer to do this. Are you doing that?

As to the scratchy pot, that sounds like DC on the pot somehow? Could it be your chassis is not at ground but has some DC component?