Silvertone 1484 Burnt Choke

[RWood]

I took this amp in yesterday. The owner was told by someone else that it needed a new OT but wanted a second opinion. The OT, at least, ohm'd out OK but this lil guy is hurtin':



It got me wondering what would fry a choke, as this one. It is short, btw, rather than open. The choke supplies the power tube screens so would a short there result in an event here? What do you guys think?

RWood

[Gtr_tech]

Its possible that the damage was due to a short to the core. That would draw lots of current, and someone prolly put in a huge value fuse in an attempt to "fix" the problem.

[RWood]

Maybe, like many components on these amps, the choke is under-spec'd.

[Gtr_tech]

Well if it shorted to itself (adjacent windings) it would just cease to function as a choke. It wouldn't draw current and roast...

[RWood]

I just noticed that one of the (replaced) power tube's locator pin was gone. I wonder if the tube, plugged into the socket wrong, might have done that. The choke feeds the pin 4 screens, but I don't know if or how that tube was plugged in wrong.

[guitician]

If a 6L6 was plugged in so the filaments were across the high voltage, that would short out the PSU and blow the fuse, or try to blow the wrong value fuse, cooking the choke, and maybe the power transformer.

[RWood]

PT was fine. I wish I had noticed how the power tube was inserted, but I didn't.

I replaced the choke when I did the filter caps & power tubes and the amp was returned to a happy owner. He had been told it needed an OT and was quoted what he (thankfully) considered to be an exorbitant price to fix it. That led him to me.

[guitician]

I may work, but if the insulation burned at all anywhere inside the power transformer it may eventually fail. Only time will tell.

[meanderingorange]

Say... I have the exact problem with my silvertone 1484.... where did you get the choke and what is the spec for chosing the right choke to replace with. (Sorry, just starting to understand the innards of a tube amp)

[guitician]

Here ya go...

This MojoTone one is cheap....and the specs are http://www.mojotone.com/core/media/m...e414b&_xt=.pdf
Mojo Transformers » Filter Choke for 6L6 Amps

There are lots of surplus, reclaimed stuff too. But, that takes more time and effort to find the right one. Good Luck

[guitician]

I looked at the original schematic and it shows the choke dropping 140 VDC at idle. This would mean a fairly high DC resistance like ~1K ohm. This would also explain why these chokes are always burning up. You could experiment with a underrated choke to see if that works to maintain the proper pre-amp voltages, or just add some series resistor to the choke. This Hammond my work....
155J Hammond Manufacturing Common Mode Inductors (Chokes)

[RWood]

The choke is not dropping that much voltage. The node for the screen supply that runs through the choke does not go through the full voltage tripler circuit. It is taken off an earlier point in the power supply.

The replacement part I got was from AES:
P-CF22707

The 2" centers on this choke fit nicely with no modification to the Silvertone chassis.


RWood

[guitician]

Oops, yes I see now that it's more like a 20 volt drop and a 285 ohms DC resistance coil. I should have looked closer at the drawing. Thanks

[davohilts]

I've got two 1484's and both chokes have failed over the last 5 years. Just working fine one day and the next, no sound and the culprit was the choke.

[Ptron]

Another one here. I'm in the middle of fixing a bandmate's 1484 that has an open choke. Just got the replacement today, actually. I got one of these. 7H, 40mA, 340 ohms, 400V(DC). That is an adequate replacement isn't it?