Silvertone 1485 from the dumpster

[t_byrd]

As the title says, I found a Silvertone 1485 at my town recycling center in the electronics bin, just the amp, no speaker cabinet. Schematic is here: http://www.prowessamplifiers.com/sch...ertone1485.pdf

I've done a few tube amp recap jobs but I didn't realize how different this amp is with the 2 output transformers etc. Whoever the last owner was did a really bad job rigging up some sort of speaker connection, and it looks like they (poorly) replaced the brown power tube socket, and you can easily see 2 of the socket pins are shorted together and the 68ohm resistor is visibly fried. Other than that, the amp looks to be stock and no other signs of anything that has been damaged, so I'm hoping I can restore it in some way. Attached are a few pics showing the bad tube socket and bad/taped together output wiring.

I'm not really interested in the original speaker cabinet or recreating that some way. I'd be fine with running this with either 1 or 2 4ohm cabinets since I have them already.

A few things that I am not too sure of after reading some other posts about this amp:

I have read these output transformers have 2 taps, 1 2.6ohm tap and 1 4ohm tap. If the red/yellow wire that is already hooked up to the speaker terminal is the 2.6ohm tap, then the yellow wire that is not hooked up to anything would be the unused 4ohm tap? And it would be better to use that if I'll always be using a 4ohm cabinet?

Can I just pull 2 of the power tubes and use only 1 of the outputs with a single 4ohm cabinet? Before I put any work into restoring/recapping this, I'd like to just do some simple tests to confirm it's alive, so this seems like the simplest option without rigging up 2 cabinets etc.

I plan on fixing the short on the bad tube socket, replacing the 68ohm resistor, pulling the 2 tubes from the left transformer, and hooking up the right transformer to a new output jack that I'll connect to a 4ohm cabinet, then bringing it up slowly with a variac. Is this a good plan or am I missing something?

Thanks!

[Jon Snell]

Before you start, remove C31 and fit a three core mains lead for everyones safety!
A balanced pair of output valves are required to be inserted as they form the output stage of the phase invertor.



T2 & T3 are designed to work a 3.75 Ohm load so 2.66666666 etc load will be near enough. One wire is ground and the other goes to the speakers along with the ground wire.

[Enzo]

How does that work? All I see is a classic split-load phase inverter - the lower half of the 6CG7. That feeds the two power stages that are in parallel to the grids. If I pull say the right-hand pair of 6L6, all it does is make T3 sit there like a brick. But there is no NFB, and no feed back to the PI, and the other pair and T2 continue along. so I don't follow you.

[Jon Snell]

Whoops, I meant T2!!!
Thanks Enzo.

[Helmholtz]

Whoops, I meant T2!!!
Thanks Enzo.

No good.
That would leave the remaining 6L6 pair without load.

[Helmholtz]

T2 & T3 are designed to work a 3.75 Ohm load

How do you know?

[Jon Snell]

Old school values.
15 Ohm, 7.5 Ohm and 3.75 Ohm were the standard.
Abreviated and adjusted to 16 Ohm, 8 Ohm and 4 Ohm in modern times.

[Jon Snell]

No good.
That would leave the remaining 6L6 pair without load.

The remaining pair are to be removed.

[Helmholtz]

The right pair works on T3, the left pair on T2, right?

If you remove the right pair, T3 is useless, but the speakers should stay with T2.

In other words, your first proposal was ok.

[t_byrd]

Thanks for the input everyone, but I am a little confused at what is being suggested.

The first proposal looks correct to me, the right pair of 6l6 is connected to T3, so I would remove these and only hook up my 4ohm cab to T2. This would be ok?

I could hook up a TRS jack to both T2 and T3 and use a splitter to 2 4ohm cabs, but at this point I'd just like to confirm the amp is working and the 1 cab setup would be more simple.

[g1]

Left and right pair is a bit vague. I think these are being mentioned with reference to the schematic.
Which pair actually corresponds in the physical amp may or may not agree.
As long as you remove a pair and connect spkr. to the OT which is connected to the installed pair, you should be fine.

[Enzo]

I guess I am guilty of that,but I assume we mean schematic since there is no standard for which way we face a chassis, or even if it is upright or inverted.

[t_byrd]

Left and right pair is a bit vague. I think these are being mentioned with reference to the schematic.
Which pair actually corresponds in the physical amp may or may not agree.
As long as you remove a pair and connect spkr. to the OT which is connected to the installed pair, you should be fine.

Thanks, yes I was referring to the pair as shown in the schematic, but just had a look at the amp and can see which pair of 6l6 goes to which OT.

[Mick Bailey]

That's an unusual standby arrangement on the PI output. They don't seem to have just followed what everyone else was doing with this amp.

[Jon Snell]

That's an unusual standby arrangement on the PI output. They don't seem to have just followed what everyone else was doing with this amp.

Vox used a 2n2 cap and a 100k pot in that position, they called it treble cut or tone.
I agree, the valves are drawing current all of the time, so not realy standby. More like mute.