Silvertone 1464 solid state 100 amp

[DrGonz78]

On a side note with this amp as am I waiting on the output transistors. I have to replace the 120vac pilot light and just don't know much about these old bulbs. I have an incadescent from an old Yamaha amp that is definitely 120vac. I am not sure what the resistor looking part is on the old pilot light, black with white stripe. I have this other bulb that is a tiny bit smaller and has a 33k resistor attached. Do you think this bulb will work as a replacement?

[Jazz P Bass]

That resistor usually indicates a Neon lamp.

Try the one from the Yamaha.

[J M Fahey]

Agree.
Those are not incandescent )filament) but Neon.
The black/white resistor is probably OEM made as "resistor for neon lamp" and they didn´t care to paint the nstandard colour bands, but you may measure it anyway.

[DrGonz78]

Cool thanks for the advice guys. Yeah I didn't even think of neon bulbs, pretty cool. I installed the yamaha one and it worked like a champ!

[DrGonz78]

So I got the MJ2955 transistors today and installed them. I brought up the amp to line voltage via 40watt bulb limiter and measured between 480-480mvDC on the output. Switched over to 60watt bulb and measured 520-530mvDC. The bulb never shined bright and no major shorts. I basically wanted to see how much voltage is on the output with out a limiter and bringing the variac up to 90vac it's about 1volt DC on the output.

I don't really want to push the amp up to 120vac yet as I am not sure that would be good idea. What should I do next as far as troubleshooting this DC voltage on the output. To me it would suggest something else is shorted somewhere but not a huge current draw. Perhaps some of the pre-drivers? Would putting a load on the amp be a good idea at this point, via bulb limiter? What's the safe direction to go from here?

[DrGonz78]

Update: Hold the phone I decided to bring it up to 120vac without a bulb limiter just now and actually it is sitting around 560mvDC on the output. A bit higher than you might want but not horrible. Not sure why it was sitting at 1 volt or so as I brought it up to 90vac before but could be that the caps are drained now(the whole amp wakes up stupid concept). So should I be tweaking that bias resistor that I replaced (10 ohm) to get the DC offset a little lower? Also, I need to test it with a load as well so I might just do that first to see what DC offset I get with a dummy load attached.

[DrGonz78]

Okay this is going great!! I just hooked up an 8 ohm load and brought it to full line voltage. I am measuring 15.5mvDC on the output, so it was wanting a load to balance itself out. It is pretty late so I will have to continue testing the amp tomorrow with an actual speaker. This much so far is a very nice start. Thanks so much everyone for helping out to get this one fixed! I will let you know how it turns out tomorrow.

[DrGonz78]

Well the amp is passing signal but the repair is not over yet. I am pretty sure these electrolytics, that are around 46yrs old, are totally suspect, lol. The amp does work though and that includes the reverb & tremolo. So I am first going over all the solder joints and cleaning the pots. Thanks again to all that contributed to the thread here!

[DrGonz78]

So just thinking about it now I am not too sure if the preamp section is at fault. Of course I cannot prove that thought but what if all I am hearing is some cross over distortion? Perhaps I need to think first about tweaking the bias of the output transistors? My thoughts now are that it did sound like cross over distortion and I do need to scope the output to make sure still. What do you guys think?

[Enzo]

You are asking our permission to scope your output? I'd have done that before even posting.

[J M Fahey]

A little crossover distortion is acceptable in that case, because it's the safe setting.

An amp sounding sweet at low volume will probably overheat or runaway when played loud, because they do not have thermal compensation, so go for the safe setting .... and use it live or in rehearsals, accept that at bedroom levels it will sound somewhat rough or "grainy".

In the old days NTC thermistors were used to compensate, but they are quite unavailable today.

[DrGonz78]

I am about to go out to the shop to start working on it a bit more and test it out again. It may not be all cross over distortion and it might actually be really old caps in the signal path(preamp). It sounds pretty bad on all inputs and it kinda sounds better at higher volume. Still it has a grainy nasty light distorted sound even turned up louder. Off to scope it a bit...

[DrGonz78]

So here are some images of 1khz sine wave signal at different amplitudes on the scope. So it looks like some cross over distortion on the output. As you turn up the volume or increase the signal the distortion is always present. Like in the case of cross over distortion it is worse at lower amplitudes, but still it is really nasty sounding at any volume level.

[Jazz P Bass]

Scope the input to the power amp & compare them.

That will decide whether it is a preamp distortion or power amp.

(or both)

[J M Fahey]

Yes, they certainly look like crossover distortion.
And it can be (mostly) dialled out, the point is to do it in a safe way.
And not having a preset pot there but relying on resoldering resistors will be a time waster ... but it can be done.

First preheat the amp (50% continuous power for at least 15 minutes into a load is fine) so your adjustment is valid for a live performance, and recheck (scope) it.
*Maybe* when hot, that biasing is fine !!!!
And if not, raise it in 50mV steps by adding, say, 1 or 2 ohms resistors (do the Math) in series with each biasing one until you get just a hint of x.o. dist. visible.
Don't overdo it.

And anyway remember it's an old Silvertone, using a power amp structure which has been obsolete for ages ... for good reason.

Absolute worst case, gut the power amp and put a chipamp there (LM3886 would be fine), for an excellent and low cost solution.

After all you already have the power rails available, and might even bolt the LM3886 to the existing heatsink (which probably is just the chassis or a flat aluminum plate)

If possible, post some picture showing the power amp area.