The Amp Garage

In fact it was the voltage selector. Marshall used those funky selectors on the 2150 (maybe other models?), the impedance selector had already gone bad on this amp, I should have known but I'm kind of slow sometimes… Anyway all is well, the PT is totally fine, I hardwired the 120v tap as I hardwired...

sluckey wrote: ↑Sun Oct 03, 2021 9:01 pm I believe this will work...

https://www.hammfg.com/files/parts/pdf/290HX.pdf

Thanks, I was looking at this one on CEDist, they even have the same wire colours as the original.

It looks pretty much the same as my '73 SL:

You need to find a replacement PT. Yeah, unfortunately… Any idea of how this PT died? I mean I get things work, and then don't, but the amp was working just fine. This is a peculiar Marshall model briefly made, but it is a 100w model, mine has the US 4x 6550's and 3x 12ax7's, can I assume a replace...

sluckey wrote: ↑Sun Oct 03, 2021 6:48 pm PT is possibly bad. Check for AC voltage between F4 and F5. Then check AC voltage between F3 and ground.

Thanks, I get nothing, no AC, between F4 and F5, and nothing either between F3 and ground.

So, with the mains on, when I turn standby on, meter on pin 3 of one of the power tubes sockets (no tubes in) after a 1kV surge, it goes to around 500v and then drops down slowly, I stopped looking at 27v.

Yes, the power switch lights up, what else could I measure to check the PT?
The amp was working fine this morning, and then no more a few moments later when I turned it back on, no smoke, noise or anything burnt that I can detect, weird.

Thanks Phil, no AC with or without tubes.
I just unsoldered the PT heater wires from the fuses PCB to isolate any issue with the wiring and/or PCB, there's no AC voltage between them, center tap is connected to ground.
Is my PT gone? Is that something that can be fixed?

The amp was working and sounding great, today turning the amp on, power switch lights up but tubes don't. I checked all the fuses for continuity, all good. No AC voltage between the heater wires. I've never faced that issue before on any amp and would be grateful for any idea! https://i.imgur.com/gt...

I seem to recall that Brimisters were a few hundred ohms when cold. Perhaps they were fitted to protect the rectifier diodes from the initial switch on surge required to charge up the reservoir cap? Consider something like a CL-90 NTC inrush current limiter, maybe even 2 in series. What value are t...

Somebody suggested I replace the 2x resistors with a thermistor, anybody tried that?

I bought that amp not long ago and finally got to replace the electrolytics and clean up a bit. A bunch of replaced components, I used the vintage looking Mods caps to replace the poly replacements, so to speak, and left the replaced resistors in place. It's super quiet and is a great sounding amp, ...

pdf64 wrote: ↑Fri Feb 05, 2021 8:55 pm If it works then it works. And I guess many originals got done that way.
But personally I’d fit a 4way tagstrip try to control the 0V return path by using a dedicated wire.

Will do, thank you for your help!

Thank you for the responses, I removed the bypass cap and wire going to ground on the chassis and it helped a good bit with the noise.
Now the TB is grounded solely from its bracket to the chassis, is that OK?

Your hum, no doubt, comes from your multiple ground points. I see the middle lug of your three-position standoff is grounded to the chassis, and at least two wires are grounded there: red and brown. That same brown wire appears to be grounded to the main strip board, true? Where are the new 12AX7's...