Marshall JMP Super Bass 100 1969 problem

[Karl]

I have got a Marshall JMP Super Bass 100 1969 on my bench. The amp was working but needed new filter caps and tubes.

And now after that has been completed the amp is making squeeling noises when either volume knob is turned up but volume 1 sounds worse. You can turn the volume up more on the second volume and if I use the low input I can turn the volume up more until the squeel and hiss is present.

I have noticed that the impedance selector is causing crackles and the selector plug itself is not held tight.

B+ is 520V ??

Please help me if you can.

[selloutrr]

did you confirm the amp worked prior to the parts being replaced?

are the new tubes confirmed tested good?

have you gone back and checked caps have correct orientation and all solder joints are solid.

were any other wires unsoldered and possible reconnected wrong?

[PCollen]

I have got a Marshall JMP Super Bass 100 1969 on my bench. The amp was working but needed new filter caps and tubes.

And now after that has been completed the amp is making squeeling noises when either volume knob is turned up but volume 1 sounds worse. You can turn the volume up more on the second volume and if I use the low input I can turn the volume up more until the squeel and hiss is present.

I have noticed that the impedance selector is causing crackles and the selector plug itself is not held tight.

B+ is 520V ??

Please help me if you can.

The early JTM100 Model 1992 Super Bass , the bass version of the JTM100 Model 1959 Super Lead, had a plate voltage on that amp spec'd at 560 Vdc, so the B+ feed to the OT C/T would be a little higher. Wall voltage and tolerances may effect that value, as would bias point of power tubes, O/T primary DC resistance and a few other factors. Later JMP100 amps had lower voltages, around 475 Vdc B+ and todays higher wall voltages would tend to raise that value closer to 48-490 Vdc I'd think. So with new caps, tubes, and higher wall voltages than in the 60's, 520 Vdc sounds like it may be OK.

[Jana]

I would sort out that intermittent impedance selector ASAP. Do that before you do anything else, please.

[Karl]

Thanks for the tips. There are no letters visible on the selector for the impedance but the plug is connected between center to hole at 12 o clock.

I forgot to tell you guys that I changed the rectifier diodes to MUR ultra fast diodes. I will compare all connections with the pictures I took before starting to see every connection is the same as before.

And also sometimes when you flip the standby switch on there are no squeels or problem but when you play nothing comes through unless you play hard then it comes through. Similar to a fuzz with a gate control.

[selloutrr]

if you post the transformer model number I can probably give you the color code for the Ohm Taps. so you can test continuity and know which contacts are what on the impedance selector.

While the amp is discharged it might be a good idea to clean and tighten the tube socket pins.

I'm guessing it's a small oversight in wiring or a versed cap and the amp will be good as new. Are you sure the replacement parts test good? it's rare but not unheard of to have a bad new part.

[Karl]

if you post the transformer model number I can probably give you the color code for the Ohm Taps. so you can test continuity and know which contacts are what on the impedance selector.

While the amp is discharged it might be a good idea to clean and tighten the tube socket pins.

I'm guessing it's a small oversight in wiring or a versed cap and the amp will be good as new. Are you sure the replacement parts test good? it's rare but not unheard of to have a bad new part.

I measured all the caps before installing. I tried swapping preamp tubes for known good tubes but have yet to try swapping power tubes for known good.

[Karl]

The amp is working again. Someone had moved the NFB to the wrong tap and correcting that fix the problem.

[JamesHealey]

I used to have a 69 super bass.. miss it a lot was a real nice amp!

[Roe]

moving the nfb (but not changing nfb or tap) easily leads to squealing or noise. the nfb wire is very sensitive in these amps. does the el34s redplate with this high voltage btw?