troubleshooting M40 Echolette with loud hum

[bea]

... one 1M resistor that was drop at 960k on Rö3,...

But You are aware that this fully within the tolerance? These resistors were +- 5% or even +- 10%

You should consider to change *all* coupling caps. Despite of that - that seems only part of Your problem.

[notea]

Yes i feel stupid for this resistor.. Anyway i changed all the 0.1uf cap and it's way more better, now the amp is alive!
i still got current noise that it's affected by the volume control, don't know if it's a ground issue, but i'm really happy, we are making progress here.
I've got a spare unit from where i took off the PT, i wanna study and follow the ground path in it and see where there could be a problem.

-I don't remember if the PT should be completely isolated from the chassis via the paper joints on the nuts?
if yes that could be a part of my grounding problem

[bea]

Yes, i know, old thread:

A fresh M40 is a rather loud proposition all said and done seing the rather limited power by the 4pcs of EL-84´s. That goes for the entire line of Klemt amps,right up to the later Winstons (the GA/BA and PA-200).

Do You have an idea what makes them so loud?
The stack below is so loud (with the M40) that i could even have used it open air as a bass amp instead of the G-2000 or even a Hiwatt DR201 clone.

The M40´s fail in one minor detail and that is that SOME of them do not apply to "the golden rule". It is imperative that the grounding of the first hit e-lyte hits the exact same spot as the centertap for the HT winding,in the case where a centertap is applicable,or the amp WILL hum. NOTHING else is to be attached to that point. 1mm away..ok,but NOTHING else to that exact spot.
It is not uncommon to run into amps that has seen previous work where this has been altered with.

Thanks ver much for that hint!

This typically might look like here (the M40 shown above, but also with my other one). ou see the mass connections of center tap and that into the circuitry nicely in the middle.



I fixed it in my other M40 and also doubled the capacity of the reservoir cap of the bias circuit. Should still be safely within the limits of the diode and reduces the residual AC on the grids by 3dB. Now the hum of that device is gone. The other M40 will soon be fixed; it is still in my rehearsal room.

[bea]

Just in order to summarize this visually - note the different positions where the wires are soldered to the caps:

Hum:



No hum:



really impressive.

[danielalberich]

Hello, I rescue this topic a few years later, because I have the same problem. When I turn on my Echolette M40 a constant hum begins and it is not affected by the master volume and, although my guitar is heard above the hum, it is very annoying to play with that noise.

Researching through the guts of my amplifier (with very little experience) I have seen that there is continuity between the amplifier output and the ground. That seemed very strange to me because there is a link between the positive and negative of the output jacks. I would like to ask you if you consider if that is normal or if it should not be the case.

Sorry for my bad English, and I will be very grateful for any responses. Regards, and thank you very much.

[The Dude]

You are reading across the output transformer which will be very low resistance. This is normal. The most likely cause of hum is bad/old filter capacitors.