Marshall 100W hum

[drewl]

This is bugging me....

Got a real mint 1971 Marshall 1959 that has a 60Hz hum around the phase inverter output.

Owner recently replace a bad power trans and all the filter caps and added a three prong grounded cord.
I cleaned and checked all ground connections, hum is still there when V3 is pulled.

Measuring DC voltage at the junction of the .02uf cap, 220K bias feed and 5.6K grid resistor (on the 82K side) knocks hum way down when meter is connected to ground, measuring DC and only DC, otherwise hum is still there

PT has some audible radiated hum.

Unless hum is being fed back from output trans...hmmm, what am I missing here.

[alexradium]

filament heater secondary not referenced to ground or missing one of the 100 ohm resistor?
power tubes ok?
screen resistors?

[drewl]

Heater center tap is grounded.

Checked screen resistors, grid resistors, every resistor for tolerance.

I think the output tubes are definitely contributing to the problem.

[g1]

So maybe the meter resistance is doing something when put on that PI side after the coupling cap? Any chance of bad power tube mismatch?

[doombass]

I'm thinking you might have a hum as a nature of the beast and if the V6/V7 side of the push/pull circuit is dead (or mismatched like mentioned in the post above), you will not have the normal cancellation of hum in the output transformer. You wrote that the hum stops when measuring the 82k side and that's V4/V5. What about the 100k side?

[drewl]

Hmnm, seems I was wrong.

It's coming from V2.

60hz still there with output tubes removed.
Remove V2 and it goes away.

[drewl]

I mentioned the owner replaced the filter caps, right?

Looks like they are wired wrong!

[drewl]

Strike that, choke measures 1.3 Ohms.

That don't sound right.

[drewl]

Yup, bad choke.

Subbed in a 100 ohm resistor and hum is gone.

Maybe what killed the PT, or vice versa.

[alexradium]

Yup, bad choke.

Subbed in a 100 ohm resistor and hum is gone.

Maybe what killed the PT, or vice versa.

probably filter caps went short and pulled too much current,that heated up the choke windings,going short,then HV secondary.