Weird howl--Marshall

[Valvehead]

Sorry Helmholtz and I are getting a little sideways on your driveway

You've made some headway. Is there still excessive noise or hum? I've seen hum chasing go from fixing excessive hum to an attempt to eliminate ALL hum. Even in designs that are prone to a little noise or hum. That doesn't mean things can't be improved further. Just that there may be a point of diminishing returns for the effort.

yes--somethings not right , it is excessive . Its the same sound in the video, but about 75% less in volume after i grounded the speaker jack . Amp works and sounds great beside that.

2 of the 100uf / 500v filter caps are mounted on the pcb , next to the rectifiers..Im going to try and move those or get some can caps .

Just throwing some ideas out there...the heater wires on the pre tubes are twisted tight..but the power tube heater wires are not twisted together at all .... would that affect anything ??

[nosaj]

yes--somethings not right , it is excessive . Its the same sound in the video, but about 75% less in volume after i grounded the speaker jack . Amp works and sounds great beside that.

2 of the 100uf / 500v filter caps are mounted on the pcb , next to the rectifiers..Im going to try and move those or get some can caps .

Just throwing some ideas out there...the heater wires on the pre tubes are twisted tight..but the power tube heater wires are not twisted together at all .... would that affect anything ??

Are the heater wires wired in phase so they will cancel out hum as they are twisted together? I would twist the power tube heaters also, but that's just me.

nosaj

[Valvehead]

i tried swapping them. no change

ill go ahead and twist them

[Chuck H]

i tried swapping them. no change

ill go ahead and twist them

Well, the thing about swapping the filament leads is that you shouldn't swap them for all the tubes. If the filaments are wired so that the push pull circuit isn't cancelling hum then it's because one side is wrong, not both. If one is wrong and you swap both then one is still wrong.

[Chuck H]

Is it possible you have a noisy rectifier (or that it isn't wired correctly) or that someone stuck the wrong type of bulb in the pilot lamp?

[Valvehead]

Is it possible you have a noisy rectifier (or that it isn't wired correctly) or that someone stuck the wrong type of bulb in the pilot lamp?

the power switch light didnt work before (had hum).....i put in a new switch --the marshall style red rocker.

the diodes look stock and appear to be wired correct--the B+ is 480v i believe

the heater wires = ill check tonight but i think they are power tubes pin 2 to pre tubes pin 4/5............pwr 7 to pre 9 . I put the twisted ones on the pre tubes sockets..... the power sockets were already done . .maybe i got 1 end wrong at the pre-tubes ?...lets say for example , one wire is going to pin 9 and one is going to 4/5...what if somewhere down the line one wire is on the wrong side of the tube ? would that create a hum ?

[Chuck H]

I think we're just punching in the dark now because you mentioned that removing the PI helps a lot. So that's not the filaments on the power tubes. And you mentioned that removing the preamp tubes helps very little. So that's not the filaments on the preamp tubes...

Rectifier noise might be snubbed enough past the PI node so that pulling preamp tubes doesn't help much. Since the push pull circuit would be trying to cancel rectifier noise I would think that pulling the PI would be the most effective at reducing it. So I'm still wondering if you don't have a noisy diode or two in the rectifier.

From another angle,.. Have you listened to the amp plugged into a cabinet with NO tubes in it? Not even power tubes. Could be revealing, probably not, but it's something to check.

[Valvehead]

ill try that....no tubes

i probably have some 1N4007s here i can throw in it to eliminate that

let me pull each 12ax7 out , 1 at a time , and ill post back exactly whats going on

[Valvehead]

ok----pulling all 3 pretubes has no affect on the hum

[g1]

the power switch light didnt work before (had hum).....i put in a new switch --the marshall style red rocker.

Can you clarify? The switch was humming physically? Or adding hum at the output?

[Valvehead]

Can you clarify? The switch was humming physically? Or adding hum at the output?

the switch wasnt humming...

it had nothing to do with the hum i still have--it just had a bad bulb

[Valvehead]

ok..i found a problem--1 of the heater wires at v3 (pi) was wrong...some idiot missed that ( ME) .

fixed that and I have the same low hum....

is it possible ....that i hurt the 6.3vac tap in the PT ?? i basically had it "shorted" i guess...but why did the amp function ? Pin 9 wire from v3 wasnt there..it was on pins4/5 , instead of pin 9 ...so the 2 ac filament leads were connected this whole time

[Chuck H]

ok..i found a problem--1 of the heater wires at v3 (pi) was wrong...some idiot missed that ( ME) .

fixed that and I have the same low hum....

is it possible ....that i hurt the 6.3vac tap in the PT ?? i basically had it "shorted" i guess...but why did the amp function ? Pin 9 wire from v3 wasnt there..it was on pins4/5 , instead of pin 9 ...so the 2 ac filament leads were connected this whole time

It's completely impossible for me to interpret how the amp was wired and working with the scenario you propose. If you draw up a diagram of how it WAS wired wrong, maybe.?. But as it is I don't see how the amp could operate without detriment with half of the filament wire shorted. I'll absolutely need to see a diagram before I understand.

That the hum didn't change is also telling. I outlined above why I didn't think the filaments were the cause of the main issue (not that there is no problem with the filaments, but it was't the thread topic). I think you need to focus on the rectifier and the early power supply stages or their grounds.

[g1]

You had both heater wires going to the same tube pin? The amp would not work and I'm pretty sure you would have seen it?
Maybe it was mis-wired some other way?

[Valvehead]

ok--sorry chuck.



heres how the heaters were wired wrong --look at pic-------i have corrected it .

i just checked grounds and wiring again.

i also moved the 2 , 100uf /500v filter caps---they were smaller caps glued next to the rect on the pcb. They are now mounted like can caps ..in their holes . For reference , all filter caps are brand new .

i also went back through the amp with a chopstick moving all wires around. ...again ..

next step - throw 4 new 1N4007 in it i guess