Express rewire "correct" sockets still some hum

[rooster]

KGW - Oh. I know we have a blizzard going on this week, But I am in Portland, Oregon. If you ever get down this way, bring your amp and we can A/B it against mine. I have 35 watter and a 25 watter here. Rawnster stopped by last week and it was a good experience. Anytime, PM me for my phone number.

Those bleed resistors can't be the cause of the hum. MF or CC, the function here is out of the audio circuit. Look elsewhere. You have adopted the Alan Philips ground method and I know that this works. For me, I stuck with the common ground for all B+ caps but separated the Presence control ground as he suggested. Oh, where he suggests tying the OT ground to the Presence control ground?, I did not do this. One is stock KF at the OT using the metal speaker jacks as the ground, the other two are using Cliff plastic jacks and grounding to a point between the two pwr tubes. Eh, no audible difference to this ear. In all cases, again, I do isolate the Presence control ground. Alan is a smart man, I have no doubts and I played his Express clone a few times by now - Killer, absolutely Killer.

[geetarpicker]

Well that confirms the ground scheme is different, though also sounds like it works also. Just a thought though. My amps only have 2 ground lugs, the one near the bleeders and the main ground for the AC cord input. The power tube grounds are tied to lugs integral to the output tubes. The input and output jacks don't have any more ground connections near them other than the bolt itself. Maybe Ken either liked to take the chance on those, or liked having the chassis be the flow of the ground instead of ground wires.

[KGW]

I did the out of phase wiring just to try it (as on Frannie).

As for the grid stop on V1, that will reduce the gain a bit, but for a high gain amp like an express, I think that no resistor is what is needed. Grid stoppers also help to control parasitic oscillation, but mainly in the power tubes. So more of a tone/gain thing than a hum thing. I am pretty happy with the tone at this point.

Everybody in our house has the flu now. I'll try to record some clips and do some hum measurements when we're feeling better.

Hum is a bitch to cure. I like the band (Hum) though.

[Firestorm]

Hmmmm. (Or Hummmmm). Can you tell if the hum is 120Hz or 60Hz. It's not easy because they sound very close (ears are bad measurement devices). But if the hum is 120, it's a DC power supply problem, if it's 60 it's an AC power supply problem or a ground loop.

If 60 cycles, make sure no signal wire gets anywhere close to the input power. Then try biasing the heaters up to 40 or 50 volts. That will eliminate any heater hum. If it still hums, it's a ground loop. The Express, all Fenders, most Marshalls are one big ground loop, but the random grounds tend to equalize it on most of them. Once in a while, things don't even out and you get hum.

If you want to go random ground, make sure every ground really is a ground; a bad solder joint or a bad physical connection from a pot to the chassis will wreak havoc.

If you want to be absolutely correct, isolate everything and run one ground for preamp circuits and a second ground for the power amp (plus the safety ground).

I don't know if it applies here, but I've seen a lot of 70s Japanese amps that would not stop humming until you went to the dual star ground.