'72 Ampeg SVT 60 HZ hum problem

[booj]

Polarity switch has no effect. AC plug ground is connected to chassis. the guy took it to the shop I work at and we could not find a problem. then he comes to the shop with his epiphany speaker cab and hooks it up. HUMMMM.... So its intermittent? I think this is a weird problem so I'm posting it. I'll keep y'all informed.

[g1]

I've had problems with the shield wire in the wiring harness to the preamp with these, or the connectors.

[nevetslab]

I've had problems with the shield wire in the wiring harness to the preamp with these, or the connectors.

THAT ^^^^^^^^^^^. I too have had issues with the shield connections in the wiring harness. Tedious to chase down with all the discrete wiring.

[booj]

Anybody got a clue on where this mystery hum is coming from? Grounds seem to be solid. Perhaps inside the preamp? Looks like good connections to the plug.

[Leo_Gnardo]

Some more clues might be found here:

http://archive.ampage.org/threads/6/...um_long-1.html

Also, boost or cut midrange often brings in some hum, which is picked up by the inductor in the midrange circuit. Some relief might be found by rotating the inductor to a position where it picks up minimum hum when mids are cut or boosted.

[loudthud]

Does this version have a Hum Balance pot ? Is this more of a buzz type hum, or a low Organ note type hum ? Check the pot if it's the low organ note type.

[The Dude]

^^^^^^ That. I've seen the pot burned up in several of these. I'll usually remove the pot and just install a couple of 100 ohm resistors for the artificial center tap.

[SoulFetish]

^^^^^^ That. I've seen the pot burned up in several of these. I'll usually remove the pot and just install a couple of 100 ohm resistors for the artificial center tap.

Yup.

[booj]

The problem was dirty tube sockets. I replaced them all. Of course now one channel is out, but the hum issue is resolved.

[boroman]

Glad you found the main problem.

I have got few early 70s SVT over the years and few later ones. The pre-'75 hum more than later ones in my opinion. This is a flaw in the design, that high voltage wire goes along with signal wire next to it (in the molex connector). To avoid that you would need to drill a hole in the chassis and put the signal wire from preamp to chassis and below, in a separate way from the molex. I did it once and it took away 50% of the hum. Also, it's good to replace the molex for the new one (they are available), make new connections with new wires. That always helps in those amps.