Ampeg V4 Standby Problem

[stereomonostereo]

Hey guys,

I'm working on a shipwreck of an Ampeg V4. Right now I'm trying to sort out why I'm getting B+ voltage at my power tubes and output when I've got standby switched off. The switch is working, and the standby light comes on when switched, but there's no effect on the voltage. (It definitely needs caps, as there's quite a bit of 60 cycle hum.) I've attached the schematics and some photos of what's going on in the amp.

http://www.ampegv4.com/images/schematics/V4.jpg

[g1]

The standby in these amps is in the ground line. Check for grounding mods that have effectively shorted out the switch.

Edit: It will probably have to do with retro-fitted filter cap ground connections.

[Leo_Gnardo]

I'm working on a shipwreck of an Ampeg V4. Right now I'm trying to sort out why I'm getting B+ voltage at my power tubes and output when I've got standby switched off. The switch is working, and the standby light comes on when switched, but there's no effect on the voltage. (It definitely needs caps, as there's quite a bit of 60 cycle hum.)

Ampeg's standby switch breaks the ground connection to the - (minus) end of the hi voltage power supply, unlike most standby that break the + end. Your meter will read hi voltages all right, and unexpectedly high voltages on the bias supply too, but can't deliver power until the power supplies' negative pole is connected to ground. What's more the power-supply-to-ground connection is made through a rectifier diode, and if not done that way brings on a lot of hum. So I'd say - don't worry about it, everything's as it should be. I'm sure there will be lots more to take up your attention in your V4. And for safety's sake don't assume the circuitry is safe to work on with the power on but in standby. I'd guess there IS enough energy there to give you a helluva shock so don't take chances. Remove the amp's AC plug from socket and discharge all hi voltage caps before sticking your fingers & tools in there.

Besides the usual suspects, hum in this series of Ampegs results from a damaged hum-balance pot. Failing output tubes often destroy this pot and sometimes it shows no external evidence of damage.

[stereomonostereo]

I just gave it a look, and low and behold, one of the tube retainer springs was touching the side of the main filter cap and taking it to ground. I just corrected it and the standby is back to normal. Thanks! Now on to the next horror story in this thing

[g1]

Is this topside? Do you mean the main filter has exposed metal but is somehow isolated from chassis? If so, the entire can should be insulated with something (electrical tape, heatshrink, something). Those caps have cardboard/paper insulating sleeves when stock.

[stereomonostereo]

The metal can is mounted on an insulator. I've since put a cardboard sleeve over it.

[g1]

That could have been a big ouch for someone!

[stereomonostereo]

Without a doubt!