HUM! Need some help...

[MHProd]

So I removed a VVR and PPIMV from a JTM45, reconnected the bias circuit and power stage and now all I get is a nasty hum when I turn on the Standby switch (with that switch off, the hum is gone).

Triple checked all connections, re-soldered everything, nothing has changed...

I tried the amp on a variac and the hum makes the B+ jump up and down between 300 and 370(?)VDC (that's with the Variac at 85V). The regular B+ for this amp is around 440V.

Checked the ground, continuity, OT center tap and choke are on the mains 32uf... I can't find where it's coming from...

Tried to get some reading with the tubes in. Brought the variac to approx 115V, turned the standby switch on and blew the mains and HT fuses instantly.



Any ideas?

[Littlewyan]

Got any photos of the inside so we can take a look? Bit odd that its doing that without any valves. Is the power transformer getting warm? Could be shorting out somewhere.

[Stevem]

Pull all the tubes and install a 1/2 amp fast blow fuse and if that goes up in smoke when you power it up you likely have a direct short on the secondary side of the power transformer!
If so remove the transformers wires feeding the rectos and see if it blows again and then report back!

[MHProd]

I tried without the power tubes and the fuses are fine. I looked under the board and there aren't any solder blobs or bare wire that I can see...

[Littlewyan]

Ok just to confirm it only blows the fuse with power valves installed?

[MHProd]

Seems like it, yes.

[Littlewyan]

Have you tried different power valves? If it only blows the fuse with the power valves in then its most likely a dodgy valve.

[MHProd]

Yes I tried. Not the valves... If I bring the amp up slowly with a variac (up to around 80V) I get what I think is a continuous "hum" as soon as the plate voltage on the tubes reach around 280V. I don't think it's motorboating but it could be something else than hum...

Any suggestions on what to do next?

[Littlewyan]

What is the bias voltage set to? I'm assuming this is the first time you've powered up the amp so the bias voltage should be set as high as it can go. Thats the bias voltage not the bias current. Also can you post some gut shots.

[MHProd]

First time I powered the amp after removing the VVR and PPIMV. Was working perfect before. Pretty sure that's not where it's coming from. Must be the OT CT.

[Littlewyan]

Well its back to basics I'd say. Follow the schematic and check all connections in the amp. Double check nothing is shorted to ground. Also we cant help much if we dont have photos of the amp.

[Stevem]

You need to confirm that you have at least a minus 38 volts on each pin 5 of the output tubes with the tubes out and the amp on.

If you find that you have less than 25 volts than its very likely that you now have have two bad output tubes also !

[matt h]

(deleted)

[Littlewyan]

Of course the master volume circuit. Are the valves red plating by any chance? I suspect the grid leaks arent hooked up to the bias supply.

[MHProd]

Acutally, power tubes are fine as long as the PI tube isn't in. Tried a different tube: same result.

Strange since I haven't changed anything to the PI...