Ampeg SVT CL loud hum

[xtian]

Owner complains of bad hum. When first powered up, I noticed a power tube redplating, along with loud hum, even with bias controls set coldest. I tested all six 6550s in another amp--all OK except the one that was redplating is dead. With any combination of two power tubes installed, even with the PI and 12AU7 driver tubes removed, amp hums loudly when tubes start conducting. With all power tubes pulled, PREAMP out sounds fine--no hum, tone stack fine, etc.

All screen resistors (220R) and grid resistors (47K) measure OK.

Ideas how to proceed?

Ampeg_SVT-CL.pdf

[Stevem]

I would check and see if you have positive voltage showing up on pin 5 of any of the output tubes from a leaky PI coupling cap.
Anything over + 200mv is a sign of a leaky cap to me.

[xtian]

I would check and see if you have positive voltage showing up on pin 5 of any of the output tubes from a leaky PI coupling cap.
Anything over + 200mv is a sign of a leaky cap to me.

All pins 5 have -ve bias voltage (about -55v unloaded, with bias controls at full CCW). The bad hum I'm hearing is not tubes running away.

[Stevem]

How much ac power supply ripple are you seeing one the output stage screen and plate supply?

[Phil_S]

I think that Steve is asking about leakage -- a/c voltage could be present even with -55VDC.

Have you tried running a jumper from the PI output to a known good power amp?

[xtian]

Thanks guys. Yes, I see 29vAC on the grids (pins 5) of power tube sockets! I guess that means C10/C11 have failed. The plates have < 3vAC, screens < 1vAC, so B+ filtering seems OK.

ampeg.jpg

[martin manning]

What about the -180V supply?

[xtian]

Yes! Martin, I was just looking at that. That's actually more likely to be the source of the large AC ripple, because the 356vDC B+ supply has only 3vAC ripple.

[xtian]

Oh, yeaaahhhhh. C23 visibly bulging, and measures "unknown part" on my LCR.

[Stevem]

That will do it, yup!