SVT Classic Power amp issues

[65ampman]

Hey Guys,

I just inherited a repair from another tech. Its been sitting for quite a while after a failed attempt to repair. Original problem was that it would not come out of standby(like all SVT CLs) but the LED did not flash, just stayed red. I discovered that the standby switch was opened. After replacing the switch, The LED would now flash red and green. I then discovered the usual, opened screen resistor(s). I replaced all power tubes and while replacing the screen Rs, I discovered that all of the plate inductors were missing. I subbed them with 1 ohm 1W Rs temporarily just to test. The amp now came on an passed signal but went into protection after a few seconds. Shutting main power off and powering back up has it functioning again. The bias LEDs work normally. Both sides are easily adjustable and the signal is symmetrical and sounds right until it is pushed hard. I noticed one of the power tubes arcing and then the relay clicked and went into protection/fail mode again.

Is the plate inductor critical to operation or is there likely a bad new tube? Also, how much current should this amp draw at idle? The meter on my variable auto-transformer shows 2.5A at idle and peaks to around 5A when pushed hard. I don't recall seeing this much current draw from any amp at idle. Is this an indicator of a problem?

Thanks in advance!

[Enzo]

Before anything else, I always check the small tubes. You have two 12AU7s and a 12AX7. I often see amps come in with three 12AX7s, or three 12AU7s, and certainly I find even with the right tubes, they are in th wrong sockets.

The "inductors" are just ferrite beads, the amp would work without them.

[The Dude]

And,...... If you see a tube arcing, the tube is probably bad.

[65ampman]

The PI tubes are good and in correct sockets. I've worked on many of these amps but have never seen an inductor/ferrite bead fail. Not sure why they had been removed but since they are in series with the plate of each power tube, something must be installed. I figured a 1 ohm resistor would be a good sub with little/no effect on tone. I guess I shouldn't be surprised to get a bad 6550 from Sovtek. Just used them to test the circuit. Will recommend JJ KT88s if customer has the budget for them.

[Chuck H]

I discovered that all of the plate inductors were missing.

WTF is a plate inductor on an SVT?!? Can a schematic be provided?

And,...... If you see a tube arcing, the tube is probably bad.

Since all the power tubes were "replaced" (can we assume known good, not just "new" tubes were used?) I would add,.. Or the operating conditions are wrong.

[The Dude]

Yep. It could well be the circuit. Put the arcing tube in a different socket and see if the arc follows the tube or the socket.

[65ampman]

Chuck H, as Enzo explained, it's just a ferrite bead designated L1-L6 on the SLM era SVT Classic schematic. I dont know how to attach the schematic with my phone. Will try tomorrow when I'm back at the shop with access to files on our PC.

[The Dude]

Ampeg_SVT-CL.pdf

[Enzo]

Ferrite bead is just a piece of wire with a speck of ferrite material around the middle. If I didn't have them, I'd use a piece of wire in their place. It is just a tiny inductance.

[65ampman]

Thanks!

[65ampman]

Any experience with this model or similar 300W tube amp to know normal idle current? 2.5A seems high to me but I dont have another to compare.

[The Dude]

I'll have one on the bench tomorrow to check against your number. That said, it doesn't seem at all unreasonable. You've got quite a bit of filament current alone with 6) 6550's . Add the other filaments, idle current, fan, etc.

[Enzo]

Flip it to standby, what is the current draw then? That should be mainly tube heaters, yes?

Any idea what you idle currents are at bias? 20-25 watts per tube? So 150 watts idle dissipation for those tubes alone.

1.6A heater per tube? times 6. About 60 watts worth of 6550 heaters.

2.5A is 300 watts, I think.

SO I get about 210 watts for just tube heaters and 6550 dissipation. Would the rest of the thing draw 90 more watts? That does sound a bit high.


But then have you measured tube dissipation?

[65ampman]

Very little current draw in standby. Will take measurements tomorrow. Thanks for your calculations!

[65ampman]

Thanks! Im guessing this is normal for 6, 6550s. If anything else was drawing that much current, there would be heat. I didn't smell or feel anything unusual so far and there was no excessive ripple on the B+ and it is pretty quite for a 300W tube amp. Still, it would be nice to have confirmation by comparison.