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cancon 11th April 2013 04:03 PM

Ampeg SVT CL fault

I have a late 90's SVT CL on the bench. The owner says the amp quit during practice (auto-standby), and he may have smelt something burning.

I fire it up and it works plays, I let it stay on for about 1hr and I've had no luck re-creating the problem. I've opened it up and can't see any suspect components.

However, the two middle tubes (out of the 6x 6550's) are not conducting at all. Plate voltages are there (and out of range on my 600vdc meter), the tubes check out on the tester, and the position of the tubes in the amp doesn't matter. The bias controls can be centered for least amount of hum, but I can only get the red lights to go out - green's won't come on at all. At that point in the bias' pot's swing I get the outer pairs running at 10-14mA each (which is really low).

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Any tips?


cancon 11th April 2013 07:27 PM


Screen grid resistors on those tubes have seized up, reading infinity... schematic reads 22ohm, service bulletin corrected those to 220, that's what I have here in front of me. Any reason these would have gone, and is there anything I can do to prevent it from happening again? Also none of the screen resistors have diodes installed in parallel as seen on the schematic, any merit to add them in?

Enzo 11th April 2013 08:08 PM

They usually burn open when a power tube fails. the cure is to put good tubes in the socket. It might be possible for a bias problem to cause such a failure, but since the grides are all wired to the same bias source, I tend to doubt that cause.

You also want to MAKE SURE you have two 12AU7s in the correct sockets. I find bad 12AU7s as about the most common failure in these amps, and when people try to service teir own, I also find things like three 12AX7s or the two 12AU7s are in the wrong sockets. The two sockets behind the two bias controls are for 12AU7.

cancon 11th April 2013 09:06 PM

Thanks Enzo.
I double checked the 12AU7s are correct. I measured those 3 driver tubes as well as all the output tubes on my tester and they have no shorts and measure good mutual conductance. Also moving the output tubes between sockets sees them conduct just fine in the 4 outer positions.

Also, just thinking out loud - the amp plays fine with these two resistors out of commission, maybe this is what's left of a previous repair (after which tubes were replaced?) and the most recent problem is due to something else?

cancon 12th April 2013 06:57 PM

Replaced all the screen resistors with 1W 220 ohm pieces.
Green bias lamps are now lit and tubes are pulling in the 20mA range. Better!

cyclecamper 16th April 2013 12:24 PM

Without knowing the history it's hard to say whether the tubes were replaced since the resistors blew. He might not have noticed a 1/3 power reduction?? You could burn it in at high power overnight into a correct impedance dummy load and see whether the fuses blow again. Since they're also working as fuses, you didn't change the wattage rating of the resistors I hope...same value and wattage as you took out??

cancon 16th April 2013 07:56 PM

Actually, I didn't know they are used a fuses and of course upped the wattage! Dang...

cancon 16th April 2013 09:10 PM

Ok, just checked the schematics and saw that ampeg used 22ohm 1W screen resistors for the original 6550 svt. I know this amp is a different beast, but the power section is more or less the same. The original drawing for the CL shows 22ohm 1/4w screen resistors and later changed to 220ohm 1/4w.

Enzo 16th April 2013 10:48 PM

More or less the same...

One huge difference is the tubes. In 1969, the 6550s were good GEs or other fine brands. The 6550s of today are cheap imports that do not hold up to abuse like the old ones. The protection of higher value screen resistors helps in this regard.

cancon 19th April 2013 10:32 PM

That's a good point, but what about vintage SVT amps fitted with re-issue import tubes?

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