Ampeg SVT CL fault

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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?

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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?
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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.
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?
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??
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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.
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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.
What about them? The new tubes are just not as robust as the old ones, put them into any amp you like - new or old - and they will not hold up as well or be able to push as hard.

Point being that all things equal, the current production tubes need to be babied more than the original American tubes. That is why the old design is updated to more benign component values like we are discussing.

If you had an old 1969 SVT and wanted to put new 6550s into it, you would probably do yourself a favor to increase those old screen resistors to modern values for the same reason Ampeg did.
I just had one of these re-issues on the bench. It had a couple bad tubes, 2 output and 1 12AU7, and IMO the modern import tubes are junk compared to NOS. I constantly see fireworks and problems you just never used to see with old tubes. Anyway, I pulled the unit apart to check the resistors, none of which were bad. It's a beast (85 lbs ?) and I'm glad my workbench is built like a BSH.
Enzo, my point was that old amps with high wattage screen resistors are running repro tubes and there isn't an epidemic of problems... I'd guess that you wouldn't see many amps that are retroactively modded with lower wattage screen resistors just because. I completely agree about new tubes v.s. old tubes and that they don't make 'em like the used to. Given that these 1/4w resistors blew, but tubes tested OK, and now the amp is running a-ok with 1W resistors, is it possible that 1/4w resistors were just too sensitive given the nature of new tubes?

The new SVT amps are lighter than the older boys. My pride and joy is a 1973 svt running great with n.o.s. preamps/drivers and well matched 4year-old Tung-Sol re-issues in the output. I've been gigging with it for 2-3 years and don't have a single complaint. It is the best rockn'roll bass amp ever, period. The svt vintage re-issue sounds very close to the originals, but I don't think too highly of the SVT Classic, both sound and construction.
AH I see where we are passing each other. I was not thinking of watts, I was thinking of 22 ohms going up to 220 ohms. I wouldn;t remotely suggest lowering the wattage of resistor on the old amps, but I was suggesting it would be prudent to raise the resistance. NO, I don;t feel comfortable with 1/4w, I am old school and expect higher wattage screen resistors, whether they are needed or not, I suppose.

I wasn't thinking about a run of blown resistors, I was thinking that new tubes just won;t last as long as old ones, and they will fail more often. On average. Individual instances may vary.
The Tube Store located in my home town and I deal with them often. I just had an interesting conversation about tube failure rates across the board of everything they offer, bottom to top of the line. Basically, all the brands they carry have an equal failure rate. One good thing that I've noticed in my experience (and they agree) is that the bulk of tube failures occur within the first few hours of a tubes operating life (usually within the warranty period). Then again, a gigging amp (and a heavy one at that) puts more stress on the tube's construction as its usually thrown around a fair bit.

I'm happy to leave the 220ohm 1W resistors in the CL for now. What do you recommend for screen resistor values in the future (both new and old amps)?
Oh, I tend not to argue with what they came up with, I see the value of the higher resistance, beyond that I don;t get very creative in the repair process until I run into a problem requiring it. Some guys see every amp problem as a "design error" and go off to modify things rather than just fix them. I am not one of those.
SVT-CL Got one of these in power fault , found a burned filament connector on the output tube board (bad crimp). Cleaned and re-soldered. All diodes and resistors fine and resoldered suspect connections. K1 still comes up even with outputs pulled with a good filament voltage. Czeched output tubes, all are OK and matched (close). Can't find schematic on the protection/fault circuit. Anybody run into this before?
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