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Old 19th April 2013, 11:08 PM   #11
Enzo is offline Enzo  United States
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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.
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Old 20th April 2013, 12:28 AM   #12
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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.
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Old 20th April 2013, 12:50 AM   #13
Enzo is offline Enzo  United States
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85 lbs sounds about right.

Count your blessings, it could have been a Peavey Classic 400 head - 93 pounds.
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Old 20th April 2013, 03:49 AM   #14
cancon is offline cancon  Canada
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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.
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Old 20th April 2013, 05:46 AM   #15
Enzo is offline Enzo  United States
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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.
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Old 20th April 2013, 06:36 PM   #16
cancon is offline cancon  Canada
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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)?
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Old 20th April 2013, 08:12 PM   #17
Enzo is offline Enzo  United States
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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.
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Old 10th November 2013, 01:14 AM   #18
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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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