Hi All,
I have a pair of Unicorns, er I mean HH Scott 240's (seen one lately?). I found them in a thrift store 30 years ago for $12, and packed them around until I actually fixed them a couple years ago. I fitted them out with Electroharmonics tubes because they were relatively inexpensive. Everything sounded great for two years or so of casual listening, maybe 200-500 hours of on time when they started to pick up harsh harmonics.
I looked around and found a couple of threads from guys with class A amps asking about short tube life. Of course there were folks with AB amps saying that tubes last a long time. I thought of the operation curves and tubes biased for class A sit at the maximum power point at idle, unlike relatively low power AB idle. I can just imagine a heavy flow of highly accelerated electrons tearing up the surfaces of the plates and grid wires in the output tubes.
I shopped around and tried a quartet of Shuguang Treasures I found a deal on. They really looked spiffy, so I bought them, but with probably 50 hours of run time they're deteriorating. My crazy Russian co-worker has pointed out I'm an iddiot for trying the Chineese tubes (probably correct) and insists on Svetlana for the next set.
Since the outputs on these amps appear to be a consumable item, I'll stick with new or NOS cheap tubes, not sought after NOS Amperex or Mullard tubes. My first impression is to get either more Electroharmonics or Svetlana's because the Russian tubes seem to be pretty rugged and can take the heat.
Any other suggestions?
Thanks,
M
I have a pair of Unicorns, er I mean HH Scott 240's (seen one lately?). I found them in a thrift store 30 years ago for $12, and packed them around until I actually fixed them a couple years ago. I fitted them out with Electroharmonics tubes because they were relatively inexpensive. Everything sounded great for two years or so of casual listening, maybe 200-500 hours of on time when they started to pick up harsh harmonics.
I looked around and found a couple of threads from guys with class A amps asking about short tube life. Of course there were folks with AB amps saying that tubes last a long time. I thought of the operation curves and tubes biased for class A sit at the maximum power point at idle, unlike relatively low power AB idle. I can just imagine a heavy flow of highly accelerated electrons tearing up the surfaces of the plates and grid wires in the output tubes.
I shopped around and tried a quartet of Shuguang Treasures I found a deal on. They really looked spiffy, so I bought them, but with probably 50 hours of run time they're deteriorating. My crazy Russian co-worker has pointed out I'm an iddiot for trying the Chineese tubes (probably correct) and insists on Svetlana for the next set.
Since the outputs on these amps appear to be a consumable item, I'll stick with new or NOS cheap tubes, not sought after NOS Amperex or Mullard tubes. My first impression is to get either more Electroharmonics or Svetlana's because the Russian tubes seem to be pretty rugged and can take the heat.
Any other suggestions?
Thanks,
M
You could try RFT NOS, made in East Germany. I don't know how widely available these are over there. They may be branded Brimar, Siemens, Mullard or many other names. Some are even branded RFT!
Valves ageing is mainly due to the cathode losing emission, or the getter being used up. High current or overheating are your enemies. You may be able to reduce quiescent current and make them last a bit longer.
Valves ageing is mainly due to the cathode losing emission, or the getter being used up. High current or overheating are your enemies. You may be able to reduce quiescent current and make them last a bit longer.
Class "A" operation definitely limits the service life. The SED (=C=) EL34 is a nice tube, but how long it will last in your amps is TBD. The EH "fat bottle" 6CA7 is a decent tube too. More expensive, but maybe worth a shot, is the "reissue" KT77 that's made by New Sensor at their Saratov, Russia, plant.
A look at the schematic shows the "finals" being used at the upper limits of allowed voltages. Current production is not as tough as the Mullard original. Perhaps 220 Ω g2 stoppers will improve things.
Unfortunately, it may be necessary to alter the O/P tube operating conditions, in order to obtain decent service lives. An easy way to bring the B+ rail voltage down is to replace the 5U4 with a "potato masher" 5R4.
A look at the schematic shows the "finals" being used at the upper limits of allowed voltages. Current production is not as tough as the Mullard original. Perhaps 220 Ω g2 stoppers will improve things.
Unfortunately, it may be necessary to alter the O/P tube operating conditions, in order to obtain decent service lives. An easy way to bring the B+ rail voltage down is to replace the 5U4 with a "potato masher" 5R4.
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