Opinions/knowledge re KT88/6550?

[Prairie Dawg]

Somehow or other I have ended up working on more local SVTs than I'm happy about and I have this theory that the biggest problem people have with them is general unreliability of power tubes and how that triggers the fault detection circuitry a/k/a the "Honey, I've got a headache tonight" system.

I built a test rig to do a better analysis of them, but I think the real issue is that the SVT is running the power tubes at the maximum rated level for 6550s back when reliable tubes could be gotten from the real Tung Sol and GE back in the day.

The current crop is pretty much hit or miss, so I guess what I'm looking for is reliability, longevity, and stability. Is there any advantage to using KT88s for this purpose-particularly stability in service? The set of winged-C 6550 tubes in the one I have on the bench was allegedly matched when purchased 6 months ago, but now there's a 24 per cent mismatch, lowest to highest, and I think it's the cause of random fault detection trips.

I figure this group should have more on-the-ground experience than any other group of savants around.

[Mr. Foxen]

I think KT88s can draw a bit more current, that might make the power supply unhappy, especially with variable quality/voltage from the wall. It is more to do with good/bad valves than valve types.

[AwkwardLoudness]

KT88s would likely not work in SVTs due to the higher current draw. Some amps with 6550 tubes can also use KT88s if the power transformer is able to handle them and the bias is adjusted accordingly.

Tubes sometimes drift over time. Current production tubes are hit or miss indeed due to lower quality control compared to vintage tubes which had to be high quality; militaries and hospitals relied on them inthose days.

If I recall correctly, Lord Valve recommends Sovtek 6550WE tubes (among current production tubes) for SVT amps. Of course, the best power tube for SVTs is the vintage GE 6550A.

[Jhtjon]

I have had great luck with the Chinese 6550's from AES/CE. They are well matched and generally hold up well to what ever I throw at them (including 500+ Volts)

[beans-on-toast]

I was measuring the plate voltage on my 71 SVT just the other day. With 118VAC line voltage, my reading at the plate (pin-3) was 673 volts. Looking at the SVT-VR schematics, they show 660 V with a 120 VAC line voltage. The newer amps are the same as the old ones with respect to plate voltage.

If the line voltage in your area is higher you are going to see a higher plate voltage. 125VAC is not uncommon these days. What plate voltage are your seeing?

One point to remember about tube data sheets, the design ratings were often specified, despite their printed warnings, to provide optimal tube life. The NOS tubes can often take more than what is specified without problems. It isn't just Ampeg that pushed the tubes in their amps, Fender and others did it as well. The Winged C tube data sheet has a specified plate voltage of 660V, same as the GE 6550A. Somewhat lower than the 800V of the KT-88.