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I came into some 6550s and was wondering what fellow inmates think about using them in the Knight KB-85 amplifier?I've converted mine to fixed bias and it works gangbusters with the Russian 6P3S-Es.
I got 8 of these 6550s. I bought a turntable from a guy and he threw them in. They're Russian, not sure who makes them, possibly Svetlanas.
They sound great. Almost as nice as the 6P3S-E, but a bit punchier. There was some problem with redplating initially, and had to adjust the bias range to cool them down a bit.
The only thing is, the 6550As have a heater current rated at 1.6A apiece. That's a far cry from the 900mA or so for the 6P3S-Es. However, the stock EL37s were 1.4A, and folks run EL34s at 1.5A, so I'm hoping it can handle the extra current.
Would I be able to bias them for 25W, not to blow my OPTs? What should I set the quiescent current to?
I'm trying them overnight to see if the paint softens on the power transformer(!). I've got a thermometer on it.
But if you all told me to definitely not use them for whatever reason, I'll pull them right now.
Thanks a bunch.
Edits: 07/19/15Follow Ups:
Hi, not only are you jeopardizing your output transformers you are also risking the health of the power trans. It may work for awhile but i don't think the long term outlook is good. Your power trans needs to deal with an extra .8A of heater current and almost .1A of extra B+ current. Since the transformers are potted the overheating will not show up for awhile so a thermometer may not indicate how hot it is in the center of the PT. Unless you know what the specs for your power transformer is you should probably stick closely with the el37 or 6l6gc. Try to find out the specs for the power trans and you can see how much leeway you have. Also remember the AC voltage specified back in the 1960s was 117vac. Most line voltages today are 120 to 125 volts. Just my opinion. regards, Dak
Thanks DAK, for the suggestions.
Precisely my concerns as well. The whole thing seems to be running a bit too hot for my liking. I think I'll put my old tubes back in and chalk it up to an interesting experiment.
All the best,
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