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Old 6th July 2009, 05:28 PM   #1
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Default Simple SE - Ei KT90?

Has anyone ever plugged a set of Ei KT90 into a Simple SE? I happened across a pair and I'd like to try them out in triode mode. They're the type that has all the little weld spots down the edge of the plates, so I think they're the "good" kind. I found a set of triode plate curves for them. Can anyone confirm if these are close to how they actually perform?

Based on that curve data, I'm guessing with a 560 ohm cathode resistor I'll get about 72 mA through the KT90 for an overall dissipation of ~27 watts. The KT90 should be able to handle a lot more. I'd like to consider using a smaller cathode resistor, but it's a pain to change it out and things are already getting close to the limit of my Hammond 374BX power transformer (rated 175mA total at 50 Hz?).

Any comments?

edit... Just thinking out loud - I bet what I would need to do to make this work is to drop down to 510 ohm cathode resistors, and go to solid state rectification to get the extra volts on the B+.
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Old 6th July 2009, 05:57 PM   #2
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You might consider fixed bias.
Minus 40 volts at the grid.
Will sound better too.

Pieter
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Old 6th July 2009, 06:07 PM   #3
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Very good idea. The 374BX has a 50 VAC bias tap. I should be able to rig up a suitable bias supply, provided I can find some room under the chassis.
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Old 6th July 2009, 06:20 PM   #4
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hey-Hey!!!,
You've got the type 2; *VERY* nice tubes. If you'd ever get tired of them, do let me know. I'd like some more of those. In comparison to GEC KT88, those are rare tubes...
cheers,
Douglas
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Old 6th July 2009, 06:20 PM   #5
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Be sure that the bias is there before the plate voltage.
A clever trick is to put a relais in the bias supply.
The relais will block the HV (plate) winding of the power supply transformer when for some reason there is no bias. Good to safe tubes.

Pieter
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Old 6th July 2009, 08:13 PM   #6
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I have a set of EH KT90s and I liked them a lot in the SSE with my midrange-lacking speakers. I intend to try EH 6CA7s at some point to see how they compare.

However I seem to remember that you strongly dislike the EH tubes, so I guess this post is useless. Have a nice day!
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Old 6th July 2009, 08:30 PM   #7
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Quote:
Originally posted by rknize
I intend to try EH 6CA7s at some point to see how they compare.

However I seem to remember that you strongly dislike the EH tubes...

Me? No. The first time I fired up my Simple SE, it had a pair of 6CA7EH in it. I thought they sounded rather good. I've got some minor complaints about the stability of their grids (tiny jingling noises), but I wouldn't say I strongly dislike them.

I did have some problems with a set of EH EL34 and glowing screens. Maybe that's what you are thinking of?
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Old 6th July 2009, 08:33 PM   #8
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I was specifically thinking of the EH KT90, but maybe I'm thinking of someone else.
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Old 6th July 2009, 08:46 PM   #9
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Quote:
Originally posted by pieter t
Be sure that the bias is there before the plate voltage.
A clever trick is to put a relais in the bias supply.
The relais will block the HV (plate) winding of the power supply transformer when for some reason there is no bias. Good to safe tubes.

Pieter
hey-Hey!!!,
Making the bias from the plate winding tap leaves only a failure of the rectifier diode to worry about; if the plate supply has power, so will the bias rail.
cheers,
Douglas
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Old 6th July 2009, 08:50 PM   #10
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Quote:
Originally posted by Bandersnatch
Making the bias from the plate winding tap leaves only a failure of the rectifier diode to worry about; if the plate supply has power, so will the bias rail.
What is the typical failure mode for a silicon diode? Open, or short? In other words, should I parallel two diodes for reliability, or wire them in series?
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