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Old 16th March 2009, 03:17 AM   #1
ben_f_m is offline ben_f_m  United States
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Default Substituting GU50 tubes for KT88

I'm interested in substituting Russian GU-50 tubes for KT-88 tubes in a push-pull amplifier. In looking at the pin-out for the Gu50, I find that the screen is brought out on pin 7 and there are three grids. The KT88 appears to have two grids and the screen connected internally to the cathode. In my substitution, what should I do with the GU50's screen and additional grid?

I will very much appreciate some help here.

Data sheets for the GU50 and KT88 can be found at these sites:

http://tubedata.milbert.com/sheets/163/k/KT88.pdf
http://www.jogis-roehrenbude.de/Russian/GU50.htm
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Old 16th March 2009, 03:21 AM   #2
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Once you get past the pinout confusion, the bigger issue will be in not exceeding the screen voltage specification of the GU50.
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Old 16th March 2009, 08:52 AM   #3
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2-d & 3-h /GU50/ grids to plate !
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Old 16th March 2009, 10:45 AM   #4
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I currently use GU-50 in Push-Pull with 850V plate and +300V screen (solid rock regulated). Such approach needs also to regulate control grid bias. Sound pristine clean. With 10K P-P I get relaxed mode, 80W of constant sinusoidal power from 20 Hz to 20 KHz. Nice to drive any speakers, from phased arrays to planars.
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Old 16th March 2009, 10:23 PM   #5
ben_f_m is offline ben_f_m  United States
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Thank you, Ty_Bower. You are exactly right that the screen voltage is a problem. The amplifier has about 550V on the screens of the KT88s. Lowering the voltage isn't technically very difficult, but what are the implications of doing so?
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Old 16th March 2009, 10:45 PM   #6
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Quote:
Originally posted by ben_f_m
Thank you, Ty_Bower. You are exactly right that the screen voltage is a problem. The amplifier has about 550V on the screens of the KT88s. Lowering the voltage isn't technically very difficult, but what are the implications of doing so?
Less distortions, especially when well regulated.
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