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In Reply to: RE: " NOS TS between 70 and 80ma " posted by FenderLover on April 29, 2010 at 00:28:36
Steve
the original citation 2s run at 460 and they were running 100ma thru each tube. Then,they dropped it to 90ma. The 600v plate,has 300v on the screens and that I would go no more than 62ma.
If the power supply waveform isn't pretty,neither is the sound in most cases.
Follow Ups:
Mike, 100mA at 600VDC or 60 watts per tube! If I ran that on an Ampeg SVT you potentially get 360 watts! Until it blew sky high. LOL! As it is, the 120 watts or so is SO LOUD it's crazy.
Mike's post isn't clear and the numbers got confused.
The original Citation II biased at 100 ma. and had 450 volts across the tube. That was 45 watts - that was really pushing it, agreed. But back then tubes were relatively cheap and plentiful so it wasn't as big a deal. In spite of that, Stu Hegeman & Harman-Kardon later reduced the bias current spec to 90 ma., which dropped the dissipation to 40.5 watts.
With my kits I recommend 80 to 85 ma. With today's higher line voltages, etc., that gives 37-39 watts dissipation.
No amp I know of ever (deliberately!) ran 600 volts and 100 ma per tube.
One thing Mike pointed out is definitely worth remembering - in a pentode or beam power tube the screen voltage has much more influence on current than the plate voltage does. You can run very high plate voltages with lower screen volts and it'll work well (if it's done right obviously).
Finally, keep in mind that when you measure cathode current you are measuring the sum of the plate current AND the screen current. So 85 ma. cathode current on a Cit II is actually (it varies from tube to tube some) about 78 ma plate current and about 7 ma-ish screen current. That means you have a plate dissipation of 78 ma. x 460 plate volts, or about 36 watts; and you have screen dissipation of 7 ma x 460 screen volts, or about 3.2 watts. Do keep in mind plate and screen current varies some from full load to idle - I chose sort of a "mid-point" above.
A 6550A or KT88 typically have a plate dissipation limit of 42 watts and a screen limit of 8 watts using the Absolute Maximum rating system, the least conservative rating. The next more conservative rating is the "Design Maximum". The Design Maximum rating for the plate is 35 watts.
----No amp I know of ever (deliberately!) ran 600 volts and 100 ma per tube.
Let's see...:) RCA's MI-9355 ran well over that voltage and quite comparable cathode current( no g2 in an 845 ). My 4E27 amps ran near 600V and idle at 110 mA. I'm sure GM100 amps exceed that idle current/plate voltage.
I suspect you speak of 6550/KT88 consumer amps...heh-heh-heh( and in that you'd certainly be correct ).
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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