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Old 13th January 2019, 05:03 PM   #1
Grumpish is offline Grumpish  United Kingdom
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Default Tubelab SE - low output, high B+

The checkout was fine, B+ at 345V, B- at 200V, filament voltages all good, and plate voltages on the 5842 tubes to to 175V - but the problems started when I installed the output tubes and set the bias. I expected the B+ and B- to drop quite a bit, but they didn't, the B+ only dropped by 5V to 345V and trying to play anything gives a very weak and distorted sound. I have checked and rechecked the board, I can't see any bridged tracks or dry joints, the resistors all look good and have the correct values, but it seems like the output tubes are not drawing any power - any ideas where to start looking? Also, the heatsink for the CCS devices appears to be live, with around 250V on it, is this normal?
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Old 13th January 2019, 05:34 PM   #2
dtaylo3 is offline dtaylo3  United States
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What tubes are you using?
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Old 13th January 2019, 05:40 PM   #3
Grumpish is offline Grumpish  United Kingdom
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Quote:
Originally Posted by dtaylo3 View Post
What tubes are you using?
45's - checked the filament voltages at the tube base with them installed, steady at 2.48V
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Old 13th January 2019, 09:35 PM   #4
Tubelab_com is online now Tubelab_com  United States
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Does the output tube draw any current? does this current change when adjusting the bias pot?

Remove the output tubes and measure the voltage on the grid pin. It should adjust from around -30 to -90 volts.

There have been two or three board / tube set combinations in 14 years where the voltage was just too negative to turn the 45's on even with the bias set for maximum current. This can be fixed by reducing the value of R11 and R22. Test the voltage swing before messing with resistors, because this is vary rare.

I have a TSE on my bench right now and the voltage adjusts from -25 to -115 volts My 45's seem to want about -80 volts to get 27 mA, but I have 340 volts B+ fully loaded because my line voltage is 127 volts!
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Old 14th January 2019, 11:41 PM   #5
Grumpish is offline Grumpish  United Kingdom
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Originally Posted by Tubelab_com View Post
I have a TSE on my bench right now and the voltage adjusts from -25 to -115 volts My 45's seem to want about -80 volts to get 27 mA, but I have 340 volts B+ fully loaded because my line voltage is 127 volts!
That is about the range I get. and I have managed to get the B+ down to 330V by reducing C4 to 4.7uF - but I suspect that may be about as low as I can get it, my line voltage is higher than it should be as well. The low distorted output turned out to be a red herring, that was down to bad connections from the output transformers to the speakers. I have a good output now apart from an annoying low level choppy buzz on one channel. Getting there, slowly.

Also, the 3.5A Sharp regulator does work, at least with 45 tubes - it gets a little bit warm, but nowhere near as hot as mosfets - and holds the filament voltage steady.
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