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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 31st December 2017, 04:29 PM   #1
KinklessTetrode59 is offline KinklessTetrode59  United States
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Default B+ increases to unloaded value while 300B plate current goes to almost zero

Seems i have a silicon problem - on Tubelab SE i have the following parameters:
B+ about 330 on R30
B- about -230 on R7
Hammond 276X wired on the Black wire (My house voltage is solid 120VAC)
Filament Voltage on Regulator is 5.25VAC
Regulated DC Filament voltage is right about 5 VDC
Grid on 5842 is about 175VDC
Plate Current after a few minutes of warm up on 300B (new Genelex Lions) is about 60ma
Using UBT-3 output transformers
Temp of silicon does not go much above 70 C (i added extra heat sinking)
Sound is excellent and no hum until i get runaway

After about 30 mins or longer or shorter its not the same, B+ goes to unloaded value (around 400 VDC), plate current on 300B goes to about zero and of course music stops. It seems more unstable as i increase B+ and plate current (I have on variac), works best if B+ stays lower. I decreased R4 to 120ohm to get higher B+ (150ohm gave me about 305 VDC), problem seems to have started but i am not sure after change of R4, i was still in check out mode.

Plans: Verify good solder on pins of silicon i got a couple that do not seem to go through the board, put 150 ohm back for R4 and reduce B- to closer to -150 with smaller R7.

Am i missing anything?

Thanks guys
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Old 31st December 2017, 11:29 PM   #2
Tubelab_com is offline Tubelab_com  United States
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Watch the filaments in the 300B in a dark room when the music stops. If they go dark the filament regulator is overheating and shutting down to protect itself. A bigger heat sink, or better air flow is the solution.
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Old 31st December 2017, 11:55 PM   #3
KinklessTetrode59 is offline KinklessTetrode59  United States
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Its the regulator - it hates being at 50+C, put another heat sink on it and a small fan and it appears to be resolved thanks for the input.
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Old 1st January 2018, 03:25 AM   #4
Tubelab_com is offline Tubelab_com  United States
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My very first TSE did the same thing. The heat sink liberated from a Pentium 1 chip (the green thing) fixed it up.

That amp is stuffed into a very small box with near zero airflow.
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