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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 3rd January 2020, 01:59 PM   #21
PDL is offline PDL  United States
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I would use the fuses you show in your second picture. I don't think I would go up to 4 amps, as George explicitly stated in this thread that he recommends 2 amp slow blows in the US. The fuses in your second picture look like they would handle the inrush better.

What is your C1 value?
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Old 5th January 2020, 03:34 AM   #22
johnzm is offline johnzm  United States
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C1 is 47uf 500 volt Jensen
C2 is 220uf 500 Volt Jensen
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Old 12th January 2020, 06:06 PM   #23
johnzm is offline johnzm  United States
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i've been pretty quiet, i have been testing numerous configurations with this light bulb installed.

so currently i am getting the lightbulb coming up to a brightness, and good volume out of the speakers. and then about 5-10 seconds later, the bulb gets a bit brighter, with the left channel fading out entirely (with possible sound reduction in right channel as well, but cannot verify).

i swapped tubes and the issue stays with the left channel so its time to start digging on the board i suppose.

the one thing i have not yet done is swap in a new pre tube (currently a 12ax7), but i would be interested in hearing any ideas you might have
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Old 13th January 2020, 04:02 PM   #24
Tubelab_com is offline Tubelab_com  United States
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It's doubtful that a 12AT7 can blow a line fuse since it's fed a constant current.

I would try lifting one end of the coupling cap on the left channel and testing the board again. You are having a runaway issue which is usually tube related, but it wouldn't be the first time it turned out to be a coupling cap....and the last time it happened, the builder had the exact same coupling caps I see in the picture in post #1.

Lifting one end of the cap will kill the sound in that channel, but should have no effect on the power consumption. If the runaway goes away, the cap is likely leaking.

It is also possible that a bad cathode bypass cap can cause the same condition, but I have only seen it happen when it was installed backwards, or abused because of a bad tube or severe overvoltage (me experimenting with a heavy hand on the power supply knob).
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Old 13th January 2020, 10:10 PM   #25
johnzm is offline johnzm  United States
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Quote:
Originally Posted by Tubelab_com View Post
It's doubtful that a 12AT7 can blow a line fuse since it's fed a constant current.

I would try lifting one end of the coupling cap on the left channel and testing the board again. You are having a runaway issue which is usually tube related, but it wouldn't be the first time it turned out to be a coupling cap....and the last time it happened, the builder had the exact same coupling caps I see in the picture in post #1.

Lifting one end of the cap will kill the sound in that channel, but should have no effect on the power consumption. If the runaway goes away, the cap is likely leaking.

It is also possible that a bad cathode bypass cap can cause the same condition, but I have only seen it happen when it was installed backwards, or abused because of a bad tube or severe overvoltage (me experimenting with a heavy hand on the power supply knob).
i was the one with the issues last time with the same caps
i returned those and these are replacements (jupiter was easy to work with as well)
i measured both coupling caps (volt meter on one side of the cap, then the other)

on the bad channel im seeing 234 volts, on the other 228.

both have around 2-3mv on the other side of the cap.
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