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Power tube filament issues - need help!

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B

Bola

Member
Joined 2011
Good day folks out there,

I am completing my Tubelab SE with 300B and I am having a weird problem. Without the power tubes (300B), I can read 5volts for both filaments. When I insert tubes, everything works for one or two minutes and then filament power shot down. I turn off the amp for a few minutes (until B+ and B- go down to 0volt), then I power up again and everything works for one and two minutes and shot down again, etc. etc.

B+ and B- remain OK (B+ = 415V and B- = 295V).

When I don`t put any power tube, filament voltage stays on forever, well I less 15 minutes.

I tried 50ma on tubes, same result with a lot of distortion. With 60ma, distortion seems to be gone but I have no scope to validate.

Any suggestions where to look at?

Alain
 
Last edited:
Tubelab_com

Tubelab_com

Member
Joined 2005
tubelab.com
I will add a better heatsick like George is recommending.
Click to expand...

Yes, depending on the ambient temperature and the airflow around the amp it can sometimes even shut down with 45's. The Lexan amp I made is about the worst possible case. The case is too small, there is no airflow, the power transformer is too small so it gets too hot to touch. On a hot day I had it shut off with 45's using the little heat sink. I added the big green one seen in the pictures and it will run all day with 300B's. It was borrowed from an old Pentium 1 motherboard.

That amp lost a power transformer in its first year, but autopsy revealed water damage due to hurricane forced water intrusion because I left the amp near a windowduring the storm. Yes, the window was closed. The replacement transformer has been in the amp for 6 years. It gets too hot, but doesn't blow up.
 
B

Bola

Member
Joined 2011
tubelab.com said:
Yes, depending on the ambient temperature and the airflow around the amp it can sometimes even shut down with 45's. The Lexan amp I made is about the worst possible case. The case is too small, there is no airflow, the power transformer is too small so it gets too hot to touch. On a hot day I had it shut off with 45's using the little heat sink. I added the big green one seen in the pictures and it will run all day with 300B's. It was borrowed from an old Pentium 1 motherboard.

That amp lost a power transformer in its first year, but autopsy revealed water damage due to hurricane forced water intrusion because I left the amp near a windowduring the storm. Yes, the window was closed. The replacement transformer has been in the amp for 6 years. It gets too hot, but doesn't blow up.
Click to expand...

George,

As a note, my board was not mounted in a chassis yet. I conclude that the IC is probably not well mounted on the heatsink. Anyway, I have larger heatsink that I plan to install this week end.

Thanks for the advice.

Alain
 
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