Power tube filament issues - need help!

[Bola]

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

[evanc]

maybe the filament regulator is overheating.
try blowing a fan at it and see if it stays powered up longer....

[rknize]

Yup....pretty common problem with the stock heat sink in the 300B configuration (the instructions mention this). The amp was originally designed for 45s.

[tomchr]

It sounds like the voltage regulator for the 5 V filament supply is going into thermal shutdown. Try with a larger heat sink.

~Tom

[Bola]

It seems to be a concensus. I also read the same from a different blog. I am going to try to cold it down with a fan.

I confirm I only have the stock heatsink - no add on.

[tomchr]

Here's another thing to try: Pull one of the 300B's and operate the amp as a mono amplifier for a while. It should take longer for the regulator to reach thermal shutdown then.

The idle/quiescent current of the 300B has no impact on when the regulator goes into shutdown.

~Tom

[Bola]

You are all correct. With a fan on the IC, everything is back to normal.

I will add a better heatsick like George is recommending.

Thanks everybody for the quick help!

Alain

[tubelab.com]

I will add a better heatsick like George is recommending.

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.

[Bola]

Originally Posted by tubelab.com

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.

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

[rknize]

My TSE is also just sitting on a bread board. With 45s, convection was enough to keep it working (but it was still very hot). With 300Bs, I got 10 or 15 minutes of playing before it gave out. I clamped an aluminum "squangle" to the heatsink to get it to stay happy until I re-did the bread board.