Newly modded dynaco st-70 (vta with pentode/triode switch)

Hi everyone. I just finished upgrading my st-70 with the VTA board (original- not octal) and installed the pentode/triode switch. I am currently running some JJ E34L power tubes and can bias them appropriately to the 40mA as suggested.

The issue I am having is when I power on sometimes I get a quick glow from the left and center driver tube. It is exactly what happens when I use a dim bulb on a amp I'm testing that is working normally. The light glows bright and quickly dims down.

I'm sure this is not normal since the right driver tube does not do this and I didn't have this issue before. I checked all voltages and they are within spec and like I said I biased the power tubes easily and they stay accurate.

I listened to the unit for several hours today and it sounded fine to me.

Would appreciate any insight into how I can solve this matter. Thanks.

mjstriker5 wrote:Hi everyone. I just finished upgrading my st-70 with the VTA board (original- not octal) and installed the pentode/triode switch. I am currently running some JJ E34L power tubes and can bias them appropriately to the 40mA as suggested.

The issue I am having is when I power on sometimes I get a quick glow from the left and center driver tube. It is exactly what happens when I use a dim bulb on a amp I'm testing that is working normally. The light glows bright and quickly dims down.

I'm sure this is not normal since the right driver tube does not do this and I didn't have this issue before. I checked all voltages and they are within spec and like I said I biased the power tubes easily and they stay accurate.

I listened to the unit for several hours today and it sounded fine to me.

Would appreciate any insight into how I can solve this matter. Thanks.

If you are talking about filaments that "flash" , it's completely normal. Some brands ( phillips derived) does this. Don't worry be happy.

If you want to "solve it", then install 3 JJ ecc82 tubes or some other 12au7 that is not made
by a phillips or related faktory. But it's not needed, if / when the tubes eventually fails it's
not for filament reasons.

Take a look at this YouTube post by Mr Carlson.

https://www.youtube.com/watch?v=bX0o3jAzSBs&index=4&list=PLwJ536x-nypK1886AlE_vLLRL5qI2xfSx

pichacker wrote:Take a look at this YouTube post by Mr Carlson.

https://www.youtube.com/watch?v=bX0o3jAzSBs&index=4&list=PLwJ536x-nypK1886AlE_vLLRL5qI2xfSx

It's a moot issue. Phillips tubes has _always_ flashed at power on and i have yet to
see one that has bunt the filaments or in any other way got damaged by this flash.

My Audio Research (Amperex) preamp and Berning (Sylvania) tube amp both had filament flash at power up. I don't like how it looks but there is nothing "wrong" with either device or the tubes.