6550WE Turned bright red on my ST-120

Hello All,

I built my ST-120 about 5 years ago, it has been running beautifully for all these year.
Yesterday, I played a Vinyl and one of the left channel 6500WE started glowing bright red.
I switched off the AMP immediately , waited for it to cool down, then switched it on again
and everything seems to be fine. The overheated 6500WE has turned a little black but seems
to function perfectly, no sound distortion the quality of sound just as good as ever.

Any ideas ? I am afraid this will happen again.

Best Regards
Sandeep

sandeep dutta wrote:Hello All,

I built my ST-120 about 5 years ago, it has been running beautifully for all these year.
Yesterday, I played a Vinyl and one of the left channel 6500WE started glowing bright red.
I switched off the AMP immediately , waited for it to cool down, then switched it on again
and everything seems to be fine. The overheated 6500WE has turned a little black but seems
to function perfectly, no sound distortion the quality of sound just as good as ever.

Any ideas ? I am afraid this will happen again.

Best Regards
Sandeep

I suggest that you swap the tube that red plated with the other one on the left side. Verify bias levels after the swap.
If it happens again to the same tube then you know that tube is defective and can't be trusted. Otherwise it is a socket issue.
Let us know if it happens again.

sandeep dutta wrote:Hello All,

I built my ST-120 about 5 years ago, it has been running beautifully for all these year.
Yesterday, I played a Vinyl and one of the left channel 6500WE started glowing bright red.
I switched off the AMP immediately , waited for it to cool down, then switched it on again
and everything seems to be fine. The overheated 6500WE has turned a little black but seems
to function perfectly, no sound distortion the quality of sound just as good as ever.

Any ideas ? I am afraid this will happen again.

Best Regards
Sandeep

There is a high risk that this tube will redplate again, it should be replaced.

BTW, 6550 needs a grid resistor of at most 50k to be safe from redplating.
KT88 is a better option for this amp.