ST35 Glowing Screen

[HiFiGuy]

I have an ST35 board amplifier that I have been listening to for a while. I stored the amp for about 3 months then dusted it off to use again. I put in a set of tubes and fired it up into a load and adjusted the bias on all the tubes to 35mA per tube. I was about to button up the amp and put it into service and realized one of the screens of the power tubes was glowing bright. I shut down the amp, set the bias pots to 400 ohms as per the manual and tried another set of tubes. To my amazement, the same socket as before had a glowing screen on a known good set of tubes. I did some research and found that if you have an ultralinear amp without resistors between the screen and the OPT that there could be arcing events. I installed resistors between all screens and the OPT, still the same socket is has a glowing screen on the tube. I checked the voltages on the pins and nothing looks out of order. Has any one else had this problem? How did you fix it?

Thanks

[dcgillespie]

I know you've had two tubes glow in the same socket, but a glowing screen is not entirely unusual for some tubes -- particularly true pentode tubes like the EL84. Most tubes won't do it under quiescent conditions, but some will. It is a product of manufacturing tolerances in the tube -- basically one of how much exposure the screen has to the current flow.

I would do more checking and see if the two same tubes you've found to glow in the one particular socket, will also glow in other sockets when set for the same current flow. My guess is that they will, and visa versa. That is, tubes that didn't glow in other sockets likely won't glow in the original socket either, when biased the same way.

Dave

[Geek]

It could be losing B+ to the plates.

Cheers!

[EWBrown]

The Z565 OPTs and the modern clones of it, use paralleled windings in the primary windings, and if one of these opens, the DC resistance will rise to twice the original value, but the amp will still work, the plate current will be reduced and the screen grid may be trying to take up the slack. That would definitely light it up. The SG is usually good for a watt or two dissipation, as compared to the 12 or 13Watts for the plate. In general correct operation, the SGs usually consume just a few mA, and stay well under their dissipation ratings.

Having the glowing screen in the same socket position, rather than following the "suspect" tube around, is a good indication of this or some other connection problems to the OPT. If you used the screw terminal strips and crimp lugs, just make sure you still have a solid connection, nothing is loose or corroded. Otherwise check the soldering around the "suspect" tube socket.

HTH

/ed B

[Ty_Bower]

The Z565 OPTs and the modern clones of it, use paralleled windings in the primary windings, and if one of these opens, the DC resistance will rise to twice the original value

Interesting theory. There is some data on the expected typical resistance of the various taps on the Z565. Find it in this thread:
http://www.diytube.com/phpBB2/viewtopic.php?t=4169

[20to20]

I have an ST35 board amplifier that I have been listening to for a while. I stored the amp for about 3 months then dusted it off to use again. I put in a set of tubes and fired it up into a load and adjusted the bias on all the tubes to 35mA per tube. I was about to button up the amp and put it into service and realized one of the screens of the power tubes was glowing bright. I shut down the amp, set the bias pots to 400 ohms as per the manual and tried another set of tubes. To my amazement, the same socket as before had a glowing screen on a known good set of tubes. I did some research and found that if you have an ultralinear amp without resistors between the screen and the OPT that there could be arcing events. I installed resistors between all screens and the OPT, still the same socket is has a glowing screen on the tube. I checked the voltages on the pins and nothing looks out of order. Has any one else had this problem? How did you fix it?

Thanks

When you say "screens" do you mean the plate. The tubular outside metal plate?

[HiFiGuy]

This is distinctly the middle (2nd) grid thats glowing. Its not an orange its a hotter yellow glow. I will try to check if its the transformer this weekend. Although I checked the resistances of the primaries and both sides are very close in resistance. I am using the reissue dynaco transformers I bought on triodeelectronics.com. I really hope its not a bad winding.

[20to20]

This is distinctly the middle (2nd) grid thats glowing. Its not an orange its a hotter yellow glow. I will try to check if its the transformer this weekend. Although I checked the resistances of the primaries and both sides are very close in resistance. I am using the reissue dynaco transformers I bought on triodeelectronics.com. I really hope its not a bad winding.

OK. I guess I've never seen a glowing screen grid on a 6BQ5. Is it the electrode end that sticks up through the mica that you see glowing? I can't see the guts in my tubes.

[HiFiGuy]

I am looking through a hole in the plate, but you can see the glow through the mica. Its like a light bulb inside the plate structure.