AB165 Bassman…high voltage…

[Wally]

This was a new one for me, so I thought I would share. A 1965 AB165 Bassman came to me…dead…no fuse. So, recap, general service. Replaced the fuse holder during the AC power cord work. Replaced a treble pot that was spinning freely. Checked it out on the current limiter…good to go.
checked voltages without tubes…good. Installed tubes and fired it up….output was there. Tubes were burning without anything alarming…no redplating. Time for voltage check, and all was fine …except…..the plate on the V5 6L6 was showing over 1000 VDC…rising…rising…and then came down to 458VDC. All other voltages in that tube were good Even during the high voltage moments. It did this a couple of times, so I switched the power tubes around…the problem stayed in the socket…when it did occur.
So, I started looking closely at the socket….past the wires and dirt. There was something Odd around pin 3…the plate pin. Yep…dirt covered silicon. I got the wires off of the socket and then could see red material around the pin on the socket. I pried the pin a bit, and the side of the socket broke free from the ‘adhesives’ someone had use to hold the pin upright. I replaced the socket, and all is fine.
My question is…..with what was that plate pin interacting that caused that very high voltage? The other voltages in the socket were still correct???
h(ere is the socket with the break totally revealed….

 

[Jasonpatrick]

Could the silicone be somehow intermittently conducting to the chassis causing a short…

 

[Jon Snell]

I would suggest that a bad connection to your meter caused the iffy reading as with 1000volts across 750volts worth of smoothing capacitors would have been catastrophic. Especially older electrolytics.
Silicone , if it was silicone is inert when cured and doesn't conduct at all however, some modern gasket goo does absorb moisture and start conducting.

 

[Dacious]

Wow weird one Wally. I think like Jon says there's going to be some sort of weird cross connection. Unless it's some sort of arc happening - that's possible with a broken socket with some sort of conductive glue I'd guess.

 

[Paul G.]

Screen voltage added to plate voltage in series?

 

[peteb]

iffy reading

i agree with this.



It seems like we have seen this here recently on the TDPRI.


an unbelievable measurement that wasn't.

 

[bwacke]

Could be the plate pin was arcing. If silicone got into the inside of the socket and a thin coat was insulating the plate pin, the inductance of the OT winding would cause a "flyback". The high voltage appears when the arc was off (i.e., no spark) and comes down when the arc is on and feeding current to the tube.