Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jordan86 on January 12, 2021, 06:19:51 pm

Title: Check Plate Voltage from top of socket? (death wish?)
Post by: jordan86 on January 12, 2021, 06:19:51 pm

This pertains once again to my 1965 Dual Showman that I recently acquired and have talked about on other threads. I've entertained pulling 2 of the 4 6L6 tubes which gives me 50W at 8 ohms. To be clear, I've actually done this already, but this was when I had the chassis out of the head cabinet. Now that it's back in the head shell its a major pain trying to do this and reset my bias accurately.

BUT....

I took lots of notes on where the plate voltage needs to be when the bias in in the correct range. The good news is that I can get to the bias pot easily to adjust that without pulling the chassis. So wondering if its a major safety concern to read the plate voltage from the top/outside of one of the unused sockets?

1. Pull tubes in sockets 1 and 4.
2. Get a multimeter lead into pin 3 tube socket on one of the unused sockets.
3. Adjust bias pot

I presume the biggest risk would be shorting pin 3 to the chassis with the MM lead? I have some lead tips that cover the entire lead except for the very tip end. Thinking I'd go that route.

Title: Re: Check Plate Voltage from top of socket? (death wish?)
Post by: pdf64 on January 12, 2021, 06:39:31 pm

DrZ used to suggest that method on his site. Its major drawback is that it doesn’t account for mains voltage variation. eg if the mains is 2% high, the amp’s HT will be 2% high, and so you’d tend to set the bias far too hot (excessive plate current) in oder to load it down.
DrZ took that method off his site a few years back.
Unfortunately there’s no short cuts to doing it properly. If you don’t want to bother getting the chassis out, use a bias probe or put jacks for 1ohm cathode current sensing resistors on the back panel or on the underside of the chassis.

Title: Re: Check Plate Voltage from top of socket? (death wish?)
Post by: sluckey on January 12, 2021, 07:24:13 pm

I don't see any danger in probing the tube side of the socket. If you are lucky your probes will fit snuggly in the socket and even hang upside down, hands free. But you don't want to adjust bias by monitoring plate voltage.

I would add a couple 1Ω resistors to the inner tubes. Now monitor pin 8 to set the bias current. Monitor Plate voltage at pin 3. Do the math.