Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: labb on October 03, 2017, 01:40:18 pm

Title: Need help with a AB763 Deluxe Lite
Post by: labb on October 03, 2017, 01:40:18 pm

Amp is a version of Steve's AB763 Deluxe Lite. Amp has been operating for several months. Failure was a power tube (EH 6V6) failure. Tube shorted screen to grid. The 470 ohm screen resistor and the 1500 grid resistor were burned. Arc on the pins.. Pulled the socket and cleaned and checked it. Replace the resistors and a new set of matched JJ 6V6's. Now I can't get the tubes to conduct. These are the voltages I am reading. They will all be high because the tubes are not conducting.
Plate voltage at pin 3  501 VDC

Screen voltage at pin 4   500 VDC
bias voltage at pin 5       -44 VDC   I can adjust this as needed
Filament voltage is 3.5 VAC to ground on each leg.


I can see the filaments glow in each tube.
Checked both 1 ohm resistors in the cathodes to ground. They check good.
Both tubes are like this.
I pulled the new tubes and had them tested. They both test good.
I have pushed on every connection and can't find anything...
Man, I hate it when the smoke gets out. Always a problem...


note I got the tubes to operate a couple of times and then would lose the cathode current.

Title: Re: Need help with a AB763 Deluxe Lite
Post by: sluckey on October 03, 2017, 01:53:00 pm

Quote

Filament voltage is 3.5 VAC to ground on each leg.

How much voltage do you measure ACROSS the filament pins, one probe directly on pin 2, one probe directly on pin 7.

Title: Re: Need help with a AB763 Deluxe Lite
Post by: labb on October 03, 2017, 02:10:03 pm

O volts measuring pin to pin..6.8 measuring leg to leg at the terminal strip where I have the two 100 ohm resistors..I am drawing a blank. What have I wired wrong? And thanks Steve.

Title: Re: Need help with a AB763 Deluxe Lite
Post by: sluckey on October 03, 2017, 02:30:04 pm

Probably a broken wire between that terminal strip and the first 6V6. Most likely right at a solder joint at the t-strip or socket. I bet you used solid wire?

Title: Re: Need help with a AB763 Deluxe Lite
Post by: labb on October 03, 2017, 02:52:54 pm

Yes on the solid wire. Back at you when I find it and thanks again...That is the reason I was getting the same voltage reading leg to ground on each leg isn't it.

Title: Re: Need help with a AB763 Deluxe Lite
Post by: labb on October 03, 2017, 03:52:46 pm

You were correct. Thanks a bunch. :worthy1: :worthy1: :worthy1: :worthy1: ..This was one where I just got too close to the problem. Couldn't see the forest for the trees.

Title: Re: Need help with a AB763 Deluxe Lite
Post by: sluckey on October 03, 2017, 04:26:55 pm

So, how you gonna check heater voltage from now on?  :wink: