Help with a bias tester setup

[AZJimC]

I finally got a couple of ampenol octal sockets and plugs, they type that fit through a hole in chassis and a small spring ring snaps on the back to hold it in place. I measured and with a bit of grinding both parts fit nicely into a sawed off PVC 3/4" coupling.

Now for the schematic, I want to be able to measure the plate voltage with a second lead coming off the adapter. Which pin should I measure from, and I want to measure with a voltage divider so I don't have full plate volts coming out of the shell, so the first question is, which pins and what are the best resistor values & wattage to use to get say 1/10 of the plate volts out of the socket without any interruption of the normal operation of the tube?

I have seen the ones offered with the divider set up so plate volts reads in millivolts, same as the 1w resistor in series with pin 8, but I think this would reduce accuracy in proportion to the rediction in plate volts read, ie.: 1000:1, so I am guessing I would place a divider of 1:10 ratio across which pins.

Next question, would this work with most of the octal power tubes, 6L6, EL34, etc.? or do any of the popular ones differ in plate volts pins?

finally, would these pins differ on tubes running in Pentode or triode modes? if so how can I easily compensate for this?

Due to using the type of plugs I have, I can make the adapter longer so I have room inside for electronics, but I still want to keep it as compact as possible to avoid pushing tubes into something in certain amps, maybe use a 45 degree L for the adapter body? lol

I will likely eventually hook these up to a small box with panel mount meters, with a-b switching, or maybe even 4 meters, 2ma, 2 volts, to avoid switching.

I know I can find all the info needed to calculate this myself, but I hope someone has most of this info off the top of their head, especially the pentode-triode thing and pinouts of other tube types etc. Once set up, I would be glad to post a schematic here of the final setup for posterity, unless there is already such a thing, and I have just failed to find it online.

Thanks much again guys,
Jim

[jhadhar65]

>Which pin should I measure from...

Pin 3 is plate/anode (a) on most common octals we see, such as EL34, KT66, KT77, 6V6, 6L6/5881, 6550/KT88, etc.  It's Pin 7 on EL84 types.  Depending on the actual circuit, the screen (g2) voltage may be more important.  Screens on common octals tubes will be Pin 4 on octals and Pin 9 on EL84s.

I don't know about your divider question.

>Next question, would this work with most of the octal power tubes, 6L6, EL34, etc.?

A properly wired bias checker will work on any output tube, but they're not completely universal.  See pin differences above.

>...would these pins differ on tubes running in Pentode or triode modes?

Nope.  However, they 'could' differ in terms of voltage and that might matter in pentode mode if the screens are at a much different potential than plates... like 600Va (plate) vs. 300Vg2 (screen).  I always just worry about screen voltage and use that as if it were plate voltage to calculate bias.  I only check plate voltage as a confirmation.

BUT...

Personally, I wouldn't even go there.  I'd focus on constructing it like this.  With most power tubes that I've come across, you can either gently tilt it in the direction away from the plate pin (EL84) or wiggle it out enough (octals) to touch your red probe to the pin and get a Va reading.  Here's some points I observe:
*  DON'T SHORT THE PIN TO CHASSIS.
*  All you need is a 3/32" gap or so for most probes.
*  You don't have to expose much of the pins to see which one is the right one anyway.
*  DON'T SHORT THE PIN TO CHASSIS.
*  Try exposing the pin first before you power it up.
*  Be careful when wiggling tubes with a key.  Some are more easily snapped off than others.
*  DON'T SHORT THE PIN TO CHASSIS.
*  If your probe has a plastic collar around it to keep fingers off the metal tip, just file down one side so you don't have as much of an angle to deal with, but you should be more careful with that probe from then on.  My real feelings on that is if you really need that collar to protect you from inadvertently touching something you shouldn't to begin with, find another hobby as soon as possible.

And...
*  DON'T SHORT THE PIN TO CHASSIS.

It should be all gravy from there, but as always... your mileage may vary.

[Rick48]

Hello, I am working on a fender twin 1972 and will be checking the bias. I have a question about leaving a 1 ohm resistor connected from pin 8 to ground for bias testing later. Would this affect the amp in any way? tone power ect. ?

[tubesornothing]

Hi Rick, welcome to the board.  Its best if you have a new question to start a new topic, that way your question will be seen by more people, and this thread will remain on topic, with answers for the original poster (Jim's note).

All that being said, having a 1 ohm resistor left in the amp for bias checking is just fine.  Lots of people do it.

ToN

[sluckey]

Hello, I am working on a fender twin 1972 and will be checking the bias. I have a question about leaving a 1 ohm resistor connected from pin 8 to ground for bias testing later. Would this affect the amp in any way? tone power ect. ?

No problem to just leave them in place.

Did you just ask this question on the Yahoo Magatone/Valco group?