How to test tubes

[Chris65]

Interesting! Found an extensive (if not complete?) guide to Compactron tubes & can't spot any others where pins 3 and 8 are connected. Maybe they are some sort of 'mutant' as PB says .

Let us know the results of your re-wire.

https://frank.pocnet.net/other/Compactrons/CompactronSurvey.html

[ejb14]

Did the re-wire today to use the mutant tubes...and it works! The amp passed the resistance and the voltage tests within specifications with all four tubes.

Sounds very good up to 1-2 o'clock on the volume knob. Starts to get a little sloppy after that - I don't think the 6FJ7's got that sloppy at the upper levels of volume. Listening to Dire Straits right now through them.

I re-did my input signal trying to get it as close to 500 millivolts as possible using audacity - ended up with a 60hz tone output from DAC at 485 millivolts. There is probably an easy way to get exactly 500 mv, I just haven't found it yet.

Mutant tube outputs with this input signal at full volume 2.63, 3.02, 3.32, 3.48 volts.

A while back I made a little jig to test amps with two very large 100watt, 8Ohm resistors, and added little resistor network to it to convert the speaker level to line level so I could record 'what the amp' sounded like direct.

I ran this through the RMAA software analyzer and the attached PDF is the results. Looks pretty good, though things got real sloppy at full volume with these tubes - had to back off to unity level (1pm on the volume control) to get good results with the software.

The PDF shows the Schitt Modi direct to the recorder for a baseline, the 'best' 6FJ7 tubes I have in the amp before I did this re-wire, and the two more power Mutant tubes.

[ejb14]

I would be very interested in knowing whether the amp works with these tubes and this mod, just to see if these are some kind of mutant 6FJ7 or just mislabeled 6FM7s.  Your assistance would be greatly appreciated!

-PB

Hi Paul -
Let me know if there is anything else you wish me to do with these tubes before I switch back to my 6FJ7's. I could also send them to you if you want them, since I am not sure I really have any use for them.

[Paul Birkeland]

Nah, I would send them back to the seller.  I'll have to go through my stash and pay careful attention to where those pins are connected.  Thanks for your help!

-PB