Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on March 21, 2023, 10:25:43 am

Title: Vibrato issue on old Danelectro Commando
Post by: markmalin on March 21, 2023, 10:25:43 am

Hello guys.


I'm debugging a non functioning vibrato on a Danalectro Commando.  The schematic in Doug's library shows a 6AU6 on the vibrato circuit, but this amp uses a 6SJ7 (the amp has 6SJ7 stamped on the chassis and the wiring matches a 6SJ7, not a 6AU6).  The plate resistor is reading around 300V DC on the B+ side, but at the tube's plate I see only 12V DC.  I've seen this sort of thing happen with 12AX7's when I've had a bad triode...just wondering your thoughts on possible causes, or how to further troubleshoot?


Thanks
Humbly - Mark

Title: Re: Vibrato issue on old Danelectro Commando
Post by: shooter on March 21, 2023, 10:52:00 am

did you pull the tube and ohm the plate resistor?

Title: Re: Vibrato issue on old Danelectro Commando
Post by: markmalin on March 21, 2023, 11:05:01 am

I did.  Actually, I just tack soldered a new plate resistor in there and it's reading more reasonably now.  I'm using an isolation transformer so the voltages aren't always as high as they should be (at least with my iso transformer). 


Now to move on - still no vibrato :(. but that's one issue solved at least.

Title: Re: Vibrato issue on old Danelectro Commando
Post by: sluckey on March 21, 2023, 12:14:25 pm

Sub a known good tube.

You should remove the original 68K plate resistor. It may still be a factor. Also measure the screen voltage. Check those 390K screen caps and the .5µF screen cap. Be prepared to replace all three.

Check for correct resistance of R57, R58, R59, and R60. Be sure they have a good connection to ground.

Still no good? Replace C21, C22, and C23. Check R53 and C19.

Be sure the Foot Switch jack does not have a switch terminal that is connected to ground.

Title: Re: Vibrato issue on old Danelectro Commando
Post by: markmalin on March 21, 2023, 01:17:54 pm

Quote from: sluckey on March 21, 2023, 12:14:25 pm

Sub a known good tube.

You should remove the original 68K plate resistor. It may still be a factor. Also measure the screen voltage. Check those 390K screen caps and the .5µF screen cap. Be prepared to replace all three.

Check for correct resistance of R57, R58, R59, and R60. Be sure they have a good connection to ground.

Still no good? Replace C21, C22, and C23. Check R53 and C19.

Be sure the Foot Switch jack does not have a switch terminal that is connected to ground.

Thanks a bunch, Sluckey!   I'll go through those steps


Mark

Title: Re: Vibrato issue on old Danelectro Commando
Post by: markmalin on March 22, 2023, 08:11:53 pm

Just as a followup - there were several issues I found.  On this particular amp the 0.5 uf wax/paper cap kind of disintegrated when I removed it.  I ended up replacing several resistors which had drifted WAY high, and the final fix was to swap out all 3 of the 0.02 uf caps.  Tremolo is now working nicely :).  Thanks again for the help.
Mark