Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on May 15, 2022, 09:27:52 am

Title: Mesa 50/50 Ch. A very low output
Post by: JustMike on May 15, 2022, 09:27:52 am

 This amp is an earlier rev than the one in the schematics library. Mine doesn't have a choke for one. I've got good +VDC at node C (450v) , but at the plate on the other side of R282 (82k), I've got 126v and the schemo shows 316v.
 Any ideas?

Title: Re: Mesa 50/50 Ch. A very low output
Post by: shooter on May 15, 2022, 09:42:11 am

what's happing on the other plate?

Title: Re: Mesa 50/50 Ch. A very low output
Post by: tubenit on May 15, 2022, 10:23:48 am

Do you have a 12AX7 or some other 12A_7 in that position? 

Title: Re: Mesa 50/50 Ch. A very low output
Post by: PRR on May 15, 2022, 12:20:56 pm

Quote from: shooter on May 15, 2022, 09:42:11 am

what's happing on the other plate?

And the common cathode?

Diagnosis by checking just one point in a circuit with more than two parts requires brilliance or great luck.

Title: Re: Mesa 50/50 Ch. A very low output
Post by: Keppy on May 15, 2022, 12:49:03 pm

If you need a schem, try contacting this poster (https://el34world.com/Forum/index.php?topic=27531.msg302788#msg302788). It looks like he hasn't been active in about 6mos, so DM me if you really need it and can't reach him through the forum.

Title: Re: Mesa 50/50 Ch. A very low output
Post by: Latole on May 15, 2022, 12:56:16 pm

Remove V2 ; how many volts ? Did you get the 316 V ?

Title: Re: Mesa 50/50 Ch. A very low output
Post by: JustMike on May 15, 2022, 02:02:29 pm

Thanks all. It was a bad P.I. tube. BUT the owner just told me that originally the amp was redplating. He replaced the 4 output tubes and then noticed that channel A was not working. So now I have to monitor for redplating.