Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: theundeadelvis on November 16, 2019, 12:21:39 pm

Title: Dr. Z Maz 38 issue
Post by: theundeadelvis on November 16, 2019, 12:21:39 pm

Hello all!


I've been away from building for quite some time (although I do have a PR build barely started), but I've picked up a couple used amps recently. One being a Dr. Z Maz 38. I've had it maybe 3 weeks and have loved it so far buuuut... 


I flipped it on the other day and all of a sudden it makes an awful static noise when I take it off standby. Super loud when you first flip the switch, then it slowly fades. You can hear the guitar but it's very quiet. Flip the switch to put it back into standby and the static happens again and fades away.


Here's a gif with audio of the noise: https://imgur.com/a/myaeoXZ


I pulled the chassis today and did some tube pulling and swapping. Power tubes (4x el84) had no effect, pulling and swapping v1 and v2 had no effect, pulling the PI made it go away (but swapping a good tube in and it came back).


Any thoughts on where to go next to track it down? Thanks in advance!

Title: Re: Dr. Z Maz 38 issue
Post by: SILVERGUN on November 16, 2019, 12:56:48 pm

I've experienced a similar symptom and wound up finding a failing snubber cap across the PI plates.

Title: Re: Dr. Z Maz 38 issue
Post by: theundeadelvis on November 16, 2019, 02:51:54 pm

Thanks SILVERGUN! Is the snubbed cap in a LPT the cap that goes from the tail to one of the grid leaks?

Title: Re: Dr. Z Maz 38 issue
Post by: theundeadelvis on November 16, 2019, 03:35:29 pm

Scratch that,  I see it it goes between the plates. I'll take a look at that guy tonight. Thanks!

Title: Re: Dr. Z Maz 38 issue
Post by: theundeadelvis on November 16, 2019, 05:01:22 pm

Well, no luck. It has a 47pf cap there and I lifted one leg of it and it still made the noise.

Title: Re: Dr. Z Maz 38 issue
Post by: theundeadelvis on November 16, 2019, 07:05:28 pm

Well. I was poking around taking voltages on the PI and now it works fine? So I've either got a bad solder joint or a socket issue maybe? But we're getting there!

Title: Re: Dr. Z Maz 38 issue
Post by: theundeadelvis on November 17, 2019, 10:26:31 am

Not fixed.  :help:


I poked around the PI with a chopstick and didn't get much feedback, except for the wire coming from pin 2 I really microphonic. Also if I take a voltage reading from that pin or here it temporarily fixes the issue:
(https://i.imgur.com/k4S6D9A.jpg)

Title: Re: Dr. Z Maz 38 issue
Post by: shooter on November 17, 2019, 10:31:25 am

PI pin 2 or PA pin2?

Title: Re: Dr. Z Maz 38 issue
Post by: shooter on November 17, 2019, 11:20:52 am

looked up tubes, both 2's are grids I believe which are problematic probing in a good amp.

sounds like you are "completing" the circuit by probing, I'd heat up the turret, pull wires n make sure an R hasn't cracked, wire isn't broke, pin sockets aren't sloppy.

if none of that works, probably time for DC analysis, then on to AC

Title: Re: Dr. Z Maz 38 issue
Post by: theundeadelvis on November 17, 2019, 12:45:18 pm

Thanks Shooter! I reflowed the solder joints around that part of the circuit and tightened the tube socket. So far so good. Fingers crossed it stays working!


Thanks again guys!