Dr Z 38 Maz Sr reverb repair

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

Post Reply
User avatar
pompeiisneaks
Site Admin
Posts: 4222
Joined: Sat Jan 14, 2017 4:36 pm
Location: Washington State, USA
Contact:

Dr Z 38 Maz Sr reverb repair

Post by pompeiisneaks »

Here's a new video on a recent amp repair I did on a Dr Z 38 MAZ Sr



Thoughts?
tUber Nerd!
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: Dr Z 38 Maz Sr reverb repair

Post by pdf64 »

A friend's EZG50 kept flipping into full power oscillation :shock:
It took a while to identify but the root cause was poor 0V to chassis connections, made via the control pot casings. I think that the aluminium chassis surface may tend to oxidise over time, and the pot bushings just couldn't keep a reliable connection, even if they were re-established a time or 2.
I solved it by establishing a new chassis connection in the area with a high tensile bolt ensuring that a solder tab was retained under very high pressure against the chassis surface.
Years later it's still running ok.
Dunno if this is a common problem.
User avatar
M Fowler
Posts: 14017
Joined: Wed Jan 16, 2008 2:19 am
Location: Walcott ND

Re: Dr Z 38 Maz Sr reverb repair

Post by M Fowler »

All of the Dr Z amps I've fixed had broken or bad grounding at or on the pots.

A touring EZG50 I maintain had rectifier tube problems changed JJ to Sovtek recto and added diodes.

That reverb section is going to have some interaction when tapped but should not be that bad.
User avatar
pompeiisneaks
Site Admin
Posts: 4222
Joined: Sat Jan 14, 2017 4:36 pm
Location: Washington State, USA
Contact:

Re: Dr Z 38 Maz Sr reverb repair

Post by pompeiisneaks »

Great info thanks.

I got er going but forgot to film more. Basically the dead resistor was a major problem causing feedback, howling, screeching etc, that went away and some sensitivity still existed at the turret going to the tube etc, but it was way better.
tUber Nerd!
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: Dr Z 38 Maz Sr reverb repair

Post by pdf64 »

M Fowler wrote: Thu Sep 21, 2017 3:18 pm All of the Dr Z amps I've fixed had broken or bad grounding at or on the pots.
Relying on the pot bushings for maintaining good contact to the aluminium chassis may not be a reliable method then.
M Fowler wrote: Thu Sep 21, 2017 3:18 pm...A touring EZG50 I maintain had rectifier tube problems changed JJ to Sovtek recto and added diodes.
On to a bug bear of mine :roll: Don't blame the tube if it fails in a dodgy circuit!
I think it may be due to the unfortunate standby arrangement used, ie hot switching the reservoir cap on to the rectifier.
Why oh why is that done? It's a certain recipe to pull damaging surge current through the rectifier plates.
Why oh why even have standby with a GZ34 equipped amp? They provide a lovely smooth HT ramp up over about 45 seconds.
I strongly suggest to avoid using standby on hot switching, tube rectified amps.
But yes, with a tube rectifier series diodes are cheap insurance / resilience, can't see why they aren't standard fitment.

Sorry to go on, it's just that they are otherwise really nice amps.
frankdrebin
Posts: 131
Joined: Wed May 14, 2014 11:40 am

Re: Dr Z 38 Maz Sr reverb repair

Post by frankdrebin »

I wouldnt get any prescription from this doctor,Sorry.

Inviato dal mio Redmi 4X utilizzando Tapatalk

Post Reply