Dr. Z Maz Sr. with VERY light reverb

[eddie25]

I've got a Dr. Z Maz Sr. that a friend bought second hand and it has extremely low reverb. You can put the reverb on 10 and it's about like a normal Fender on .5 to 1.

The problem is that everything seems to check out. Tube change and reverb tank change do nothing, I get what seems like healthy noises when poking around the reverb section. For example poking around the grid of the input of the reverb section gives me what seems like more reverb on the pop than ever is on the guitar signal. Shaking the tank also provides what seems like normal a amount of noise.

There's a pretty healthy amount of ground references on the return section, It almost seems as if this amp was meant to be like this, is that a possibility?

I haven't started to get deep into it because I don't want to de-solder anything if I don't have to. Anyone got a Maz Sr. that could confirm that this is not right in the first place?

[Stevem]

If you unplug the rca from the output side of the pan and tap on its tip with the reverb up full you should hear a very strong buzz, if not then the amp has a issue with the recovery side of the reverb section, if you do get a good buzz sound then the pan is not getting fed enough reverb drive signal .
Does this reverb circuit use a drive transformer like a Fender?

[eddie25]

It does use a transformer for the reverb.

I get more of a squeal when I touch the wire going to the return section. It's very loud. The reverb pot does effect it. If the reverb is all the way up touching the plate on the return tube is extremely loud. Voltages all seem to be normal.

[Stevem]

Ok so on this amp the return gain stage is always wide open and the only the signal level driving the pan is adjustable it would seem.
If you have a rca to 1/4" adapter you can place it on the end of the input cable to the pan and plug it into another amp, it should if all is well have enough signal level to drive that other amp to a loud volume, this of course assumes that the input rca cable is good.
If the drive tube is good along with the voltages on it then that drive transformer is suspect !
Do you have a schematic, or can you confirm for us that the reverb level pot is on the drive side?
If it's on the recovery side then the pot could be open, so just for kicks jump it out!

[eddie25]

It's wired very much like a Fender AB763 with tweaked values, except that the send stage is only one triode and the recovery stage is parallel triodes, which is opposite a Fender. So the pot is one the recovery side and I tried jumping it already with no luck.

I noticed that the reverb transformer uses the same colors as a Fender would except it has the red and blue wires reversed (red goes to the plate and blue goes to B+). Could this cause the problem? I still have a weird feeling that this is some sort of wiring error.

I can eventually try to set up an amp beside it and run each half of the reverb stage from each to see if I can attempt to pinpoint which section it is.

Thanks for the input.

[tubeswell]

I get what seems like healthy noises when poking around the reverb section. For example poking around the grid of the input of the reverb section gives me what seems like more reverb on the pop than ever is on the guitar signal. Shaking the tank also provides what seems like normal a amount of noise.

This indicates that the recovery stage is working fine.

Sounds like there is an issue in the reverb driver stage. You might have a fried reverb transformer or dodgy driver tube. (But try replacing/swapping the send cable first). What are the voltages on the driver stage?

[Stevem]

It's wired wrong , blue goes to the plate and red goes to the B+ node.

[eddie25]

It's wired wrong , blue goes to the plate and red goes to the B+ node.

I'll try this. Hopefully I can get to the blue wire easily enough without having to remove anything else, as it's directly under the board.

Thanks.

[M Fowler]

Doc would repair this probably free if it is indeed wired incorrectly from factory.

In my experience there are several versions one is non loop and other is with loop. Reverb is subtle.

Mark

[eddie25]

Doc would repair this probably free if it is indeed wired incorrectly from factory.

It's really more of a gut feeling that I am not looking for a bad part.

The reverb is pretty subtle even at 10, is that your experience? I would consider it beyond subtle, at 5 you have to pay attention to be able to tell it's there, but I can see how someone might consider it to be working because it is there.

Thank you so much for the pictures. That confirms that the wiring to my transformer is correct since I can see the red wire there. I'm a little disappointed in that actually.

There are some differences in the one I have. After the master volume but before the reverb I have a 2.2M into a 2.2M as opposed to what looks like a 1M into a 100K (the other pic does look like it has a 2.2M).

Not sure still, may have to give this one back as I'm not really confident about what I should be hearing and it appears to be working.

[JazzGuitarGimp]

Is the customer asking for more reverb? (I could well be incorrectly assuming this is a repair job)

[M Fowler]

You should be able to hear enough good reverb as in the first Dave Baker video in the attachment below scroll down to see it 2nd video.

http://drzamps.com/amp/maz38sr/

Could be the reverb driver if wired same as photos. In a case like this I would call Dr Z and talk it out, he gladly helps.

I know from a video I bought from Dr Z Mike Zaite likes subtle reverb not the swampy reverb a Fender typically has.

Mark

[Stevem]

What does the input and output resistance of the tank read?

[eddie25]

in is 217 ohms and out is 1.2 ohms. It came with a 8AB2A1B, and that's also what I swapped out to test.

[didit]

in is 217 ohms and out is 1.2 ohms. It came with a 8AB2A1B, and that's also what I swapped out to test.

You might recheck input vs output. Quick search says nominally it's Input Impedance 10 Ohms, Output Impedance 2,575 Ohms, which given your measured static resistances are close if plausibly a bit low --- but reversed.

Best .. Ian