A look inside a Dr. Z Maz Jr.

[DuaneOh]

This week I was working on a Dr. Z Maz Jr. 18 reverb. This amp has 2 Clestion Vintage 10’s and great clean sound, especially with single coils. The amps design is a bit unusual and I was wondering what you guys thought about some of the design choices. I found a hand drawn schematic online but this amp was slightly different so I drew my own schematic.

It has an unbypassed cathode resistor for the EL84’s. I’m wondering if this was a mistake? I hooked up my signal generator and with a resistive load I could only get about 8.5 clean watts out. (master maxed, treble – mid – bass centered, reverb and top cut off) From what I understand with an unbypassed cathode, as the amp goes into class AB the gain will be reduced due to negative feedback and the output will start to distort by rounding over the waveform, which is what I saw on the scope. So, does this mean that it is really class A?

I tried jumping in various caps around the cathode resistor and at different frequencies, it seems that with a 100uf it was fully bypassed, and it went to 17.5 clean watts.

With the resistor unbypassed, like I said it sounded really good with my Strat, very jangly. The transition from clean to distorted was very subtle. When I bypassed the resistor, the bass was more pronounced and it lost a bit of the jangle. I tried playing with the tone controls but it just wasn’t the same. But for deeper distortion, especially with a humbucker guitar the bypass capacitor got a smoother sound. I think that it sounded best with a 33uf cap.

It also has a shared 2.2k G2 resistor. Would that make for a more constant G2 voltage? Would the large G2 resistor would increase compression? A bit of negative feedback? What effect would it have on the sound?

The master volume doesn’t get a great overdriven sound when the master is turned down and the volume dimed, it gets a bit of crunch and not much more. I’m wondering if a PPIMV might be better suited to this amp?

There are other interesting things like paralleled 12ax7 input, the reverb drive, recovery and mixer. Looking forward to hearing your input. Duane

[mresistor]

Must be nice to have capacitors named after your company.

[brewdude]

I built a cathode biased amp that sounded best without a bypass cap—It is my current favorite.

[Willabe]

Must be nice to have capacitors named after your company.

 

[jordan86]

Must be nice to have capacitors named after your company.

Haha! My Z from 2008 has the same caps. Dr Z has said that he used to special order certain spec caps through CDE/Sprague (whoever it was back then). I think he had to order them by the thousands. I guess they put your name on them, when you buy enough 

[DuaneOh]

Must be nice to have capacitors named after your company.

Haha! My Z from 2008 has the same caps. Dr Z has said that he used to special order certain spec caps through CDE/Sprague (whoever it was back then). I think he had to order them by the thousands. I guess they put your name on them, when you buy enough 

He doesn't make his own caps?    From the size and shape I'd say they are 6PS orange drops. Smaller ones are 715's and 225's.

I built a cathode biased amp that sounded best without a bypass cap—It is my current favorite.

Yeah, the unbypased cathode has a very nice sound. I can now see how a cathode cap switch could be a useful addition. If you are in the region where your getting some distortion without the cap and you switch it in, it would clean up and increase the volume. Kind of like a clean boost?

[mresistor]

He doesn't make his own caps?    From the size and shape I'd say they are 6PS orange drops. Smaller ones are 715's and 225's.

Ah I am referencing the .1uf cap on the right, the others could very well be Sprague. SBE was a manufacturer in Vermont. 
https://vermontbiz.com/news/2019/december/06/sbe-inc-barre-shuts-down-new-company-emerges
Was SBE a division of Sprague?  I don't know.  I think they were under an Indian company.  Can't find that link now.


Ok looks like the SBE plant was origially a Sprauge plant.  And now Sprague is part of Cornell Dublier.  All in the family :-)
https://www.linkedin.com/company/sbe-inc.

[DuaneOh]

I was looking for some more info on the Maz and found an interesting post on The Gear Page from the Doctor:
https://www.thegearpage.net/board/index.php?threads/dr-z-maz-18-jr-mods.1906964/

"But the big enchilada is the change to the Reverb and Master Volume circuit.
Prior the reverb was driven by the split Master Volume signal, so depending on were the Master was set it would effect the input signal to the reverb circuit.
This caused a less then dynamic reverb response and some nasty critics from Reverb lovers.
Now the Master is my tweaked PPIMV , which gives excellent linear volume and drive control.
And the reverb gets a strong preamp signal which in turn gives a lush reverb with very low noise floor.
That alone is worth the upgrade to many MAZ owners."

So it sounds like a PPIMV would be an improvement. And the Reverb is very subtle, it needs to be turned up about half way to get a real noticeable reverb sound.

[pullshocks]

Must be nice to have capacitors named after your company.

Haha! My Z from 2008 has the same caps. Dr Z has said that he used to special order certain spec caps through CDE/Sprague (whoever it was back then). I think he had to order them by the thousands. I guess they put your name on them, when you buy enough 

I have some Orange Drops labeled Torres......

[pdf64]

A shared screen grid resistor tends to be the arrangement that Mullard suggest https://tubedata.altanatubes.com.br/sheets/129/e/EL34.pdf
I’ve seen it used with Traynor, Hiwatt and the JTM45.
In the latter, it seems to help linearity at high power outputs

I like the inverting feedback mixer amp.

I don’t like the hot switching standby or the fuse or switch in the neutral.

[PRR]

Is that voltage 12.7V silent or LOUD? Or does it vary?

[DuaneOh]

Is that voltage 12.7V silent or LOUD? Or does it vary?

12.7v was silent. Without the bypass cap the voltage would get up to 13.1v at max clean signal, with a 100uf cap it got up to 15.4v. The voltage would get larger as I used larger caps but maxed out at 100uf with a 150hz signal.

[mresistor]

I wonder why Dr Z chose to use that big carbon composition resistor for the screen V.

[pdf64]

I wonder why Dr Z chose to use that big carbon composition resistor for the screen V.

To make toast when a valve shorts?
I think a HT fuse would be a good idea.
CC should be noninductive, maybe that was a consideration?

[jordan86]

I wonder why Dr Z chose to use that big carbon composition resistor for the screen V.

Are you sure that’s the stock resistor?

[DuaneOh]

I wonder why Dr Z chose to use that big carbon composition resistor for the screen V.

Are you sure that’s the stock resistor?

The owner said that it had never been worked on before and all the solder joints looked original, he bought in the early 2000's. There was a mix of carbon comp and metal oxide / film throughout and a couple of wire wound.

This amp had some custom appointments, the control panel was surrounded by figured wood, looks like curly walnut, and had vox diamond grill cloth. I should have taken a photo....

[mresistor]

To make toast when a valve shorts?

Makes sense.. I've seen quite a few cc screen resistors in vintage fenders that blew right in the middle quite neatly.  They looked like they were sawed in half of course there were the burn marks telling the tale.

[DougGuy]

I have the same MAZ Jr Reverb amp, not the MKII and it's one of my all time favorite amps.  I'm using two JBL K110s and the amp REALLY sings with these, and it lost that nasty peakiness that the Dr Z speakers had.  These are not clean amps, not really, but their drive is very smooth and the whole amp is musical and user friendly no matter what guitar you use, I find it responds better to hot pickups, like a tele with 10k bridge and 8.3k neck sounds awesome, you run your guitars with volumes rolled back from full out, and it becomes very expressive to the touch on the neck.  No other amp I ever owned did this nearly as well as this Maz Jr.   The reverb in this amp is also slick as snot on a glass doorknob, it is elusive, it's there but smooth and laid back, not in your face like a blackface Fender, I don't know how he does it but it blends into the background, very subtle, VERY cool.

You guys know this stuff inside and out, values, caps, all the math, I just know the tone and dynamics when it hits my ears.  I found that running a Fulltone Deja Vibe on a really slow setting gives the amp a super cool 3D effect to the tone, it gives it a texture and depth and you don't really hear the vibe at all, just the very dimensional sound that this amp puts out.

If only Sluckey could come up with a tremolo to retrofit, it would then be THE perfect amp.