Dr Z Therapy.

[wozt]

I was hearing great things about the DrZ Therapy, so I ambled across to Dr Z's. I am looking for a front end that's pedal friendly for a 6V6 power section and thought I might get some leads for the blurb. In the blurb it states "because the Therapy DOESN'T use a cascaded front end...etc"
There is a picture of the amp layout. Tracing this shows the signal goes to v1a - to volume pot (with a 50% voltage divider following it) - into v1b - to TMB tonestack - into v2 PI. So why is this NOT a cascaded pre amp?

Thanks for alleviating my confusion.

[J M Fahey]

It's even more confusing for us because we are not seeing the layout.

[tedmich]

I'm guessing its a sales gimmick based on the fact that it only has one 12AX7 serving as the preamp, while 12AX7 #2 is the PI. Course its 2 gain stages for the one pre 12AX7, but maybe they're parallel?

Check how many extra holes the chassis has (?!)



maybe he got a universal chassis made, but people do not pay per tube for a Dr Z, they're super simple!

[Jazz P Bass]

Hmmph!
"Special attention was used to select the coupling caps. We chose Jupiter hi end hand rolled coupling caps for the banana yellow Astron tone found in early Fender tweed amps"

[Justin Thomas]

I'm perfectly okay with an explanation of "we picked these because they sound really really good." And, I know it's a small thing, but open socket holes are kind of a peeve of mine, ESPECIALLY for a commercial amp! I wouldn't want to have to pay $30 extra for socket covers, or make my own...if you can punch your own chassis or have someone do it for you, you can afford the extra touch of class.

Like I said, small potatoes, and I still like Dr. Z amps. But PLEASE don't succumb to the same hype and compromises that have infected so many! It's a beautiful piece of work; don't let these niggles mar it.

Justin

[J M Fahey]

Hmmph!
"Special attention was used to select the coupling caps. We chose Jupiter hi end hand rolled coupling caps for the banana yellow Astron tone found in early Fender tweed amps"

I bet the Marketing guy hand rolled one of these before writing that piece of sh............................eet of Advertising literature:

[The Dude]

......I wouldn't want to have to pay $30 extra for socket covers, or make my own...if you can punch your own chassis or have someone do it for you, you can afford the extra touch of class......

Agreed. It would cost them a couple of bucks an amp to cover the holes and this is supposedly "high end". After all, they use "hand rolled" caps.

[The Dude]

I bet the Marketing guy hand rolled one of these before writing that piece of sh............................eet of Advertising literature:

Soaked first in snake oil, of course.

[olddawg]

I wouldn't get bent over the socket holes. It might even allow a little better air circulation.

[The Dude]

Also easier for booze, dust, smoke and everything else to get in.

[J M Fahey]

Really don't know how many amps a Year does Dr Z make.

Personally I love the simplistic and elegant approach, but if they offer, say, 10 models and minimum reasonable order is 100 units of anything, simplifying that supply nightmare (for a very small Company) into maybe 3 or 4 chassis models which cover the full catalog becomes kind of unavoidable.

That said, they might order a couple thousand socket covers in 2 basic sizes, noval and octal, for peanuts.

I mean peanuts, 1 or 2 bucks each at most.

Chassis on the other side can be very expensive.

Cutting/punching/bending/spot soldering/riveting sheet metal to produce stuff, from beer bottle caps to complex car frame parts is very very economical and efficient ... when you run into the tens of thousands.

In small amounts it can be prohibitive because of the fixed setup fees.

I paid for a custom cut and punch die set to make metal corner protectors, similar to what Acoustic and Sunn used, long ago.

No big problem with cutting, if the male punch overshoots the female die by 1 or 2 mm, it's the same, but the one which punches the flat 3 arm "star" into a finished corner protector has a very critical sweet spot measured in tenths of a mm.

You set it too short, the 3 legs are not exactly perpendicular to each other, it does not perfectly wrap the cabinet corner, but worst is that metal shows some "ripples" caused by deep deformation.

Too long and you can destroy die, machine, or both.

Sweet spot: the part gets "ironed" and comes out perfect.

Now, to reach that point the machine has already spit around 800 stamped parts , quite usable but slightly substandard.

Minimum production to justify that run: 4500 to 6000 parts (which are cut and punched in a couple hours).

End of the story?

I haven't made metal corner protectors for some 8 or 10 years; even making them myself I can't beat the low cost of Chinese plastic protectors (similar to what Hartke uses in cabinets) so I use these instead.

A couple friends are making beautifully finished tube guitar amps here in Argentina, and have the chassis made one by one in water jet CNC machines , no dies involved.

What's the problem?

They pay for a single chassis about half the price for which I can sell a finished 100W head (SS of course) .

[Chuck H]

As Juan has said, amalgamating designs for economy is almost unavoidable for low production of each model. And re: hole plugs, it's just another potential chassis rattle to chase down as far as I'm concerned. The Dr. Z amps are more about function following form. That ideology seems to be making it's way into production as well as just the circuits. It makes enough sense to me and I'm just not bothered by it if I don't have to see it evident on the visible panels. JM2C on that.

As to "cascade", I think the pitch is directed at players, not techs. Looking at the layout I'm pretty sure it's just as wozt noted. That is, one triode amplifying the guitar into another triode feeding the TS and then to the PI. I suppose this could be looked at a couple of different ways. Is the designer disregarding the triode feeding the TS as a "function" circuit and NOT part of the preamp? Nah. That's crazy making because the TS is indeed part of the tone shaping that IS a preamp. Maybe because no additional signal gain is generated they felt it didn't need to be counted? Nah. There are plenty of cascade preamps that do this sort of thing. Maybe there is some semantic difference between "cascade" and "series" in the designers mind? Nah. We all know that's apples to apples. So, all I can come up with is that the designers chose to create their own definition. *HENCE FORTH, ALL SERIES CIRCUITS INTENDED TO GENERATE ADDITIONAL CLIPPING SHALL BE KNOWN AS CASCADE AND SERIES CIRCUITS SHALL NOT QUALIFY AS CASCADE UNLESS SUCH ADDITIONAL CLIPPING IS PRESENT!* So sayeth Dr. Z

[Daver]

Enlarge those pictures. The inside chassis shot looks like all the holes have black plastic plugs in them. That's typical of Dr. Z amps.

[wozt]

There is a mica cap across V1 plate resistors, any idea what these are for?
The capacitor leading into the phase inverter is mica, has this been done before. I presume it is only 1n.

[tedmich]

though these amps are quite simple, there is little point in discussing the circuit without a schematic, IMHO.


Mike Zaite should make a multi gain stage version and name it THERAPIST, with a nod to Sean Connery on Jeopardy (SNL)...