Dr. Zed Remedy tone loss when guitar is rolled off

[ViperDoc]

I have a Dr. Z(ed) Remedy--a 40/20 Watt quad- 6V6 Plexi style amp that sounds really good in general. When my guitar volume is on 10, all works as you'd expect. When you roll the guitar volume down as I would normally do, even past 8, the high end and general tone is utterly gutted, much more than any other amp I've played. It is not the guitar. I have guitars with treble bleed mods that I've played into this amp and others into other amps that take guitar volume changes like a champ. Not this amp. What would you do to fix something like that?

[pdf64]

Have you checked that the tubes are good, especially tried a known good tube in the input stage socket?
If not a tube then try adding a bright cap to the vol control, or if there already is one, increase its value
In the absence of a public domain schematic, suggestions beyond that aren’t really feasible.

[ViperDoc]

Thanks, I’ll check that out. I can’t find Dr Z schematics either, but perhaps I could open it up and draw it.

[nworbetan]

My super vague semi-educated guess is that there's a larger than typical capacitor from the input of the amp, to either ground or the positive end of the input valve's filter cap. A drawing of the input valve circuit would be very helpful.

Also you mentioned a treble bleed circuit, and you'll need to include the guitar's wiring diagram if you want to see the entire low pass filter that's being created at the amp's input.

[ViperDoc]

Here's are some shots of the inside. There is a passive effects loop added by Dr. Z after the high gain pot (jacks on the back between V1 and V2).





The resistors on the input jack are 1M to ground and 68K on the input.





It might take a while, but I may draw the amp up as best I can.

[Todd Hepler]

The resistors on the input jack are parallel 1M and 58K.

Seems like an odd way to get 54.8K, eh?

Is the 1M supposed to be the 1M to ground on the input? could the jack be mis-wired causing your issue?

-T

[ViperDoc]

I’ll have to look up that particular jack, I’ve never used that type.

[ViperDoc]

The resistors on the input jack are parallel 1M and 58K.

Seems like an odd way to get 54.8K, eh?

Is the 1M supposed to be the 1M to ground on the input? could the jack be mis-wired causing your issue?

-T

I was mistaken. The 68 K is on the input and the 1M is to ground.

[nworbetan]

The jack looks like a Switchcraft 113B, or similar: https://marvac.com/products/switchcraft ... r-enclosed

Can you get a pic a little closer to the jack? Maybe with a flash?

The low hanging fruit I'm looking for are:
1) it's probably just the light playing a trick on my eyes, but the green band on the 1M is hard to see, it's really 1M, right?
2) is there a capacitor hiding under the jack where the core of the shielded cable meets the 68k?

It looks like the 1M is probably connected to ground and shorting to the tip like it's supposed to, but the other end of the grid stopper isn't visible in the pics.

If those check out good, did you try a known good tube in V1 yet?

[M Fowler]

On Z amps you really need to clean them when repairing them. Tighten all jacks and pots. Clean all jacks and pots. Look for broken ground wires on the posts that is common on repairs I have done.

Mark

[ViperDoc]

Thanks, Mark! I actually just threw this back in the head cab without messing with it. Opening it satisfied my curiosity. I’m more interested in the 18 watt I just built which sounds WAY better. I’m also looking at a TW build next.

[Travis_HY]

How long is the cable you are using and what type of cable is it? That might be playing a role in the high frequency roll off.

Additionally the coupling caps on those first two gain stages look kind of odd to me for their respective channels but it's kind of hard to tell from the photos. It appears that the smaller .001uF ( ) Mallory 150 film cap is feeding the "Normal Loudness" pot and the larger valued high voltage .0022uF (?) ceramic cap is feeding the "High Loudness" pot. That seems like it would be the opposite of what is typically in a Marshall (usually the "Normal" channel has a Bass spec with the .022uF/820R+320uF combination, but maybe he was trying to "lean" up that coupling cap a bit, maybe it was an error, I dunno). It also looks like the brown and blue plate wires on that first tube are reversed color code-wise from the output wire colors on the coupling caps which is another possible error. The cathode arrangement is also suspect from other photo I could find. Actually, your amps looks very different from the other photo I could find:



It looks like the amp above has an un-bypassed "Normal Loudness" cathode resistor and a .022uF coupling cap instead of your amp, which appears to have some kind of bypass cap across the 2.2k cathode resistor.

Honestly, because everything looks stock in your amp, I'd send some gut shots to Dr. Z himself and ask what's up with the strange component discrepancies. It could also be exactly how it was designed and they changed the spec, so I'm just throwing these differences I noticed out there. But from my experience, it appears your particular Remedy needs a remedy when comparing it to another!!!

I hope that helps with something!

[ViperDoc]

Thanks for looking at that. When I used this amp for a few gigs, I used a pedal board with separate pre-input, buffered gain stages and time/space effects in the loop. I still noticed massive high-end tone loss when using my guitar volume pot. I've never seen that with any other amplifier. Using the same guitar with other amps never showed that type of issue, so I ruled out a treble bleed mod, especially on a Strat.

I wish the V1 bypass cap labels were more visible in my photograph. I have no idea what those are.

[Travis_HY]

Ah, I see. Given that info, probably the biggest issue I can see is the loop. If it is simply a passive loop coming off the 470/470-470pf mixer resistor network, it's going to be high impedance and low current, so the longer the cable from the "Send" of the loop, the more high end loss will occur. Ideally, you would have a low impedance, high current "Send" like the output of the Treble control driven by the cathode follower. But first let's try to narrow down what the issue is so we can isolate the problem and then try to solve it.

First, the amp doesn't look "correct" in the front end to me-I would still send some gut shots to Dr. Z just to verify that the amplifier has no wiring errors to remove that variable if it is one. Second, eliminate all the external electronics from your signal chain and just plug a 10 foot cable into the front end of the amp to check that the guitar volume pot is not incurring high end loss. If that is all good, then the third step would be to integrate the effects you were using in the front end (ie not the the time based ones in the loop). Check the guitar volume pot's high end loss that way with nothing plugged into the loop. And if that is good then finally move on to the effects loop to see if the guitar's volume pot's high frequencies are lossy with that arrangement. Add one device at a time to the loop if you need to, but I have a feeling that even if you just plugged a 5 foot cable from the Send to the Return, you will have signifcant high end loss. 10 feet will be drastic and 20 feet will be nearly un-usable.

I have a hunch that the loop is the biggest issue. My other hunch is the wiring errors (again, I'm not sure what the deal is with this spec and verifying with Dr. Z would clear up that up) would only make the amp sound less than ideal, but they shouldn't be causing such drastic issues with the guitar volume pot unless you were maybe using a really long (>25 foot) cable. Usually a Marshall preamp with the correct spec is pretty spicy in the top end when you roll down the volume pot with a 10 foot cable so I would start there. Oh, and NOT a 10 foot coily cable; just a regular old boring 10 foot cable would be fine.

Hope that helps!

[ViperDoc]

Much appreciated. I did, in fact, test all that out and the high end drop with the guitar volume was still an issue. The loop is not a standard inclusion on this particular amp, I had them build this amp with it added. I would think a Metro loop might be better sounding and off the treble pot like you mentioned. I'll see what I can find out about the wiring.