Gregory "mark X" Tube Guitar Amp

[Paul Marossy]

Well, I bought this amp last week. Paid a little more for it than I should of, but I was really curious about this amp, electronically. It's a unique sounding amp. I have no idea what circuit it is probably a copy of.

Here's what I have on it so far:

1x12 combo amp, approx. 10-12 watts (AB push-pull?)

(2) EL84/6BQ5 power tubes

(2) 12AX7 preamp tubes

(1) 6CA4 dual anode rectifier tube (noval)

Controls: Volume, Tone, Tremolo Speed and Strength

3 input jacks (!)

Point-to-point wiring using terminal strips

Manufactured in mid 60's?

It appears that the tremolo is the power tube bias type. How do I verify that? Also, anyone know of a source for a schematic? There's one in the bottom of the enclosure, but it has suffered water damage and it mostly illegible.

I'm going to clean this thing up and probably tweak it some. Any information I can round up on it would really help, although, it seems that info on this amp (or any Gregory amp) is mighty scarce...

[lovekraft]

Score!

That sounds like a great setup, very Supro/Valco cool! A lot of those little amps used a 12AX7 phase shift oscillator to vary the power tube bias to produce tremolo. I've never heard of Gregory amps, but it should be easy enough to trace the circuit since it's PTP. Replace the electros (but you already knew that) and any dead tubes, and you've got a Jimmy Page style stack killer! I'll check around and see if I can find any info. Great find!!

[Paul Marossy]

Score!

That sounds like a great setup, very Supro/Valco cool! A lot of those little amps used a 12AX7 phase shift oscillator to vary the power tube bias to produce tremolo. I've never heard of Gregory amps, but it should be easy enough to trace the circuit since it's PTP. Replace the electros (but you already knew that) and any dead tubes, and you've got a Jimmy Page style stack killer! I'll check around and see if I can find any info. Great find!!

←

Yeah, it's kind of a cool amp, for something I never heard of!!

It sounds like the type of tremolo that modulates the power tube bias.

[lovekraft]

Sorry, Paul, couldn't find any info in my reference stuff, and my only internet hits were a few HC reviews. Just a semi-educated guess, but from the tube lineup and the controls, it sounds like a slightly lower powered Gibson GA-8T "clone" with 6BQ5s instead of 6V6s in the power section and a different rectifier setup. Does it have a long tail PI, or is it a single triode Cathodyne? At any rate, it sounds like a great new toy! If you find out any more about it, please let me know! And you know we'd love some sound clips!

<EDIT>Almost forgot - the Vox Peacemaker was also very similar, so you might want to take a look at it also. I think I've got the schematic somewhere around here (I'll just have to dig it out), so if you have any trouble finding it, holler, and I'll zap you a copy.

Edited May 6, 2005 by lovekraft

[Paul Marossy]

Thanks for the tips lovecraft. I looked at both of those schematics, and they look similar to the schematic in the bottom of the Mark X. The Vox Pacemaker is very similar, right down to the three input jacks.

If I understand what a long tail phase inverter is, looking at the schematic in the amp, it does not appear to use one. I think it's the phase shift oscillator type. Check out the schematic for the Pacemaker: http://www.blueguitar.org/new/schem/vox/pacemakr.gif

I took some pictures of the thing. I'll post a link when I get the webpage put together.

[Paul Marossy]

For some reason, I haven't been able to upload anything to my website lately because of some internal error, so I made a temporary page up on this amp. Hopefully the people hosting my website will get this problem fixed soon - this is going on day three now (that I know of).

Anyhow, you can check it out here: http://home.att.net/~psalmist/MarkX.htm

Sometime I'll try and do a schematic for it. The wiring isn't that easy to follow because it's sort of a rat's nest, but I think I'll give it a shot.

[lovekraft]

You're not kidding -it's almost identical (electrically) to the Pacemaker! Very cool amp, and it's in incredibly good shape for its age and quality. Can't wait to see you restore it!

[bluespresence]

Can't wait to see you restore it!

You must have missed this link at the bottom of his page......

Here

Looks great! I like the unique look of the covering.

[lovekraft]

My bad! I read down to the "Coming Soon" tag on the schematic, and didn't look any further - it looks great!

[Paul Marossy]

Thanks guys.

My intuition usually serves me well. It's a really nice sounding amp, and it's kind of cool - like a retro 60s look. I'm making a footswitch for it tonight, and finishing up the rear chassis cover, too. It wouldn't surpise me if it is identical to the Pacemaker, except they substituted the hi/low tone control for a single tone control.

[Paul Marossy]

OK, here's a schematic I have drawn up using the schematic attached to the amp as a guide - which was a PITB. I still have to fill in a few missing values, but I will verify those in the next few days. Let me know if you see anything I might have screwed up!

http://www.diyguitarist.com/PDF_Files/MarkX-Schem.pdf

Very simple little circuit. Sometimes the simple ones sound the best!

EDIT: It's not quite an exact copy of a Vox Pacemaker, but it's very similar.

Edited May 11, 2005 by Paul Marossy

[Paul Marossy]

Ok lovecraft, here it is:

I pulled the chassis out last night and verified everything. I think that I have it all worked out now, except for a couple of things: I forgot to verify the plate resistor on V2A. Would it normally be the same value as the cathode resistor? Also, I forgot to write down where the wire from the wiper of the "strength" pot goes. IIRC, it was to Pin 1 of V2B...

Here's the schematic: http://www.diyguitarist.com/PDF_Files/MarkXSchemR1.pdf

The plate voltages all look a little high to me - 392V on the plates of the EL84s! I think that the 6CA4 is not the original rectifier tube used, though. I believe that it was a 6V4 originally. And, it's kind of strange that inputs 2 & 3 aren't grounded with nothing plugged into them...

Anyhow, I think this might be worth building for anyone interested. It's got a pretty warm, clean tone.

[lovekraft]

Great effort, Paul! That was quick work! It usually takes me days to trace even the simplest stuff. I think you're right about the plate resistor on V2B - it should be the same as the cathode resistor IIRC. V1B sure is an odd duck, isn't it? Anyway, many thanks for this, it'll give me something else to put on the endless build list. How am I gonna convince the little woman I need to build another amp?

[Paul Marossy]

V1B sure is an odd duck, isn't it?

You mean with that 1M plate resistor? Yeah, that is kind of odd.

Great effort, Paul! That was quick work! It usually takes me days to trace even the simplest stuff.

Well, if I didn't have the original "schematic" to go by, it would have taken me quite a while, too.

I have just a few more little things to verify, but I'm basically there. I need to correct myself on my above post - I have 342V on the plates of the EL84s, not 392V. So that is reasonable. Those voltages are very similar to what are inside my Matchless Spitfire clone.

[Paul Marossy]

Just for fun, I did a chassis layout that follows how the amp is wired. I kind of like the way it's laid out, it seems pretty practical. It can be seen here: http://www.diyguitarist.com/PDF_Files/MarkXChassis.pdf

Just FYI.