Wednesday, April 29, 2009

Gregory Mark V / Gemini 700 practice amp

Once in a while a repair comes in that's a real surprise. This Gregory tube practice amp is one of them. When it was fixed up it really sounded fantastic. It's not as aggressive as most small amps. The treble is tamed and the distortion is fairly smooth and laid back and the clean sound is very sweet even at very low volumes.

I was really sorry when I had to give this one back!

Since the result is so nice, I thought I'd spend some time going though some of the oddities of the circuit.

Here's the schematic:

Gregory Mark V / Gemini 700 Schematic

There's a number of unusual things here.
The first is the input stage.

A run of the mill 12AX7 triode input would look
something like this:

triode input

But the 12AU7 in this Gregory has two more elements in the preamp tube.

Here's what it looks like in the schematic:

12AU6 tube - pentode input

The plate, cathode and control grid are elements found in a triode tube.

I've labeled in red the additional elements the additional elements found in a pentode tube:

pentode tube elements

Since it has five elements instead of three, we call this tube a pentode. Pentodes are common as output power tubes. The EL34 is one. But pentode inputs on guitar amps are pretty rare birds (one example is the 6SJ7 in the early Fender Champs with the 5C1 circuit). The pentode input is certainly not sole contributor to the sound of the amp, but it makes me curious to try a 6SJ7 in the Champion 600.

I'll go through some more of the circuit in future posts (and hopefully get some pictures from the owner - I forgot to take some myself before I sent it back).


Dan said...

I am interested in building this circuit. I have the tubes, and I have a chassis (from an Ampro Stylist projector). Did you build a copy of one of these? Other than adding an isolation transformer is there anything you would suggest before I get started?

Have a good one,

akavalve said...

If you're fairly familiar with building you shouldn't have too much trouble and it will sound very good as it.

The circuit is VERY sensitive to grounding issues, so test as you go and make adjustments as needed.

The original value has it's charm, but you'll get a fuller sound by changing the 100K resistor a the input to 1meg.

Does you chassis have an OPT for the 50C5?

Dan said...

Thanks for the info. I do have an output transformer, but it is push pull. For some reason I thought the Gregory was also a push pull. Mojotone has surplus single ended transformers for about $12, so if I go ahead with this I will order one of them unless you know how to use a push pull transformer in a single ended circuit.

I did a quick search and see that Mesa Boogie has a patent describing switching between push pull and single ended, I guess to be able to drop the output:

One other thing I am considering is lifting the vibrato from this circuit and adding it to a push pull circuit using two 50C5s:

This is the schematic for the Ampro Stylist projector, which is my "donor" for this project. I can drop one of the three 50C5s as it is there to drive the exciter lamp. If I can also lose the 6BH6 then when I add the vibrato 12AX7 I end up with a string that wants 124Volts, which I think will work out nicely with modern wall voltage (through my isolation transformer).

Thanks again, I keep an eye on your blog and have learned quite a bit from the past articles.

akavalve said...

I can't remember off hand what I measures the OPT's impedance ratio at, but going from the 50C5 datasheet that Mojotone surplus OPT should work with a 4 ohm speaker load.

I've read about that Mesa Boogie arrangement in a couple other places - sorta surprised that they've got a patent on it. In any case, it's pretty simple and should work fine if you want to go that route.

I'm used to seeing less than 120 VAC service. Even if you eliminate the standard dropping resistor I think you'll end up starving the heaters a bit with 2 12AX7's and 2 50C5's. You're right in you're observation regarding the original heater string - the dropping resistor should be adjusted to suit modern line voltages.

Best, Andy

My Sisters Diary said...

I have had a Gregory Mark V I bought used for about 30 years. About 10 years ago I replaced the big capacitor can when it stopped working and I replaced it with separate capacitors of the same voltage and value. I stopped using it because it has never had an isolation transformer. I read a page where a guy installed one that was 50VA and it got too hot. I know how to install an isolation transf. and fuse but need some guidance on which isolation transformer and fuse to use?