Does this obscure amp have any mod potential?

[joe lp]

I have this cheap old gregory amp with the right tube compliment and speaker size to be a nice amp for small clubs. Unfortunately it is a little short on treble, volume and gain while being a little too heavy on the bass. In fact, to this point I've only used it for recording bass parts and unconventional guitar sounds. I found a schematic online and was hoping that someone could tell me if they see any potential in this circuit. It has 2x EL84, 2x 12ax7, 1x EZ81. Apparently the tremolo runs by using "a split load phase inverter and a phase shift oscillator which modulates the power tube bias." Basically, the tremolo isn't so hot and if I could improve the sound of the amp by bypassing it I would have no problem with that. Anyway, if anyone has some ideas for this little amp I would be very happy to hear them. Thanks in advance.

If you scroll to the bottom this page there is a link to the schematic in .pdf form.

The Gregory Mark X Tube Guitar Amp

[Chuck H]

As the article point out, these were not top of the line. My guess is that the lowish volume is due to a slightly cheesy output transformer and an inefficient speaker. The speaker could also be part of the poor high end performance. At 390+ plate volts you should have no trouble getting 17+ watts out of a pair of el84's. Have you tried plugging the amp into a different cabinet?

The whole circuit is a little goofy for a guitar amp. The output transformer should be moved to the outside of the chassis and overall layout could be improved. I would gut the thing and rebuild it to a different circuit. I would keep the output transformer too. Some cheap little transformers can sound great, even if they aren't that loud.

Chuck

[Mark Black]

Another thing the article mentions is that Paul did like the sound of the amp with that different speaker.

So based on that I might start out with a different speaker and making sure the amp is functioning correctly as per its design. A fresh set of tubes, possibly replace electrolytic caps, check resistor values, check voltages, and so forth.

If it still sounds bad to you when working correctly and with a decent speaker then as Chuck says you could always gut it and rebuild.

Just my take...

[joe lp]

Sorry, I forgot to mention that I had to replace the speaker when I picked it up because the cone was rubbing pretty badly. So it has a jensen c12q in it right now (not my favorite speaker but it was just sitting around unused). I have also recapped and retubed it. Maybe I should clarify: the amp actually doesn't sound all that bad as it is, it just seems a little dull compared to my ac30 (apples and oranges of course) and I'm trying to tweak it into something that I can feel good about using in smaller clubs. I appreciate your help guys.

[olddawg]

Sorry, I forgot to mention that I had to replace the speaker when I picked it up because the cone was rubbing pretty badly. So it has a jensen c12q in it right now (not my favorite speaker but it was just sitting around unused). I have also recapped and retubed it. Maybe I should clarify: the amp actually doesn't sound all that bad as it is, it just seems a little dull compared to my ac30 (apples and oranges of course) and I'm trying to tweak it into something that I can feel good about using in smaller clubs. I appreciate your help guys.

I would gut it and convert it to an 18 watt Marshall variant. Everything is there. It's just a matter of making a new turent board and wiring it up. As stated before you might want to get a new OT. Check out a Heyboer for an 18 watt.

[pdf64]

It needs a proper tone control to add some sparkle.
Try lifting the one from the Harvard
http://www.ampwares.com/schematics/harvard_5f10.pdf
In fact, the pre-amp biassing is all dodgy grid leakage type, try building all the signal circuits as the harvard.
Weird how they spent money on dual pots which weren't needed (single track would have done the job). Peter.

[joe lp]

Ok, so I opened the amp up for the first time in about a year and the first thing that jumps out at me is that the pots values are all different from what is specified in the schematic. It looks like someone changed them all to 300k at some point so I think my first move will be trying 1m in all positions. After this I think I'll try the harvard mods recommended above. Does anyone have any thoughts on removing the tremolo circuit? Any quick and reversible ways of doing this? Thanks guys, lots of help so far.

[Regis]

Hell, looks like AC15 time.

[tedmich]

I would gut it and convert it to an 18 watt Marshall variant. Everything is there. It's just a matter of making a new turent board and wiring it up. As stated before you might want to get a new OT. Check out a Heyboer for andn 18 watt.

+1, maybe the minimalist 18w, and a decent Edcor XPP series OT only costs $15-30, for a true lowball, while a Cadillac CXPP25 would run $57.