Matamp/Electric amp schematics

[Nutjob]

If the PT is anything like the OR120 PT's, the secondaries should be something between 180-0-180 to 200-0-200 and with the voltage doubler giving something between 500-550 VDC unloaded. Voltage point A in an OR is usually around 450 to 475 VDC in the ones I've seen. I would use a PT with at least 350mA on tap to be safe, pushing 120W. I used a junkpile PT I had with 400-0-400 @ 500mA on the secondaries, and didnt use a voltage doubler. Bias circuit will depend on the available voltage at the bias taps of whatever PT you use, and yes there are 4 individual bias pots. Hadn't thought of using a effects loop, so i cant comment on that.

[Zozobra]

Hi, first post.

This is an interesting thread, thanks for taking the time to draw up a schematic! I have a salvaged MM basmman 135 PT with a 178-0-178 secondary which looks like it could be a good candidate and it looks chunky enough to handle the additional heater draw of EL34s. Of course it would be nice to get the specs but MM aren't forthcoming with those. Any ideas on the OT primary? I'm guessing something in the region of 2K(ish).

[Zozobra]

Actually that produces a pretty scary looking load line!

[sburck]

Were you able to record any clips?

[Nutjob]

Gonna try n do some quick video clips tonite. Audio will probably suck, but the band is going to be doing some recording soon, so I should get some decent clips up soon

[Nutjob]

EAMV120TEST.AVI - YouTube

Quick video clip with a small digital camera. Not the best quality, but if you're familiar with the vintage Orange or Matamp sound in a Doom/Stoner Rock context, you'll get an idea.

[Nutjob]

Most Recent Update 10/28

[Zozobra]

One thing I was thinking is that it might be worth adding V1B back into the circuit in parallel with V1A and then perhaps dropping the plate load resistor a fair bit. Might make it a little more stable perhaps. I do find tying the shield to the anode to be a very weird thing to do!

[Nutjob]

Yeah. I thought of paralleling the unused triode too, but i wanted to stay as close to the actual amp as possible. I mean, if it can be done that way, then i want to do it that way. Besides, parallel triodes would lower gain considerably. I think most would agree that a hot shield at the input is dangerous, I chose to keep it that way, but I made sure to use shielded Teflon wire with both ends heatshrink wrapped. but as with anything of this nature, and amp making in general, "DO AT YOUR OWN RISK".

[Zozobra]

I'm not sure why adding a tube in parallel would drop the gain? I thought that the general rule was that for the same plate load you got about 30% more swing and an improved snr. Thus my thinking of adding it parallel and dropping the plate load a bit to try and increase the stability. I may well be off the mark here of course!

I'm not entirely sure why adding the shield to the anode cures the oscillation here. Is it just the added capacitance? Seems a bit of a ghetto solution!
The clips you posted sound pretty sweet too. Forgot to mention that earlier.

[Nutjob]

Hmm, I dunno, perhaps it's dependent on the rest of the circuit. Ive done the single/parallel input on a supro thunderbolt clone before, with one input with the parallel, and the other with the single, and the parallel had a little less gain, but was definitely quieter, and the single triode input was noticeably a bit louder with more grind. Maybe it's worth a try, although I don't see that in any of the gut shots, so there must be a reason.

[Zozobra]

I suspect the bias needs changing a little to get the benefit. I'll have a flick through my preamp book later to see whats said there.

[Nutjob]

I just took a look at Merlin's book myself, and I have no idea why I said that. Gain should remain the same if the anode resistor is halved, right? Probably mixed up with something else. I'd crack the amp open right now and give it a try if it were not for the fact I have a gig this Friday, and I dont want to botch anything poking around in there, since it performed flawlessly through this weeks rehearsals, at blistering volume. I am beginning to suspect the problem I had with oscillation had more to do with the amount of space I had in the chassis used (17"x7"x3"), I had everything kinda cramped together compared to the actual EA amp (19"x10"x3"). I am currently building a 50-watt version on an old Univox I pulled out of my junkpile, with the components and turret strips spaced out more, so I may give the parallel triodes a try on that, once I get it fired up.

[Zozobra]

I'd leave it alone too give the circumstance. Kudos for getting it all in a chassis not much bigger than a JTM45 chassis!
Hopefully a larger chassis and paralleled input with a smaller plate load will cure the oscillation as I really dont like the idea of having the anode connected to the shield in case the insulation gives out. I'm at a loss to think of a suitable failure protection mechanism too given the severity of the failure! I'd prefer to avoid putting high voltage onto the tip of the cable! A zener from the input to ground perhaps but I'd need to think about that more.

[sunburnedgoat]

Hi there. I was referred to this place from a friend, and very glad to find it. I own an Electric MV120. I'd be happy to take as many details pictures as needed if you need to verify any more values on your schematic. Your videos sound pretty damn spot on though. Killer work.