trinityamps.com

I'm looking for a schematic or layout for something along the lines of a mini z or the bad cat. I don't really need tone controls. Any suggestions?

THANKS

Go to Hoffman's website, click in the library of information. In the projects and mods section, you'll see the "Schedule 40" project. It is based on the old Gibson GA5, but with an EL84 instead of a 6V6. I would recommend using something with a lot less high voltage than the PT that Hoffman uses if you're going with an EL84. You want high voltage of 240-0-240 @ 40mA with a 2a 5v winding and a 1.5a 6.3v winding. I think the Hammond equivalent is the 270X. I used a small OT, I don't know it's specs, other than it came from one side of a single-ended EL84 stereo hifi amp. Also in the Hoffman pages is the version I built using a 6V6 and the transformers from his site, along with a layout. Oh one last hitng, I left off the NFB on both of my GA5's. They are both awesome. CB

I havent see too many SE EL84 really, but the AX84.com project has a number of variants you might consider. Not sure they sound lke a mini z or bad cat tho.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

The classic SE EL84 amp is the VOX AC4 or Domino Normal. The circuit is the same but the Domino has a more point to point construction(Domino was the budget line from VOX).
Take a look at their schematic:

http://www.voxamps.co.uk/downloads/ac4.jpg

These amps have a nice clean tone but they don't really sceam, without a pedal, like I suspect the amps you mentioned do.
But that can be remedied with a few component changes. These amps were choked back in several ways. The input mix resistors are 100k not 68k (not a big difference). The EF86 screen resistor (R4) is 5.5 meg. Change that to 1M for more gain. Also, this schematic doesn't show pins 3 & 8 connected on the EF86 as they should be. For more low end, you can increase the value of the EF86 decoupling cap (C4) to 16uF.
If you don't need a tone control leave it off or use switchable coupling caps for the tone control . This will give a bit more gain.
You could move the EL84 plate supply to the other side of the "choke" resistor (R15) to increase the voltage on the EL84's plates, as well as the voltages throuhout the amp ( without the EL84 plate current draw through R15 all the voltages past R15 will rise) . Be sure and leave the EL84 screen supply on the same side of R15 as it is on the schematic.
The overall voltage of the amp can be increased by substituting an EZ81 for the EZ80 tube rectifier.
I like 300v on an EL84's plates, at idle, in SE. That could be accomplished just my moving the EL84 plate supply to the other side of R15, but still using the EZ80. Or just leaving the circuit like on the schematic and use an EZ81 tube rectifier.

Or you could use the circuit from a Domino Bass amp. This is like a Fender Champ with a cascaded 12AX7 and an EL84 in SE. There is a schematic for it at Mitch Colby's website here:
http://members.tripod.com/mitchcolby/Sc ... o_bass.htm

mark durham

You know, I have a generic amp in the shop with an EF86, EL84 and 6X4 rectifier and no tone control. Point-to-Point. Handful of parts. Just sitting there. Waiting. I always wanted to play with it. I think maybe I should turn it into one of these - when time permits.
Thanks for the inspiration. Now, the time...

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

If you want a cheap platform to tweak off of, there is the Epiphone Valve Jr., which runs $120 or less retail. BBQ speaker, single 12ax7, EL-84. Solid state rectifier, but the only difference on SE is supposed to be voltage drop or lack of it.

There are pages full of tweaks for the Valve Jr., largely about fixing noise problems from bad PCB design that sometimes gets problematic. Usual solution is to switch to DC filaments, but it's likely better to drop in a fresh turret board and do good wire layout from the start.

Bear

yeah I picked one of those up $99 at GC, I was thinking about putting a new board in it.

It's just hard to beat a Single Ended Class A Amp. I love em.

That Tube Jr is now available as a Head and has DC on both the Preamp and Power Tube. Haven't tried one though.

CR ><>

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WoodCross Amps

http://www.geocities.com/woodcrossamps/

I love Hotrodded Single Ended Class A Amps.

CR ><>

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WoodCross Amps

http://www.geocities.com/woodcrossamps/

SPAM!!

There's this amp in the shop, a real no-name brand with an EF86, EL84 and 6X4 rectifier, but it's missing something: tone control. Its design is simple, point-to-point with only a few essential parts. And yet, it sits there idly, waiting for someone to give it a new life. I've always been intrigued by it, aching to get my hands dirty and tinker with its inner workings. Maybe now is the time to take this forgotten amplifier and shape it into something better. Thanks for inspiring me. Now, all there is to do is find the time... mini crossword