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 Post subject: Single ended el84
PostPosted: Thu Mar 16, 2006 5:08 pm 
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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


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PostPosted: Thu Mar 16, 2006 8:58 pm 
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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


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PostPosted: Thu Mar 16, 2006 10:57 pm 
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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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 Post subject: Re: Single ended el84
PostPosted: Fri Mar 17, 2006 1:27 am 
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teletroy wrote:
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


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


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 Post subject:
PostPosted: Fri Mar 17, 2006 8:03 am 
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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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 Post subject:
PostPosted: Fri Mar 17, 2006 10:45 am 
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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


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 Post subject:
PostPosted: Fri Mar 17, 2006 2:15 pm 
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yeah I picked one of those up $99 at GC, I was thinking about putting a new board in it.


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 Post subject:
PostPosted: Tue Mar 21, 2006 2:36 pm 
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8)

It's just hard to beat a Single Ended Class A Amp. I love em. 8)

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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 Post subject:
PostPosted: Tue Mar 21, 2006 2:37 pm 
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8)

I love Hotrodded Single Ended Class A Amps.

CR ><>

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 Post subject: Re: Single ended el84
PostPosted: Mon Jul 23, 2018 5:31 am 
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SPAM!!


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