Wherein John Frum Goes Single Ended

J. Frum

J. Frum

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Since acquiring the 604-8G, I've been exploring single-ended tube amp options. Build a JE Labs Simple 2A3? Take a chance on the JBH Tube EL34? I almost made a deal on a Decware Zen clone, but that fell through...

Last night on Facebook Marketplace (by far my best honeyhole over the last two years), I saw an ad for a derelict Akai Terecorder reel-to-reel.

One hour and $50 later, I was at home extracting the two SEP EL84 monoblock amplifiers!

Looks like the folks using these amps for hi-fi are mostly utilizing the mod recipe laid out by AKer tracker-x. I think this job is within my wheelhouse as a neophyte tube DIYer, and I'm excited to get started.

Look for updates here as they develop.

20180912_130239.jpg20180912_125601.jpg20180912_130120.jpg
 
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I've got a set of these as well, John. They've been sitting on my 'To Do' shelf for a couple of years now...
 
Used the Eico AF4 SEP EL84 (and a very similar Monarch SEP EL84 before the Eico) to drive the Stax SRX III Electrostatic Headphones.

The SEP EL84 amp that really shines is the wonderful 4 watts/channel Almarro A205a featured in many previous posts.

@Ernie has one and after hearing @Prime Minister @billfort and him talk about it in very glowing terms, I grabbed one when it became available on eBay.

Easily the best sounding EL84 SEP amp I've heard. Runs with the Big Dog SET pack. Just a great sounding little amp.

Drives the Belles, La Scalas, Lowthers and other HE speakers with ease.

And you ought to hear the Almarro driving the Stax SRX III. ;)

My opinion and YMMV. :)
 
The more I look at these amps, the less convinced I am that leaving the original design intact is a good idea, with its messes of six-decade old switches, pots, and extraneous circuitry.

Instead, I think the correct course of action is to, behind the faceplate, strip the chassis to iron and tube sockets, rebuilding from the ground up.

I'm especially drawn to Alex Kitic's RH84 Rev2, and it looks like it could be easily implemented in the Akai amps.

RH84 - rev2.png

The good news is that I'm pumped about this project, and I feel like I'm ready to do the construction.

My concerns are as follows:

1) Adapting Kitic's Classic CLC Power Supply design (300-0-300 secondary, 5R4, 10H/100mA choke) to work with what I have (250-0-250 secondary, 6X4, 6H/60mA choke).

RH84 - rev2 CLC.png

2) Adapting amplifier component values as necessary to accomodate the custom power supply.

3) Figuring out how to wire in the lighted VU meters.

4) Adding a pilot lamp to each chassis.

5) Figuring out mounting solutions for RCA inputs, speaker terminals, and power cord.

Specifically, I'm reaching out for help on 1-3!
 
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Been getting help from a similar thread Somewhere Else. Planning on making a parts order soon.

Update:
  • Got somewhat up to speed on PSU Designer II. Using thefragger's iron measurements, I'm leaning towards a CLC using a 30 uF C1 and 100 uF C2. This should get me 267 V B+ with 21 mV ripple.
  • Sticking with RH84 Ver2 schematic, except in accounting for the meters, and only supplying 85% of the called-for 315 V B+.
  • Looking at using Soviet surplus K40Y for the coupling caps, F&T and Nichicon electrolytics for the balance of the capacitors, and Vishay BC 1/2w and 2w metal film resistors.
  • Replacing the long tag boards with turret boards.
  • Utilizing the 1/4" phono jacks on the front panels for signal and speaker connections.
Am I missing anything obvious before I go crazy ordering parts?
 
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I like where this thread is going and I wholeheartedly endorse the plan!

The only thing that gives me pause is that B+ -- 267V seems a wee bit lean for SE EL84, though I suspect your actual voltage might end up a bit higher, since the power transformers are wound for 117v, and your wall power might be a few volts higher than that.

Here's where I start spinning a bit, though: if you're going to strip that much out of the original chassis, and replace that many parts, at which point are you better off just robbing the good bits (in this case, the iron!) and transferring everything to a new chassis (or chasses), with new parts elsewhere? This is how I ended up scratch building most of my stuff...
 
mhardy6647 said:
cannot speak for the OP, but I will note that some of us aren't so amenable with the use of the appropriate tools (hand or power) to fashion... shall we say... aesthetically pleasing chassis and/or cabinetry.
Click to expand...

Well, there's that...

But, really, I'm not opposed to fabbing a top plate and scratch-building. It's just that I think the Akai amps are cool, and I'm not sure I want to take them too far from what they are. Once the appropriate clutter is cleared away, they don't even look too bad to work on.

I like the idea of keeping the 6X4, but I'm getting a lot of advice to just ditch it for a pair of solid state diodes.

I presume all the iron was spec'd together, and the designers never intended for these amps to be anything other than 6X4-rectified single-ended EL84 that don't run their power tubes very hard. I'm wondering if by juicing the B+ with solid state rectification, all I'll accomplish is getting the OPTs to saturate sooner. Is this concern potentially valid, or am I being too antiquarian in holding onto the 6X4?
 
I like those Akais too, and I'd love to see them "reimagined" in their original chasses.

I would keep the 6X4 and try to make it work, but I'm a sucker for anodes and glow ( <--- don't bother clicking, it's a too-obscure Ween reference. Great song, though!)
 
mhardy6647 said:
I think there's still lots to be said for vacuum tube HV rectification (FWIW).
Click to expand...
Yep. I can't quantify it or prove it, but I can prefer it.

If you ultimately can't get the B+ up to where you want it to be, you could tear the whole thing apart and reconfigure it with a 46 output tube; 285v is the sweet spot for that one, and it wouldn't require many additional parts!
 
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