The SE EL84 "All Out" Build Challenge thread

[JRTA]

From the OP, page 1, his first post. If he stated something else later, my apology.​

"The challenge:​

Build an "all out" single-ended EL84 amp"

"What if you pulled out all the stops and built an SE EL84 amp from scratch without any constraints?"

cheers

pos

Then kindly contribute with a design or a schematic . . . we're waiting with abated breath.

 

[JRTA]

The Lundahl 2766 seems worth exploring as an output transformer, but I wouldn't commit to constructing a chassis without doing some measurements of a breadboarded circuit. In general, it would be a good idea to see a 5K OT with at least 40H of primary inductance and less than 250 ohms of primary DCR (this adds in series to the plate impedance of the EL-84, so you don't want it to be substantial).

With doing some more research and testing, it seems indeed that a transformer with greater than 36H primary is going to be needed. Even than its going to be hard to get some half decent performance below 40 Hz.

 

[Positron]

Then kindly contribute with a design or a schematic . . . we're waiting with abated breath.

I already posted, more than once, a larger coupling capacitor as the first stage with CC losses
some 30% of bass at 20hz. The CC/grid resistor loss forms 6db/octave filter. You should be able to
calculate and prevent any possible low frequency oscillation.

cheers and all the best.

pos

 

[Nerdorama]

Using the MPS 5k SE outputs I measured -3dB in the low 30's using 6V6's at approx 4W, which is max output before clipping on my amp. The parafeed version was -3dB at around 15Hz at 4W.

 

[paul_b]

Here are some results from the schematic I published:
More Amp Construction

 

[tubegopher]

The linearity of the tube itself and the iron quality are rather important.

Hey Redboy, I have to agree with you that an old EL84 amp from a console can sound better than it should with "teensy" output transformers. Here is an example of a Motorola that I have and now use on my lower deck to drive a pair of Bang and Olufsen RL60.2 speakers. My wife and I were literally "blown away" with the sound quality from that tiny amp. I use a rebuilt Dynaco PAS for the preamp. I have attached a couple of pictures. One of the amp and one of the speakers.
My main system is currently Rega NAOS speakers driven by a DIY 300B amp that is modeled somewhat after the old WE 91A. I had some design improvements provided by Paul Birkland who lives not far from me. (What a blessing!) Paul put my 6SJ7's in triode mode and switched to dc filament voltage so no hum pot needed.

 

[paul_b]

When I finally make it over to your place, I can bring by the blue EL84 amp if you'd like.

 

[tubegopher]

When I finally make it over to your place, I can bring by the blue EL84 amp if you'd like.

Hi Paul, we definitely will make it happen this year! I want to come by your place sometime to follow up on your comment that you could show me some tips on the de-solder gun Hako FR315. Perhaps in late Feb. early March.
Cheers,
Don

 

[MartinV56]

Mullard EL84

 

[MartinV56]

Eico AF-4 Great single ended amp

 

[paul_b]

27dB of feedback across an airgapped output transformer does not make for a great single ended amp.

 

[MartinV56]

27dB of feedback across an airgapped output transformer does not make for a great single ended amp.

You've heard it?

 

[paul_b]

I have extensive experience refurbishing Eico equipment and I am very familiar with their shortcomings.

I'll stand by what I said though, 27dB of feedback around a SET amp output transformer is a recipe for disaster. You could cause some really interesting problems if you happened to hook up electrostatic speakers to an amp like that...

If you don't need a lot of power and you don't need much in the low frequency department, the AF-4 could be useful.

 

[MartinV56]

pleasant memories of AF4 ,I never connect electrostatics to it, I always use full range with my single ended amps, greetings

 

[MartinV56]

Pablo, af4 was my gateway to the world of set amps, I have great affection for that flea.