EL84/6P14P Headphone Amp

[andrew Ivimey]

Maybe valves themselves are wrong on many levels but oh I love mercury vapour rectifiers and thoriated output valves.

Onwards!

[Cressy Snr]

OK, so I came to the conclusion that the EL84s were being thrashed mercilessly by too high HT voltage.
New production valves can't be abused like the old ones, so they say. This HPA sounds too good to be replacing valves every 5 minutes, so something had to be done to get the HT down and reduce the dissipation somewhat.
Anyway, I changed the mains transformer to a 240/240V, 100VA toroidal isolation transformer and the rectifiers to a hybrid bridge with two EY500A colour TV booster diodes:

EY500A.jpeg (67.33 KiB) Viewed 10618 times

I had been meaning to use a couple of 6D22S, but the top-caps were too short for the deep skirted bakelite top-connectors to hold onto properly. EY500A however have taller top-caps, which the skirted connectors gripped perfectly.

We now have 279V off the end of the power supply, 271V at the EL84 anodes and 9V of cathode bias. The output valves are now dissipating 8.6W out of a 12W maximum. I'm a lot happier with that.

EY500A_TwinRects.png (63.03 KiB) Viewed 10618 times

I needed three heater supplies as my existing heater transformer with two 6.3V, 4A windings did not have enough capacity to feed a pair of these current hungry rectifiers. One winding fed the EL84s and the other one would have been right on the edge, having to provide 4.2A to a pair of EY500A or 6D22S. The answer to the problem came with a giant Danbury heater transformer with 3 x 6A windings our Ant gave me. This transformer runs nice and cool.
Of course, to incorporate the two new transformers, the entire power supply had to be rebuilt but that's the nature of the hobby.

[Cressy Snr]

The fume extraction now goes through an external wall and exits via a louvred vent outside.

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[Cressy Snr]

Using the mask and the fume extraction, I’m in the last chance saloon.
Hopefully this belt and braces approach will work.

[Paul Barker]

Interesting. I’ve been working on an off grid flat, so I bought a small power supply which is a bit small for the job. So I set up my SE el38 in the mian room to show it to my workmate. It is better sounding than the pp kel84. I just chill for an hour enjoying blissfull se. So I discovered if I turn the variac down to about 350 volts at the anode of the el38 I still get sweet sound but the whole amp is consuming 112 watts only and I get more than 2 hours of listening. Which is an hour longer than I ought to be chilling out listeningnto music instead of getting on with the convertion. But since I am the boss of the project thats OK. The goal is to sell the house and downsize. Put all my valves gear and everything into the cellar which is huge well ventilated and dry. Bring up to the living room whichever system I wantbto listen to.

I take the power bank home after a days work and chargenit at home. Utility companies are robbing us blind at the moment so Im off grid doing the work. Have all battery tools.