audio-talk

Schematics:

I’m afraid a 20uF polypropylene capacitor is going to be big, so it’s difficult to suggest anything that will fit, to be honest. That’s why they used electrolytics. My 10uF ones only just fitted.

Cheers Paul,
I always use bus bar earthing. Star earthing sometimes makes more noise than busbars, probably because I don't do it properly, but there you go. In the future I'll lower my voltages. There's no reason for high voltages in a headphone amp. At least I've learned something.
With the 1626s ...

Well eventually I finished up reverting to the 1626, driven by an ECC82 as these can, of course, be configured for 12V heaters. It's about the best I could do given that I couldn't find another 6EM7 that would remain stable, even when the grid resistor was reduced to 220K. I wasn't going to risk ...

Yes, but that’s the least of my worries!
Now a cathode resistor on the power section of one of the valves has burned out. The valve obviously ran away. Maybe a leaky coupling cap. But either way it’s out of action. The grid resistor of 500k was well within the 2meg limit.

Too many old components ...

Picture of the inside:

IMG_20250715_152518658_HDR~2.jpeg

Diagrams will follow shortly.
I’ve managed to sort the hum. Still a bit of residual noise but that’s valves for you.

I’d need regulated HT and regulated DC heaters for silence. If you can’t be arsed with that malarkey (which I can’t ...

OK, so I’ve finished it.
Using both sections, of the 6EM7, there was far too much gain as you’d expect, but with the power tube section on its own there was not enough to play vinyl decently.
So using both sections and a ton of global feedback wrapped around it, I got the overall volume about right ...

And we have heaters:

OK, I’ve finally started on a proper 6EM7 headphone amp rather than a lash-up. Theres a long way to go but it should be good (I think.)
Here’s the box made up with General Electric, coin base 6EM7 our Ant found during his house move. Rectifier is EZ81.

Well I tried to put DC heating on the amp and what a disaster! A right bleed'n racket of buzzing.
It all went silent when I touched the top caps of the EL38s.
Looks like the RF field coming from the onboard SMPS DC supply is being picked up by the anode leads.
I can't be arsed with anode stoppers ...

10k

So less distortion and you hVe plenty of power anyway. I love my el38 se. I cant beat it with el84 pp


Cheers Paul,
Yep, the EL38 is a great valve. Out of all the combinations I’ve tried, it sounds the most natural and just gets out of the way of the music. There’s plenty of gain using it ...

EL38s have returned to the fold:
EL38.jpeg

This time as a single valve amplifier.
5k output transformer tap on the primary side and the 32 Ohm headphones connected to the 16 Ohm tap. That's as far as the OPTs go.
What impedance that reflects to the primary I don't know. Can't remember the ...

I bought a pair of the General Electric, 6C5, metal tubes Paul mentioned in an earlier post.
At £7.50, you can’t go wrong. I might get around to doing something with them before Owston; we’ll see how things go.

OK,
Heres a pic of the headphone setup as it is now: Looks like the monitor has air extraction
It’s there to get prevent one posting too much hot air.

simon wrote: Sat May 31, 2025 5:58 pm
You mean you're not going to try something else between now and September?

Probably not. Don't want to tempt fate with the respiratory stuff. Can't be doing with the unpleasantness of the symptoms.
However, I will look at the 7A4 option for when I feel up to doing a bit more.