EL38 Triode Connected HPA Pentode Drive

[Cressy Snr]

The postie brought these handsome devils this morning:

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[Cressy Snr]

After a bit of re-wiring in a series/parallel arrangement off the 12V DC heater supply, we now have 6.3V at the EL38s and 6BR7s:

[Cressy Snr]

Well, it's playing music as I write and what a superb sound!

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Gain is a bit high at the moment so I might have to do triode mode on the 6BR7 input stage and forego the pretty little glow tube.
I found a nice set of curves for the 6BS7 in triode mode on Ale Moglia's site. The 6BS7 is the same valve as the 6BR7 except that it has a top-cap grid, so those curves will work perfectly with the 6BR7.
I'm thoroughly enjoying the sound of this amp. It's the best one I've built...lovely

[Cressy Snr]

Here's the audio cct:

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And the PSU:

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I've gone triode on the input stage in an effort to reduce the gain, but there's going to have to be a rethink in that department in order to pull the gain down some more.
Nevertheless the sound is great. I can certainly see why Paul likes the sound of this valve.

[Paul Barker]

Yes.

[Paul Barker]

My operating point is 60mA -20v bias 400 plate volts. So b+ 425v 5v opt primary voltage drop. You can add 8watts dissipation for screen to that if you want max, which goes to 5 watts. Ive got mine at 4.5 watts. The curves available don’t allow for screen dissipation.

[Paul Barker]

Glad someone else has built one.

[Cressy Snr]

Yep, they're bloody good valves!

I've solved the gain problem by using 1/2 an ECC82 per channel. It is fine now in terms of volume control travel.
I've wired the valves so that triode 1 of one envelope is active on the left channel and triode 2 of the other envelope is active on the right channel.
When one wears out, swapping the valves over will activate the good triode in each one, providing the cathodes on the inactive triodes haven't been poisoned by then.

[Cressy Snr]

There was a bit of residual ripple in the earpieces of my phones, but a mod to the power supply sorted that one:

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Voltages are lower but there is no difference to the sound that I can hear. It's still fab.

[Paul Barker]

If it didnt have the top pip it would be the best of the bunch.

[Cressy Snr]

A bit more work on the power supply and we're within 1V of the previous bias of 10.5V on the power tubes:

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[Cressy Snr]

Newest audio circuit:

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[Paul Barker]

I’m sure you are really happy with this. After All since I built mine Ive kept it.

But I did the maths , you’re getting 0.9 watts. I’m itching to know how the 6EM7 sounds.

Its just that you arent getting the most of the el38. Im getting 4.2 watts on 5k, and if I add the 8w screen dissipation even more.

Whereas the 6em7 is 1.25 watts into 5k, which is well suited to youre headphones. The 6em7 has the best soundstage of any iht Ive heard and also better than any dht for imaging. So ……..i wander if em7 images in headphones…..

Ill get my coat.