Heptode audio preamp? ECH83

[engels]

Originally posted by ulibub
Yes, the first screen grid (g2) - between the two regulator grids (g1 + g3) - protects g1 from capacitive couplings of the oscillator frequency, which is fed into the mixer via g3, to avoid oscillator frequency being radiated away by the antenna, which is fed into g1. It also accelerates the electrons to be fast enough to penetrate g3 (as in the old space-charge tubes), so it forms some kind of "virtual cathode", an electron cloud directly in front of g3. The rest of the system (second screen, g4, and suppressor, g5) acts as a simple pentode. That's the usual mixer operation of these tubes.
I only can tell from the ECH81 or ECH41/42 in radio sets, where the screen voltage always has to be much lower than the plate. I don't know how it is with these low-voltage tubes. Just try, what sounds best...

Uli

thanks a big lot!

I think I will make a switch for the third grid to connect it to the screens or to the control grid.

Moving the grid bias resistor before the grid stopper seems 100% right.

As for the voltages in low voltage tubes - I've got an amplifier with 12DL8 which is a space-charge grid tetrode. The space charge grid is placed between the control grid and the cathode (not between the grid and the plate as in a "normal" tetrode). The space charge grid is wired directly to 12V and the plate has something like 6V on it. It works fine. The ECH83 heptode is not a space-charge tube so it doesn't need that potential on the screen but who knows... those tubes always work - even if you put 100V on screens and 1V on plates... they're tubes.

[ulibub]

Hi engels!

Nice to see someone is working with space-charge tubes. I don't remember ever having seen these tubes in use troughout Europe. So I know them only from theory. How are they performing? Do you have a circuit diagram of that amp?

And yes, low-voltage tubes doesn't mean they're built exclusively for battery operation, they work equally as fine with "normal" tube voltages. They're only designed still to function at low voltages, too. In my phono preamp I use an ECC86, also a low-voltage type, in SRPP with 50 Volts on each triode. Works perfectly.

Greetings, Uli

[engels]

yep, I'm addicted to weird tubes. Space charge tubes are pretty cool - cheap and the iron is even cheaper. The only iron you can freely buy today at almost any electronics shop - 12V power (for fluorescent lamps) and line output trannies.

If you're interested, here's the amp I've made -

Space Charger amplifier - schematics in PDF format



It's the greatest thing I've done. I play guitar and bass and at the moment this is the only amp I record with. Very quiet but sounds huge.

I would not call this a hi-fi amplifier. The preamp tubes preform well but for some reason they don't sound too hi-fi. I've listened to the preamp only and it cuts some of the high frequencies. It might as well be the capacitors in the chain, I'm not sure. The resulting sound is very pleasing so I've left it like that. It's more like a germanium transistor that gives everything a nice vintage flavour.

Now I'm thinking about making a spacecharge amp with PP output... but the PI is a big question.

[aletheian]

Originally posted by engels


It's the greatest thing I've done. I play guitar and bass and at the moment this is the only amp I record with. Very quiet but sounds huge.

I would not call this a hi-fi amplifier. The preamp tubes preform well but for some reason they don't sound too hi-fi. I've listened to the preamp only and it cuts some of the high frequencies. It might as well be the capacitors in the chain, I'm not sure. The resulting sound is very pleasing so I've left it like that. It's more like a germanium transistor that gives everything a nice vintage flavour.

Now I'm thinking about making a spacecharge amp with PP output... but the PI is a big question.

The sound clips of the amp don't work on your website

[ulibub]

ok, I understand. That's a guitar amp, not meant for HiFi puroposes.... I was thinking of a true HiFi amp with space charge tubes....

[Alastair E]

Ive recently been fooling with various 'Standard' triodes at 12-20V +B. Take a look at the scheme I posted around halfway down this page....

6SL7 or 6SL7GT for broken Tubalizer

Ive tried 6SN7, 5687, ECC88/6DJ8, and 7586 Nuvistors. All work reasonably well, The Nuvistors particularly well, being very transparent, providing you keep the input below 1V RMS, if your supply is 13.6V, as it runs out of 'headroom'

I have not tested for distortion/noise etc, But it Does Sound pretty good, with no really apparent 'noise' or flaws-In fact, running it via my OTL,Ii have difficulty in telling whether its switched in or out of circuit, so the dist/noise figures cant be too bad.......

IF you have a 15-20V supply, the circuit performs extremely well, and does not have input limitations as on 13.6V It has a gain of around 4-5, and is happy running into 'loads' of 47K-100K.- Great little 'scrap-box' project

[planet10]

Originally posted by ulibub
he EF 89 ....

Thanx for that ... i probably have 30+ of these (and some very pretty). Would be nice to find a use for them

dave

[ulibub]

I would start with "pseudo-triode" mode as I described above, wired as a true pentode, screen with its own resistor and NOT connected to the plate, but WITHOUT screen grid condenser. I haven't tried it so far, but would start this way. And this should apply to all remote-cutoff-rf-pentodes, like EF 9/11/41/85/89/93/183...., just to mention the most frequent ones.

I am very interested in your results! Don't have the time at the moment to try it myself...

Uli

[Cursor]

Wow - perfect! Looking to explore space charge tubes (and tubes in general) I've acquired a single ECH83.

Would there be any problem running filament and B+ from the same supply? It can do 12V at 5A, and I can use a hefty 6V regulator for the filament.

I really want to build one of these but I'm having trouble sourcing the tubes.

[ulibub]

Hi Cursor!

No problem running the heaters from the same supply.

But looking at the diagram on your link - I get some headache looking at the 100 kohm resistors in the cathode lines of the 12U7 tubes - if they are correct. Using about half or one third of the plate resistor voltage drop for bias alone doesn't leave much voltage across the tube - I wouldn't expect neither a high amplification nor a good overdrive reserve.

For the triodes you could also use the double-triode ECC 86. But don't know at the moment about its plate and bias resistors. Don't know about the space-charge power tube - EF 98 is a low-voltage (car-radio) pentode wich also stands some plate dissipation.

Uli