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Old 8th March 2007, 11:39 AM   #1
engels is offline engels  Israel
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Default Heptode audio preamp? ECH83

At some point I've bought a couple of ECH83 and EBF83 tubes and now I'm thinking about how to use them. They're european parallel to american space-charge tubes: low voltage anodes.

ECH83 is a triode-heptode with 6.3V/0.3A filament and 6-50V plates.

ECH83 datasheet

I've seen the heptode might be also used for rf/if amplification, so I've tried to think of audio amplification (from my experience most of RF tubes may happily amplify audio). I've never made a heptode audio stage so this is a funny experiment for me. Here's an attempt to design something -

Click the image to open in full size.

whatcha think? will this thing smoke?
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Old 8th March 2007, 12:21 PM   #2
ulibub is offline ulibub  France
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Hello!

No, it won't smoke certainly ;-), and most certainly it will work, too. But think of noise level.... The more grids in a tube, the higher the noise level.... hexodes, heptodes and pentagrid converters are quite noisy. Also, microphony might be an issue with all these grids. So you were right putting the triode before the heptode.

Many years ago I built a similar amp with an AK2 - a pre-war octode with 4 Volts heating, just for fun. Impressive bulb, quite noisy and incredibly microphonic... (the tube was in the first stage...) Even got acoustic feedback from the speaker.... But - it amplified.... ;-) But with those modern types, you shouldn't have these problems almost anymore.

I have two questions:

1. biasing on common cathode - are the two simple grid leak resistors sufficient? Especially the 47 k on the triode grid most certainly will be much too small - I would suggest 1 Mohm at least. Its low value comes from its operation as an oscillator where grid bias was generated by rectification of the rf input signal on the grid-cathode diode.

2. Why do you use the second regulator grid of the heptode as a (3rd) screen grid? Why not using it in parallel to the first grid as a second regulator grid, as it usually has been intended to? Thus enhancing sensitivity? Would perhaps be worth trying.

Let us know about your results! I also have a lot of these tubes lying around....

Good success!

Uli
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Old 8th March 2007, 12:30 PM   #3
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I can't say if it will work, but that looks interesting as I have a few "unusual" tubes myself that I'd like to use. Quite a few ECH41 (triode-hexode) and UAF42 (diode-pentode), and even if only one tube section is usable it would be fun building something with them.
And if someone knows if something could be built using an EF9 pentode, please let me know
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Old 8th March 2007, 12:51 PM   #4
ulibub is offline ulibub  France
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Quote:
Originally posted by showdown
And if someone knows if something could be built using an EF9 pentode, please let me know
Hi!

The EF 9 is the almost identical type to the EF 89, only with different bulb and base, it is a remote-cutoff-pentode with gliding screen voltage (reduction of distortion) for variable rf or if amplification in radio sets. But you also can use it for af amplification. If I remember well I even have seen a music dynamics compressor/enhancer which was using the variable amplifiaction (depending on bias voltage) of this tube. I think it could be used similarly to the EF 83, which is the remote-cutoff-version of the EF 86, both types specially crafted (low hum sensitivity) for af amplification service.

I would try the EF 9 in similar layouts to the EF 86, just playing around with cathode and screen resistor values - both certainly smaller than with the low-current EF 86. I would suggest 100 k plate, 330 k screen and 270 Ohms cathode - just to start with. Grid leak 1 MOhm. And also trying different grid bias settings to obtain different amplification factors (cathode on ground and bias fed from a variable voltage source).

Triode operation also should possible.

Good success!

Uli
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Old 8th March 2007, 01:14 PM   #5
engels is offline engels  Israel
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Default hmm

2 ulibub:

thanks for the reply!!!!

yes, 47K grid leak resistor looks small. Still the datasheet specifies it as the bias resistor and I think it's reasonable to try it like this first. If the signal comes from something between 600 Ohm (microphone) to 8K (pickup) it should work (actually the triode will work in grid leak mode with 600 to 8000 ohm resistor).

now I don't know a thing about grids in heptode! The datasheet says that if heptode is operated as rf/if amplifier, Vg2+Vg3+Vg4 = 6 to 25 V. I thought that means they're all connected to the B+.
If I connect the control grid to the g3 there's going to be a positive grid (g2) between the g1 and g3 - and how the hell this thing should speed up electrons on their way to the plate? Backwards?
I think I'm going to read some more about heptodes.
The way it's wired in the schem it acts like a pentode.

The questions I have are more about the resistor between the stages (100K) - I'm not sure I can connect the triode's anode to the grid of the heptode without a resistor because of the common cathode. If I could eliminate it I would certainly get a louder chain - if the heptode won't distort too much.

Also the load resistors are a question - the little schem in the datasheet shows the heptode with the g2 and g4 wired to B+ and the plate a lower potential. I've put a 150K load on the "screens" and 100K on the plate to make the plate more positive. Am I wrong? Should I wire the screens straight to B+ ?
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Old 8th March 2007, 01:15 PM   #6
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Quote:
Originally posted by ulibub

I would try the EF 9 in similar layouts to the EF 86, just playing around with cathode and screen resistor values - both certainly smaller than with the low-current EF 86. I would suggest 100 k plate, 330 k screen and 270 Ohms cathode - just to start with. Grid leak 1 MOhm. And also trying different grid bias settings to obtain different amplification factors (cathode on ground and bias fed from a variable voltage source).

Triode operation also should possible.

Good success!

Uli
So something like THIS could be possible with the EF9? I would certainly look pretty cool, especially with the AZ4 rectifier I have here...
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Old 8th March 2007, 01:30 PM   #7
ulibub is offline ulibub  France
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Simply try it!!!

It certainly will work, the question is HOW GOOD it will work! And instead of much discussion, just build it and see the results! Thats one of the big advantages of tube technics against semiconductors... It still works, even if it might be far from optimal. Don't know how triode operation is behaving with the remote-cutoff characteristics. It depends largely on the gliding screen voltage to reduce distortion - these tubes otherwise have quite some nonlinear characteristic. So I would prefer pentode operation.

Or, use "pseudo-triode" service without condenser on the screen grid, only fed by its resistor to keep the gliding voltage characteristic. So you should get quite low-distortion triode operation.

But - simply try it and find the best operation mode!

Uli
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Old 8th March 2007, 01:35 PM   #8
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Heptodes are indeed something like a two-quadrant multiplier and therfore they were actually used for ALC in IF stages and as very linear mixer stages in superhets.

For those who like to experiment they might indeed be used for compressors exciters etc.

Regards

Charles
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Old 8th March 2007, 02:36 PM   #9
engels is offline engels  Israel
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Default compressors, exiters

Any suggestion on how to make a compressor/exiter/whatever tremolo noise maker with a heptode?

I've heard similar ideas not once, but haven't seen any actual scheme with a heptode for that purpose.
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Old 8th March 2007, 10:00 PM   #10
ulibub is offline ulibub  France
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Default Re: hmm

Hi Engels!

Quote:
Originally posted by engels
2 ulibub:

thanks for the reply!!!!
:-)))

Quote:
now I don't know a thing about grids in heptode! The datasheet says that if heptode is operated as rf/if amplifier, Vg2+Vg3+Vg4 = 6 to 25 V. I thought that means they're all connected to the B+.
If I connect the control grid to the g3 there's going to be a positive grid (g2) between the g1 and g3 - and how the hell this thing should speed up electrons on their way to the plate? Backwards?
I think I'm going to read some more about heptodes.
The way it's wired in the schem it acts like a pentode.
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.

But with all these grids you also can configure special conductivity conditions depending on the wave angles of the different ac voltages on the different grids relative to another. - the "Enneode" (type EQ80)- with 3 regulator grids, is such a type and was used as an amplifying FM demodulator. In this way, ECH83 often were used as amplitude filters in old TV sets to get the syncronisation pulses out of the video signal.

Quote:
The questions I have are more about the resistor between the stages (100K) - I'm not sure I can connect the triode's anode to the grid of the heptode without a resistor because of the common cathode. If I could eliminate it I would certainly get a louder chain - if the heptode won't distort too much.
Simply take it away (ot put the grid leak in front of it, to avoid the voltage divider) and take an ordinary grid stopper directly in front of the grid - 1 to 10 k should be sufficient.

Quote:
Also the load resistors are a question - the little schem in the datasheet shows the heptode with the g2 and g4 wired to B+ and the plate a lower potential. I've put a 150K load on the "screens" and 100K on the plate to make the plate more positive. Am I wrong? Should I wire the screens straight to B+ ?
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
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