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Old 13th April 2007, 02:11 PM   #1
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Default Hybrid amp: ECH83 + TDA2030A

Hi all,

This is based on Engels' idea of using the ECH83 as a preamp (a low-voltage triode/heptode with a B+ of only 12V).

For my first valve project I'm playing with Engels' design, building a hybrid practice amp using a TDA2030A as the power amp, with the stock datasheet circuit. It's running on +20/-20 split rails feeding a 12" 8 Ohm Celestion. That's working fine, plugging my SG straight in gives a good loud clean tone.

I've wired up the triode stage of the ECH83 as per Engels' circuit. The plate then goes via a .0473uF to one end of a 100K pot. The other end of the pot goes to ground, wiper goes to power amp input. I have a seperate 6V tranformer providing DC heater voltage through a rectifier. My B+ is the +20 from the power amp PSU.

Even in this simple config with one triode stage I can hear the valve - subtle warm clean tones! Some mains hum issues though. Currently reading -0.4V on the grid and +2.9V on the plate with respect to ground - does that look OK?

Theory-wise I've been learning from a book found in my attic (Foundations of Wireless, M. G. Scroggie, 1944, 4th ed.) and the awesome funwithtubes. I'm going to try cathode biasing instead of zero biasing; then when I'm happy I'll hook up the heptode section. I'm having trouble finding a reference for using a heptode as a voltage amplifier; I suspect the key is to get it acting as a pentode and using a standard pentode circuit?

Am loving my first valve experience, thanks so much to Engels and Ulibub! I've also just acquired a couple of 12K5s and a 12U7 for more low-B+ fun

Any comments/advise most welcome. I'll post a schematic when the design's more finalised.
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Old 13th April 2007, 03:28 PM   #2
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Just to add, I've now put in the heptode as per the original schematic (except with all caps .0473 and omitting the 100K resistor before the heptode grid. The output goes into a 100K lin pot then to the power amp input.

The sound is far far better than I expected (I should add I'm going for valve colour not transparency here) and really loud.

I'd like to run some calculations to prove to myself I'm hearing the preamp distortion and not just distortion caused by going too hot into the TDA2030A. Any advise on this would be much appreciated.
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Old 15th April 2007, 04:12 PM   #3
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Default Progress and problems

Hi all,

I tried Ulibub's suggestion of using the grid on pin 7 of the ECH83 (the middle grid of the heptode) in parallel with the heptode control grid, but this just gave me oscillations.

I'm a tube newbie and could really use some advice

My current schematic and voltage readings are as below:

Click the image to open in full size.

As many have said, the 100n filter caps have to be as close as possible to the TDA2030A to avoid it oscillating. Once that was sorted I've had no problems at all with the power amp. Really punches the Celestion.

I'm pretty happy with the sound already but I don't think I'm handling the gain or volume controls correctly at all. The gain control produces no sound below a certain level, full sound thereafter - making me think I should replace it with a 680K grid resistor and look elsewhere to add a gain control.

The volume control is working, but for the bottom 20% of it's travel a distorted tone is present underneath the clean tone. This distortion is very pleasant but it goes away suddenly as the volume reaches 20% or so. I'd really like to keep this distortion throughout the whole volume range if possible. The closest analogy I can find sound-wise is that it sounds like the way a JFET stuggles when it's on the point of starvation, except much richer and warmer (bear in mind I'm coming from JFETs to tubes - I liked the sounds the runoffgroove.com guys were getting so much I wanted to check out the real thing). I'm aware that tube distortion generally comes from overdriving them not starving them!

Oh, just to reiterate, this is a GUITAR amp

Many thanks for any help you more experienced designers can offer.
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