Low voltage tube overdrive.

[Beach]

Dear forum

Recently I was donated a box of valves (tubes) from a good friend. There were some well known valves in it, a few lesser known and a few obscure ones. However, soon my eye caught a few ECH81 and ECH83 valves. The ECH83 is a triode/heptode combination intended for use as a mixer in carradio sets. It is specified at low voltages. Could these be utilised in a guitar pedal?

I did find another ECH83 project with promising results, so I dived into the datasheet and tried to design a circuit from there.
(Sorry, the following is a bit mathematical, but required to get a grip of the working of the final circuit. And also: I might have made a mistake, but in the end the pedal works pretty good.....)

The (condensed) spec for the triode at 6V3 are:

Va = 6V3
Vg = Obtained by grid current biasing with 47k
Ia = 0.3mA
S = 0.8mA/V
u = 14.6

With a 12VDC power supply this would yield an Anode resistance of:

Ra = (12-6.3) (V) / 0.3 (mA) = 19 kOhm

Output resistance triode: Ro = u/S = 14.6/0.8 = 18k25
Req = 19k//18k25 = 9k3
Av = Req*S = 9k3*14.6 = 7.45

I set the valve up with these values and did some measurements:

With Ra = 20k → Av = 7.8
The theory said Av=7.45 So that's pretty good.

I found the highest amplification with an Anode resistance of 27k

Va = 5VDC → Ia = 7V/27k = 0.26mA
Av = 9 (in 200mVtt out 1.8Vtt)
BW > 200kHz

At low voltages the input impedance of the grid is far from 'very high'. Electrons emitted from the cathode are not accelerated that much to the Anode and a bunch of them tends to 'float' around the grid (spacecharge), resulting in a non zero grid current. The datasheet recommends a 47k from grid to ground. The grid voltage is then some -320mV.

Vg (with Rg=47k to GND) = -320mV → Ig = 6.8uA

Since this current is actually generated from the valve, one could also 'bias' the grid voltage from the positive voltage rail. A much higher resistance than the 47k can be used then. This sounds counterintuitive, but the grid current comes from the spacecharge from the valve itself.

12v32/6.8uA = 1.8Meg to 12VDC

I Measured the input impedance in this setup to be ca 15k Ohm.

In the final design I split the 1.8Meg into 1Meg and 820k with a decoupling to GND as to filter out any unwanted noise from the power supply.

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Now for the 2nd stage of the overdrive, the heptode part:

Datasheet:
Heptode (as RF or IF amplifier)

Va = 6V3
Vg2=Vg3=Vg4 = 6V3
Vg1 = Obtained by grid current biasing with 1Meg
Ig2+Ig3+Ig4 = 0.08mA
Ia = 0.11mA
S = 0.35 mA/V
Ri (Internal Resistance) 0.6 Mohm (probably output resistance).
Req = Noise resistance


Design: Ra = (12V-6.3V)/0.11mA  = 52k → 56k
Rg2g3g4 = (12-6.3)/0.08 = 71k → 82k
Av = Ri//56k * S = 17.9

We'll make Ra=56k and Rg234 = 82k because these are readily available.

Measurements:

Av = 17.5 (in 200mVtt out 3,5Vtt)
   theory → 17.9

Rinput = ca 112k (ECH83) (measured with 2 other tubes ech81: 80k-120k)
BW = 120kHz

Enough with the theory! Let's get to the schematic:




I added a tone control around R7 (Anode of the heptode). Could be omitted, then use 56k as anode resistance.
Further, I divided the output voltage with a 470k/50k (pot) combination as to reduce the amplitude at the output (not sure my amp can handle the full 12Vtt signal)
The heaters need 6V3V. An LM7806 will do the trick (Be aware: needs cooling!).
     My 'bin' did not held a 7806 so I used a 7805 with 2 diodes in the ground lead resulting in 6.4V heater voltage
I added D1 as a safety for wrong polarity power supplies
I added a LED in the valve connector, tied to the 3PDT switch, now it lights up when I switch the overdrive 'on'
It is true bypass.

I put everything in a black metal case I had lying around.

And I shot some footage: Judge for yourself. Pls give it a thumbs up if you like it
(pls forgive the superlatives in the description, you need to stand out in order to get views)

I call it the AVTube overdrive.

[iainpunk]

sounds good!
i have some ECH81 and ECH83 tubes in my collection, i think i have to try this in the near future!

cheers

[Beach]

The original clip had some copyrighted material and I took it down.

Replacement: