• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EC80 as preamp tube.

These triode are intended for common grid circuits at high frequencies...
They are very noise resistant, u=80 mutual conductance (Va=250V) is 12mA/V.
Tube contains only one triode with interesting and good looking construction.
I was wandering, has anyone used them as preamp tubes either in guitar amps or HiFi amps?
I'm always for a challange and this one looks pretty good.
Only important thing is "how do they sound?".
Calculations and projecting circuit does not bother me...
Thanks in advance.
 

artosalo

Member
2010-02-16 9:00 am
I have tested with soviet 6S4P-EV.
It also has four grid connections but it works well without any special precaution.

How some "tube sounds" depends on the used ciruit, operating point and the person who is listening. The tube also has some effect, but it is not the most effective factor.

EC80 has so high mu that the gain without negative feedback is quite impractical for typical preamp.
 
TheGimp: shield on tube would do the job... PS is not a HP antena :)
DF96: No need for that, my eares are not scope, I can't hear 100kHz (reduced gain does not bother me neither)
I wrote low noise couse I tested for noise on scope, not seen in datasheet( i think that 4 grid connection makes it better, not worst) ...
I'm just afraid of bad things that "ideal" transistors brought, they killed natural distortion.
I don't want this tube to bring me same tone from input to output, I want tube sound. That was my question, are they good for audio application as ECC83's and similar?
If yes, I'll finish my preamp, post it here (schematics, recordings), write troubleshoots,
help poor and save the world :) :D
 

Shoog

Member
2002-08-15 10:16 pm
Eire
It does seem rather silly to attempt to make a tube do something it was neither designed for or is good at. As others have pointed out, such a high gain tube should only find use in extremely small signal duty - such as a phono pre-amp or a mic-preamp.

Attempting to use it in any normal line amp application means that significant feedback will be needed and then it will sound just like any other feedback dominated preamp (think solid state sound).

Find a better tube for the job.

Shoog
 
the same applies for the ECC88 then. tell that to the audio guys..

if you use grid stoppers and ferrite beads. you can likely tame the oscillations maybe even add some capacitance to decrease BW

heck, these tubes where invented, and built whit low inter electrode capacitance.

could several pictofarad from grid to ground, tame this tendency to oscillate ?

v4lve
 
EC80 is designed to run with its grid solidly grounded for RF. That is why it has multiple grid pins, so it can have multiple low inductance grounds. As it is intended for grounded grid operation in fairly low impedance circuits it does not need particularly low internal capacitance.

It may be possible to tame it for audio purposes, but its high mu and poor linearity means phono input is about the only application. It might make a cathode follower too.
 
is it imperative to put grid stoppers on all 4 grid connections at socket?

For best immunity to oscillation:

1. Remove the socket-contacts for G1 pins 1, 7 and 8. (ie those nearest the anode). This minimises the unwanted coupling to the grid. You could even cut off the valve's pins, if you believe that you can do this without disturbing the glass->pin seal. But the socket receptacles must certainly be removed.

2. Feed G1 through pin 2 (nearest Cathode).

3. Use a "stopper" resistor 3.3KΩ to 10KΩ or maybe more, depending on the circuit. Stopper of carbon composition would be my choice, but experiment if you wish.

4. You could also try ferrite beads in addition (or instead of) resistors. These can be very effective at VHF-UHF, and can be used in the anode as well. The currents involved are so low that you can usually pick a ferrite bead with the highest available impedance from 10MHz to 200MHz+.

5. Try to minimise the grid wiring, especially unshielded runs. Don't run any anode wiring in any way parallel to the grid wires.

6. Minimise the anode wiring. Have the load resistor (etc) nearby, and the stage's power decoupling capacitor near the load resistor. The decoupler's negative terminal should return to the cathode resistor, and be bypassed with a stacked film polypropylene or polyester (FKP MKP MKT etc). I like the WIMA FKP here, (up to 1nF).