EF86 VCA

Hi,

i built EF86 VCA for synth use from link. For now it is in test bed in form of an old chasis for tape machine controls, with several tube sockets.
https://i.pinimg.com/736x/8d/40/a0/8d40a0d501d2ce82b9ff4d54ccabdad9.jpg
Control signal come from DSP based module with TL07x output, audio from similar output which is ok as far as output impedance. I might add potentiometer to control audio input signal to grid 1.
The problem is output from anode because of high impedance and weak drive. It needs something like AC coupled cathode follower as here:
http://www.valvewizard.co.uk/accf.html
Inputs are also TL07x based with 10-20k impedance, so follower should drive them. I thought of using FET instead of ECC82, the problem with this is need of additional trafo so tube will be cheaper.

PSU trafo has 230V/20mA and 6,3V/1A, winder always leaves some margin. I think simple RC filter will be ok for both tubes (even cap multiplier if needed), heater with 2x100r resistors too.

This is my first take on synth modules with tubes, please let me know if i could change something (gain staging, etc.), or add more controls. It might benefit from 1k pot after follower for output control, i don't see a problem with it.
No one makes pentode/triode very similar to this two tubes, i would like to only use tubes in production for ease of retubing.

My3gger said:

The problem is output from anode because of high impedance and weak drive. It needs something like AC coupled cathode follower as here:
http://www.valvewizard.co.uk/accf.html
Inputs are also TL07x based with 10-20k impedance, so follower should drive them. I thought of using FET instead of ECC82, the problem with this is need of additional trafo so tube will be cheaper.

Click to expand...

You could use a high-voltage MOSFET in source-follower. Check Fig 15 there
http://sound.whsites.net/articles/followers.html#s9

You show "2mA" power demand, but 150V/270K won't ever be more than a half mA.

The 100V cap on CV In is at risk from 150V control signals.

Have you tried this in context of a synth?? It looks to me like the range of gain will be small, and limited by overload at low CV (G2) voltage.

The version shown here:
http://www.electronicpeasant.com/projects/tubesynth/tubes.html

(see VCA section)

Is also from Eric Barbour, and is used in the Phattytron. It has a few extra bells and whistles. Also, this being a Metasonix thing, it might be that "clean" was never in the spec  8)

This is the link to the original article (via Wayback):
https://web.archive.org/web/20180211082210/http://www.cgs.synth.net:80/tube/vca.html

Thanks a lot to both for replies. A friend and i are very excited about tube/hybrid synth module.
FET should work well here, totaly forgot HV ones as followers. I'll have do some more reading on them to have better understanding.
Headwize is still online, have to go over it because there are some interesting things about them. Iirc, PRR had ideas there too.

I noticed "low" HT, high anode resistor. So yes, gain and Ia are very low. Lack of experiences with synth modules make me want to understand CV and screen grid voltage interactions first. It is essential.
Solid state modules friend uses have low psu voltages, so i don't expect CV In to be anywhere near 150V. If it was C1 needs 200V at least.
For now i can make EF86 stage that passes signal and set its gain. CV connection and typical voltages are unclear to me despite really searching; like that -150V connection for which cap might need higher voltage, etc.
Here is another schematic to study with same HT and anode resistor (VCA1, bottom left), G2 CV input and some other things are a little more clear to me, "C1" has 250V as expected:
http://www.electronicpeasant.com/projects/tubesynth/tubes.html

I will get Barbour's VCA along some other files from my other computer tomorrow after work, there should be more information. This is example of Metasonix VCA specifications (no need for 600r drive here) which gave me a few hints like controls, gain, CV, use of second pentode:
http://www.metasonix.com/index.php/rk2-xs-vca-new



Quote from: Jarno on Today at 10:46:36 PM
The version shown here:
http://www.electronicpeasant.com/projects/tubesynth/tubes.html

(see VCA section)

Is also from Eric Barbour, and is used in the Phattytron. It has a few extra bells and whistles. Also, this being a Metasonix thing, it might be that "clean" was never in the spec  8)

This is the link to the original article (via Wayback):
https://web.archive.org/web/20180211082210/http://www.cgs.synth.net:80/tube/vca.html

Thank you, second link has information i couldn't find online. We heard Barbour's VCA and it is sound for him Have to get some sleep now.

PRR said:

You show "2mA" power demand, but 150V/270K won't ever be more than a half mA.

The 100V cap on CV In is at risk from 150V control signals.

Have you tried this in context of a synth?? It looks to me like the range of gain will be small, and limited by overload at low CV (G2) voltage.

Click to expand...

I have only tried this with audio signal and generator. Module synth output levels are in general around +10dBu, depending on waveshape. A few other tube VCAs have gain of about 20-30dB, so much lower than many similar pentodes wired in ussual way. Wouldn't gainn of 150 as per text and input signal be way too high?
This is Eric Barbour's paper for this schematic, i learned a bit although it is very different from what i've seen in preamp where i really like this tube:
https://web.archive.org/web/20180211082210/http://www.cgs.synth.net:80/tube/vca.html
Now i understand filter at the input of G2, R2/R3 with -150V connections are still unclear.

We talked with a friend about high distortion and sound when made as on schematic. Original has cathode follower output, i don't see any reason not to use FET as suggested since EF86 dictates distortion. He likes this sound and asked me to build into test bed for him.