6j6 twin triode running at low voltage (18V)

[rankot]

I still didn't put 100p capacitors in parallel with anode resistors. I want to check if they affect noise or not. I have also put R10 and C16, but it may also work fine without them?

[rankot]

Q1 is 2N5088 or similar, Q2 is BS170 or 2N7000. D10 and D11 are there just in case, to protect later equipment - maybe not necessary at all?

[amptramp]

The current source may bring back that multivibrator effect since a current source is high impedance.

[rankot]

I have tried both ways to power cathode (that's why I have installed the switch) and no oscillations at all.

[granite]

 I have also built one of these, using basically tca's schematics. Sounds really good!

The minor differences are: mine uses 9V power so I put a 7805 with 2 diodes to generate the 6.3V heater.

Also, the gain is trimmed to a level where the guitar volume has a chance to get into overdrive.

Schematics:



Ignore the blue LED part. I figured it would look great to get the tube lit from below with a blue LED. Since the socket I have did not really allow any light through, I just skipped that part.

Inside with a small solderboard and the voltage regulator screwed to the aluminum case - it does get warm:



The finished Tube Driver pedal:

[rankot]

It's much simpler to use a resistor to drop voltage from 9 to 6.3 V. See my schematics.

[tca]

Looks great!

[granite]

It's much simpler to use a resistor to drop voltage from 9 to 6.3 V. See my schematics.

Yes, when you have the big beefy ones. I only had the accurate but low power ones so they I went with the 7805 (of which I have a bunch). Those resistors are dissipating over 1W if I am calculating it right.

[tca]

If using a resistor you'll need to take care of the generated heat, bigger box, fan... simple use a voltage regulator (invented for).

P.S.
I see... 9V Ok for the resistor (I was thinking 18V).

[rankot]

I have used 1R8 + 4R7 2W resistors to drop from 9V to 6V, and they are fine. No need for a heatsink, which is, I suppose, necessary when using 7805. 1R8 can even be 1/2W if space is a problem.

[rankot]

Also, what do you folks think about this way for polarity / overvoltage protection as shown on my schematics here http://www.diystompboxes.com/smfforum/index.php?topic=111622.msg1097263#msg1097263? Will this zener diode do the job, or it will blow if more than 11V is present at power input?

[granite]

I don't think the zener would fry itself. I believe it would let the max current through it is designed for and keep the voltage at 11V .

I don't know what zener you have in there but this one (the 11.4V one) will let close to 400 mA through - which I guess is close to 5W:

http://www.mouser.com/ds/2/308/1N5333B-D-102336.pdf

Back to the 7805, it is like a dumb resistor of course, so it will have to dissipate (9 - 6.3V) * 450 mA ~= 1.2W

I guess the benefit of this is that the aluminum box gets nice and warm around it, so if you are playing outside in a blizzard, you could just rest your foot on the pedal and warm up 

[rankot]

One question to all of you - if you take a look at my version of this schematics, there are R10 and C16 between volume pot and the second triode. Are they necessary or not? I put them there just in case, but maybe I don't need them at all? Help appreciated!

[PRR]

> Are they necessary

Can not be sure. Why don't you try it? Short C16. If fine, remove C16 and R10.

In higher-voltage work, they are not needed. Tonestack blocks DC, also Vol pot provides a DC return.

But at low voltage sometimes tubes "work" only because of large constant grid resistor making a significant grid bias. Also in OVER-drive work, sometimes a consistent R-C recovery time matters.

[rankot]

You're right - tonestack blocks DC. I have already put them on PCB and it works fine, so I didn't want to remove them and risk PCB damage. However, I will try without them just to see what happens. I also installed switch to try constant current source and resistor/cap cathode connection, but there is no much difference with that.

[rankot]

This is my last build, after lots of tweaking:



C2 is redundant, but I left it on PCB since it was there from start. I will try without it, just to hear if it will change distortion sound. C4 and C19 are there just in case, to protect circuit from radio noise.

R4 can also be omitted. Tone bypass switch could be DPDT, but it also works like this.

My real build is without R9, but maybe it is safer to have it there, especially if R4 is omitted. I have also tried without C16/R10, but I had problems when bypassing tone stack, so I left them in place.

R1 can be 100k too.

When gain is at 12 o'clock or less, this can work as bass preamp with really nice sound. More gain will distort. I don't like this distortion too much, maybe C2 makes it suck. But I really like it as a bass preamp!

I will soon post images of the whole build, currently in the painting process.

I was playing a lot with this on PCB and in LTspice, and simulation shows that R2 can be 22k, with R13=100k and R1=470k for optimal gain on both stages (I am using 27V as B+, generated from 9V DC with LT1054 voltage tripler). I've tried that resistor combination too, it distorts less, but it seems that it is humming a little (or I am just tripping cause I'm tired right now), while this combination shown on schematic is dead quiet. However, some help from experienced tube builders about this resistor choice will be appreciated. 

[rankot]

Now I've checked again with R2=22k and R1=470k, noise level is the same, but distortion is much nicer, so I will leave it that way. I also removed R4 and put R9=33k as a second triode's grid stopper.

[rankot]

And now, some pictures - ready for assembling:



Final:



I named it after Tiago and myself, I hope properly: Thiagosaurus Ranx

[rankot]

Somehow, image links are missing from previous post, here are they again: