Simple tube boost/OD idea:

[Pott]

Still just a little sparkle in my head but I noticed that the torroidal transformer in my little Madamp G2 amp is fairly small. The primary is 230V (for Europe) and secondaries 6.3 (heater) and 185V (plate).

Now 185V is a little above average for a 12AX7, so it'll run with a lot of headroom. Right there would have a footswitchable or switchable resistor to change this.

We could have a drive pot after the first triode, and a level after the second. Though the drive would influence the level a lot so both would be highly interactive.

Switchable bias resistors for the triodes.

 

It'd have to remain simple but versatile. Mostly an extra gain stage, a real boost from a genuine tube. But above all, a fun and educative project mostly for me.

 

Now the questions... Does the classic 68K resistor still need to be there at the input? These frequencies would normally be taken care of by the same resistor in the amp, but since they'd be amplified by the pedal, getting rid of them earlier may improve the sound quality?

Also, assuming a very straightforward unit with a minimum of parts, would there be a huge level boost between input and output, or would it be properly controllable via a correct 'level' pot?

[olcircuits]

Pott,

 

The 68K resistor at the grid of the first stage is there to prevent (or at least minimize) radio interference. That 68K should be soldered as closely to the grid as possible. If you're using off-board wiring to a socket, solder it to the grid pin's socket lug.

 

I have been working on a similar project for several months (however, mine is a starved-plate design). I have 22dB of clean volume boost available (with an ECC81) and a volume control is neccessary (in my opinion). However, I had 23.5dB of boost before I added the volume pot.

 

Feel free to email me if you want. I'd be happy to help you in any way I can.

 

[Pott]

Woa man that looks cool as hell, and pretty much EXACTLY as I pictured mine hehe...

Thanks a lot for the help! I want to design this one all by my sweet self though. Trial and error, add or take aways things from here and there... well if I don't kill myself first.

What made you chose a ECC81 if I may ask? Less gain, simply enough? I thought of the 12AX7 because it could be used as an OD, or by decreasing drive or increasing plate voltage as a clean boost, and is readily available. Would there be a huge difference in the circuits between the two?

 

And thanks for the tip about the radio interference, I didn't know that!

Oh and what is starved plate design..? Less voltage, to use with a battery if needed?

[olcircuits]

Pott,

 

Thank you for your kind words.

 

ECC81 vs. 12AX7: My pedal will accept an ECC83, but the tone darkens quite a bit due to Miller capacitance. It also gets a little crunchy and I'm going for pure clean boost. There's a lot of work in the tone stack to get +8dB treble (for a glassy-like tone), and the ECC83 was killing it.

 

I went with a starved-plate design because I don't want people injuring themselves by opening it up and poking around inside. By placing the bias points inside - effectively inviting people to poke around in there - I felt it was better to play it safe.

[preservation]

we gotta find someone to make an easy kit to put that old Acoustic bass head's fuzz into a stompbox a la DFA1979.

[Pott]

hey Mark! Thanks again for more help.

 

How do you get the tube to run on 9V though? Do you use a battery and 9V power supply, then a transfo with a primary of 9V and secondaries of 6.3 and whichever V you chose for your tube?