Overdrive based on low voltage & high Mue Tube

[gto]

Hi guys,
I'm new in the tube components and need some help in choosing a tube for building an overdrive,
I'll need a tube with high amplification factor & low operating voltage

any ideas??   

Thanx

[rutabaga bob]

check out renegadrian's 'tube star'...a 12at7 running on 50v +/- for the plates.

[brett]

Hi
I'm no expert, but I think that low voltage and high amplification are difficult to squeeze out of tubes.
The volts provide the "pulling power" for electrons from the cathode.  Without lots of volts, transconductance will be low.  I think this means that the output impedance will have to be very high high (approx 1 Mohm).
High output impedance can be got with high value plate resistors and following with JFET stages, but this further reduces the plate voltage, so you are in a downward spiral. 

If you mean 50V or more and mu of only 30 (12AT7, 12AU7), then these problems don't apply as much.  You can always use two stages of 30 for gain of 900.
cheers

[zambo]

Hi Brett,
There is a mega thread on the valvecaster called tube boost running at 9volts. Very educational for new tube guys like us. 12au7 fires up on 9volts every time. Everything else is luck of the draw. If you are truly new to this then I would start with 9 volts but build my circuite with parts rated for 100volts or soso that you can modify it later.(your on here so you will modify it later i am guessing  ).There are a number of designs hidden in that thread. While I havent built the tubestar I do know renegadrians stuff always gets great reviews and his layout skills for vero are a thing of beauty. If you want to go higher voltage I have a few designs based on 9 and on up to 45. They all use at least two tubes though. I am more than happy to share if you want. Cheers, G

[Dragondreams]

My latest build was based on the late Fred Nachbaur's "Real McTube2". It uses cheap components and operates a 12AX7 at around 150 volts. The power supply comes from a pair of dismantled wall-warts. The sound is incredible.

I have a couple of low voltage overdrives running in starved plate mode. And they sound just like transistor overdrives. There just isn't enough juice to get the valve cooking properly.

As long as you're careful and patient, it's a straightforward build. I found the longest part of the job was cutting out the metalwork to house the parts.

Might be worth checking out: http://www.dogstar.dantimax.dk/tubestuf/mctube.htm

Paul.