question on safe voltages on little chinese 12au7 valve booster?

[bobster]

hi guys ,
i wanted to ask to see if i can safely run a little valve booster on a higher voltage than what it is designed for
i got this from e bay  http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=290928704180
and it runs on 12v.
i tried it with 24volt today and it seems to still work ok but the valve gets much hotter than at 12v and burns brighter inside the envelope also.
there are only a few resistors and caps in this little circuit....no opamps or anything else i can see other than an led
im not sure what circuit it is based on ..........
any advice appreciated!
bob

[davent]

Well if the circuit expects to see 12v it's probably set up for 12v on the heaters. If you suplly it with 24v it's possible you have thatvoltage on the heaters and will quickly kill the tube. Without a schematic we can't tell how the heaters are set up, depending on how the tube is wired, the heaters expect 6.3v ±10% or 12.6v ±10%, 24v  = death.

dave

[Mike Burgundy]

Running *anything* on a higher voltage than it's designed for without any further knowledge of what's happening in there is a coin-toss at best. There's often no real benefit, and a very real risk of damage. Heaters are as said quite specific on what they want to be supplied with in order to work and not die, if there is a charge pump in there to up the anode voltage that would surely have died (or you got very lucky), electrolytics might have ratings that are exceeded which will lead to failure as well. Not a bad find at a buck-fitty, though. Over ten times the cost in postage 

[bobster]

hey ,
theres 7 resistors 3 electros and 3 film caps in this thing  , plus the two in out jacks , power in and an led + switch
3 pots , vol 100k , tone 100k and gain 50k , thats it

good point re the heaters and stressing them though

thanks
bob

[PRR]

Tube circuits have TWO power supplies.

The *heater* must stay near 12 Volts (whatever the tube is rated, 6V is common). If a 12V heater is run at 24V it will burn-up (like a light-bulb) in 1 or 2 hours.

The *plate* supply is more often 50V to 300V. 12V operation of plate is a stunt. It works, it sucks, "sucks" is often what we want for Distortion.

The 12AX7 plate can take 300V directly; in typical circuit the plate only gets part of the supply voltage and 450V supply is sometimes done.

But this booster has just one power supply connection. To split heater from plate supply you need to open it up and understand it, then cut stuff, and not screw-up.

Also a low-low-price booster is likely to have low-voltage capacitors and low-Watt resistors. So it needs real study.

And overall a high-volt tube amp tends to be CLEAN, near as clean as an opamp, which makes a much cheaper booster.

Keep it at 12V and enjoy the distressed sound.

[GibsonGM]

+1   Running  a tube on low voltage is also called "Starved plate" operation...it causes very much more non-linear action from the tube by putting it into that non-lin region of its transfer characteristics.  That's where the distortion lies!  For low voltage, single stage work, that is.  Otherwise, you need several (4+ often) stages and REAL high voltage to get back into where you're clipping stage outputs by the next stage...a different way of getting tube distortion found mainly in amps.   
   
Increasing the voltage will take you out of this 'unpredictable' (distortion-generating) region into linear operation, so you'll get more "amplification" and less "crunch".    Sometimes it's hard to wrap your head around the fact that a higher voltage won't get you 'more' of something, and maybe even less (cleaner)...there's an inherent desire to crank it up, ha ha!