tube question

[digthisbigcrux]

is there an article or some information i can obtain somewhere about running tubes at lower voltages, and when it can/should be done?  ive been toying with an idea revolving around adapting a distortion circuit from an amp into a pedal and didnt know if i would be better off adapting it to run at lower voltages.  ive got plenty of experience working with high voltages so its not a question of being uneasy (though you should always be a little uneasy).

the tubes it uses are: 1/2 6jw8, and 1/2 12ax7. 

[Johan]

the PAIA website has a good article explaining starved tube circuits, what happens and so on.
J

[tubelectron]

Hi,

I had a very interesting article about using tubes in amplification at low plate voltages, but the hell I can't find it anymore, and the hell again I never tried anything in lo-voltage tube overdrive pedal. The topology of the circuit is nonetheless way different from a tube overdrive on HV : it seems that to obtain good results, an IC preampfification is required, etc... And many folks on this forum have made trials, with mixed results, AFAIR.

I stayed on HV for mine : The upper one is probably one of the first HV tube overdrive commercially available (1978-1982), the lower ones are my HV DIY models, powered by an external 12VAC adaptor.



A+!

[digthisbigcrux]

veddy nice!

i was strongly considering staying hv and just running a power chord out of the pedal and putting a transformer inside with a rectifier of course and filtering.

[tubelectron]

i was strongly considering staying hv and just running a power chord out of the pedal and putting a transformer inside with a rectifier of course and filtering.

It's perfectly doable. The main issue is : how much do you want it to be compact ? On very compact units (like the one I dispayed before), there are 2 critical points :

1 - finding a compact Xfo which has the required voltages. I know that it exists under differents brands and/or suppliers, more or less compact - I do not remember instantly the details.

2 - avoiding the hum and hum radiation of the transformer. This is more difficult, especially if you consider AC mains powered transformer : proximity of the audio circuit with the transformer induces 50-60Hz hum and their harmonics, not easy to get rid even with shieldings. It's by the way the issue which affects somewhat the Westbury W-20.

I turned out the problem with the external 12VAC adaptor, feeding an internal very compact and common 3VA / 12VAC to 230VAC (I am in France) which gives the HV. See the schematic below :



This is one of the hum-quiet solution for HV tube pedals...

A+!