Bias adjustment sweep on Smallbox

  • Thread starter Thread starter napalmdeath
  • Start date Start date
napalmdeath

napalmdeath

Well-known member
I want to put some Tungsol EL34B's in my Smallbox, as I bought it used and previous owner said the tubes were a couple years old. Question is, in my experience with the Runt 50, the tubes were always rated in the high 30's, and not a lot of range to get cooler tubes up to snuff. These Tungsols are rated 26. I'm concerned they may be too cold.. I always had to order tubes 34-39 to get them in range.
 
Not sure what the question is. You'll have to put them in and see where the bias lands. If you can't get a tube hot enough with the bias trimmer, the bias resistor value will have to be reduced.
 
RedPlated said:
Not sure what the question is. You'll have to put them in and see where the bias lands. If you can't get a tube hot enough with the bias trimmer, the bias resistor value will have to be reduced.
Click to expand...
Yeah, I couldn't get them to bias up. Maxed out, I only got it to 26mA. I put in some hotter JJ 6CA7's, and it sounds fantastic! Got them to 32mA, which is still cold, but in range. PV was 469.
 
napalmdeath said:
Yeah, I couldn't get them to bias up. Maxed out, I only got it to 26mA. I put in some hotter JJ 6CA7's, and it sounds fantastic! Got them to 32mA, which is still cold, but in range. PV was 469.
Click to expand...
Yeah that's always a bummer. I really wish the bias trimmers had more range.

You could pretty easily parallel another resistor over the current bias resistor to increase the range a bit.
 
Yes just add a resistor across the series resistor feeding the bias pot
 
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