Uber Tuber question

[zgrav]

I built a working Uber Tuber from the original version when it was released.  It has a warm clean sound with very little distortion unless the incoming signal is high.    I noticed today there had been a 1.1 revision to the build doc not long after it was released, and saw that two trimmers were put in to replace the 100K resistors for the plate voltage.  The build doc suggests setting the trimmers to bias at about 12 volts, but there are comments that most of the tubes work OK over a bit wider range.

I checked the plate voltage on my board and they are both running over 17 volts using the 18 volt Dunlop power supply.   Would changing the resistors to drop the voltage to 12 change the sound from the tube to possibly add more use to the drive/cut/sat controls?  If not, is there a reason not to leave the board as-is with the higher plate voltage?

[madbean]

Yes I suggest trying higher values. What I found after putting in the trimmers and testing a bunch of tubes is that there is a pretty wide operational range on the plates so 12v is just a guideline. Try 220k, 330k or similar. You should be getting a decent amount of dirt like in the demo video.

[zgrav]

thanks.

[zgrav]

I cut out the 100K resistors and replaced each one with a 1M trimpot.  By maxing out the resistance I could get the voltage down to between 13 and 14 volts for each test point.   I was surprised it would not drop the voltage more than that, but that still made a big difference in the sound.  With those lower voltages the distortion in the tone and adjustment with the drive pot is great.  Before it was a pretty clean signal that could be overdriven with a boosted input, and now there is some nice dial-in distortion without upping the level of the input. 

[madbean]

You should be fine up to 14-15v. I'd just measure the trimpot after adjusting and put in the closest value resistor. Anyway glad you got it sorted.

[zgrav]

thanks again, and for the quick replies today.

[madbean]

thanks again, and for the quick replies today.

Sure thing! As an aside, last week I was testing out a 6021 @ 110v. They can deliver some serious output when you give them a lot of juice. And they sound great.