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Re: Got my Tube driver going, got a couple questions


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2/1/2001 3:58 PM
Doug H
Re: Got my Tube driver going, got a couple questions
I think you will need higher grid-to-ground resistors for higher B+. Also be careful of those 68K plate resistors. With a high enough plate voltage I suspect they could cause problems if they are not increased. But I'm not sure, maybe an amp expert can give some advice here.  
 
 
 
In any case, this thing is really tuned to run at the low voltage. Try sticking a 470K grid-to-gnd in there and you'll see what I mean. For a moderately higher B+, say around 60-70V check out the PAIA stack-in-a-box circuit for ideas. Much higher than that and you might as well start thinking about implementing the preamp section from a guitar amp. I think it's Allen Amps that sells a transformer made for building HV tube pedals (Aron pointed me to that.).  
 
 
 
Doug
 
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2/3/2001 1:49 AM
KB

Most of these tube starving devices use a Cathode follower after the op-amps which means that using a higher voltage is not going to give you anything but more noise as unity gain is all you get from the tube. It's output is low impedance so it drives the input of the tube amp very well. However, if you rewire it and take the signal off the plate using a 100K plate resitor and a coupling cap to the output jack then change the probable 10K cathode resistor to 820 ohms and bypass it with whatever cap you want as the rolloff frequency then the voltage will give you something. IMHO I don't like the way op-amps go with tubes and some JCM model Marshalls use back to back zener diodes as clippers to square the wave a little then Cathode folowers to drive the high impedance inputs of the next gain stage. The sweet sustain and smooth distortion comes from cascading several gain stages and some Audioheads decouple the power nodes in between for good clean plate voltage. Dc standoffs for the heaters will reduce the hum reduction by several dB and eliminate noise. All of this in a little box ain't gonna happen. Either build a mean ass tube distortion box or stick to the stock pedal with Dougs mods as I would be willing to bet if you bypass the tube you will like it even more.
 
2/4/2001 5:45 PM
Doug H

The BKButler circuit uses a super-high gain opamp stage followed by 2 lowZ low gain tube stages, no followers. The tube cathodes are tied directly to "ground" (-15V, actually). The "warmth" comes from the low input Z of the tube stages, I'm convinced. The opamp stage has to be really high gain to drive the tubes at all. At these low voltages, that seems to be an effective way of doing it. Adding more gain or higher input Z to the tube stages causes severe clipping and fuzztone, not "tubey" at all. That's why they use opamps for this kind of stuff I think. They give the gain needed to drive the tubes, without the tubes being gainy themselves and the headroom exhaustion that goes with such a low voltage.  
 
 
 
Just my theory, I could be completely wrong...  
 
 
 
Doug
 

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