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Why are most tube Overdrive pedals run on low voltage?


 
3/20/2005 1:20 PM
Humbucker
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Why are most tube Overdrive pedals run on low voltage?
I just picked up a used Tube Works, Real Tube Overdrive pedal and I must say that I really like the sound, very crunchy vintage tube tone IMHO.  
 
When I first looked at it, I noticed that it's got a 120volt power cord rather than the more common 9-15volt jack. So I wrongly assumed that the tube was running on a high voltage rather than the "starved plate" scheme found on most tube Overdrive pedals but when I opened it up I saw that there's actually a 120/12volt stepdown transformer in there and the tube is powered with 12 volts AC. That kind of bummed me out even though the pedal sounds great, so I guess I shouldn't worry about it..  
 
I have a Tonebone Classic and it runs on 15volts, and a Mesa Boogie V-Twin pedal that I had before that was the same deal. Why is it that so many companies design their tube pedals with a low voltage? Is it cheaper to build or could it be a safety issue?
 
3/20/2005 3:39 PM
JHS
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Because they are cheap to build and the tubes last nearly forever when driven on low voltage.  
 
Most "tube" pedals are built with I/O buffer and a lot of IC stuff around and sound like real solid state pedals even with the tube inside.  
 
The tube could be replaced by another IC stage but it seems in terms of marketing and tubehype that pedals with an integrated tube sell better.  
 
If you want real tubesound and dynamics build a real tubepedal with 200-300V on the anodes.  
 
JHS
 
3/21/2005 1:34 PM
anonymous
Some of these designs actually have two xfmrs in the circuit.  
One steps down the voltage for support to IC based tone circuits and filaments etc.  
A second xfmr steps the low voltage back up to power the plates. It may even include a doubler.
 
3/21/2005 5:22 PM
KB
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Doublers are indeed used in these circuits if necesary but some SS boxes do run their plates at high voltage but not many. The starved tube plate is the most common and least expensive. An op-amp like the 4558 if cascaded can have gains in the 70's and greater. The tube after is used as a Cathode follower as off the plate would yield extreme noise problems. Cathode followers have less than unity gain and sometimes buffers are used after them to boost the gain back up a hair before the output. The tubes do have a smoothing effect on the overall sound but as I stated in the other post not close to getting even order harmonics.,  
 
KB
 
3/21/2005 7:27 PM
coledavis
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Re: Why are most tube Overdrive pedals run on low voltag
I had a Guyatone tube OD that was beat to hell (and sounded like garbage) so for kicks, I re-wired it, removing the tube from the circuit, replacing it with a couple of red LEDs to ground. Guess what, it sounded 10x better that way!!!!
 
3/22/2005 11:13 AM
KB
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quote:
"I re-wired it, removing the tube from the circuit, replacing it with a couple of red LEDs to ground. Guess what, it sounded 10x better that way!!!!"
 
 
I did the same thing in the Rockmaster I modded. LED's have a much smoother clipping and maybe the smoothest than say a 914 silicon or even zeners. Germaniums are smooth but have serius temperature conflicts and hey I live in the South! A 12AX7 wired as a diode with compliance and reference voltages can really round the signal nicely and zeners with a tail resistor can have somewhat the same effects. Of couse it also depends where in the chain you do it and a low impedance source driving it seems to work best and right before the tone stack or eq. Whatever you do to round the wave off instead of a square is a step in the right direction as far as preamp distortion goes if your looking for smoothness. What happens after that is a total different ballgame and it starts (in tube power amps) with the peak inverter, it's asymetric imbalance and the whole output section,tubes and output transformers as a reflection of the different topologies used to incorporate it.,  
 
KB
 
3/22/2005 3:07 PM
Humbucker
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I just changed the board-mounted footswitch on my pedal.
I've been having problems with the stompswitch on my Tonebone Classic, at first it would only cut out once in awhile but it just kept getting worse. I informed Radial about this and they were good enough to send me a new 2pdt footswitch which I replaced myself.  
 
Well, it was a lot harder job than I thought it would be! I ended up having to cut the old switch in pieces with a pair of side cutters while it was still mounted to the board, then I had a chance to pull each leg off. I never did anything like this before, so it was a learning experience. I hope that this latest switch lasts longer than the three months that the first one did. But, on the plus side, I repaired the pedal without any damage and it works like new again. 8^)  
 
BTW, while I had the Tonebone opened up I tried swapping the 12AX7 tube with some other brands that I had laying around the house and I couldn't hear any significant difference between using one tube or the other. And while I was at it, I tried swapping the original 12AX7 tube in my (Tube Works) Real Tube overdrive pedal and again the same result or lack of result.  
 
My Real Tube Overdrive pedal has the original tube in it(1993) and it still sounds ok, so I'm guessing that these types of pedals can almost go on forever with the same tube. They recommend changing the tube every 2-3 years but I can't see it myself, since a new tube didn't make the pedal sound any better than the 12 year old tube! They run at such a low voltage and barely even get warm.
 
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