Questions about BK Butler Tube Driver

[jaytee]

I was thinking of building one of these. I was looking at how the tubes were biased and wondering how it works. I'm guessing the supply for the tubes will be about 30volts? The cathodes are connected to -ve and grids to -ve via the 10k resistors. So where does the bias come from? Grid leak? With only 10k? Does it not need bias because the voltage is so low? There's a mod that BK Butler does for variable bias, does anyone know about that? Thanks in advance

[ranchak]

You can build the Real McTube from the Tonepad site or Paul M. has a similiar design (Aron's Shaka Tube) posted at:

http://www.diyguitarist.com/PDF_Files/ST1590BB-PCB_9-9-06.pdf

The link to the article showing the build is:

http://www.diyguitarist.com/DIYStompboxes/ST1590BB.htm

Paul's design does have a bias adjustment. I built the Real McTube, it sounded great, but it did have a hum from the transformers. I'm going to try another build of it with 12VAC input power to eliminate one of the transformers. I recently built the Tube Driver from the GGG site, but have a problem with it that I can't find.

[sfx1999]

I've heard the Real McTube isn't all it's cracked up to be.

I believe the tubes would be run at a lower voltage than 30. It appears to be a grid leak circuit, but you might be on to something. On the Fender schematics I was looking at, the resistor was more like 5M.

If you are wondering how it sounds, Eric Johnson uses it.

[jaytee]

I believe the tubes would be run at a lower voltage than 30. It appears to be a grid leak circuit, but you might be on to something. On the Fender schematics I was looking at, the resistor was more like 5M.

If you are wondering how it sounds, Eric Johnson uses it.+

I found another thread where the measured voltage was +13v. -10v. So it's running on 23v. I heard a grid leak resistor has to be big, like the 5M you mentioned or the charge won't build up. The shakatube schematic I have has 1k cathode resistors so it's not the same thing although very similar. Looking at the Real McTube it only has 2 stages, no op amp to help the gain and no tone control, maybe it could be played with a bit.
I just had a look at a datasheet for a 12ax7. It looks like at 23 volts and 0v bias it would pass about 750uA so maybe thats why it works like that although the curve looks a bit odd. At 150 volts it would pass over 3mA.
Maybe a dual gang 1k pot as cathode resistors would give a bias tweaker? The bias adjust on the shakatube looks like it would give mostly positive bias. Is that useful? Can you run a tube with positive bias? There's nothing on the datasheet for that.

[ranchak]

I built a Real McTube and it's sounds great through my JMP Marshall. One thing I have noticed is amps have their own personalities and what is good for one amp will suck with another. A prime example is the tube screamer. I built one and tried it with an old Randall tube amp that I have, it was terrible. I recently bought a Standard Marshall combo, it's a bit dark sounding so I built another tube screamer and it sounds great with it. I have tried some of the more sophisticated distortion circuits and they (in my opinion) do not work well with my Marshalls. All I am looking for is a booster to mild overdrive, so a number of the pedals do not fit my criteria. Again this is just an opinion and is not meant to imply that a certain pedal is not worth building. If you are thinking about building the Real McTube go for it, you will never know until you try. What type of amp, guitar and music do you play?

[sfx1999]

What are the specs on your power transformer? A 12AX7 heater draws 150 mA at 12.6V.

[ranchak]

I used two Triad transformers that I got from Jameco, part#104387 (if I remember correctly), Triad part# FD612. I'm thinking of using only one of the transformers on my next build. I will supply the 12VAC from an outside power source and step up to 120VAC inside the pedal. I tried using 240VAC, but the pedal sounded very harsh, I was hoping to get more headroom and have a "cleaner" boost. I didn't mess with it to see were the fault was, I figured that Fred didn't design the pedal to use high power so I would re-build it with only one transformer. It was very quiet using a EH Toriodal tranny that I got from Aron. The pedal has a fair amount of hum using the two tranny setup. One thing I did learn is that it's not to important as to where the power supply is located in relation to the tube and pots. The biggest difference is using good quality sheilded wire for your input/outputs, both from your input/output jacks to your DPDT switch and also to the PCB board. Sheilded jacks like the Marshall style help to reduce noise as well. It is best to keep input/output jacks and switching at one end of your enclosure and the power supply, on/off switch and fuse at the other end.