Advice Needed - 2 tube McTube 2 build

[bungusbeefcake]

Hi guys,

Basically I'm building a McTube 2 design footswitch/pre-amp, but I'm thinking of augmenting it and sticking in an extra tube to give it some extra girth. Unlike the original I'm running it on 230V (due to irish voltages) but I'm looking for a bit of advice on the set up I'm going to do. I'm putting in an extra switch for the extra tube (both tubes are the 12AX7-A tubes) to select it on or off but I'm not sure whether this set up will work at all.

My gut is telling me that the 1st set up will generate hum when the 2nd tube isn't in use. What I'm looking to do is to have the ability to use 1 or 2 tubes and then a bypass. I've got 2 DPDT switches so was looking to keep it to the 2 of them and if anyone has any suggestions regarding layout or switch placement or even that the whole thing isn't viable then I'd really appreciate any feedback.

All the additions are circled in red.
For convenience the original schematic is at the bottom.

1st Layout



2nd Layout



Original Schematic

[PRR]

1st Layout has multiple problems.

I'm not sure what the "use" is for a box with total gain of 60,000 (!!) (96dB!!!!) and four gain knobs. Yes you can amplify cosmic background hiss to a billion Watts, but acoustic feedback is an issue, and who wants that much hiss? Yes if the four pots are Log you can set it for unity or small gain, with small and possibly un-musical distrotion residue. You can offset the pots to get one stage freaked-out, and with a lot of experimentation maybe two at the same time. There will be many different sounds in there, not so many "good" sounds.

Also a key secret in "musical" distortions is EQ before and after. This does not even have treble-trim caps.

Manageable multi-stage distorters are usually alternation of fixed-gain stages and fixed-loss attenuators with selected boost-cut caps.

[tubegeek]

I would try breadboarding this idea to see if it really gives you much that is of any use, PRR's comments are spot-on.

But be careful breadboarding at multi-hundreds of volts!

If it were me and I were looking at using 2 x 12AX7 (I hate those tubes so I wouldn't be) I'd look at Merlin Blencowe's excellent tube preamp book and try to implement some of his cathode-follower distortion ideas, there is a lot of very interesting stuff in there.

Another crazy/interesting idea would be to drive the crap out of a small transformer with the first part of the circuit and get some iron distortion instead of gain/clipping distortion, might sound weird/cool. Then again, might not.

Why doesn't anyone ever use octal tubes anymore? 6SL7 is a high gain tube that sounds about 50 times sweeter than any 12AX7 ever did, and don't get me started on 6SN7 - that's a tube that is almost impossible to get a bad sound out of.

I guess they and their sockets are a little chubby in size but other than that, they are very good tubes. And I am not in a position to criticize chubbiness.

6DJ8 anyone?

[zambo]

+1 to both of those replies. Check out your grid leak resistor/ac signal ground reference on those layouts. if that goes away when switched i believe you get runaway bias or something fatal like that. Not totally sure but i know its bad.

Have you built high voltage tube stuff before? Ive been messing with them a bit. Their are some cool preamp designs that do what you want but they are a bit more complex ( not to bad though ) . Your design has a lot of gain but not a lot of eq or high/low pass filters. will probably have some hum, squeal, oscillation issues with that many gain stages. whats your overall sonic objective if i may ask? High voltage at the front of the stage scares me. relay switching a preamp rack unit would be safer and easier to layout and control. If you just gotta build a stomp box have you looked at higher voltage valve casters or the gtfo builds? good stuff on those threads.

[PRR]

I would not "breadboard" this, not in the sense of nailing it up on a cutting board. This much gain probably needs a closed metal box, maybe with a partition halfway down, and probably a lot more treble-cut caps to deter ultrasonic oscillations.

> runaway bias or something fatal

Because of the large plate resistor, here a loss of grid reference won't be "fatal". (Is very different in Power amplifier stages.)

However there will be a large plate voltage shift, maybe 100V. And when you switch back, another shift. POP!! So it is generally good to keep your grids tied to some nominal voltage.

[zambo]

not to side track, but would that put 100 volts across your footswitch in that instance? That cant be a good thing. I should stalk all your posts. I learn a lot from them. thank you   

[bungusbeefcake]

Thanks for the advice guys, after reading the comments I'll probably leave the 2nd valve out! I think I'll also put in some tone controls, the schematic is below and I'll probably put them after the valve just before the output I think.

@tubegeek: Would love to get those tubes, however its quite difficult to get any tubes here in Ireland. Still feels like the middle-ages over here when getting some electronics stuff. What did you mean by getting some iron distortion. I'm using 2 trannies at the start, one step down and one as a step up (230-12V and 12-230V). Oh also, the main aim is to get a moderate amount of gain then over-drive/distortion out of the tube and hopefully a bit of sustain.

@prr: That 100V pop sounds like it would not be ideal, to say the least. Would you have any suggestions on how you would rectify it?

Thanks again guys for all your comments, I really do appreciate any advice!

Tone control schematic:

[zambo]

Usualy a 1M resistor from the grid pin to ground stops the 100v ( or whatever) pop. The iron distortion mentioned is referring to output transformer distortion. Two tubes can sound quite lovley by the way, you just have to make a design that really highlights the use of both tubes effectivley. THe gtfo thread is good, uses two tubes. I have made pedals like what your wanting with 3 tubes. No one is saying dont do it, just learn a bit and design smart so you dont hurt yourself or make something that you really cant use and wastes your hard earned cash.

[PRR]

I don't see a serious *switching* flaw in your 2nd Layout. It keeps grids referenced to ground.

I do think you should plagiarize all the HI-gain channels you can find. Peavey 5150 is a classic. I recall another with *six* stages. Both are riddled with loss and EQ circuits.

[bungusbeefcake]

@PRR: I had a look at those schematics for the peavey. Are you suggesting I change the basic input to one similar to the high gain input of that amp? I've changed the schematic to set the input as a "high gain" input and have added the tone controls after the 2nd tube. Let me know what you think, please?

2 Tubes, Hi Gain Input and 3 Tone controls added:


Sorry, image edited due to my oversight!

[bungusbeefcake]

I've edited the schematic again to use a 3PDT switch to give me true bypass when hit as there was a problem with the last setup that it wouldn't bypass if tube 2 was on. Could anyone please have a look at the switching and let me know if they can forsee any problems with anything,

Thanks

Schematic V4:

[PRR]

Switching:

V1A always gets signal so there will be a huge/nasty signal at SW3A even when switched to "clean"; this big signal will leak all over and contaminate the clean.

V2A grid gets un-grounded, and is liable to pop when switched in.

The "clean" switch setting has the source (possibly a naked guitar) trying to drive the tone stack. This is a heavy load and a major loss for raw guitar.

Advice Needed

and-

...I'll probably leave the 2nd valve out!

You ask for advice but are not absorbing it.

Sometimes the best education is Try It And See.

Not the best plan for Rocket Science or Brain Surgery (altho it *has* been done).

Go ahead and build it. Nobody will die. It won't work right the first time, but tubes on turret-board is *easy* to modify. "Risk" is a few hours of happy soldering and some bucks worth of parts. Up-side is a serious *practical* education in high-gain circuits and switching affairs.