Real McTube power supply - stretchable?

[GibsonGM]

I was wondering if anyone's played around with the McTube power supply?  I have 2,  2A,  120V-12V transformers that will do the job for the single tube in the thing just fine.  I'm thinking of using them to make a dedicated 'test box' to check out all sorts of preamp ideas.  The question is, can the PS deliver enough current to drive 2 12AX7's rather than just 1?  It seems it should, maybe with some output sag.  I'd like to add a 3rd gain section so's to be able to work in real tone stacks. 
Any experiences appreciated!   

[SG6505]

Hi, Gibson.

This is one of my favourite projects. I Built a real mctube years ago. Just after I finished it I started to think about building a whole preamp, just like you and it works very well. I made a fender topology preamp with two 12ax7's, tonestack and all and used the spare triode as an extra gainstage. I played around with it and modded the voicing of it and was very pleased. Just two weeks ago I finished another project with an old amp that was broken that was given to me. It has a 4 tube EL34 output stage and had a terrible transistor pre. I built a "dual rec" clone pre with a beefed up real mctube style power supply to it. I am very pleased with this project aswell.
My suggestion to you is to use only one of the two 2A 12V trannys to supply the heaters and get a smaller 200-300 mA tranny for B+ because the 12ax7 draw only about 3 mA per stage if I remember right. Save the other for your next project, I am sure you get hooked  .

Also, I live in Europe so I get 220Vx1,41=310V for B+ after the rerctifier wich is good to start with. I assume you live in the US (110V?) so you might consider a choke in your PSU so you don't get too much voltage drop. You are going to need at least two extra filter stages.

[Salvatore]

With 2A you can use 6 12ax7's, so you are well within the specs.
(If I remember well 1 12ax7 needs 300 mA)

I heard some nice things about the McTube.
There are a bunch of mods for it to.

[GibsonGM]

Thanks guys, good info!  Yeah, they will draw .3A each if in parallel for the heaters, and the plate current is negligible.    Seems a good base to start with for preamp stuff, even if it's low-voltage compared with a 'normal' B+.   I've done an 18W and enjoyed it, now it's time to tweeeeeeek.....

What luck, an old amp with a transistor pre and EL-34 output...what WERE they thinking?  LOL 

[SG6505]

Just for the record, 12ax7 draws 300mA in parallell/6.3V wiring and 150mA in series/12.6V wiring wich is the case here so you have some juice to spare .

Let us know how how you progress

Cheers

[GibsonGM]

I should at least have the supply together by the weekend...I'll hook up the heaters (or a load to simulate them) and measure B+.   Yes, I'm in the US, so I'm hoping for the 140V or so the schematic shows.  Having 240V at the wall socket must be nice!

On that note, to beef up the filtering on B+, would one just add another cap? I'm using a 47uF/470R/47uF at the moment...I have 1 more 47uF rated high enough - seems like I need to use another 470R resistor?  And, should I add a 4th filter stage, too, you think?  Might as well do this before I box it.

So, in running the heaters, I would tie them all in series (4 to +, 5 to + on both, with the 9's to ground?)

Thanks! 

[SG6505]

You add a resistor and a cap for each stage. I designed my power filter as the amps i "cloned". Same value caps and resistors as this affect frequency response. Too big caps can lead to motorboating, oscillation in bass frequency. Especially this applies to high gain circuits.

You connect the heaters + to pin4 and - to pin5 and no connection on pin9 with 12V supply.

[GibsonGM]

Cool, I'll just add another section per the schematic, then. If it needs more afterwards, a 4th section could be added later.

On the McTube, he's got the center tap (pin 9) grounded.  I thought that was more for a 6.3V supply, myself...I'll try it both ways and see what happens!  lol

I'm thinking about whether one of those diode & cap "voltage doublers" could work with such a small preamp, to bet B+ out of the 100's and up into the 200's...will try that over the weekend, too.  One thing I have many of is rectifier diodes....

Thanks again,

Mike

[GibsonGM]

Ok, got it together!  157V at B+ no load,  and 18V no load on the heater supply....a tad high!  I put a 50 ohm load on it, got my 300mA for 2 tube heaters, but the voltage was still at about 14V...B+ staying up in the 150's.  Probably will use a series resistor on the heater tap to lower it, especially if I only use 1 tube!  Need to bring that voltage down.
But with tweaking this thing has potential.   

[brett]

Hi
160V on the B+ sounds cool.  Are you aware that it will help with ripple rejection if you put an in-line resistor (say 1k), and caps (10uF electro and 0.01uF film) to ground between the B+ and the plate resistors of the 12AX7s.  You'll only lose a few volts (about 5), but gain several dB in noise rejection.

Also, for lowest noise, you should definately go for regulated heaters.  One option is to use an LM7812 for every 2 12AX7s.  If the input voltage (under load from the heaters) to the 7812  is way too high (eg 18V), use series diodes (1N4004 or 1N4007) to set the voltage down.  Each diode drops 0.7V and they can handle about 1A without melting or burning.  For your setup, the voltage under load might be 15 or 16V (if it is 18V without load), so a couple of diodes would be in order (remembering that the LM7812 needs at least 13.5 V of input).

just my 2c.

Happy pre-amp building.

[GibsonGM]

Hey brett,
Some good tips there.  I'll throw a circuit together and see how noisy it is...I added a 3rd filter stage after the rectifier, but the series 1K and caps is a good idea to pass to gnd any hash that might get through.

I used a few power diodes last night to drop the heaters to 6.14V; worked great!  I'd thought of the 7812 and it is really a good idea, but more than I want to do with this thing right now...It's enclosed in an old VTVM metal cabinet, already mounted the PCB, etc.  Voltages seem to have less than a few percent ripple, if I can believe my calculations.

A big problem I was having was the voltage soaring if you removed one of the tubes from the heater circuit (which I might do when using this rig)...I will switch in a 10R/10W resistor if only using a single tube - worked fine when tested.   I'm now running them the "right way", pins 4+5 to "+", and center taps to "-".   

Does anyone see any value in trying out a diode & cap voltage doubling circuit for something like this?  Or would that be pushing it?  Not really sure how that would react to the current draw from a 3 or 4 stage preamp circuit, although I'd expect the draw to be pretty low (the heaters being the big one, and B+ is pretty stable with heaters on).  It would be nice to crank 'er up above the 160 I am now getting!

Thanks again,

Mike

[petemoore]

  I've been going through all this stuff this week too.
  7812, 14vac converted to DC, filtered and 7812'd, that works as good as the battery supply IME.
  160V on the B+ sounds cool.  Are you aware that it will help with ripple rejection if you put an in-line resistor (say 1k), and caps (10uF electro and 0.01uF film) to ground between the B+ and the plate resistors of the 12AX7s.  You'll only lose a few volts (about 5), but gain several dB in noise rejection.
  That's a 1k/10uf to ground from the plate ['new thing here']...worth a try ! ! !
  thanks for helping out here Brett !
  I tried elevating the heater supply with a resistive divider across the HV DC, thanks Gus for mentioning that one..
 

[biggy boy]

How has the project been coming along?
I've started working on making one. Have the board layed out.

will be etching it tonight.
Glen