160v tube PSU

[HeavyFog]

Hello everyone, looking for some advice for a project iv'e been working on. I'm working on a single 12ax7 preamp that needs 12v for the indicator led and heaters, and 160v for the plates and i'm stuck with figuring out how il do this. I have seen 160v power transformers with 6.3v winding for tubes but i'm not sure i'm ready to work with 120vAC wall power as i have no experience with such. That said i'm not ruling it out as an option and i do have a few 6.3v fender pilot lights lying around i could use instead of leds.

What i was thinking of was something along the line of RG's tube wah power supply
http://www.geofex.com/Article_Folders/tube-wah/tube-wah.htm
This would be best because i could have 12vAC in from an adapter and rectify the 12v line for the leds and heaters and do the same for the 160v. Only problem is i'm not sure how to pick out a suitable transformer and how to set it all up properly. I think a 1:13 transformer is what i need to get around 156v from a 12v source. I could maybe use a real mc tube power supply for 140v but it would be best to get 160v for authenticity, something il elaborate on later.

If none of that works perhaps some sort of charge pump might work or getting a voltage higher than 160v and dropping it with a resistor? Any ideas?

[GibsonGM]

1) get 2 12V transformers and connect them back to back like the McTube.  Authentic enough.  The 12v. "in the middle" runs your heaters.

2) make a nixie supply.  Used in the "GTFO" tube preamp (search here, net for Nixie Supply).  Use a 16V DC wall wart to power it; regulate that to 12V for heaters.   This will run more than 1 tube, by the way....and the higher voltage (tho scalable back to 160) is more 'authentic' for what a 12AX wants.  I like this approach and use it...simply, compact...

3) get an actual tube transformer, 12V secondary plus a 160 one.   Then rectify/filter.

4) build R.G.s supply. 

[thermionix]

Use the transformer and wall juice.  It's not difficult to do safely, just pull your hands out before you plug it in.  Seems like you could run an LED indicator off of the (rectified) 6.3v, but a #47 bulb is fine too.  An added bonus is that the heaters on a 12AX7 are humbucking when run parallel at 6.3v.

[thermionix]

Also, what is that transformer rated for?  A 160VAC secondary does not equal 160VDC on the plates.

[davent]

This article details how to do what you want using a wall wart (12.6/6.3VAC) and a backwards wired 120/12v (or 6.3v) transformer. Details on sizing the two for the current demands of your circuit.

http://www.audioxpress.com/assets/upload/files/stamler2874.pdf

dave

[HeavyFog]

I kind of feel silly now. If i were to use a 12-120v transformer (1:10 ratio) but with a 16vAC power supply instead of 12v in theory i could get 160v if i change the cap and resistor values?

I should also add that when i said i wanted the 160v for authenticity its because the project is based on the tube preamp from the binson echorec from an echo machine and i'm trying to get it close to the original.

[davent]

Rectifying the 120vac with a bridge will give you ~1.4 times more or around 170vdc, probably a bit higher as it appears most people report their wall voltages as higher then 120vac.

dave

[GibsonGM]

I don't think you'll be able to hear any difference if running a few 10s of volts + or -  ....

[HeavyFog]

Looks like the real mctube 2 power supply puts out around 150vDC. I think you're right, i probably wont notice a 10v difference. Looks  like it might just be the right choice for this project! Now i just have to track down a suitable transformer and draw up a schematic! thanks everyone for the help s far! Once i get a prototype built il report back with my results!

[PRR]

> If i were to use a 12-120v transformer (1:10 ratio) but with a 16vAC power supply...

...you blow-up the transformer.

The rating of a transformer is essentially the MAXimum voltage it can safely take. There's a 10%-15% margin for line variation. Your proposed 33% over-voltage is quite high. The iron saturates, loses inductance, starts taking large excess current, runs hot, melts or burns.

I have seen small over-voltage "work" on very small transformers because they are so lossy they "can't" suck a lot of excess current. Still not a good idea.

And as Dave says, the peak-catching rectifier tends toward 120VAC*1.414= 169V of DC. Which is probably why 160V was suggested. No, tubes don't much care 140V or 180V.

[HeavyFog]

Alright here's what I drew up. Its pretty much just the psu from the real mc tube 2. Planning on powering it with a 12vAC 500ma wall wart. Just thought I'd run it by here just incase I've made some glaring mistake.

[thermionix]

-nvm-

[amptramp]

I have the schematic beside me for my Trio KW-55 stereo receiver which uses a number of 12AX7 stages.  The phono preamp runs from a 135 V supply and the plate voltage on the first stage is 65 and the second stage is 85.  The plates on the FM multiplex amplifier is 97 V.  The tone control driver runs from a plate supply of 196 volts and the plate is at 105V.  The driver for the phase splitter runs from a 263 V supply and the plate voltage is 95 V.

Triodes are not that critical for supply or plate voltage.  A lot of 5-tube AC/DC radios used a 12AV6 which is a double-diode triode where the triode has identical characteristics to each triode in a 12AX7 and they run from a 130 V supply and the plate voltage is around 82V.  In this circuit, people use plate resistors from 220K to 470K with little difference in performance.  These are all linear amplifier stages.  I wouldn't obsess over getting a specific voltage.

[bluebunny]

here's what I drew up . . . some glaring mistake.

I think that LED will burn your retina.      Consider upping that 470R by an order of magnitude.  Or wear these: 

[thermionix]

glaring

ba-dum-pshh!

[HeavyFog]

Hahahaha!
Well played!
I don't think the brightness will be an issue. I actually plan on mounting a single green led under tube socket to give off a similar vibe to the magic eye used in the echorec, so having it very bright will be a bonus. Plus im planning on having it set up as preamp on top of my amp with a footswitch on/off with its own separate led. Might be a while before I get the chance to build the preamp but when I do il definetly report back on the results. This one looks promising!

[bluebunny]

Seriously, you might well burn the LED.  That's a very low value for a rectified 12V AC power source.

[davent]

If you rectify the 12v for the heaters you going to have ~16v on the heaters and a very unhappy tube, it's wanting to see 12.6 +/- 10%.

Why not run them using VAC as the vast majority of tubes in guitar gear are.

I've some super bright green led's that needed 47K current limiting resistors to tame them on a 9v source.

dave

[HeavyFog]

I think il change the 10ohm resistor back to 12ohm 2w (same as the tonepad project). As for the led resistor il mess with it to get the brightness right without killing it

[thermionix]

I still think this would be way simpler if you just use the one transformer you mentioned originally with the 6.3 and "160v" secondaries.