Power supply for from-scratch preamp

[chipaudette]

Hi All,

I'm thinking about building a from-scratch guitar tube pre-amp to explore high-gain ideas.

I've done a lot of fiddling/modding of my existing tube amp including cascading gain stages as well as tweaking out interstage elements and cathode elements. That's been a lot of fun. But, the amp that I'm working with just doesn't have room to add more stages or to add more switches. I'd also like to experiment with different elements/topologies than is possible with my current amp. So, I'm thinking about building a stand-alone box with plenty of room to do these things. It'll be my sandbox.

My major unknown is...what do I need for the power supply for this stand-alone pre-amp?

What kind of power transformer would you recommend? I've never spec'd a PT before. Clearly I need a high-voltage tap (somewhere 300-400V) for the plate voltage and I also need a heater tap (6 or 12 V). Assuming that I'll want up to, say, 8 12AX7 (or whatever), what current rating do I need for these two taps? If I want to include relays or op-amps/transistors in my system, will these run off the voltage of my heater tap, or do I need a third tap on the transformer to provide another voltage for these auxiliary devices?

How about the rectifier? Would a solid state rectifier (diode bridge or whatever) be easiest? Do I need to look for a certain current rating here? How about a rectifier to provide DC for my relays/op-amps/transistors? Or, for these auxilliary elements, should I just use an off-the-shelf 9V (or whatever) AC/DC power supply.

Thanks for any advice that you have!

Chip

[Steve Conner]

The classic power supply for a home-made tube preamp is two wall-wart transformers back to back. You step the voltage down and back up again. The connection between the two transformers also supplies your heaters. Depending on the transformers you choose, you can get 6V or 12V AC or DC. 12AX7s will take either. If you choose 12V DC heaters, you can run your relays and things off that supply too.

In 240V land, you use a bridge rectifier on the output of the second transformer to get around 300V DC: in 120V countries you use a doubler, or get a 240V transformer for the second one.

A PT from an old tube radio or record player might be suitable too.

[Bruce / Mission Amps]

Hi All,


What kind of power transformer would you recommend? I've never spec'd a PT before. Clearly I need a high-voltage tap (somewhere 300-400V) for the plate voltage and I also need a heater tap (6 or 12 V). Assuming that I'll want up to, say, 8 12AX7 (or whatever), what current rating do I need for these two taps?

Thanks for any advice that you have!

Chip

You certainly do not need a 400vdc power supply for this.
You can use any PT with around 320vct to 400vct... which will give you about 225vdc to 290vdc using a FW solid state rectifier.
Don't bother with a tube rectifier for an "all preamp tube" circuit.
Any PT like this that can muster 30ma-40ma at high voltage will be sufficient for this crazy project.
The 6.3vac winding will need to be around 3 amps for 8 preamp tubes, which by the way, will be absolutely ridiculous.
Now, I don't know exactly what you have in mind there, and you might be thinking four different channels, of which two 12AX7s are used in each but, I can tell by your questions that you are a bit of a novice.
IMHO, if your are cascading the triodes, all you end up with it a preamp circuit that will be nearly uncontrollable, toneless and massively distorted in a very non musical way.
For a single channel preamp project, stick with something like two or maybe three 12AX7s (one or two being cathode followers) in series, with less gain per stage and lots of bandwidth but attenuated... even that will be hard to control.

[chipaudette]

Bruce,

Thanks for the recommendations.

No, I was not planning on cascading all 8 tubes in series. 16 gain stages is completely rediculous. I tossed out the number 8 as an upper limit to make sure I got a sufficiently sized transformer for any possible crazy thing that I think up.

Most likely, I'll start conservative. I'll probably start with a Fender blackface pre-amp using 2 stages (one tube). This is like the amp I have now, which means I have a good example to follow (and probe if my build doesn't work). Once that works, I can do what the Boogie Mark II did by following the blackface pre-amp with another two stages of overdrive (another tube). After debugging that build, I should be experienced enough to get more bold.

Then, because the topology is so completely different, I'll probably want to build an independent 2nd channel to attempt to recreate the DualRec/Soldano/Bogner overdrive (4 stages + CF, 3 tubes). I recognize that that's a lot of gain with a lot of opportunities for oscillation and microphonics and who knows what else. But, I'm here to learn and have fun.

So, altogether, my two channels have used 5 of my 8 tubes (BTW, the Dual Rec uses over 400V on the plates of its pre-amp tubes...that was my motivation for asking about PT that might go this high).

Then, if I can get that stuff to work, I can let the real fun and wacky experimentation begin. I might be interested in trying out some parallel gain elements or some cascode elements. I know that they are rarely (if ever) used in production amps...but I'm curious to hear what it sounds like. Maybe then I'll try some phase inversion shenanigans. Maybe I'll go for 3 channels. Who knows? I figured that 8 tubes was in that category of you'll-never-possibly-want-more and, therefore, was a safe upper limit.

How about you all out there...any wacky pre-amp configurations that I might have fun experimenting with? Did any actually end up sounding any good?

Chip

[chipaudette]

Steve,

Thanks for your comments. I don't think that I understand how to get the 300V using the wall-warts.

I could understand using a wall-wart to get 9V (or whatever the converter is rated for). I could see wiring two in series to get 18V. And, from your next paragraph, I understand what a diode bridge is and does. But, I don't understand your words on how to go from two wall-warts all the way up to 300+V.

I searched the web a bit on the use of wall-warts for tubes. This article on the "Real McTube II" had a nice schematic and discussion (see link below). But, the schematic shows that they only get up to 140 V. What is that you are recommending for me to get another factor of 2+ out of this configuration?

http://www.dogstar.dantimax.dk/tubestuf/mchowit.htm


I also found this other link (below) where a guy powered his tube Hi-Fi pre-amp off a laptop power supply and a DC-DC converter (a handwound transformer?). I don't think I'm really interested in winding my own transformer.

http://diyaudioprojects.com/Tubes/Beauty/index.htm

As a reference, my only understanding of AC-to-DC power supplies is from studying the one built into my Fender Deluxe Reverb Re-issue. It's, of course, got a tube rectifier and filtering. I can easily understand swapping a diode bridge for the tube. That would seem very sensible for this project. I also understand that maybe a giant power transformer is not necessary for this pre-amp project. But, beyond that, I'd probably need to be pointed to a schematic for me to study for a few minutes in order to understand what you might be suggesting.

Thanks so much for your help.

Chip

[Jag]

Basically it's this.

Sorry for the quick and dirty drawing.

[chipaudette]

Jag, that is pretty sweet. Regarding the voltage doubler...are you talking about a device as described in the link below?

http://www.play-hookey.com/ac_theory...ltipliers.html

Oh yeah...and what wattage should those 100Ohm resistors be? 1/2W, 1W? Higher?

Chip

[chipaudette]

OK, I've lost my understanding. From your schematic, you step 120VAC line voltage down to 6-12VAC for the heaters. Fine.

Then, you step it back up to 120VAC to send to the voltage doubler and then off to be my B+ for my tubes.

If we started at 120V and ended at 120V, why did we need the two transformers? How does using two transformers help me with getting my B+? Why can't I just use the voltage doubler on the line voltage right from the wall? Is there some safety benefit or something by using the two transformers?

Sorry these are such basic questions.

Chip

[chipaudette]

Hi Again Bruce,

This reply is regarding the 300V-400V needs for the PT. I had originally remarked about a 400VDC supply because Mesa does 400+ on their pre-amp tubes for the Dual Rec. I'm not a huge fan of the Dual Rec, but if I'm going to be making a pre-amp sandbox, I might want to try it out.

My Fender Deluxe Reverb Reissue, by the way, runs about 260V. That seems like a pretty big difference -- 260V versus 400+V.

I know you have tons of experience. What is the effect of plate voltage on the sound? Why would boogie go high (400+) while the old-style Fender would go lower (260)?

Thanks for your thoughts.

Chip

[Enzo]

You need the second transformer so your mains are not directly connected to your B+. That would be very dangerous.

That someone used 400v for the preamp doesn't make it necessarily desirable.

Two wall warts is cheap, but then you have wall warts, and for this exercise, your 6VAC wall wart needs to run at several amps. Pretty large for a wall wart. I use this trick for something small like a one tube pedal circuit.

Two transformers back to back is still a viable idea though. If this device is to be at least semi-permanent, you can find chassis mounting transformers that are not expensive. In 120V areas, you can still get the 240VAC out if you use split primary transformers - the Universal 120/240 type. Tun one off the 120 mains, that provides the 6v for the heaters, and then run another backwards using the 240v primary as the B+ winding.

On the other hand, a basic tube supply transformer should be easy to find. Get most any with a few amps of 6v, and enough B+ winding voltage to get 300 or so DC. The preamp only needs a couple milliamps per triode, so B+ current ought not be an issue.

Note that Mesa might run a 400v B+ but that is not what is on the tube itself.

[Steve Conner]

any wacky pre-amp configurations that I might have fun experimenting with? Did any actually end up sounding any good?

My Toaster amp has served me well over the years. It has two gain controls and a three-position switch to rearrange the gain structure. Each position switches in one more 12AX7 stage, which lets it go all the way from completely clean to Boogie-style distortion.

http://scopeboy.com/toaster.html

The whole thing is completely my own design. I used to think it was nothing special, but we have been gigging it a lot, and everyone who played it likes it, either as a bass amp or a guitar amp... We also recently recorded a whole album of punk rock with it and it came out great.

I recently added a gain boost footswitch, but it's unusable on the clean gain structure, since it also shoots the volume through the roof. It would need a second master volume, like the SLO-100 has, to fix that. It also needs a presence boost in the power amp, as it sounds pretty dark.

If I were doing it again, I'd change the circuitry around the first tube to a standard Fender/Marshall front end.

BTW, as Enzo said, the reasons for stepping the voltage down and back up again are:

1) It isolates your B+ from your mains, which is absolutely vital if you want to connect the thing to any other equipment, and not wake up in intensive care after trying to plug a guitar into it

2) 120V:6V, 120:12, 240:6, 240:12 transformers can all be bought cheaply off the shelf from various electronics outlets, and a pair of them may well be cheaper than a custom PT for a tube circuit.

The doubler that I referred to is what this guy calls the "Greinacher doubler": http://www.kronjaeger.com/hv/hv/src/mul/
Fender used it in the 300PS and 400PS.