Small power supply for tube Preamp project

[Guitar-Tube-1969]

Hi all, I want to build a tube preamp for use with a guitar and several power amps. I had build Sluckeys AB763 lite amp and want to use the preamp section including V1 and the tone section. Now I‘m searching for a small power supply for that purpose. Diode rectifying is ok, any ideas? Thanks Axel

[sluckey]

Tis may give you some ideas...

     http://el34world.com/projects/tube_box.htm

[ALBATROS1234]

surprisingly i used a 12volt wallwart to completely run a 12ax7 based overdrive .just a really simple input ...hit the 1st grid... go out the plate thru a cap into a pot into the next grid... out the plate a coupling cap... into another pot again then an output jack.1st pot is gain second is volume. i just took the negative from the wallwart and grounded it and split the positive en route to the plates and pin 4 of the tube.ground pin 5 and done. it actually sounded halfway decent. mainly because it increased the gain of my guitar signal and pushed my 6SL7 preamp tube in my amp into od. it did sound better when i used a separate 32volt wall wart just for the plates and a 12volt just for the filaments but it can be done with just the 1st option. if you are really interested i will draw up the schematic and post it. i also have used 2 12v doorbell transformers back to back thru a basic filter section which gives you around 160 to 170vdc and you take the filament from the junction btw the 2. i built a pentode od with that ps with a 6sj7 that sounded pretty good as well.

[labb]

Take a look at this project. Might find what you want.
http://www.frontiernet.net/~jff/SonOfAlembic/SonOfAlembicF2B.html


http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreamplifier.html

[Guitar-Tube-1969]

Hey, thank you guys for your hints!

The one by Steve seems to fit best at the moment.

I watched the clever power supply solution but thought for me it is a bad idea to step down my 230VAC to 12VAC and step it up to 240VAC.

So I spend a little bit more space and think about the following power supply which is similar to Steve’s one from the deluxe lite.

Here it’s a toroid PT (https://www.tube-town.net/ttstore/Transformatoren/Netztrafos/Ringkern/Ringkern-Netztrafo-50VA::527.html) which gives me 6,3 VAC for the filaments and 250 VAC sec. After that I would use four 1N4007 diodes for rectifying and some filter caps.

Will that work for 1 or 2 12AX7?

In addition I‘m a little bit confused to calculate the B+ values for A, B, C and D.

Could you help me out?

Thanks - Axel

[PRR]

> it is a bad idea to step down my 230VAC to 12VAC and step it up to 240VAC.

Can't be totally bad. Your power company generators run 2,000V, stepped-up to 100,000V for long transmission lines, stepped-down to 19,000V for local distribution lines, stepped-down to 230V for houses. In a large grid, many more steps for combining and switching. Another couple steps won't hurt much.

Here your double-step 230-12-230 is only the plate power, which for a couple 12AX7 is the least of your load. Say 4 Watts of heater, 1.5 Watts of HV.

Yes, a one-lump solution is neater and more efficient.

[MFowler]

I've build a few preamps and it all depends on one preamp tube or more for mA rating needed and filament supply needs.


Also, the height of your enclosure.  The top tranny is 2" tall so it worked great in a small Hammond enclosure.


https://www.tubesandmore.com/sites/default/files/associated_files/p-t261_p-t262.pdf

[Guitar-Tube-1969]

I've build a few preamps and it all depends on one preamp tube or more for mA rating needed and filament supply needs.

I want to build a preamp like that V1 circuit from Steve’s AB763 Lite amp (see att.). It would be great, if the PT could feed an optional second 12AX7 for later use.

Your PT looks good, but I need a PT for european voltage on the primary side.

Do you have an advice?

[sluckey]

The transformer and circuit you show in reply #4 will be perfect for this project. If you prefer a different style transformer consider the Heyboer Reverb unit Power transformer (part # Mojo779) on this page...

http://hoffmanamps.com/MyStore/catalog/parts1.htm

Or consider the Hammond 369AX.

My choice would be that toroid you chose.

[Guitar-Tube-1969]

Thanks a lot Steve!

I will make my orders ready and keep you posted.

[Guitar-Tube-1969]

I just tried my power supply and I'm wondering, why I've got the same VDC at A, B, C and D. The values are:

A= 378 VDC
B= 378 VDC
C= 376 VDC
D= 376 VDC

I would assume that the D is much lower.

The sec. from the PT is: 276 VAC

What do I have done wrong?

[sluckey]

Voltages are all the same because there is no load on the power supply. No load means no current flow. No current flow means no voltage drop. Connect a 100K/3W resistor between node D and ground and you will have current flow and voltage drops across the 10K resistors.

[Guitar-Tube-1969]

Oh, thanks! I only have 100K/0,5W or 10K/2W 1M/2W. Which one can I use instead?

[sluckey]

Oh, thanks! I only have 100K/0,5W or 10K/2W 1M/2W. Which one can I use instead?

I wouldn't use any of those. The 100K and the 10K will burn up very quickly. The 1M will only give about a 4V drop across each of your 10K resistors. Just trust me on this one. When you connect the rest of your circuit to the power supply, the voltages will drop.

EDIT... Extra info... That 1 tube preamp will only draw about 2mA of current, so expect to see about a 20V drop across each of your 10K dropping resistors. If you desire to have more voltage drop (ie, less plate voltage) then increase the size of one or both of those 10K resistors. My node D voltage is 308, which is fine for a Deluxe Reverb AB763 preamp. 378V is also fine for an AB763 preamp. Super Reverb and Twin Reverb use 410V.

[Guitar-Tube-1969]

I trust you always!! I will proceed then, thanks!!

[sluckey]

I added a bit more info to my previous post.

[Guitar-Tube-1969]

Thanks Steve, I've got all the things together in a temporarily enclosure because I'm not quite sure which one to choose and I want to experiment a little bit with this circuit.

Everythings works finea but I have much hum coming out so I think there is a grounding problem. In addition:
1. I'm not sure if the master volume is in a good shape because its noisy when you use it and the hum changes when putting your fingers on it
2. I'm not sure if the wiring of the input and output jacks are correct

[davidwpack]

Is that cardboard?

[Guitar-Tube-1969]

Yes, it is

[sluckey]

Since you are using a cardboard chassis you will have to provide dedicated wires to connect all your grounds together. IE, connect a wire from the pots/input jacks buss to the ground on your preamp board. Connect a wire from the preamp ground to the ground on the power supply board. And connect a wire from the metal plate for the tube socket (I'd use a ground lug attached to one of the screws for the socket)to the ground on the power supply board. This should take care of most of the hum, but you should still expect a bit of hum just because the cardboard offers no shielding.

How do your power supply voltages look now?

That transformer and the preamp circuit would fit nicely in a rack mount chassis.

[Guitar-Tube-1969]

The power supply voltages are:
A, B: 373 VDC
C:     357 VDC
D:     340 VDC

What was not grounded was the metal plate of the tube socket, I completed this. The result is unchanged and the hum is still there.

I had already grounded Mid, Vol-Pot together with the in/out-jacks before.
 
1. Do I have to ground bass and treble pots as well?
2. Does it makes a difference if I connect the pots ground together with the pots ground directly to my ground lug instead of connecting this first to my preamp board and then to my ground lug?

Additional note: When touching the vol pots shaft, the hum inceases and there are certain cracks when I turn the vol up at the beginning. It seems that it's not dust because it did not go away when turning it up and down. I assume the pot is buggy?

[sluckey]

Assuming the circuit is wired correctly... The layout and the long wires are a big factor in the hum. I would swap places with the PT and the power supply board. Then look at the three blue wires going to the tone controls. Twist those wires together and route them straight across the back of all the pots. When you get to the volume pot make a 90° turn down to the board. Keep these and all other signal wires as far away from the PT as possible. I would also rotate the tube socket 90° clockwise so the socket pins are facing the board and no more than 1" from the board. Then make those wires as short as possible. And I would connect that pot buss directly to the ground on the preamp board then run a single ground wire from the preamp board to the power supply ground. Eliminate that black terminal lug near the PT and keep the B+ and ground wires away from the PT and the AC line connections. In fact, I would twist the black and red wires that run from the power supply board to the preamp board.

[terminalgs]

Make sure the 6.3V filaments have a reference to ground.  artificial tap, hum pot, etc.  If you don't, lots of hum.


Also, use C&D for your triode B+, not  B.   


Valve Wizard's grounding guide is one to read from beginning to end.     I've had great success with hum & noise free preamps thanks to that guide, in particular, see 15.10: Ground Lift section and the split dropping resistor detailed in Fig.15.7 of the same guide.

[Guitar-Tube-1969]

Thanks to Steve and terminalgs for your advices!. I've ordered a small chassis and will fit the components in there. I will keep you informed. Thanks!!

[PRR]

You seriously can't run high-gain audio amplifiers in a cardboard box. The air is full of hum (any place you have power) and the amplifier picks it up and amplifies it.

You generally want a near-closed metal box. For some reasons, many guitar amps work "OK" with a 5/6 closed box (bottom open), but that makes a big difference from "nothing" (cardboard is zero shielding).

Yes, there's ways to do non-metal boxes. Build very tight and right, and preferably switch to transistors working with small-value resistors, and maybe some metal tape at key points. (Or use batteries a mile back in the woods.) But the time-honored tube amp is a metal box.

[Guitar-Tube-1969]

Hi all,

I've got my new chassis and here it is.

I've had worked on the grounding and now there is absolutely no hum. Thank you all for your help!!

The power supply voltages (after changing the resistors to 22k each):
A, B: 389 VDC
C:     353 VDC
D:     319 VDC

The next step is to add a reverb kit: https://www.tube-town.net/ttstore/Bausaetze/Sonstige-Bausaetze/Kit-LNDVerb::6830.html

I'll keep you posted.

Thanks - Axel

[sluckey]

Now that's a good, clean, logical layout. 

[Guitar-Tube-1969]

Thanks for your support!

[jojokeo]

Nice job GT69! Okay, now that you've got a taste, when will the power tubes' build come next???  BTW, the cardboard box was classic! I've seen cake and bread pans, ammo cans, etc. but never a cardboard box! Awesome...

[Guitar-Tube-1969]

Hi guys,

it lasts a little while but I've got some updates:

I've built a little Frankenstein-like Amp out of the preamp, a class d power amp and a reverb kit. After that I assembled all in a custom made cab made from paulownia with a Jensen Blackbird P12/100. The overall weight is less than 12 KG or 26 pounds.

The sound is very fat and warm to my ears.

You can hear the results on two videos:

Let me hear what you think.

Thanks for your advice.

Have fun - Axel

[sluckey]

Sounds great. Enjoyed the pickin' too.

[Guitar-Tube-1969]

Thanks Steve!

[tubenit]

Great amp design! Well executed project. Beautiful guitar playing.   

THANK you for sharing the project with us. 

With respect, Tubenit

[Guitar-Tube-1969]

Thanks a lot!

[pompeiisneaks]

Outstanding tone, and man your jazz style is impeccable.  I'm really starting to dig jazz guitar as I get on :D 

~Phil

[Willabe]

       WOW!!!!!!!  Just beautiful!!!!!!

[mresistor]

Wow   nice !

[jojokeo]

Again great job GT69! Those are modular supplies along with the toroid? Separate heaters, plates 12ax7) and collectors (power amp trannies? Marrying to tube preamp?

[Guitar-Tube-1969]

Thanks for all of your comments.

@ jojokeo: Yes, I started with the tube preamp first as I want to build a small tube preamp. After that I experimented with two reverb solutions and the class d module.