Power Supply for Tube Preamp

[captainate]

Hey guys,

I'm about to pull a couple of amplifier modules from an old radio console, and I'm wondering what power supply I should get. A friend recommended an "International Power linear supply" from Mouser, but I also need to know specifically which model to get. Needs to supply 310V? Which seems like a high number to me. I'm a bit of a noob, and will be consulting experts and getting the project fully inspected before applying mains power

Attached is a schematic of the project. It's a line amplifier, which I'll be converting into a microphone preamp. Thanks for your help!

 

[crutschow]

Yes, you need to supply 310Vdc (which is why I don't like working with tubes) for the B+ plate supply and 6.3Vac for the filament supply. You can determine the dc current required by calculating the voltage drop across the resistor feeding the plates and calculate the current through them from that. For example the current feeding the three tubes on the left is approximately (310V-290V) / 6800 = 2.94mA. The B+ current required by the output 12AU7's can be calculated from the current through the cathode resistor which is approximately 11V / 750Ω = 14.7mA. The total B+ supply current is thus about 18mA. You can look up the tube specs to determine how much filament current you will need.

The power supply could be made from an appropriate plate transformer of about 220VAC output with a rectifier/filter. Such transformers often include the 6.3Vac filament winding.

 

[captainate]

Any chance you could link me to a suitable power supply? I can't find an International Power one that supplies 310V.

 

[KeepItSimpleStupid]

The PS-1 here http://glass-ware.stores.yahoo.net/vore.html is a regulator, which usually isn't used on amps, but it would make the amp sound much nicer.

I will link to this: http://www.hammondmfg.com/pdf/5c007.pdf which is a PDF on how to design an LC filter for a power supply. This is what you need for a tube power supply.

I'd be more worried about getting the input/output transformers. Here a company in the US that deals with tube/valve construction supplies. http://www.hndme.com/

Also check out: http://www.hammondmfg.com for transformers etc. e.g. http://www.hammondmfg.com/5cindex.htm

So, your going to need a 350 V transformer: http://www.hammondmfg.com/300series.htm based on the design guide above. Now, you also need to figure out how much current you need for the filaments and the secondary. (Corrected mistake) 315 = 0.9 * 350

The PS-1 regulator has some restrictions when using regulation especially filaments. You don't have to use one. I have two of the PS-1 supplies unassembled for an upgrade to an AM/SW receiver when I get the time. I also have the FM tuner section with lots of parts missing which is another project in itself.

I was dealing with this stuff when I was 12 years old, so high voltage doesn't bother me and neither does high current, The largest supples that I've dealt with in my lifetime were 15 kV at 1.5 Amps and 100 kV at 0.1 Amps. Big enough to do some serious damage, The input to the former was 208 VAC 3 phase at 60 Amps.

I have a friend who I taught some stuff to, that allowed him to build a few tube amplifiers.

 

[captainate]

I really appreciate the help, but I think you guys are giving me way too much credit regarding my electronics knowledge. I was kind of under the impression that I could buy a PSU and simply wire it up to the preamp cards I'm racking. I'm good at mechanical stuff, soldering, etc. but not so good when it comes to the math and pairing this part with that part. If it sounds like I'm over my head, feel free to tell me. I'd rather know now than later!

That said, if all I need is a PSU, a transformer, and to hook up a couple of wires, I can do that if someone holds my hand along the way.

 

[crutschow]

You might try ebay.

What's you budget for this supply?

 

[captainate]

My budget is whatever it takes to do it right. I'd like to keep the whole project under $300, which I think is pretty reasonable since all I need is a power supply, custom rack (I'm building myself) and misc. parts for the front panel. I would search ebay, but I wouldn't know where to start regarding search terms. 310V linear power supply?

 

[KeepItSimpleStupid]

I was under the impression that you were building the entire circuit from scratch, not that you just needed a power supply.

In any event, this is the tube era and that means you pretty much have to build one. You basically have to build a topology like the last figure on the right, here; http://www.hammondmfg.com/pdf/5c007.pdf We need to figure out L & C and whether or not you may need or want a regulator The 315 is a bit high, I think for that regulator circuit. You can get a 360 V transformer, but not a 350 V transformer. I corrected the earlier mistake in my post.

The PS-1 manual that I am reading suggests using a full wave choke input supply where:
(60 Hz) L ~= Vout/Iout (mA)
(50 Hz) L ~= 1.2 * Vout/Iout (mA)
Iout = Iac/0.707
Vdc = Vac*/2.2 - Vdiode-Rinductor*I2
* Entire primary voltage

Not sure if L is in Henry's or mH; Although probably mH.

But they do suggest using a 115 to 230 transformer in a rectifier/diode combo. They say that 230 VAC non center tapped xformer should result in 325 to 340 VDC, So, you might be able to get away with an isolation transformer..

The PS-1 MIGHT work at 315 V with some component changes.

What's your mains voltage? 230? 240?

 

[captainate]

Are there any kits or anything that could make this simpler? I'm a bit wary of building a power supply from scratch, and couldn't even dream of designing one. I'm a reasonably intelligent person, but I just don't have the time to devote to electrical engineering. As I stated before, if you can tell me what parts to get and where to hook things together, I can manage that. But I understand that may be asking a lot of a stranger, and it may well be a project I need to shelve indefinitely.

 

[KeepItSimpleStupid]

This might work: **broken link removed** We have to see what crutshow thinks besides a bit overkill. It supports: Power Requirements: 320Vdc - 100mA, 12Vac - 150mA, 2x 6Vac - 450m-2A, -50V - 3mA. Probably not the BEST choice, It is a kit. It could probably be improved and probably the PS-1 that I linked to would help. The PS-1 does require mods based on what you need.

This guy https://www.londonpower.com/power-supply/tube-preamp-power-supply will supply 37 mA at 320 VDC, so if the guess is 18 mA, this would be a better choice at 1/2 the cost. Again, adding the PS-1 will make it better.

Your mains voltage?

Crutshow?

 

[captainate]

Reading your reply on my phone yesterday, I missed the question about mains voltage. I'm in Oregon, so 120VAC. Pretty clean power here too.

 

[crutschow]

I would think that either of the supplies suggested by KISS in post #10 would work for you. The cheaper unit should have adequate power for your requirements. A regulated supply is probably overkill. If hum turns out to be noticeable, you could add the hum filter they also sell.

 

[captainate]

I'm just now revisiting this build... and noticed that your calculation of 18mA is for one amplifier. There are two identical amplifiers in this build, thus bringing it to 36mA. That's still under the 37mA rating for the cheaper supply but it says it's only designed for 3-6 octal tubes, and I'm running 8 pentodes. Any reason it won't work in this configuration?

 

[crutschow]

I'm just now revisiting this build... and noticed that your calculation of 18mA is for one amplifier. There are two identical amplifiers in this build, thus bringing it to 36mA. That's still under the 37mA rating for the cheaper supply but it says it's only designed for 3-6 octal tubes, and I'm running 8 pentodes. Any reason it won't work in this configuration?

I wouldn't use a power supply that close to its limits. Normal circuit tolerances could push the current past the limit.

I think octal refers to the number of tube socket pins and pentode refers to the tube internal design.

 

[JimB]

That's still under the 37mA rating for the cheaper supply but it says it's only designed for 3-6 octal tubes

That is a silly way to define the rating of a power supply, on a par with the popular way of defining things on dumbed down TV progams in terms "football pitches" and "10 storey buildings".

I think octal refers to the number of tube socket pins and pentode refers to the tube internal design.

Correct in both cases.

JimB

 

[chemelec]

Hammond Transformers in Canada, Part Numbers:

269GX = 225-0-225 Vac @ 65 mA, and 6.3 Vac at 2.5 Amps

The 225-0-225 should be about 315 VDC when Rectified and Filtered.

I was told by Hammond manufacturering in Canada, that this Transformer Should be Available from Digikey.com.

 

[captainate]

Chemelec, you think there will be a 135V drop after the bridge rectifier and filter caps? That seems utterly ridiculous.

 

[chemelec]

How do you get that 135 Volt drop?

This is a Center Tapped Transformer.

Center Tap Goes to GROUND
Than One Diode from Either Side to form Full Wave Rectification to your filter cap.
So, 225 x 1.414 = 318 Volts.

NOT a Full wave Bridge on the full output.

 

[captainate]

Ok, I see your point about it being a center-tapped transformer. Obviously there's no need for a full wave bridge rectifier in this case. But I'm not following your math. It would help me to see a circuit, sorry I'm still a noob.

 

[4pyros]

Ok, I see your point about it being a center-tapped transformer. Obviously there's no need for a full wave bridge rectifier in this case. But I'm not following your math. It would help me to see a circuit, sorry I'm still a noob.

He showed the circuit, its the top one and it is a full wave supply because the center tap is grounded.