Tube Rectified Power Supply Questions

[Icegoldnixon]

Hi All,

I've had a PA/guitar amp project sitting around for several years. My grandfather and I did some work on it, but it was all chopped up when we got  it. After it died for a last time, I completely disassembled it for a total rebuild. And disassembled it has stayed, as life has been busy and my Grandfather can't really help me any longer.

Lots of uncertainties have kept me from starting the project, since I want to do a good job and don't know tube gear that well. To boot, the voltages in this old amp are very dangerous so I don't want to be fumbling around. I have a lot of questions, but the main one is about the power supply. In the schematic, there's a 10/10/10 filter capacitor that's (apparently) rated at 600V. I cannot source any 600V multicaps, which I would prefer to use to make the layout easier. Do you think the 600V rating is necessary? 450V caps are much easier to find.

(Also, I've thought about creating a separate power supply to make room in the chassis. In that case, I'd just use the axial caps. But I suspect that would create trickier design decisions than I'm prepared to solve. I'm also thinking of removing a few channels from the preamp, maybe that would lighten the load on the PS? ) Thanks!

 

[Tubetec]

450 volts caps should be very adequate for the 3x10uf caps  on the screen supply , (265volts dc)
The main plate supply to the output tubes comes out at 465volts , thats a little high but good quality 500 volt caps will probably do fine . The other thing you can do is connect two caps in series with a pair of balance resistors , now each cap only needs to deal with 50% of the supply volts , in this case youd double the cap value for the same overall effect as far as psu noise is concerned.
dropping one or two mic inputs might save you a small amount of heat in the mains transformer ,its probably insignifigant as it looks like quite a beast of a psu with 4 rectifier tubes . 

 

[Icegoldnixon]

450 volts caps should be very adequate for the 3x10uf caps  on the screen supply , (265volts dc)
The main plate supply to the output tubes comes out at 465volts , thats a little high but good quality 500 volt caps will probably do fine . The other thing you can do is connect two caps in series with a pair of balance resistors , now each cap only needs to deal with 50% of the supply volts , in this case youd double the cap value for the same overall effect as far as psu noise is concerned.
dropping one or two mic inputs might save you a small amount of heat in the mains transformer ,its probably insignifigant as it looks like quite a beast of a psu with 4 rectifier tubes .

Thank you! That's good to hear. I'd also probably drop the inputs just for the room in the chassis.
Also, is there interaction between voltage and capacitance for this application? For instance, if I were to increase the value of the screen supply caps would I need to increase the voltage rating also? I'm looking at a 20uf multicap which would be convenient.

 

[rmburrow]

Hi All,


Lots of uncertainties have kept me from starting the project, since I want to do a good job and don't know tube gear that well. To boot, the voltages in this old amp are very dangerous so I don't want to be fumbling around. I have a lot of questions, but the main one is about the power supply. In the schematic, there's a 10/10/10 filter capacitor that's (apparently) rated at 600V. I cannot source any 600V multicaps, which I would prefer to use to make the layout easier. Do you think the 600V rating is necessary? 450V caps are much easier to find.

S? ) Thanks!

Is the multisection 600 volt can capacitor really bad?  In some cases, to keep the unit looking original, I've seen the mounting and terminal phenolic carefully removed along with the capacitor, the insides of the capacitor discarded, and new series capacitors (axial leads) with the balancing resistors (100 k or so) built and covered with shrink wrap inside the can, and the leads brought out to the phenolic terminals as marked.  In many cases, the new capacitors are much smaller and should fit.  Alternatively, you could get yourself four 20 uF 350 v capacitors, four 100 kl 1 w resistors, connect the capacitors in series with the balance resistors across each cap, and have two sections 0f 10 uF at 700 volts.  Or you could get two 40 uF 350 v capacitors, connect the balance resistors across them, and have a single section 20 uF 700 volt cap.  If you build inside the original metal can, make sure the can is grounded!  Reusing the can saves "real estate" below chassis... I like the parallel 5U4 rectifier arrangement...helps keep the "bounce" down on the HT...the fixed bias for the 6L6 GC tubes is also nice...make sure the bias supply voltage is correct and stable...

 

[Icegoldnixon]

Is the multisection 600 volt can capacitor really bad?  In some cases, to keep the unit looking original, I've seen the mounting and terminal phenolic carefully removed along with the capacitor, the insides of the capacitor discarded, and new series capacitors (axial leads) with the balancing resistors (100 k or so) built and covered with shrink wrap inside the can, and the leads brought out to the phenolic terminals as marked. 

Sadly we threw the cap away as "bad", so looking original is out the window haha.

 

[Walrus]

I see a 40/10/10/10 multicap, which the diagram show as 450v for a rail of 292volts  as measured.
This cap should work fine...  https://www.thetubestore.com/jj-can-capacitor-40f-x-20f-x-20f-x-20f-500v

 

[Tubetec]

As long as your caps are rated above( with an extra margin to spare) the voltage across them they should be fine .
Tube rectifiers have a recomended max capacitance you can put directly after it  , typically 20-50uf, further down the line such as after series dropping resistors you can afford to up the values of capacitance for lower psu hum if you like .

Lets say you try powering up the amp without the power tubes in place , your HT  will rise well above 500volts ,its not a requirement for it to be able to run this way ,but imagine you had a defective tube in there your Ht could rise to a dangerous level for even 500 volt rated caps , like was previously mentioned 2x 350 volt caps with balancing resistors and your good for anything upto 700volts ,so plenty of safety margin in the event of a problem .

Sorry was a couple of small inacuracies in my first post relating to screen supply voltage .

 

[Icegoldnixon]

What about making a separate amp power supply? Would that be a good idea, or would it be problematic? I don't mind the extra work of making a separate unit, but if it's not a good idea I'd love to know why. Thanks all! This forum is great.

 

[rmburrow]

Sadly we threw the cap away as "bad", so looking original is out the window haha.

Find another can type capacitor of similar size in the junk box and recycle the can and phenolic parts...

 

[Tubetec]

A seperate power supply is a good thing , aside from the extra cost and work

When you isolate all the power transformers and rectification off onto its own chassis you generally can get lower noise .
With an amp and power supply on a single chassis some of the Ac currents end up in your signal path ,its almost unavoidable .
Distance between psu and signal chassis  means less magnetic coupling between output and mains transformer ,probably for most speaker driving applications single chassis with good placement is adequate .

Id say if you have the will and the means to do it seperately ,go for it , what you learn will stand you in good stead later if you want to build low noise preamps . Typically hum 90db down could be done with a single chassis and top notch design  ,noise 100 or more db down is possible with seperate supply.

 

[PRR]

500V first caps is marginal unless you wire for 130V and only power with <120V. These are available.

This was a TOP-line amplifier, and will be expensive to maintain.

The output stage runs in AB2, which is unusual in audio amplifiers. It appears to exceed plate voltage rating for any classic 6L6, though this may be OK for occasional use (the first batch of 6L6 would decay in the base with >400V for many hours). It runs right AT the datasheet maximum output condition. This is a bold amplifier.

 

[Tubetec]

I never heard of that company before , but the only place Ive ever seen that kind of dual rail ,dual rectified setup was on a big old Williamson that came off some old Royal Navy ship ,most likely from the radio room as a modulator .
The above or just on spec HT volts for the tubes might mean specially selected mil grade  ,cost is no issue for their majesties.  If the amp had to spend its life on the high seas theres little point in designing for long tube life electrically if they  only ends up rattley as a bag of bolts after 3 months  on open ocean duty.
That amp deserves a fine quality pair of Russian 5881's ,they defiantely hold up at 465vdc and getting hammered down the high ways and byways in the back of a van every night of the week .
Rebuild it, treasure it , along with the memories of the good old days .

 

[Icegoldnixon]

500V first caps is marginal unless you wire for 130V and only power with <120V. These are available.

This was a TOP-line amplifier, and will be expensive to maintain.

The output stage runs in AB2, which is unusual in audio amplifiers. It appears to exceed plate voltage rating for any classic 6L6, though this may be OK for occasional use (the first batch of 6L6 would decay in the base with >400V for many hours). It runs right AT the datasheet maximum output condition. This is a bold amplifier.

If I do the separate supply I'll probably stick with the 600V caps as I'll have plenty of room. But the 130v wiring might be a smart move also.

Should I do something to drop the B+? Add a choke perhaps? Maybe make it less tube-eatingly powerful?

 

[Tubetec]

Sovtek short base 5881

https://www.amazon.com/Sovtek-5881-6L6WGC-Matched-Tubes/dp/B0002M71OE?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-ffab-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B0002M71OE

thats one of the hardiest ,electrically and mechanically ,of any tube ever known to mankind , Ive known amps to fall out of vans and burst their guts all over the road ,yet the tubes still came out good 

Maybe the reason you dont see chokes in this amp is by keeping the plate and screen supplies seperate they didnt need it , if you have the chassis real estate only very bad placement could concievably make matters worse , a choke of identical dc resistance to a standard resistor considerably improves PSU noise factor,  all else being equal . But you will have to take magnetic coupling of iron cored components  into account .

 

[CJ]

130 volt wiring will save the transformers a little stress also.

who makes those transformers BTW?

do they look like they would be easy to take apart? jus sayin...

 

[Icegoldnixon]

130 volt wiring will save the transformers a little stress also.

who makes those transformers BTW?

do they look like they would be easy to take apart? jus sayin...

The laminatations look to be bolted together, I've googled the numbers printed on the transformers before but never came up with anything. Why would you want to take them apart?

 

[Icegoldnixon]

A seperate power supply is a good thing , aside from the extra cost and work

When you isolate all the power transformers and rectification off onto its own chassis you generally can get lower noise .
With an amp and power supply on a single chassis some of the Ac currents end up in your signal path ,its almost unavoidable .
Distance between psu and signal chassis  means less magnetic coupling between output and mains transformer ,probably for most speaker driving applications single chassis with good placement is adequate .

Id say if you have the will and the means to do it seperately ,go for it , what you learn will stand you in good stead later if you want to build low noise preamps . Typically hum 90db down could be done with a single chassis and top notch design  ,noise 100 or more db down is possible with seperate supply.

Where do you think I should separate the power supply components? I was thinking of putting the PT, rectifiers, bias circuit and first smoothing caps (the 40uf for the 292 and 30uf for 465v)  from each B+ supply line separately. Having the rest of the filter capacitors close to their loads seems like a good idea, but I could also see sending just filtered DC over the umbilical. (These are the questions that have kept me from building this thing!)

 

[PRR]

Why would you want to take them apart?

You must be new here.

Some people make music. Some people smoke crack. CJ attacks transformers with fire and hammer.

He documents the carcass which gives clues for re-creating similar transformers. Or apropos your case, for re-winding to a different voltage. But that's no reason you should subject your working iron to CJ's indignities.