Power Supply Question

[Captn Dave]

I picked up an old Knight Kit amp at a yard sale. Why? I wanted to build one when I was a kid and couldn't quite swing the deal until I discovered camaras, cars and girls. Oh yea, it had a set of Mullard tubes too! A pair of 12AT7s and a pair of ECL82s. But wouldn't you know, it requires a quad of the pentode/triodes. A fair deal for fifteen bucks none the less.

I recapped the power supply, replaced a fried resistor and replaced the mising rectifier tube with diodes. Now I want to fire it up and see how it sounds but I never got around to buying the second pair of tubes.

So here is the question. Given that the power supply uses a chain of resistance to drop the voltage to the various levels required, I will not have the proper current to achieve the design potential with only half the tubes. When I power it up, it will take a little while to measure voltages to see if I have a problem. Will I damage my tubes in that short period of time?

I know what you're thinking but I don't own a variac.

 

[dddac]

can you post a circuit, so we can have a look?

 

[john65b]

Won't the output voltage going from a tube rectifier to SS be different? I would try modeling it first on Duncan Amp's PSU designer first.

I would guess that your going to need to modify the resistors on RCRC filter to drop the voltage a bit more if you were going to try to match the Tube voltage exactly.

 

[jackinnj]

put a lightbulb in series with the power line -- this will limit the current.

if you want to "gild the lilly" -- you can use SuperTex LR8N3 high voltage regulators -- a TO-92 device -- to drop the voltage and regulate the driver stage.

 

[Captn Dave]

I have attached a copy of the schematic. My struggle is that the voltage tags on the drawing are very difficult to read and this is the only copy I have been able to find in spite of an extensive search.

The voltage out of the transformer is 275 VAC per leg and 580 between legs. Out of the rectifer pair I have 277 VDC. (I am not using a bridge rectifier).

I do have a short extension cord with a series light bulb spliced in that I use to power up unknown circuits (my virtual variac) that I can use for this.

The RCA tube manual says the ECL 82/ 6BM8 likes 200 vac on the pentode plate and 100 on the triode plate and grid. I intend to shoot for those.

The regulated power is an interesting subject and I intend to investigate that with a more valuable project. My Conrad Johnson preamp uses them to good effect. I found some that were pretty cheap but they had a 37 volt limit. They float so they can be ganged but it would be better to find a proper device. How expensive are the high voltage devices?

 

[Captn Dave]

Well, here is what I meant to say

The RCA tube manual says the ECL 82/ 6BM8 likes 200 vac on the pentode plate and grid and 100 on the triode plate. I intend to shoot for those.

 

[jackinnj]

The regulated power is an interesting subject and I intend to investigate that with a more valuable project. My Conrad Johnson preamp uses them to good effect. I found some that were pretty cheap but they had a 37 volt limit. They float so they can be ganged but it would be better to find a proper device. How expensive are the high voltage devices?

Unfortunately, you aren't going to have a lot of space to do regulating, but you can change the value of the 10W dropping resistor, increasing it ~40%

The LR8N3 is about $0.68 each at mouser -- you have to wrap a pass transistor around it for any meaningful power dissipation. the device was designed for off-line switch-mode power supplies for the so-called "supervisory voltage".

 

[Captn Dave]

I found a good article on these power regulators here.

http://www.antiquewireless.org/otb/resto0504.htm

I'll get a few and breadboard one of these circuits. It seems like a very elegant and economical solution.

As for this project, I'll tweek resistor values until I get the proper voltages. I'd love to hear your opinions on the proper voltages as I'm unable to read the printed values with any certainty, and they look to be greater than the 200 & 100 vdc sited.

 

[Dave Cigna]

Here's my advice, and remember what I'm charging you for it: Get on the phone and order a quad of ECL82s and a 6CA4 rectifier. When they show up at your door just plug them in and enjoy.

There'll be plenty of time later to redesign the power supply into something more complicated than the whole rest of the amp. (Is it worth it anyway? You can put lipstick on a pig but it just pisses off the pig.)

 

[mod_evil]

I found a good article on these power regulators here.

http://www.antiquewireless.org/otb/resto0504.htm

I'll get a few and breadboard one of these circuits. It seems like a very elegant and economical solution.

As for this project, I'll tweek resistor values until I get the proper voltages. I'd love to hear your opinions on the proper voltages as I'm unable to read the printed values with any certainty, and they look to be greater than the 200 & 100 vdc sited.

If you put a capacitor from the base and the gnd, you can make a capacitance multiplier.

I think it's a good idea test this circuit with TWO TIP50 in a Darlington configuration and with the capacitance multiplier.

It can be good!

Best Regards,
F. Navarro

 

[Captn Dave]

Dave, that's some solid advice and it had occured to me but the point is to get my feet wet with tube circuitry and I'm in no hurry to listen. I have a nice el34 Jolida going right now and sound is not a problem. I'm just out to futz around with this pig. I might fit it with a rectifier tube later on because I'd like to see if I can hear the difference so often attributed to the rectifier tube.

Mr. Navarro. That comment went right over my head, but I will research your ideas. Just not that familliar with ss power suppy circuits yet. Thanks for your input.

 

[jackinnj]

capacitance multiplier

http://sound.westhost.com/project15.htm

 

[Captn Dave]

Filter cap values

I believe I will part out that Knight amp and use the tubes and output transformers on a future project and use the power transformer to make an adjustable B+ power supply for experimentation.

I've got all the pieces together and I'm ready to build the regulated power supply using the LR8 regulator and TIP50 transistor. (the schematic is posted in the thread a few post higher; post 8). I'll mount a pot so I can control the voltage.

But I do have a question. The drawing calls for using 20 and 40uf caps as filters. I have some 220uf 400V snap in caps that I would like to use. What are the pros and cons of using more capacitance that is needed in a filter role?

 

[koolatron]

Significantly increasing the power supply capacitance can lead to inrush current problems as the caps are energized. Series inrush limiters are available; I don't suggest simply replacing the fuse (you have installed a fuse, right?) with a higher-current one.

-k

 

[Captn Dave]

Ok, that is the explaination I was looking for; the con of excessive capacitance.

I have other smaller caps that I can use and will do so. I was thinking this might be an opportunity to use these larger caps.

Also, one frequently sees large caps at bargain pricing and it's always tempting to by them instead of their pricier smaller counterparts.

Thanks for your comment, And yes, I have a fuse.