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mashaffer 17th February 2016 05:30 PM

Bench Supply
1 Attachment(s)
Was going to knock up a SE garage amp from 6GV8s I have on hand but I think I may build a Bench Supply first as it will probably come in handy for other projects. Other than the 120V PT for the garage amp I have only one truly high voltage PT. I was saving it for an amp but since it is only rated for about 120-125mA on the B+ winding it might be marginal for a lot of stereo PP applications I may use it for the bench supply instead since one normally prototypes just a single channel and if I do overload it in the bench supply application it would be for a very short time.

The center tapped B+ winding has full voltage tap and a lower voltage tap (maybe intended for screen supply or just for lower voltage option) on the same winding. on one leg of the B+ winding there is also a lower voltage tap for a bias supply. The bias tap is only on one side of the CT. Not sure whether I should use those lower B+ taps to provide a second output for screen supplies or whether to use it for range selection and use a separate transformer to make a screen supply output. Since it is on only one side I would have to be sure that a bias output was not loaded very much to prevent asymmetry on the B+ outputs.

It has a 5V heater winding and a 6V heater winding. It appears that there are three taps on the primary (blue. gray and brown) which I suspect are 240, 120 and 100 (Japan). I suspect I could use any of the primaries to choose different voltage output ranges so as not to over stress the pass tubes but if I do so I would not be able to use the heater windings for the tubes in the supply. Of course the same would apply for using a variac to reduce voltage range.

If I use separate trannies for heaters I could connect the 5V and 6V windings in series to provide a regulated DC heater supply output (max 7V or so).

The chassis has 3 octal and 3 9 pin mini sockets. I figure on using 1 octal for the screen supply output pass tube and the other two for parallel pass tubes for the main B+ output. The 9 pins can be used for error amplifier tubes and bias supply pass tube. I also have a few 9 and 12 pin compactron sockets that fit where the octals are so if I need to I can use some of those tubes for pass tubes.

Possible pass tubes on hand:

6JS6C (1)
6JB6 (1)
6JN6 (1)
6L6 (2 Used)
6W6GT (1)
6LQ6 (3)
6LU8 (lots)
17JM6A (1)
17JN6 (1)
33GY7A (1)
36KD6 (1)
38HE7 (1)
22JF6 (1)

I could start with the 6LQ6s and if they prove to be in good condition I could sell them and use the proceeds to get some other less expensive tubes for this and use the remainder for some iron or other goodies. :D

As always thoughts welcomed.

BinaryMike 17th February 2016 07:38 PM

Quite a lot of effort goes into building a good variable HV PSU. I wouldn't expend that amount of effort unless the final product had some pretty serious voltage/current capabilities -- e.g. at least 500V and 200~300mA. Even then, it's awfully tempting to throw money at that problem so I can focus on more interesting ones.

lexx21 18th February 2016 12:49 AM

You might want to look at just picking up a heath kit hv power supply. I use one of those as my bench supply and it works quite well.

mashaffer 18th February 2016 01:54 AM

I see the wisdom in that but since I have virtually no money and I do have parts I might make a stab at it anyway since it might be useful, it can be disassembled later and all parts reused, and I might learn a few things along the way. Seems like all I have to lose is time. :)

smoking-amp 18th February 2016 01:56 AM

Looks hard to beat: $15 + check the shipping, heavy

ISCO Electrophoresis DC Power Supply Model 494 | eBay

mashaffer 18th February 2016 02:03 AM

I rang out the windings this evening and got the following RMS voltages ref to CT:
Input Black and Gray: Main 178V, Screen 137V, Bias 24V
Input Black and Blue: Main 322V, Screen 261V, Bias 46V
Input Black and Brown: Main 386V, Screen 314V, Bias 56V

So pretty good range of voltages.

BinaryMike 18th February 2016 10:41 PM

1 Attachment(s)
Here's a schematic that might be useful -- from my archive. Unfortunately, I haven't always been good about noting sources. It might have appeared in one of those big Markus books.

mashaffer 18th February 2016 11:03 PM

Thanks Mike. That is very similar to what I have in mind. I plan to use sand rectification and zener reference. Is it high gain that one wants in the error amplifier? Have seen the reference in various locations also so wondering why one would choose one location over another? Was envisioning zener in cathode of error amp. Wonder if mu stage errod amp would have any advantage over tandem as used here.

BinaryMike 18th February 2016 11:19 PM

Desired error amp gain is much like that of any feedback amplifier. More gain increases output accuracy, but it also pushes you toward oscillation. If you deviate from a published design, be prepared to test it thoroughly. Power supply stability is generally tested by stepping the load between two values and monitoring the output as well as internal nodes with a scope. One of the complicating factors is that most of us want the PSU to tolerate large capacitive loads, which put a drag on output slew rate and delay the feedback signal. That can cause the error amplifier to misbehave or even saturate. There are ways to deal with this, but the point I'm trying to make is that PSU design has its subtleties.

dgta 18th February 2016 11:55 PM

Agree with Smoking-Amp. It will be very expensive and time consuming to build a bench supply. Keep in mind that a bench supply is subject to errors in your prototypes so it has to be well protected against shorts and all sorts of other accidents. And hopefully protect your circuits as well, i.e. adjustable current limiting.

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