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Old 20th September 2008, 07:27 AM   #1
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Default Power Supply Question

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.
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Old 20th September 2008, 09:49 AM   #2
dddac is offline dddac  Germany
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can you post a circuit, so we can have a look?
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Old 20th September 2008, 12:53 PM   #3
john65b is offline john65b  United States
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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.
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Old 20th September 2008, 01:39 PM   #4
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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.
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Old 20th September 2008, 11:38 PM   #5
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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?
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Old 20th September 2008, 11:38 PM   #6
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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.
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Old 21st September 2008, 12:18 PM   #7
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Quote:
Originally posted by Captn Dave
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".
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Old 21st September 2008, 04:06 PM   #8
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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.
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Old 21st September 2008, 08:22 PM   #9
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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.)
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Old 21st September 2008, 08:49 PM   #10
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Quote:
Originally posted by 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.
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
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