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 The Trusty Rusty Power Supply updated.
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 Return to top of page · Post #: 1 · Written at 10:35:02 AM on 14 September 2016.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 915

Hi all, I got tired of flogging the 2A3 pass valves in this supply. I was prompted by a fireworks display in one of them when I accidently shorted the HT output. I guess there must have been a transient surge generated that stripped some cathode material off a heater and it shorted from here to there inside with blue zig zag flashes and sparkles!

Not the best way to treat what was a junk valve but know I see its a precious "Golden Ear" triode output valve with marvellous sonic capabilities................ if I could believe what I read in some audio journals.

I suppose I really should make a stereo amplifier with the 2A3's I have laying around, I could then play some old records and pretend I can hear things I never heard before! Now that's a project to consider.

Meanwhile back in the real world I stripped out the 2A3's and a lot of the circuits to do with bias and low voltage supply and made the supply just an adjustable 250 to 350 volt 100ma using 6CM5 sweep valves. That released the 2.5 volt winding to join the 6 volt winding for prototype heater supply. I also put a back bias resistor in so I have a smidgen of negative voltage for biasing if needed.

Trusty Rusty Power Supply Update

Cheers, Fred.

 Return to top of page · Post #: 2 · Written at 6:39:20 AM on 3 May 2017.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 915

An update on the supply.
After a lot of use I found the 6CM5's tended to go unstable on some load settings.
The output HT line would oscillate or "motor boat" at low frequency.
I changed the valve type and used 6CA7's in place of the 6CM5's.
The 6CA7's I used to use in guitar amps and still have a bunch of them in a box.
They show no sign of instability on any load.
I guess its a much more linear valve in this use, the 6CM5 really is a switching valve and not the best for a linear application.

 Return to top of page · Post #: 3 · Written at 12:16:31 PM on 3 May 2017.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1464

There's different types of 6CM5. The AWV version is different from Philips for example.


 Return to top of page · Post #: 4 · Written at 9:30:48 AM on 4 May 2017.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 915

Hi Robbbert, yeah I thanks for the Serviceman link, I then googled using "6CM5 as an amplifier valve" search and discovered another world of stuff and trouble I don't know about! I remember they were a popular valve in small PA amps probably being the cheapest "big" valve you could obtain then being used in every other TV set. I have about 50 of them in a box! I bet every body has the same in a box somewhere.

When I revamped the supply I test loaded it with resistances hooked across the terminals and it was fine.
Once I used it for real work with yards of wire draped across the bench and active loads reflecting stuff back into the supply it was an unhappy child. I did not bother scoping it you could hear and see the problem just like the serviceman described in his TV repair.
Just plugging the 6CA7's in and rearranging the plate wires turned it into a pussycat.

I like 6CA7's, I used to parallel then up with a 700 volt plate supply and make screaming guitar amps with "C" core power and output trannies at about 70 watt per valve pair. Then along came transistors and I could get the same output almost with one "C" core and one pair of transistors and direct coupled into the speaker! Valves were gone in a flash!

Cheers, Fred.

 Return to top of page · Post #: 5 · Written at 12:04:27 AM on 7 May 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4460

Interesting: I used a 6AU6 to control a 6L6 & as I only wanted a range to run common sets I used up a stray VR105/30 regulator not a Zener diode. To avoid flash over heater to cathode these are floated like a 5Y3 filament.


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