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Dwjames 12th February 2014 02:30 PM

CCS Mod for a KEL84, anyone got details?
Hi all,
I've had a good search and a read of the forums here and I'm interested in going for a Constant Current Source for the EL84 cathodes in my KEL84, but although I can see reference to it a few times, some of the links are dead which makes it hard to follow up on....

Does anyone here these days have experience with modding a KEL84 to run CCS using an LM317 as per the diyparadise Simple EL84 design?

I know some tried it and found good results, but with the side effect of clipping at higher than regular class A volumes, but there's even a proposed workaround for that on the tubecad blog here

Can anyone help or point me in the right direction?

pre65 12th February 2014 03:22 PM

Re: CCS Mod for a KEL84, anyone got details?
I run my 832a push pull amp with a CCS on the common cathode of each twin valve. As far as I remember it was an improvement when I did it.:thumbsup:

I believe that when run in that configuration the amp will ONLY run in full class A.

Dwjames 12th February 2014 04:03 PM

Re: CCS Mod for a KEL84, anyone got details?
Thanks Philip,
That Tubecad blog I posted above has a couple of suggestions for how to work around the limitation of only full class A thusly:

but for now, I'm not worried about that as I'd be happy with a lower output.

Does anyone know/remember how CCS is best achieved with the KEL84?

I'm ok with the principle of using the LM317 as a CCS on the EL84 cathodes and a 15 to 18ohm resistor should give about the right current, but wasn't sure on the specifics like whether the original cathode resistors should be removed or replaced with 1ohm as per the diyparadise schematic and whether the original cathode caps should stay or go

I don't mind trying a few things, but my valve circuit knowledge is limited and I don't want to release any of the magic smoke if I can avoid it...

pre65 12th February 2014 04:27 PM

Re: CCS Mod for a KEL84, anyone got details?
As I understand it you do away with the old cathode resistors and bypass capacitors.

Just connect the two cathodes on each channel to the CCS and the other end to ground.

I did use an LM317 on a small heatsink.

pre65 12th February 2014 04:34 PM

Re: CCS Mod for a KEL84, anyone got details?
I found an old photo of my CCS.

The amp is still in use with this mod and is playing sweet music as I type.:D

Richard 12th February 2014 07:46 PM

Re: CCS Mod for a KEL84, anyone got details?
Would 2 ccs be better to stop one valve drawing more than the other?

pre65 12th February 2014 11:21 PM

Re: CCS Mod for a KEL84, anyone got details?
Must admit, I've not read that "simple EL84" article lately, but I seem to remember that one valve drawing more current was not seen as a problem.

Dwjames 12th February 2014 11:44 PM

Re: CCS Mod for a KEL84, anyone got details?
Thanks guys, that's helpful food for thought and seems easy enough in its basic form, so probably worth a quick go :)

Dwjames 22nd February 2014 03:12 PM

Re: CCS Mod for a KEL84, anyone got details?
After reading around a fair bit, I don't think a simple LM317 based CCS will do what I want given how easily it can do horrible clipping, so I had a look at servo bias circuits as well as a few others, but I've yet to find anyone who's shown a specific example of it working with a parts list, with PCBs available or anything in kit form.
The best solution I've been able to find is from Tentlabs who do a Tube Bias Control module which is basically a very low noise power supply plus 4 x individual bias control circuits. Clever stuff and supposedly better than a servo based solution.

The only real downside is that it's not particularly cheap, but it's the kind of thing that can easily be moved to a different amp at a later date. I'm tempted to give it a go.
I emailed Guido to ask if he thought it would work well in a KEL84 and he said he thought it would do an excellent job.

He also mentioned that their Electronic Choke would work very well in this amp in place of the standard choke

I'm pretty tempted to put an order in...
Any thoughts?
Would this take an already v good amp up to another level?

pre65 22nd February 2014 05:06 PM

Re: CCS Mod for a KEL84, anyone got details?
I'd try plan A first, and see what actually happens re clipping. Cost will be minimal.:thumbsup:

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