CV2799

pre65 · Post by **pre65** » Tue May 15, 2012 10:20 am

I'm getting in a muddle now.

To cure the excess current should I be thinking of a cathode resistor for the CV2799 ?

If I want the current to be 70ma (per two sections) and the grid voltage +12v is it a straightforward ohms law thing ie 171R or 180R ,nearest standard value.

Paul Barker · Post by **Paul Barker** » Tue May 15, 2012 2:19 pm

pre65 wrote:Thanks Paul.

I fitted a 180R resistor from anode to grid 2.

Now the CV2799 is losing approx 40v through the OPT and that's 165ma. Varying the + bias does not seem to make a great difference in the current drawn.

And the volume level has taken a dive.

Gonna try a cathode bypass cap on the C3g to get the gain up a tad.

Well obviously now you are actually in triode mode you are on a whole different set of curves so as you have found you are miles off optimum. Maybe it doesn't any longer want biasing in A2. I would stick to 300v on the anode and try 0v on the grid. If current is still too high try some auto bias until you get the current which suits the dissipation.

Paul Barker · Post by **Paul Barker** » Tue May 15, 2012 2:20 pm

pre65 wrote:PS Is EL34 a good candidate for A2 experiments ?

NO!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Paul Barker · Post by **Paul Barker** » Tue May 15, 2012 2:23 pm

pre65 wrote:I'm getting in a muddle now.

To cure the excess current should I be thinking of a cathode resistor for the CV2799 ?

If I want the current to be 70ma (per two sections) and the grid voltage +12v is it a straightforward ohms law thing ie 171R or 180R ,nearest standard value.

you are in a big muddle.

You no longer need any positive bias.

Now you need negative bias until the current comes right. your B+ will then float back to where it was if you don't alter the dropper resistor from what you originally had.

You were in pentode mode mate!

pre65 · Post by **pre65** » Tue May 15, 2012 2:37 pm

So, where I was earlier, with the 470K resistor between anode and G2 is pentode mode.

Can you have A2 in pentode mode ?

I might go back to that for a while as I quite like how it sounded.

Paul Barker · Post by **Paul Barker** » Tue May 15, 2012 2:47 pm

the trouble is Phil we are ignorant without the curves. If that works for you at the moment do that.

You are pioneer for all of us.

pre65 · Post by **pre65** » Tue May 15, 2012 2:57 pm

Yep, if you want to know about parallel single ended pentodes in A2 I'm yer man.

Mr Ignoramus strikes again.

pre65 · Post by **pre65** » Thu May 24, 2012 11:52 am

Just turned the amp on for a session, and the LH channel only has a noise, no music.

Thinking I'd blown up a CV2799 I replaced it, but no change.

Replacing the C3g did the trick, back to normal and a bit more of Siri's Savale band. Not necessarily so this time.

Dave the bass · Post by **Dave the bass** » Thu May 24, 2012 12:13 pm

Good stuff innit. AFAIK they only did 2 albums.

I've had 2 x C3g's just stop working. Nick mentioned years ago he had a few that just just like that too.

DTB

pre65 · Post by **pre65** » Mon May 28, 2012 2:51 pm

From a conversation at the "seminar" it seems I need to decouple the positive bias supply which feeds both channels.

Is there an optimum cap size or will anything in the 47uf-470uf range do ?

pre65 · Post by **pre65** » Mon Aug 06, 2012 10:39 am

I'd almost forgotten about this amp, in fact it took a while to remember where I'd hidden it.

Anyway, it's back in situ now and playing sweet mooooooooosic again.

I've no idea how, or why it actually works, but I like what it does. Don't think it's got enough watts for Owston but it does me at home.

Who needs old fragile and expensive British thermionic valves ?

JimC · Post by **JimC** » Fri Oct 05, 2012 8:18 am

Hello,

I have got hold of a couple of these Valves and would like to use them in an Audio Amp.

I have just got a basic 'lash up' on the bench at the moment and am not 100% decided on whether to go PSE as you have done or PP.

Do you have a circuit of your Amp to share?

How did you work out what Primary Resistance O/P Transformer to use?

Cheers for now,

James.

Mike H · Post by **Mike H** » Fri Oct 05, 2012 3:28 pm

Going back over this page, not sure he did, just whatever works with what he's got available to use. I stand to be corrected of course

pre65 · Post by **pre65** » Fri Oct 05, 2012 4:36 pm

Mikes right.

It sort of evolved from another project (op amp driven 1626 in A2).

At the moment it's trioded C3g driver to interstage to CV2799. The positive grid bias for CV2799 is provided by a separate bench PSU at it's max output which is around 12.5v. The cathode is grounded.

Please don't ask me how (or why) it works because I can't tell you, but rest assured it does. I like the way it sounds but it's not that powerfull, probably about 3-4 watts I would estimate.

The OPT are 5K single ended, huge great things that I purchased for my 833a project.

If you are mad enough to want to try this for yourself I can turn it upside down and draw up a diagram. I'm using an existing PSU for my version so HT is reduced a bit from the 450v I have available.

+

JimC · Post by **JimC** » Fri Oct 05, 2012 5:59 pm

Hi,

thanks for the offer with the circuit, appreciated. However I think I'll carry on with a bit of experimentation of my own.
I want to use a Valve input stage (despite the views on Valves expressed on my 'Welcome' page

) so will see what I can get out of the CV2799 in both SE and PP.

I'll keep you informed of progress but what with other things it won't be quick.

Cheers for now,

James.

audio-talk

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