audio-talk

Max N · Old Hand Joined: 16 Aug 2007 Posts: 474

Well, I am finally making a start on the much-delayed 211 push pull project.
To start with, I bought some filament transformers. These seem like a bit of a bargain, so here are the details:
Rapid electronics, order code 88-3439, nominally 9V 50VA at 230V in. With my mains running at 240V and with the secondaries paralleled I get exactly 10V when feeding a 211. £7.94 each plus VAT, free next day delivery. Very chuffed.
The other bonus is that I was now able to test the filaments on all the 211s I have accumulated from Ebay over the last few years - and they are all good Smile

(One of them has no getter, but that's another story).
Next step is to try to get 1000V at 120mA out some spare mains trannies. More soon hopefully.

andrew Ivimey · Posted: Tue May 12, 2009 2:13 pm Post subject:

Well, there's nothing like starting at the beginning Max, but phroaaaaaaaarr as one might say -- - - -- 211PP - breathtaking!!!

About another valve..., of the several ways of getting 7.5 volts, I wonder what the best sounding and cheapest way of doing so, is.

Max N · Old Hand Joined: 16 Aug 2007 Posts: 474

OK, to put a bit more flesh on the bones of this project, it all started many years ago when I floated the idea of a 211 push-pull amp. Neal Gibbons was enthusiastic about the idea, and he spoke to Andy Grove, who agreed to design some transformers for the project. Specifically, Andy designed an output transformer and an interstage transformer. The interstage transformer is push-pull in and out (i.e. 1+1:1+1) and is bi-filar wound. It is designed to be driven by the two halves of a 5687. Neal and I split the design cost for the transformer set.
Neal arranged to get the transformers wound by Morite, which took quite a while. The project then stalled because I haven't found the time to crack on with it, and I believe Neal has been in the same boat.
Some of you (including you Andrew) will have heard the interstage transformers already - the system I cobbled together for one of the Owston meets included a driver amp which was basically the driver section of the proposed 211 amp. This was coupled to the output stage of my WAD 300B PP clone.
Anyway, long story short, I have decided it is high time I got off my butt and built the thing!
I will try and post some photos of the various bits tonight.
Oh, one more thing, I also have some Lundahl 2+2:1+1 interstages which can handle a lot of current, and the idea with these is to try 300B PP -> IT -> 211 PP. The 300B through the 2:1 step down will have an output impedance of less than 200 ohms so will be able to drive the 211s into A2 without too much distortion I think. I could also try 2A3 PP driving 211 PP.
But these experiements are for the future, the main thing now is to get something working.....

Neal · Old Hand Joined: 03 Aug 2007 Posts: 322

This is good to hear Max, I'm still in the 'contemplating' stage. I will be very interested to read how you get on! Wink

Nick · Posted: Tue May 12, 2009 4:25 pm Post subject:

Sounds like a lot of fun Smile

.

Wihich Lundahl IS's do you have Max?

Max N · Old Hand Joined: 16 Aug 2007 Posts: 474

They are LL1671PP AM Nick. I think Kevin Carter at K&K built a 2A3PP -> 300B PP using them a few years ago.

Nick · Posted: Tue May 12, 2009 6:45 pm Post subject:

Nice, I look forward to hearing it. I still remember how nice the 300b PP you had sounded,

andrew Ivimey · Posted: Tue May 12, 2009 7:36 pm Post subject:

Your 300BPP was a giant amongst pigmy ponies - 'oh its just something I cobbled together for Owston' cool!

Those 1671s are amorphous - yep, I could use them Wink

onwards Maximus!

Max N · Old Hand Joined: 16 Aug 2007 Posts: 474

Nick · Posted: Wed May 13, 2009 6:35 am Post subject:

You have started a rather foolish train of thought Max. Push Pull 211 -> electrostatics -> direct coupling...

SteveTheShadow · Posted: Wed May 13, 2009 6:58 am Post subject:

OMG !

"Run away, run away" Laughing

Toppsy · Posted: Wed May 13, 2009 8:04 am Post subject:

Nick wrote

Ali Tait · Posted: Wed May 13, 2009 8:16 am Post subject:

Sounds like a plan Nick! I can offer my statics as guinea pigs Smile

Nick · Posted: Wed May 13, 2009 8:43 am Post subject:

Nick · Posted: Wed May 13, 2009 9:40 am Post subject:

Sorry to hijack your thread Max, I will stop in a minute. But Ali/Colin thinking a bit more, even better that a 211 would be a 75tl push pull, running at 1k5 on the anode, 40ma it would be nice and easy for the valve, -120v on the grid, and it would be able to swing a lot more than the 211, say 2kv p/p. Depends on what the statics need. And I think 75tl's are possibly cheaper than NOS 211's.

Stay in class A1, so a shared cathode resistor of 1k5, b+ of say 1750. I have a pair of 3k CT transformers that would do that just fine running between choke and cap input to tune the voltage.

Maybe I should go and have a rest in a darkened room Smile