Darren 212 / 833 SE amp

Darren · Post by **Darren** » Sun May 27, 2007 2:33 pm

Hi Guys,

I'm getting the building itch again

This time I'm looking at an output platform only, no driver stages. The idea is to be able to swap output valves at will.

I have two 0-30V 10A Farnel DC supplies, bloody great heavy things they are. Really a two man job to carry just one of them. Very tidy though. They are to be my adj heater supplies.
For the HT I have a wide choice of transformers right up to a monster 3,300/4,000 V 57Kg transformer. OK this is a little on the large side so for now I'm considering a 1000/1200v 300ma job with a matching choke. Size wise a bit more manageable. Use a variac and it'll be quite versitile.
For the bias I have a 0-500v switchable 25/125ma DC bench supply.

I have all the caps rect etc etc. So I'm at the "How to lay it out stage".

My current valves I'm playing with are the 212 and the 833. Though I have many more types of power valves I wish to try later. Hence the adjustability required.

Ok, so to my question, is one bias supply OK to feed both channels?
In this link, not that I wish to copy, this schematic shows one heater supply to feed both channels of an 833 amp?? http://www.geocities.com/bobdanielak/se833nynoise.gif
Any comments on this?

Best regards
Darren

Post by **Nick** » Sun May 27, 2007 4:20 pm

I have to say I don't know abut the shared heaters, my gut feeling is it wouldn't be ideal, but just how much crosstalk you would have I don't know.

I guess as you will be using the heaters at 0v and a fixed bias it can't be that bad. Of course it would be a bad thing using cathode bias.

As to the bias, I would think you will have to make them individually adjustable, so they will be isolated.

I would have thought though with a A2 valve like the 833 the bias supply will need to handle a fair amount of current, so would need a little more than just a VR tube and a pot.

I rearly wanted to try cathode drive some time, I was going to do that with the GM70 amp for Colins electrostatics, but it got hard to find someone willing to wind the interstage, and the cost was getting a bit beyond the original budget.

Paul Barker · Post by **Paul Barker** » Sun May 27, 2007 6:40 pm

shared cathode resistor isn't bad sounding.

careful with fixed bias on such valves very stiff supply needed and you are morelimited in grid resistor size than with auto bias.

You are trying for too much flexibility.

Paul Barker · Post by **Paul Barker** » Sun May 27, 2007 7:25 pm

notice the a2 circuit you posted has no bias supply or grid resistor so solves or avoids those problems. the 10:1 it is a tall order though. might have to getit specially wound.

question? whatdo you get if you buy a new eyboard for yourlaptop off ebay?

you get the y and the 6 atlastbut loose the one between j and l and the commer the left hand shift ey and the space bar needs a hard tap and you are £9 poorer.

Andrew · Post by **Andrew** » Sun May 27, 2007 7:57 pm

Hi Paul,

Try giving it a clean with an air can, you can also as I discovered gently lever the non-working keys up and they will "click" out. You will normally find bits of someone else's dinner stuck under them.

Hi Darren,

I use fixed bias on my 300Bs, circuit originally derived from a Simon Shilton design. I found you need separate bias voltages as things seem to sound better if you can adjust the bias to suit each valve in the pair, there could be lots of other good reasons as well, I guess, but it seems matched pairs don't remain matched for that long.....

I also found, I use an interstage, that a bigger cap than you think is necessary to close the AC loop between the secondary's lower leg and bias ground. Why? Well my thinking and it seems to be on Simon's circuit as well, is that if you have a big choke/cap on the B+ it will take a while for the B+ to discharge, so you want to bias voltage to discharge at a similar rate and not just hit 0v and switch the valve full on with what B+ remains stored in the PSU's choke, cap etc. It's also possible I misunderstood Simon's intent and it could be a big cap to avoid an unwanted filter.

Of course, if the bias PSU is totally separate then switch the bias off afterwards, I think that would be OK too.

The other thing I did, for safely, again from Simon's circuit, was put a relay in series in the ground of the B+, the relay is only closed (on) when the bias voltage is applied, that way of the bias dies suddenly the B+ is removed to.

cheers,

-- Andrew

Darren · Post by **Darren** » Sun May 27, 2007 8:47 pm

Ok, a few points there so I thought what the heck, lets clip this idea together and see what happens.

Just got back from the lock-up with the two heater supplies.

Well it's a start init....

Turns out they are 0-30VDC at 20A, can't be bad. Should cover just about any valve heater I'm ever likely to come across.

I can feel an experiment coming on when we next meet Paul, GM100s !!!

Paul Barker · Post by **Paul Barker** » Sun May 27, 2007 9:29 pm

Paul Barker · Post by **Paul Barker** » Sun May 27, 2007 9:41 pm

Paul Barker wrote:
question? whatdo you get if you buy a new eyboard for yourlaptop off ebay?

Give the seller his due he has paypalled me a full refund only a short while after I emailed him to say the problem.

Darren · Post by **Darren** » Sun May 27, 2007 10:50 pm

Well that's one 212 up and running, sounds ok,

Actually not too bad at all, I'd forgotten how much power these things had at only 950V B+ Which is where it is now.

OK it's distorting a bit, but I need to play with the operating points now.

I'm only messing at the mo, so i may just stick the 833 on before bed time.

Surprisingly not much hum, if you saw the setup you'd realise why I say suprisingly

Darren · Post by **Darren** » Sun May 27, 2007 11:07 pm

Umm,

Somethings not right, it sounds better the lower you go with the HT.

ie 250V sounds better than 600 which sounds better than 950V.

I'll leave it for tonight but would suspect something isn't up to the rating.

I'm using a cathode resistor to give -36V at 950HT bypassed.

Darren

Darren · Post by **Darren** » Sun May 27, 2007 11:58 pm

What the *&%$...!!!!!!!

I just couldn't resist, I just had to plug the 833 in, jeeeezeee..!!
Paul, forget the 212 the 833 is way, way better !!!

You know me Paul, I wouldn't say it if I didn't mean it, and I mean it.
This is one very nice valve !!!!

And power, I'm only on notch one and I'll have to turn it off before the neighbor complains. But the best bit is the bass. It's beautiful. Makes the 212 sound like a 211.

Oh what a discovery, for me anyway.

Talk to you tomorrow Paul. I'll put the 212 back in tomorrow just to make sure.

Darren · Post by **Darren** » Mon May 28, 2007 12:18 am

How do I describe this sound,

I'll have a go, I find the 211 a bit cold sounding, very precise. The 212 is a bit warmer in the mids with better/more sharp bass.

The 833 is like a PX4 with power, shed loads of power.
Put it this way. I'm running it at 800V and on notch one it's louder than the 212 at 950v On notch 11 (halfway) I have 22 notches.

Add to this, I have so much detailed and very low bass I think I could remove the active pre, 200W amp and 15" sub.

I'll turn it up a bit tomorrow and see.

Guys, this is truly awsome what I'm hearing here.

Post by **Nick** » Mon May 28, 2007 12:35 am

Quick, delete the thread until we all have a chance to buy up 833's

Interesting stuff Darren, what output TX are you using?

Paul Barker · Post by **Paul Barker** » Mon May 28, 2007 8:03 am

must try it when i get time.

Dave the bass · Post by **Dave the bass** » Mon May 28, 2007 9:39 am

I wasn't sure what an 833 looked like so I did a search on't tinterweb.

Have you seen this page Darren/Paul/Nick?

http://www.positive-feedback.com/Issue9/wavac833.htm

DTB

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