10W SET

pre65 · Post by **pre65** » Wed Oct 05, 2011 2:49 pm

Dave the bass wrote: S'funny you having a go at building this cct as I mentioned the other day because my winter DIY project this year is go at 6DN7-> TZ40 (or 811a or 830B) .

We is idea bruv's!

DTB

I saw when you got the 811a off David that you had a circuit in mind, is it different to the one I'm looking at ?

Post by **Nick** » Wed Oct 05, 2011 2:52 pm

Your maths seems fine, I would guess 1ma.

Dave the bass · Post by **Dave the bass** » Wed Oct 05, 2011 3:16 pm

pre65 wrote:
I saw when you got the 811a off David that you had a circuit in mind, is it different to the one I'm looking at ?

Radically different.

So radically different I actually haven't 'designed' it yet

Another worlds best amp....

DTB

pre65 · Post by **pre65** » Wed Oct 05, 2011 3:27 pm

Nick wrote:Your maths seems fine, I would guess 1ma.

OK, 1ma it is. Thanks Nick.

On triode 2 (pins 1-2-3 ) what might I expect ?

Something like -50V / 35ma / 275v ? (EDIT -40 /40ma/240v)

Mikes Danbury interstages may be lower resistance than the Magnaquest ones so I shall need to allow for that otherwise the anode volts will be too high.

Dave the bass · Post by **Dave the bass** » Wed Oct 05, 2011 3:40 pm

I'd guess about 420V ish DC is on the anode.

Nick mentioned before, that 2nd triode is being thrashed.

The BK DC coupled cct IIRC runs the 2nd triode @ 150V 55ma or so. About 8.25W anode disp.

DTB

pre65 · Post by **pre65** » Wed Oct 05, 2011 3:46 pm

Dave the bass wrote:I'd guess about 420V ish DC is on the anode.

DTB

420V eh ? (Edit - that's too high I think)

So -75v and 25ma, no that can't be right with a 1K cathode resistor

I'm more confused now, the brain FOG is closing in.

Dave the bass · Post by **Dave the bass** » Wed Oct 05, 2011 4:02 pm

Aye, it'll only be the voltage drop across the primary of the IT that'll be reducing the HT. If the winding resistance is higher a then maybe a bigger Vdrop.

Might be worth thinking about how much current you can pull through one of Mikes IT's too, it might be higher than you'd think.

DTB

Mike H · Post by **Mike H** » Wed Oct 05, 2011 4:07 pm

Dave the bass wrote:I'd guess about 420V ish DC is on the anode.

Nick mentioned before, that 2nd triode is being thrashed.

So, reduce its current until it's within its 10-W max. by increasing cathode resistor. If dangleberry IT I suggest around 20mA.

Don't worry too much about what Voltage is on V2 anode, just settle on it being a few Volts less than HT whatever you do.

The BK DC coupled cct IIRC runs the 2nd triode @ 150V 55ma or so. About 8.25W anode disp.

DTB

Ack-shirley, if V1 anode is held down to about 100V that's what I'd do, viz DC couple V2 grid to V1 anode, keeps the Va & Watts down on V2, indeed I did exactly that with a 6EM7 in me Aurora amp once, worked fine.

Post by **Nick** » Wed Oct 05, 2011 4:15 pm

You are assuming you are starting from a working circuit. has anyone else built it?

pre65 · Post by **pre65** » Wed Oct 05, 2011 4:16 pm

All I'm really trying to do, is understand what current the designer was wanting to run the second triode at. With a 1K cathode resistor the grid volts and current (in ma) will be the same number.

Then I will have enough info to work everything I need out. (I think).

pre65 · Post by **pre65** » Wed Oct 05, 2011 4:18 pm

Nick wrote:You are assuming you are starting from a working circuit.

Yep, that is my assumption.

Otherwise I have to start from scratch, and that frightens me.

Mike H · Post by **Mike H** » Wed Oct 05, 2011 4:21 pm

Dave the bass wrote:Might be worth thinking about how much current you can pull through one of Mikes IT's too, it might be higher than you'd think.

It's rated at 5 Watts apparently but depends a lot how it's wired up; problee 50mA I would say as a safe max. before noticeable bass roll-off starts rearing its ugly head. But as DC operating point follows from what peak signal current is actually required, which follows from secondary load impedance, I doubt it needs be anything like as much as that in practice.

Dave the bass · Post by **Dave the bass** » Wed Oct 05, 2011 4:27 pm

Why not build it.... but.... make everything easily accessible so you can do proper/easy fettling and replacing ('cos it'll need it at 30W through a 6EM7!) 'scoping and learning where the desing is a bit iffy possibly?

I'll send more 6EM7's!

You could REALLY learn a lot from this cct I reckon Phil in that its got an unknown pedigree and hasn't been proved but it could be YOUR job (and thus Victory!) to put it right and tweak it so it works OK (maybe even a contender for best amp in the world...natch!)

Think of it as a challenge, you can do it, gwaaarrrrn.

EDIT. Actually thats my idea behind this years winter-project, having a go at 'designing' a simple 6DN7 amp then seeing if I can 'graft on' a different output stage just to see how it'll measure and behaves when I've had a go at doing something myself. I'm not expecting great things but it IS a great way of learning I think.

DTB

Mike H · Post by **Mike H** » Wed Oct 05, 2011 4:31 pm

pre65 wrote:All I'm really trying to do, is understand what current the designer was wanting to run the second triode at. With a 1K cathode resistor the grid volts and current (in ma) will be the same number.

Yes wow according to unit #2 graph it makes grid -50V -ish, and even if that makes Va 300 -ish (subtract Vk & IT DCR drop from HT) that's still >=50mA so Nick's right unit #2's being flogged to death at >=15 Watts.

Sorry can't make the answer come out any differently!

pre65 · Post by **pre65** » Wed Oct 05, 2011 4:36 pm

Perhaps I should do something completely different ?

Like hibernation.

audio-talk

10W SET

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