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Dave the bass · Post by **Dave the bass** » Fri Jul 17, 2009 8:56 pm

Andrew wrote:Bruce Rozenblit's book has all the equations in Dave.

-- Andrew

Well...confession time fior me Andrew!! I've had that book since I've first met all you ex WD guys at a Heathrow show, I had a sarnie with Neal and he recommended that very book and TBH I've struggled with it, number/equation phobia kicking in as usual I guess.

Having you guys holding my hand through this very thread has made the book more approachable, I know it's meant to be the other way round but being bottle-fed (valve joke, SWIDT?) by y'all is making a HUGE difference.

I've even started understanding a some of MJ's basic building blocks too

The more I do it/read it the better I'll get. The calculator is now my new bestest friend (next to you lot obviously

).

DTB

SimonC · Post by **SimonC** » Fri Jul 17, 2009 8:56 pm

Nick, Andrew & other guru's

Without wanting to hijack DTB's thread at some point during the teach-in could you give me some pointers on how to calc the expected slewing rate distortion between the driver & power stages?

I want to do some more work on the 6C33 this summer and its proving a difficult beast to crack. I've got some idea's and valves in mind but I'm struggling a bit with how hard I need to drive it.

Might be better to split off into a new thread??

Cheers
Simon C

Dave the bass · Post by **Dave the bass** » Fri Jul 17, 2009 9:08 pm

Nick wrote:Yep, that looks about right Dave. So now you have

ra = 9743
µ = 13.8 (from the sheet)
Rl = 47k

So we can calculate

rk = ( ra + Rl ) / ( 1 + µ )

Its right

Whahey!!!! Super chuffed.

Equation phobia just kicked in... I've checked with JTS and we think its this. You solve the sums in the brackets first then divide the 2 results, if so then....

9743+47000= 56743

1 + 13.8=14.8

56743/14.8=3833.98 ohms

Therefore rk = 3833.98 ohms rk being the dynamic cathode resistance yes? In that we can't physically touch it but its a factor inside the function of the valve its self?

Andrew · Post by **Andrew** » Fri Jul 17, 2009 9:11 pm

SimonC wrote:

Might be better to split off into a new thread??

Cheers
Simon C

Yes, not least 'cos I would need to think about it. Do me good to exercise the old grey matter.

-- Andrew

Andrew · Post by **Andrew** » Fri Jul 17, 2009 9:16 pm

Dave the bass wrote:
Equation phobia just kicked in...

Well done Dave, you've cracked it! I reckon its time to change the 'L' to at least a 'P'. Or even better throw away the plates completely.

PS I must admit every time I pick up Rozenblit, RHD or MJ etc I read another chunk and another layer of the 'onion of enlightenment '(how very Zen of me) is peeled away. That's what makes these writers so good, there's so many levels to these books.

-- Andrew

Dave the bass · Post by **Dave the bass** » Fri Jul 17, 2009 9:20 pm

Andrew wrote:
Dave the bass wrote:
Equation phobia just kicked in...
Well done Dave, you've cracked it! I reckon its time to change the 'L' to at least a 'P'.

-- Andrew

Tell you what Andrew, it's a deal.... BUT only when I've built another single stage common cathode amp on my own with only minimum maths help (if needed).

I'm hoping to use this thtread and it's working practises as a reference then do another valve/stage totally on me own.

THEN..... yeah! Swap the red L for a green P.

Deal?

DTB

PS. No prob's SimonC, ask me in another 6 years, I might be slewing all over the place by then

simon · Post by **simon** » Fri Jul 17, 2009 9:49 pm

Really enjoying this thread on a number of levels. You're nearly there Dave

.

I think Simon's slew rate suggestion is a good idea, I 'm not convinced I really understand it.

I don't know Rozenblit's book at all - is it worth getting as well as MJ, or does it cover similar ground in a similar way?

Dave the bass · Post by **Dave the bass** » Fri Jul 17, 2009 10:08 pm

simon wrote:Really enjoying this thread on a number of levels. You're nearly there Dave .

I think Simon's slew rate suggestion is a good idea, I 'm not convinced I really understand it.

I don't know Rozenblit's book at all - is it worth getting as well as MJ, or does it cover similar ground in a similar way?

Cheers matey, yes, slowly its going in.

The Rozenblit book (http://www.audioxpress.com/bksprods/products/bkaa42.htm) is written in an American kinda way, MJ is English natch and recently I bought a copy of Elektors 'Build yer own Valve amp' by Rainer zur Linde ( which is a european slant on the same subject giving a background history to valves I've never heard of (AD1, RE604 etc etc etc).

All good and essential IMO. In 20 years time I might begin to appreciate their worth

DTB

Dave the bass · Post by **Dave the bass** » Fri Jul 17, 2009 10:12 pm

Bedtime for me, gonna rest the brain!

We've got the Dartford festival this weekend, that means 2 solid days of Skate and BMX-ing (+ boozing and The Temperance Seven live on stage whahey!).

Catch up with y'all Sat or sundee night,

ttfn

DTB

pre65 · Post by **pre65** » Fri Jul 17, 2009 10:40 pm

Dave the bass wrote:The Temperance Seven live on stage whahey!).

DTB

Cor, I remember them !! Have fun,and give the bwain a rest.

Dave the bass · Post by **Dave the bass** » Mon Jul 20, 2009 12:55 pm

I think I pickled my brain this weekend, certainly can't feel my legs but I can see 'em though so they must be there

Onwards with schooling.

Whats next? Calculate cathode cap value?

DTB

Post by **Nick** » Mon Jul 20, 2009 1:21 pm

Therefore rk = 3833.98 ohms rk being the dynamic cathode resistance yes? In that we can't physically touch it but its a factor inside the function of the valve its self?

Yep, you have rk now, and the cathode resistor you have chosen is 2k, so work out the value of these two in parallel, then pick a -3dB point to aim the RC low pass filter to.

Then plug into the equation for a RC filter

f-3 = 1 / ( 2 * Pi * R * C )

Rearanging this to give C for a target f-3

C = 1 / ( 2 * Pi * R * f-3 )

Where f-3 is in hertz, R is in ohm, Pi is 3.1415 etc and C is in Farard.

Dave the bass · Post by **Dave the bass** » Mon Jul 20, 2009 4:26 pm

Nick wrote:
Yep, you have rk now, and the cathode resistor you have chosen is 2k, so work out the value of these two in parallel, then pick a -3dB point to aim the RC low pass filter to.

Then plug into the equation for a RC filter

f-3 = 1 / ( 2 * Pi * R * C )

Rearanging this to give C for a target f-3

C = 1 / ( 2 * Pi * R * f-3 )

Where f-3 is in hertz, R is in ohm, Pi is 3.1415 etc and C is in Farard.

OK, the formula for 2 resistors in parallel is

1/r+1/r=1/R

or
r1xr2/r1+r2 = R

(thanks to the Newnes Engineers pocket book!)

Here we go, 1/2000Rk + 1/3834rk = 1314.36ohms.

...and again to check...

2000 x 3834/2000+3834 = 1314.36ohms

DTB

Dave the bass · Post by **Dave the bass** » Mon Jul 20, 2009 4:28 pm

Hmm....a good -3dB point.

Um, sub 20hz natch, ermmmmmm 10Hz OK?

DTB

EDIT.

Working on the assumption it is (and I've got a new calculator!).

1/2x3.14x1314x10 = 0.0000121F = 12.1uF (I think).

Just to experiment 'cos I can, lets reduce the f-3 to 5Hz.

1/2x3.14x1314x5= 0.0000484F = 48.4uF (I think).

Lower cut off = bigger by pass cap (not unsurpisingly).

Are those sums right?

DTB

Post by **Nick** » Mon Jul 20, 2009 8:18 pm

Yep, looks good to me, I would aim for at least 5hz, maybe lower.

Ok, so its done, you have designed a single triode common cathode stage

Oh, and you can calculate the output impedance. You know your Rl and rl, so put them in parallel to find what rout is.

Just need to decide on what its going to drive, then you can use the RC formula again to work out what size of coupling cap you need.

audio-talk

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