kambo said:Ra 5k Rk 890r , bypassed...
i have another 6sn7 in front as micpre...
Winston O'Boogie said:OK. Your bottom triode is getting way more signal with a 5K load on the top valve. If you like it, more power to ya
I would plop a 385 ohm resistor at the top myself. Shooting from the hip and an adled & dodgy memory, I'd also start with a quiescent current from about a 400 ohm-ish cathode resistor on a 6SN7.
But each to each my friend
kambo said:i actually started with 400 ohm. but didnt like the sound at all!
i usually record in to same cubase file, and have instant access to my other micpre takes,,. ( ie: miked speakers playing drums, music,
and some talking etc! )
this setting sounding really good to my ears. very fat and very lo distortion! around 8mA is more hairy sounding!
btw, i have something like 130-150vdc(need to measure) coming from first stage to wcf input!
edit: top and bottom is 3.4mA 100th digit is only 1 off... 3.45 top - 3.44 bottom
Winston O'Boogie said:push-push
Peace.
Winston O'Boogie said:So:
Ra = rp/mu.
Ra for a 12BH7 is 5100. Mu is 16.5.
So Ra = 310 ohms.
ruffrecords said:Unfortunately those values of mu and Ra are true only at one operating point. They are not fixed or constant values. At lower plate voltages (such as in a White) u increases. Ra is very dependent on standing current. So exactly what is the design standing current in the Jensen design? Here is a datasheet for the 12BH7:
http://www.r-type.org/pdfs/12bh7.pdf
Are we sure Jensen got it wrong?
Cheers
Ian
I'm not really contesting the standing current, it's the 5K top resistor in your schem.kambo said:NYD line amp is it at 17mA too ...
edit : Jensen might get it wrong but, NYD wouldnt ;D
Winston O'Boogie said:Unless I completely lost my marbles again I'm sure Jensen got it wrong. Not as wrong as some have and, I admit, it's not as complete a disaster as elsewhere, but it's off. Plug the values into the equations I gave from any point on the 12BH7 datesheet curves and it will hover around the value I gave.
ruffrecords said:I can understand setting the plate resistor at 1/gm simply because the stage gain is gm.Ra which will be unity (as required) if Ra = 1/gm.
What I do not understand is your contention that one value will work for most anywhere on the 12BH7 curves. gm varies significantly with plate current which means Ra depends critically on standing current.
Cheers
ian
Winston O'Boogie said:Ah I see. Yes I did say that, which is not correct. Sorry. Had been up all night... have had a nap now
I meant, we have a given point on the curves which we have defined based on our standing current, hence gm is reasonably known.
We set our value for Ra based on this and, within reasonable boundaries around this point, where Rp for the 12BH7 may be 5000 or 5300, and Mu may be 17.5 or 16.5, then the Ra will be quite reasonably within a few percentage points of being exact.
So, given the operating point of the 12BH7 as used in the Jensen schematic, I say that my value of 310 ohms is a better choice than the 680 ohms they chose. There is nowhere around that point at which the reciprocal of gm is approaching 680.
At low anode voltage use (as defined in the datasheet you provided), the 1/gm is lower, in which case Jensen's Ra value is even further off.
Again, sorry for the sloppy and incorrect statement. My intentions were in the right place, brain not so much!
ruffrecords said:k
From load line I estimate a 10V swing in Vg gives a 16mA swing in Ia which gives gm=1.6mA/V so Ra = 625 ohms. Have I gone wrong somewhere?
Cheers
Ian
Edit: I used the data sheet found here:
https://www.alldatasheet.com/datasheet-pdf/pdf/230561/GEC/12BH7.html
Winston O'Boogie said:Thanks Ian. I'll check the datasheet in a while, not able to print and no computer at the moment so it's a little hard on a small screen but I'll give it a go.
There's every possibility you have NOT gone wrong. Could very well be me who is wrong. I've had to try and relearn stuff I knew quite well a decade+ ago. Not using information for a long while, along with seizures and brain damage catches me out sometimes
Again, PRR would be the one to nail this.
Back at ya as soon as...
Cheers.
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