|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
76.174.68.112
Could you point me to the math?
I want to figure out what would be "okay..in your opinion...to drive a 845. ( are "math" and "opinion" contradictory?)
I know current would also be a factor and I have a source to find that out....( Bugle article and also calculators available at vt52.com)
Thanks1
Follow Ups:
Grid current aside, the reactance of the Miller capacitance will cause a drop in the high frequencies.The math is the same for any low pass filter. -3db point = 159155 / value of cap in uf X value of source impedance in ohms
The current needed to drive that reactance to a given voltage is just ohms law. Current equals voltage / resistance(reactance)
A 845 has 11.5pf grid to plate and a gain of 5.3 so 11.5 X 5.3 = 60.95pf + the 5pf grid to filament.
66pf would need to be driven by 12k ohms (assuming no other stray capacitance) to have a -3db point of 200kHz. (The only reason I calculate for 200kHz is phase. For the phase to be undisturbed at 20kHz the -3db point of the filter needs to be 200kHz. But you can do your own math and find the -3db point for any combination of drive impedance and shunt capacitance you have.)
66pf would have a reactance at 200kHz of 12k ohms. If you were swinging the grid 140 volts peak that would take 11.7ma of current. The driver stage would have to supply that current.
If we settle for a -3db point of 100Khz then a source impedance of 24k ohms would do and only 5.85ma of current would be needed.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/23/15
"66pf would have a reactance at 200kHz of 12k ohms. If you were swinging the grid 140 volts peak that would take 11.7ma of current. "I don't think he will need to drive the grid at 200kHz, but if he does, peak current will be approximately 8mA due to power factor. More important, peak current at 20kHz - the highest frequency likely to be reproduced - will only be a little more than 1mA. That's the value he has to budget for when designing the driver.
--------------------------
Buy Chinese. Bury freedom.
Edits: 05/24/15 05/24/15
1mA? Hell, a 12AX7 will do that just fine ;)
cheers,
Stephen
"1mA? Hell, a 12AX7 will do that just fine"
You're probably joking, but this does deserve further explanation. Tre' said his goal was to have phase undisturbed at 20kHz. He then determined that in order to do that, the 66pF output stage must be driven from a 12K ohm source. It's true a 12AX7 could provide the necessary current to drive the output stage to 140V peak (the hypothetical maximum grid voltage), but it won't provide the output impedance necessary to achieve -3dB at 200kHz. Both the minimum output Z and minimum drive current capability are necessary for the stage to function as Tre' described. In this case, a 'SN7 or similar would meet both requirements.
--------------------------
Buy Chinese. Bury freedom.
Or you could roll off the lows by partially bypassing rk (if present) and with special expen$ive boutique caps and magically get adequate HF response. :-) heh heh. JH
For your time and for the calculations!
You'll want to look at Miller capacitance, and how that limits bandwidth.
IMO, driving the 845 to the point of drawing grid current may not be such an important design consideration (Tre may disagree).
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: