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In Reply to: RE: single Lundahl LL1670 grid choke for Push-Pull? posted by rongon on November 30, 2015 at 08:13:23
It should work fine. With 540hy of end to end inductance expect each signal to see 1/2 that value for calculations.
dave
Follow Ups:
Thanks Dave!
When using grid chokes on a push-pull pair of output tubes (like triode-wired 6L6, for instance), what does the driving (series) resistance need to be? I threw the grid chokes in a quick sim, and got a horrendous, high-Q peak at 10Hz. In the screenshot, the green trace is the 4 ohm tap on the OPT, the blue trace is grid of U3 (6L6).This is with a 5687 cathodyne using 10k plate/cathode load R's, 13.5mA Ia. I varied the value of interstage caps from low to way high, which only moves the frequency of the peak up or down, but not the amplitude or Q.
Is that because of the high DCR of the choke? That's 2k65 on either side of the center tap.
Edits: 11/30/15
Remember, everything is a tradeoff!
Yes, you will have that peak down there to deal with, but in the end you will have improved overload characteristics as you drive the grids of the output tubes to and past 0V (do you actually care about this?).
Another thing that folks like with a center tapped grid choke is that you can do phase inversion with the choke itself.
> Another thing that folks like with a center tapped grid choke is that
> you can do phase inversion with the choke itself.
How is that done? Like in the second stage's grid, in this circuit?
http://www.electra-print.com/docs/el34lowimd.pdf
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I found the original Electra-Print circuit, but I can't find any specs on the PSA-2 (N or D versions) such as DC resistance of the coil, inductance, etc. Does anybody have that info?
--
The higher the DCR, the lower the amplitude and Q of the peak.
Also the peak should be lower in amplitude and frequency as you increase the value of the coupling cap.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
> The higher the DCR, the lower the amplitude and Q of the peak.
I wonder... Should I try a resistor in series with each 'output' of the choke? In other words, a series resistor between the outer end of each winding to the grid of each output tube. That would be instead of loading each winding with a parallel resistor (33k).
Thoughts?
Ron
I would do neither.
To me a parallel resistor defeats the purpose of having the grid choke.
What happens if you use 4.7uf caps instead of 1uf caps?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I tried that, and it lowered the peak's amplitude just a bit, and lowered the frequency of the peak to down below 10Hz (moved the peak's center frequency down). It didn't seem to change the Q much at all (the shape of the peak stayed pretty much the same).
Weird. Could just be an anomaly of the model. I'll have to use alternate tube models to see if that makes a difference.
Aren't magnetics fun?
Great fun! ???
If the frequency is below 10Hz then I would say it's out of your way, don't worry about it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Well, maybe not out of my way, since I do play records ("vinyls" as the young 'uns call 'em). The cartridge/tonearm resonance can be right there in the 8 to 11 Hz area, so boosting it by a dozen dB would make the woofers pump quite a lot.
I tried plugging that choke after a long-tailed pair, and got the same resonance. So it's not the tubes used, or even the kind of phase splitter circuit used.
If these simulations are accurate, I think I'd use a grid choke in a single-ended circuit, but not in a push-pull circuit.
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the grid choke does the same thing in a single ended circuit. Typically the response in spice is much worse than you ever see in practice and typically a 50-100K resistor across the choke will damp the resonance nicely. This resistor is really easy to put in and out of circuit while listening and then your ears can decide.
The actual frequency where it happens is purely a function of the values of L and C and everything else just works into the Q (how well it is damped)
dave
Ah HA!
Yeah, I found that 33k in parallel with each 270H winding made everything perfectly copacetic in LTspice.
So it is expected/accepted practice to place damping resistors across the choke. Just like damping ('tuning') a transformer. I had to do exactly that with the Edcor 15kCT:15kCT transformers I was playing with. I found that a 10k resistor across each secondary winding sounded OK. 22k end-to-end also worked fine. Tune to taste...
I'm not sure why putting a parallel resistor 'defeats the purpose' of using a grid choke. Seems to me that most commercial magnetics need damping in real world use.
Good learning experience, this...
Why WOULD putting a resistor in parallel defeat the purpose of the choke? For one thing, it would limit the net grid resistance to 33K max, but is that a bad thing?
I thought the point(s) of a grid choke was having a low DCR to ground from the grid (he would still have that) and a high AC impedance to ground from the grid so the driver stage is loaded with a high impedance for less distortion (he won't have that anymore).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Based on my quick look at the proposed schematic before posting last time, if the proposed 33K resistance is in parallel with the grid choke, then it does limit the max grid to ground resistance to 33K, but how does this affect the "load" on the driver stage? (This is probably a dumb question, but I think of the load as the impedance between the plate and B+.)
The grid resistor of the stage driven is in parallel with the plate resistor and those two parallel resistances are seen by the driver tube as it's load.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Of course. Thanks.
If I load the plate of my 6sn7 driver tube with a CCS, the load line for that tube will not be totally horizontal.
The value of the grid resistor on the output tube will be the load for the 6sn7.
Using a 270k grid resistor gives a near horizontal load line for the 6sn7.
I get almost the full mu of the tube for the gain of the stage and the distortion will be much lower than a standard 33k plate resistor but there will still be some current change through the 6sn7 so the cathode will still need to be bypassed.
Any audible contribution of that bypass cap will be greatly minimized vs. a tube loaded with a normal sized plate resistor. In other words, the cathode bypass cap is not "in the signal path" nearly as much.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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