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Old 9th January 2013, 03:15 AM   #61
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Quote:
Originally Posted by Robert Kesh View Post
CC, am curious about your biasing. What voltages show on the cathodes and grids??
Good point.

If we make R3 & R6 100k, then we get 0V on the grids with the cathode @ 0.7V and the anode @ ~21V with 0.375mA quiescent, (kind of conventional) but surprisingly with the values shown, the anode moves to ~4V with the cathode and grid @ ~-7.8V, the cathode actually being slightly (about 0.1V) more negative than the grid, the quiescent current goes to ~0.8mA, but the output waveform (when hard driven), while still pretty square, is considerably more symmetrical than with the 100k's (because there's more room to swing positive I guess). When lightly driven, both look quite clean. There's plenty of gain (>35 after both stages) and the FFT looks pretty typical for a tube.

It all kind of blew me away, but I'm not really a tube guy, and I just have to go with the sim. Of course I could be operating it wrong, or there could be problems with the models.

Looking at it again, I'd probably put a blocking cap at the input, it's not hi-fi.

Perhaps you'd care to comment further, as I said, I'm not really a tube guy, I'm just old, old, old.
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Old 9th January 2013, 05:23 AM   #62
m145mcc is offline m145mcc  United States
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You do have some valid points. I did remove the 1M resistor, It did make little since after you pointed it out.(pertaining to previous post) I did include a grid stopper (R8, R9, C4), It forms a roll-off at about 4khz. I got rid of the gain at (R4,R2) because it didn't seem like a good idea to load the next stage with 30 volts, but If I can do that without damaging anything I'll remove (R4, R3) and replace with a proper grid stopper. Just to make sure I got my lingo right, a grid stopper is to filter the DC from the signal right? (R10, R11) was to drop everything down to about line level, the pot was just for level control. I do appreciate your provided schematic, but I would hate to just copy your work.
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Old 9th January 2013, 09:59 AM   #63
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Quote:
Originally Posted by counter culture View Post
Good point.

If we make R3 & R6 100k, then we get 0V on the grids with the cathode @ 0.7V and the anode @ ~21V with 0.375mA quiescent, (kind of conventional) but surprisingly with the values shown, the anode moves to ~4V with the cathode and grid @ ~-7.8V, the cathode actually being slightly (about 0.1V) more negative than the grid, the quiescent current goes to ~0.8mA, but the output waveform (when hard driven), while still pretty square, is considerably more symmetrical than with the 100k's (because there's more room to swing positive I guess). When lightly driven, both look quite clean. There's plenty of gain (>35 after both stages) and the FFT looks pretty typical for a tube.

It all kind of blew me away, but I'm not really a tube guy, and I just have to go with the sim. Of course I could be operating it wrong, or there could be problems with the models.

Looking at it again, I'd probably put a blocking cap at the input, it's not hi-fi.

Perhaps you'd care to comment further, as I said, I'm not really a tube guy, I'm just old, old, old.
Thanks. Some models aren't good at modelling grid current, which obviously sky rockets when the grid is close to positive.
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Old 9th January 2013, 06:48 PM   #64
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Originally Posted by m145mcc View Post
I would hate to just copy your work.
That's the spirit, make your own mistakes.

A grid-stopper is a series resistor placed as close as you can get it to the grid pin on the tube, often ~10k, but the value can actually be calculated to work best with the dynamic input capacitance of the tube.

Grid Stopper Resistor Calculator
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Old 11th January 2013, 04:08 AM   #65
m145mcc is offline m145mcc  United States
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Pretty sure I have everything buttoned up now. The frequency response looks good. Hopefully someone can approve this. I still got to add supply caps and a few other little things, oh and thanks for the grid stopper link.
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Old 11th January 2013, 09:21 PM   #66
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Looks fine.
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Old 11th January 2013, 09:26 PM   #67
m145mcc is offline m145mcc  United States
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Your not yanking my chain right? Thanks, ill post a few pics after I build this thing. Thanks for every ones help.
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Old 11th January 2013, 09:47 PM   #68
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Originally Posted by m145mcc View Post
Your not yanking my chain right? Thanks, ill post a few pics after I build this thing. Thanks for every ones help.
No, looks like totally standard two stage design. Even got the standard grid stoppers.

I guess you may want to be prepared to tweak the output divider to match whatever chip amp circuit you use. And remember you'll need a coupling cap after the output.

If it's of interest, I just built a tube chip amp hybrid myself, just got it working today, but in pieces and bread boards, etc. Two 6N11s and an LM3875.
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Old 11th January 2013, 10:15 PM   #69
m145mcc is offline m145mcc  United States
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very cool, and I probably will tweak the "attenuator". you should post a few pics when ya get every thing done. I would be very interested to see and maybe hear it.
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Old 11th January 2013, 11:54 PM   #70
m145mcc is offline m145mcc  United States
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Since everybody been so helpful I was wondering if anybody has an psu advice. I using a Avel Y236652 250VA 25V+25V Toroidal Transformer 122-625 and a 262F6 Hammond Manufacturing | 262F6-ND | DigiKey . I'm 65% sure this will give me what I need With "ok" ripple rejection. I'm most worried about the way I'm going to wire the tube filaments. sorry no neat simulations just pen and graph paper this time.
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