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Old 7th December 2010, 04:55 PM   #11
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Quote:
....by connecting the secondaries of the output pubes...
Should be: "for example by connecting the secondaries of the output transformers in series with the cathode resistors, (and with correct polarity).
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Old 7th December 2010, 05:10 PM   #12
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Originally Posted by artosalo View Post
...for example by connecting the secondaries of the output pubes in series with the cathode resistors...
I'd be worried about this too!
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Old 7th December 2010, 05:57 PM   #13
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Ok I have sketched 3 different options. I think the one suggested was the middle option. I seems to me that global feedback is what was suggested to lower output impedance to help with the mismatched load. To do that I think I have to introduce local feedback at either the driver stage or the power stage. take a look and see what you think. Let me know if I might need to adjust the feedback network resistors. I can adjust the caps to get the low frequency cut off that I need. I like the looks of the first network I have shown but am not too familiar with adjusting the size of that resistor for optimum in this case. Let me know if I didn't get something correct or misunderstood someone. Thanks again.
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Old 7th December 2010, 06:25 PM   #14
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I would try the first (new) one with NFB from secondary of the OPT.

Quote:
Let me know if I might need to adjust the feedback network resistors.
To adjust the proper feedback resistor is not big science.
Do this way:

1. Simply adjust your amplifier - without this NFB-resistor - to nearly max. undistorted output level and read the voltage ( I guess you have unversal meter with AC-ranges). You can use loudspeakers if you do not need resistive dummy load.
Take the test frequency, say 1 kHz from your PC's soundcard output.
There are many free "signal generator" -free SW available. (TrueRTA forexample).

2. Connect a 22 ... 100 kohms potentiometer to the place of NFB-resistor and adjust it to the voltage reading, which is between 0,3 ... 0,1 x the original voltage.
Value (0,3 x) represents some 10 dB NFB and value (0,1 x) represents 20 dB NFB.

3. Measure the value of the potentiometer and replace it with fixed resistor with same value.
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Old 7th December 2010, 06:28 PM   #15
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Quote:
2. Connect a 22 ... 100 kohms potentiometer to the place of NFB-resistor
Correction: This potentiometer needs to be smaller. Try from 2k2 to 10 K.
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Old 9th December 2010, 01:15 PM   #16
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So last night I went back with some of these suggestions to implement them. The first thing that I did was separate out the 6v6 bias. So they both now have 470uf caps and 500 ohm resistors. I then took out both cathode bypass capacitors off the 12at7's. I then fired up the circuit. It sounds golden.

I then went ahead and decided to put a filter cap before the inductor to see if I could get the high voltage up to about 350 or so. I put in a 67uf there and now my high voltage is at 460 or so. not so good on the 6v6. However not a bad place for a 6l6g right?
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Old 9th December 2010, 01:50 PM   #17
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Quote:
Originally Posted by J_Hart_14 View Post

I then went ahead and decided to put a filter cap before the inductor to see if I could get the high voltage up to about 350 or so. I put in a 67uf there and now my high voltage is at 460 or so. not so good on the 6v6. However not a bad place for a 6l6g right?
Start with a 3-5uF cap and go from there.
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Old 9th December 2010, 05:29 PM   #18
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Quote:
I put in a 67uf there and now my high voltage is at 460 or so. not so good on the 6v6.
Not necessarily bad for 6V6 since your output transformer has a bit too high turns ratio for 8 ohms load. Now with 450 volts anode voltage the tube works at higer impedance level, assuming the anode current is at right (=lower) level.
Now your bias (cathode voltage) should be some 15 volts if cathode resistor is 500 ohms. If not, adjust cthode current to 30 mA.

And, yes I know the max. anode voltage rating is 315 volts, but so what.
Main issue is that the max. anode dissipation is not exceeded (too much).
Actually 6V6 may sound great with this unusual bias - anode voltage - load resistance combination.
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Old 9th December 2010, 05:34 PM   #19
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Quote:
Now your bias (cathode voltage) should be some 15 volts if cathode resistor is 500 ohms. If not, adjust cthode current to 30 mA.
I just now looked the tube data. Your cathode voltage should be higer, some 35 V to get 30 mA cathode current.

Are you sure the supply voltage really is 460 volts when 6V6 is drawing current too ?
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Old 9th December 2010, 05:58 PM   #20
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I see. Yes that 460v reading came with the two 6v6 in the circuit and the addition of that 67uf cap. I also wish I had a couple of smaller value caps to see if that would reduce it, sadly the smallest one I have with that voltage rating is 47uf.

I see where if I had the same setup with a 500ohm resistor that I would have about 445V across anode to cathode and if it was set at 30ma that would give about 13.35W dissipation and an internal resistance of about 14.8k which would be about right for the turns ratio and an 8 ohm load. I also have a couple of higher wattage 1k ohm resistors if I end up in the wrong place current wise. I will see if I can get the circuit tamed tonight.
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