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CCS on 12AU7 Push-Pull
CCS on 12AU7 Push-Pull
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Old 28th May 2018, 09:45 PM   #1
guavatone is offline guavatone  United States
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Default CCS on 12AU7 Push-Pull

I've been working on a Differential tube mic preamp based on McCurdy au300 and Langevin 5116b designs. The output jack goes to a sowter 9900 transformer at the moment. In this design the output tube is biased so cathodes are at 10.5-11 V when using the cathode resistor and balance pot. But when I use the CCS it seems to be rather high, like 12.5-14.5V. I've tried a bandgap lm385 2.5V as suggested by Merlin B in his book. I think with this circuit, I finally got tubes be receiving about 13 mA between according to a 10 Ohm resistor on the CCS board. But the Cathode voltage seems a bit high at 14.4 and 13.2V, which makes me think that the tube isn't biasing.

My question is whether or not the CCS below will work to bias the tubes at 11V? Morgan jones and others like index seem to do their CCS with negative supplies. But I don't have a negative rail to grab from at the moment.

There should be connections from V2's plates to V3 grid stoppers via a 47nF coupling cap. and B+ is 290V-310V. R11 is omitted when using the CCS.

PS. I browse here a bit and appreciate the wealth of info. Thanks to those who maintain this site
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Last edited by guavatone; 29th May 2018 at 05:50 AM.
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Old 29th May 2018, 05:55 AM   #2
jazbo8 is offline jazbo8
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CCS on 12AU7 Push-Pull
Forget the cathode voltage, it will be as high/low as determined by the CCS to maintain the correct current.
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Old 29th May 2018, 07:05 AM   #3
guavatone is offline guavatone  United States
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Thanks so much Jazbo8. I have another thing thatís puzzling me. I have a 10 ohm resistor at the output of the CCS to measure voltage and read the current. And whatís puzzling me is that even with an Re=732 I am still reading 130 mV which tells me itís outputting 13 mA. Right? I thought I should get around 150 mV. I Ďm getting 11.75 V across the Zener. Iím wondering if I need to lower the 100K resistor going into the LED to get some more current flowing.
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Old 29th May 2018, 08:35 AM   #4
jazbo8 is offline jazbo8
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CCS on 12AU7 Push-Pull
If you don't have a negative rail, it still can work just at the expense of wasting some B+ voltage. Why 150mA?! That would kill the 12AU7, 13mA sounds about right...
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Old 29th May 2018, 09:44 AM   #5
guavatone is offline guavatone  United States
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I was shooting for 14-15 mA. But I am reading volts across a 10 ohm resistor at the CCS output expecting closer to 150 mV to show that iím Getting 15 mA using ohms law.
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Old 29th May 2018, 09:51 AM   #6
jazbo8 is offline jazbo8
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CCS on 12AU7 Push-Pull
I see, I didn't read your post carefully. I doubt the slight increase in current will make much a difference to the performance of the diff amp.
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Old 29th May 2018, 10:14 AM   #7
guavatone is offline guavatone  United States
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Probably so. This my first time playing around with current sinks for tubes, so I just find it strange that changing Re value may have little effect and the tube just takes the current it needs. Maybe I would get different readings sending signal through it.

One interesting thing is that B+ seems to drop substantially less than the resistor biasing, or my test results are off. Iíve had it on and off the bench many times over the past few weeks.

Could it be that CCS is just more efficient?

I like the sound though and what the Audio Precission is telling me at around .06% THD.

ONE thing I donít like is the delay of 10 seconds for the CCS to drop the B+ and activate/stabilize.
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Old 29th May 2018, 11:56 AM   #8
thoglette is offline thoglette  Australia
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The limit will be on how low (voltage wise) the CCS can go given that you've tied the grids to ground.

Lynn Olsen has spent a lot of time playing with differential valve amps with exceptionally good performance. He's on these forums and also on his website Nutshell (see here to start)

He tends to use the CCS to drive the plates and uses a shared cathode resistor and bypass the cathode to the B+, not ground. Although it's a little more complicated than that with a little voltage regulation thrown in too. (see the Raven or amity full schematics)
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Old 4th June 2018, 06:44 AM   #9
gingertube is online now gingertube  Australia
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You have no compliance voltage across the CCS to work with.
That 12V Zener in the bottom transistor base is killing you.
The CCS is simply not working as a CCS.
Replace the 12V Zener with another red led and set the bottom transistor emitter resistor (RE1) to 56 Ohms.

Cheers,
Ian
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Old 4th June 2018, 04:29 PM   #10
guavatone is offline guavatone  United States
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Thanks In. What do you mean by compliance voltage?

Would a ring of two be better/work correctly?

Last edited by guavatone; 4th June 2018 at 04:44 PM.
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