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CCS on 12AU7 Push-Pull
CCS on 12AU7 Push-Pull
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Old 4th June 2018, 11:10 PM   #11
gingertube is offline gingertube  Australia
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Compliance Voltage is the Voltage the CCS has to work with.
With the 12V zener and then the RED LED (1.4V) then even with the top transistor staurated the 12AU7 cathodes will sit at a bit above 12 + 1.4 = +13.4V.
With that voltage you are not getting the current you want.

You also need to note that with a diff amp then the cathodes will have half of the input voltage on them. On the negative signal peaks you still need enough voltage across the CCS for it to keep working.

With the 12V zener changed to another RED LED thenthe top transitor base will be at about +2.8V and teh CCS should work as long as the collector of teh top transistor is about 1V higher than that so I would say the CCS needs +3.8V "Compliance Voltage".

At the Currents you are setting the bias voltage for the triodes will be about +10V.
The max negative signal swing at the cathodes would then be about 6.2V allowing an input signal of up to 2 x 6.2 = 12.4 V peak (24.8V pk -pk) to be passed.

A Ring of Two CCS would also work and needs slightly less compliance voltage but is not quite as good (impedance wise) as the totem pole you show above.

Cheers,
Ian

Cheers,
Ian
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Old 5th June 2018, 12:22 AM   #12
nigelwright7557 is offline nigelwright7557  United Kingdom
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I cant make head nor tail of your circuit.
V1 goes into V2 OK but then it goes nowhere.
Then V3 seems out its own doing nothing with no input and the outputs go nowhere.
Maybe its just me ?
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Old 5th June 2018, 03:46 AM   #13
guavatone is offline guavatone  United States
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Thanks Ina. I'll give it a go. Still trying to wrap my mind around the theory. I was going by Merlin B's advise to have a high Re Resistor and higher Impedance. How are you getting 6.2V?

BTW, the original circuit calls for 11V at the Cathodes. I am also planning on trying this with a 12BH7 and ECC88 at 24-30 mA shared current.

Here is a Schematic that's more complete. The inout is a 1:10 Mic input XFMR. Actually David Garen at Cinemag sent me on this exploration of current sinks. It's been a bit of a rabbit hole.
Attached Files
File Type: pdf McCurdy CCS2p.pdf (82.4 KB, 19 views)

Last edited by guavatone; 5th June 2018 at 03:48 AM. Reason: image too big
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Old 5th June 2018, 09:14 AM   #14
gingertube is offline gingertube  Australia
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Hi RE is good because the impedance seen at the collector of the bottom transistor is approx.. RE x Beta (hfe) of that bottom transistor. So as well as high RE you want a "C" version of the bottom transistor. "A" like BC547A means hfe of 0-100, B means 100-300, and C means >300. I might not have thoe values exactly correct but you get the drift.

The problem here is to get a high RE you have put a high base voltage on that bottom transistor. More hfe and higher RE is good and the impedance (CCS goodness) is then limited by Early Effect. Early is the guys name who described this effect, nothing to do with "stuff" happening before you expect it to. The cascoded upper transistor removes that limitation.

You can not have a base voltage on the bottom transistor of 12V and then hope to get a bias voltage at the 12AU7 cathodes of 11V. That is what I mean by the CCS has run out of compliance voltage.

With the existing circuit values the CCS is acting more like a Zener diode and fixing the cathode voltage not the cathode current coz the transistors have run out of operating voltage.

The suggested fix will address that but as you have noted the CCS performance is compromised a bit by the lowish RE that is then required.

This is typical Electronics design where one of the maxims is "You can't do just one thing". Usually changing one thing leads to domino effects.

6.2V was from ASSUMING a +10v bias at the cathodes and subtracting the estimated modified CCS operating (compliance) voltage of 3.8V.

You also do not need an MJE340 as the top transistor. Any small signal transistor will do. make it the same as the bottom one. The beefier MJE340 will give some protection from tube shorts because of it voltage rating but it trades hfe to get that higher power/voltage rating.

Cheers,

Ian

Last edited by gingertube; 5th June 2018 at 09:23 AM.
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Old 5th June 2018, 09:23 AM   #15
gingertube is offline gingertube  Australia
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Hi RE is good because the impedance seen at the collector of the bottom transistor is approx.. RE x Beta (hfe) of that bottom transistor. So as well as high RE you want a "C" version of the bottom transistor. "A" like BC547A means hfe of 0-100, B means 100-300, and C means >300. I might not have thoe values exactly correct but you get the drift.

The problem here is to get a high RE you have put a high base voltage on that bottom transistor. More hfe and higher RE is good and the impedance (CCS goodness) is then limited by Early Effect. Early is the guys name who described this effect, nothing to do with "stuff" happening before you expect it to. The cascoded upper transistor removes that limitation.

You can not have a base voltage on the bottom transistor of 12V and then hope to get a bias voltage at the 12AU7 cathodes of 11V. That is what I mean by the CCS has run out of compliance voltage.

With the existing circuit values the CCS is acting more like a Zener diode and fixing the cathode voltage not the cathode current coz the transistors have run out of operating voltage.

The suggested fix will address that but as you have noted the CCS performance is compromised a bit by the lowish RE that is then required.

This is typical Electronics design where one of the maxims is "You can't do just one thing". Usually changing one thing leads to domino effects.

Cheers,

Ian
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Old 5th June 2018, 09:45 AM   #16
guavatone is offline guavatone  United States
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Thanks for being so helpful Ian. The phrase no free lunch applies so much with electronics. Yes after reading your post a couple times I then realized where you got 6.2 from. But how much free lunch is given after the 3.8V is used? Or how much compliance is needed? Say for understandings sake I was to use a 2.5 volt or 6.2 V reference on the bottom?

And also what would happen trying to CCS a 12ax7 biased at 1.5V? With 3.8V being used up already...
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Old 5th June 2018, 07:02 PM   #17
guavatone is offline guavatone  United States
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The Merlin b reference is a combo of this:
The Valve Wizard
And his books. And from the bottom page of the link it appears that 1.8V from 3 diodes is already “used up” but he’s claiming the bias is set at 1.5V. Hmm
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Old 5th June 2018, 10:48 PM   #18
guavatone is offline guavatone  United States
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Woo Whooo!!!! Ian you Rock! 12h7 output CCS Up and running at 28 mA with Re=41R, adjustable and correct Vk. I paralleled a 100R pot with a 68R for Re. Then 2 ZTX690b's. One ZTX came out at 700 Hfe and I'm guessing the other one is close giving me 20 Meg Z.

The one thing about the CCS is that they pull more current from biasing the current supply cascode.

The current circuit is a 2 stage version of the original differential preamp with the input tube biased at 1 mA per side at 1.5V, with 100K Ra and 1.25K Rk. which is probably the weakest part. but I am pretty happy with the sound of the CCS. More dynamic and lower THD by .05%

Here's a pic. The CCS PCB I made fits 600 mil resistor spacing via 90 degree 100 mil header pins. I still have a pot between the CCS and the tubes to protect the transformer from magnetization.

Click the image to open in full size.

Last edited by guavatone; 5th June 2018 at 11:14 PM.
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Old 11th June 2018, 09:07 PM   #19
guavatone is offline guavatone  United States
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Default CCS on Push pull output 12h7

Ok I got dual CCS's on the output to protect the transformer. So far any tube I've dropped in has has under a 20 mV difference across Anode. But when I wen from a shared CCS to the dual config the bass seemed to suffer a bit. It still shows a pretty flat reading on the AP but with the mic it seemed to decrease.

Merlin talks about bypassing with a 18KF cap, but says you can get decent results with a few thousand uF, in his HIFI tube book CCS section. The question mark cap is what I am wondering if it's useful and will get bass response better. Just across voltage references.

I drew I tubecad's coupling circuit with the 1 M resistor which I tried but didn't notice much. I also tried np caps between cathodes up to 33 uF not much either.

Also wondering if the 30V zener on the right would do any good.

Click the image to open in full size.
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Old 12th June 2018, 01:51 AM   #20
guavatone is offline guavatone  United States
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To answer my question. The 2200 uF ? question mark cap on both sides produced horrible buzz.
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