New Schematic - PP UL 6V6-types, CCS-fed - diyAudio
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Old 21st May 2009, 08:15 AM   #1
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Default New Schematic - PP UL 6V6-types, CCS-fed

Here it is. I'll call it the "6V35" because it uses Dynaco Z565 output transformers. The actual output tube shall go unmentioned Normally I'd just plow right ahead but this is only the second time I've ever implemented constant current sources, and the first time I've tried to design one into a new circuit.

Click the image to open in full size.

The CCS in question is a 10M45S. I have four of the TO-220 package types. I guesstimated about 65 ohms to set each tube to 37.5mA using the graph at the bottom of this page:

http://www.tubecad.com/2006/11/blog0087.htm

(I'm also considering using a 50-ohm pot on top of a 47 ohm resistor.)

Will the output stage work as described? I'd heard that it's good to bypass CCS' with capacitors, so I have done that. I also guessed 100K for the grid resistor, is that OK?

I think I'm going to need at least +-21V to drive these to full output, working from the curves here: (I just picked the 300V/37.5mA point)

http://www.polstra.com/projects/spic...les/loadlines/

11.25W is pretty conservative for 14W total plate+screen dissipation, so this amp ought to last a while. And I suppose I could always dial up the current.

As you can see, I have not decided on a driver stage. I was planning on rectifying and maybe doubling the 6.3VAC heaters to feed a CCS of some sort, but don't really have any ideas so far. Does anyone have any suggestions? I have a couple 12AT7's and a few 12AV7's that might work..
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Old 21st May 2009, 09:12 AM   #2
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If you don't bypass the CCS then you get no signal.
When I use CCS in the tails I usually take the node between the two bypass caps and reference it to ground via a 1meg resistor (which keeps them biased). The effective value halves but it forces the valves into differential mode and helps to clean up the sound.

The IXY chips are very good in this capacity (I am using therm as well), but you can expect quite a bit of variation between units - in the order of 1-3mA, so that trimmer might be a good idea.

Shoog
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Old 22nd May 2009, 09:28 AM   #3
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Quote:
Originally posted by Shoog
If you don't bypass the CCS then you get no signal.
I'm a bit confused... Serpentine (Tom) over at Geek Zone had this to say about bypassing the CCS' with caps:

Quote:
Originally posted by Serpentine
...don't those 220uF caps across the CCS at the power tube cathode completely disable the AC-wise CCS function? IOW, what is the point of those CCS' except to set idle DC? And what do you think will happen when leaving class A operation?
(http://geek.scorpiorising.ca/GeeK_Zo...46581#msg46581 )

Something doesn't quite add up.. This is the first time I'd feed an output tube with a CCS so I don't know the answer!
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Old 22nd May 2009, 10:02 AM   #4
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CCS here is as very long tail in LTP

if you bypass it for AC ( with cap ) , you loose LTP tail , and that's then plain symmetrical amplifier , without inherent balancing feedback , where CCS is used just for biasing
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Old 22nd May 2009, 12:22 PM   #5
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In order for that to work you need one CCS in a common tail (which is not what you have). If you try for one CCs in a common tail you will lose DC balancing which may or may not be a problem for you. My suggestion creates the LTP and the DC balance.

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Old 22nd May 2009, 03:10 PM   #6
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ooops - I didn't clicked on attached pic ( so I saw just a thumbnail) , so I didn't realized that there are two CCS-es .

Shoog is right ....... off course ;
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Old 23rd May 2009, 03:22 AM   #7
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Quote:
Originally posted by Shoog
My suggestion creates the LTP and the DC balance.
That would be ideal, but I'm having problems visualizing what you mean. You said:

Quote:
When I use CCS in the tails I usually take the node between the two bypass caps and reference it to ground via a 1meg resistor (which keeps them biased). The effective value halves but it forces the valves into differential mode and helps to clean up the sound.
I thought that by "tails" you meant the LTP, and not the power tubes? But then you talk about bypass caps, which are under the power tubes. I'm confused though, wouldn't the "node" between the two caps just be ground?
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Old 23rd May 2009, 09:48 AM   #8
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At the moment each bypass cap goes from the cathode to ground. What I am suggesting is taking the ground connections and connecting them together at a new "node". This new node still needs a trickle path to earth to keep them biased (ie the cathodes negative), inserting a 1meg cap from the new node to ground will do this nicely. Now the signal through the tubes can only travel through the low impedence path across the caps rather than the high impedence path to ground. This makes it functionally a LTP.

Hope that helps.

Shoog
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Old 23rd May 2009, 10:31 AM   #9
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nice tip, Shoog .
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Old 23rd May 2009, 01:47 PM   #10
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Hope you spotted the diliberate mistake - I mean't 1meg resistor not cap.

Its such a simple mod to try that you could rig up a bypass switch to short out the 1meg resistor and do a dynamic comparison.

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