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Old 3rd October 2004, 09:20 PM   #1
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Default 6L6 class A P-P CFB amp

This thread continues the thread 30W 6L6 UL amp.

30W UL 6L6P-P amp

I decided to make a new one because the topology has been changed to pure class A 6L6 CFB P-P amp.
Help is still needed and any comments, for everything until here are acceptable.

As I said to the old thread Iíd like to put at the phase splitter a CCS with the second half triode of the input tube.
I have attached a schem .Do you believe that this works?
Values of R2 and R3 depend on Vb and Vg-k.
R1 depends on how much Iíd like to be Vk.
How much to choose these values for good linearity of the current sink?

From the original schem I have change Va from 380V to 300V (Iím planning to use anode voltage of the P-P output stage about 300-320V) and anode loads of the phase splitter from 30K to 40K.
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File Type: jpg 6sn7 ltp w ccs.jpg (56.4 KB, 470 views)
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Old 3rd October 2004, 09:22 PM   #2
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And this is the original (Wayne's) 6SN7 direct coupled to 6SN7LTP driver stage.
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Old 4th October 2004, 11:58 AM   #3
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Newbie questions about DC and AC balance.
Do these circuits work?
For AC balance which of two to prefer?Or both work well?
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Old 4th October 2004, 12:00 PM   #4
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And the AC DC balance circuit.
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Old 4th October 2004, 08:34 PM   #5
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Everything shown looks like it'll work... mind you a pentode is more customary than a triode for CCS, and you won't burn as much voltage. (Although I'll have to check into that. The grid voltage divider would require current drawing an analog to the screen grid requirement, while depending on the perveance (for saturation voltage), the triode may end up a reasonable CCS.)

For AC balance, I would prefer balancing the plate resistors. Output stage grid leak will have little effect.

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Old 4th October 2004, 08:51 PM   #6
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Hi Tim
I prefer to use 6SN7 because it's the other half of the first stage.I don't like to split the first 6SN7 to L+R channel.So I decided to use it as CCS of the PS.

ok I'll balance the plate resistors.
Can I put simple bourns type multiturn trimmers over there?
What kind of multiturn pots use at the cathodes of output stage?
Over there I need >2W pot,right?
Any good source?
I found at Aquablue swiss precision, for 20euro each.
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Old 4th October 2004, 11:31 PM   #7
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Quote:
Originally posted by resident
Can I put simple bourns type multiturn trimmers over there?
If you only make it a fraction of the plate resistances, rather than the whole thing, multiturn will not be necessary. Tubes can't even comprehend anywhere such accuracy anyway, you'll need a live spectrum analyzer to null second harmonic distortion (which is what it will affect).

In 99% of cases, a balance isn't necessary, though along with independent fixed bias, it will allow using completely mismatched output tubes.

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What kind of multiturn pots use at the cathodes of output stage?
Over there I need >2W pot,right?
Any good source?
I found at Aquablue swiss precision, for 20euro each.
::blink::

No need for a $20 ANYTHING if you ask me...

In either case, whatever voltage or current present. Ohm's law is your friend.

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Old 5th October 2004, 09:01 AM   #8
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Quote:
If you only make it a fraction of the plate resistances, rather than the whole thing, multiturn will not be necessary.
What do you mean with fraction?

Quote:
though along with independent fixed bias, it will allow using completely mismatched output tubes.
Arenít self bias circuits more stable than fixed bias?
With a good choice of cathode resistors (value), I can match them with self bias,too.
Thatís why I decided to use pots. But I think, Iíll use Ohmís law.
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Old 5th October 2004, 06:53 PM   #9
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Quote:
Originally posted by resident

What do you mean with fraction?
Say each tube of the LTP needs 56k ohms. You could use a 20k potentiometer, which at middle would give 10k on either side, allowing you to use 47k plate resistors instead (47+10 = 57, very close to the required value). That would be about one fifth, as opposed to using a whole: you could use a 100k trimmer for both purposes.

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Arenít self bias circuits more stable than fixed bias?
Uh, yes...but that has nothing to do with this.

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Old 7th October 2004, 07:43 AM   #10
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Hi Tim,



I'm thinking to use a negative HT rail (-150V or -170V )at the lower end of the phase splitter-CCS to achieve about 450V total!

What do you say??Much moooore headroom!

About trim pots.
Ohm's law again and again.
Thank you for the idea.Even at the phase splitter.
Also,I don't think CFB output stages needs an ideal matched tubes.I'm not planning to use too different tubes.A pair of SV6L6 svetlana at each channel should not be so unmatched.
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