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Old 2nd June 2007, 03:07 PM   #1
bigwill is offline bigwill  United Kingdom
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Default Critique my first design

I managed to win some cheap 6C41Cs on ebay and I want to build a simple Class-A PP with them.

I came up with the attatched schematic. It's my first ever design for a tube amp but I have yet to come up with exact component values.

The input transformer is a 1:10 step-up and is to be driven from a low impedance source. The output transformer will probably be a mains toroid. No DC will flow through it because of the coupling caps (not scared of electrolytics).

As you can see it's very simple and I THINK it will also be a very hum and hiss free design as well, since the current sources on the anode offer no reasonable path for AC, and any noise that gets through will be cancelled by the other half.

It's a PP design, will it be as efficient as normal class-a PP designs? Will it work with suitable component values?

Lastly, I hope to get around 25W from this design, is this feasable?
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Old 2nd June 2007, 03:26 PM   #2
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Forget about the caps and think through the circuit
...Right, any DC will cancle in the Transformer because in is wound in anti-phase seen from the center tap. Only the out of phase AC voltage will add/summarize. So omit the CCS and put the B+ to the center tap.

25W ? Hmmm a bit optimistic

Tell us your exact Transformer data and the operating point, voltages etc. Then it can be calculated

Futhermore I don't think that your 10:1 input tarnsformer will suffice. 6S41 needs much driving voltage AND and low impendance source because of its miller capacity. Remeber, a 10:1 transformer will tarnsform the resistance by a factor of 100 !
If I were you, I would build a tube stage.

regards, Manta
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Old 2nd June 2007, 03:33 PM   #3
bigwill is offline bigwill  United Kingdom
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I agree with the sentiment about needing a tube input stage, a 1:10 step-up can't possibly drive the grids enough

As for the CCS, that is to bias the output tube without any DC going through the toroid

As for what transformer I'm going to use, I don't know I want to use a mains transformer because people have reported good results from them if there isn't any DC offset (I'm avoiding this problem entirely by havign the DC through a CCS)
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Old 2nd June 2007, 03:44 PM   #4
EC8010 is offline EC8010  United Kingdom
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Those CCSs won't work. You have the emitter and collector transposed. The collector goes to the anode of the valve. You will be lucky to get 6W. One of the advantages of an output transformer is that it allows you to swing to twice the HT voltage. You have just lost that advantage using CCS loads.
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Old 2nd June 2007, 03:54 PM   #5
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How about putting the CSSs in the cathodes and having a cathode follower output stage? (with the appropriate driving voltages)
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Old 2nd June 2007, 04:04 PM   #6
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Yeah, a cathode follower output stage is possible. But you will definitly need a good driver stage. Moreover this would be an amp which will require 200 to 250V for the 6S41 and 500 to 700V for the driver :-) Except you will use a choke loaded driver.

I would buy a good output transformer and make an old fashioned amp. Of course a mains tarnsformer is cheaper but has a much smaller primary inductance which gives a lack in the bass.
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Old 2nd June 2007, 04:15 PM   #7
Yvesm is offline Yvesm  France
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Where ever you put'em CCS innevitably asks for doubling PSU voltage, one half being vasted in heat in the CCSs.

Moving the load in the cathode does not change its value to extract the same power but you loose the voltage gain in the tubes and thus need vy huge voltage drive.
Of course, this kills Miller effect ! But designing the driver becomes a real challenge.

Yves.
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Old 2nd June 2007, 04:53 PM   #8
bigwill is offline bigwill  United Kingdom
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Basically the reason behind all this CCS nonsense is because I want to avoid messing with DC servos to balance current through the output transformer
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Old 2nd June 2007, 05:23 PM   #9
Yvesm is offline Yvesm  France
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Bah !

Put one bypassed CCS in each cathode !
Even if I don't like bypassing caps at such critical place.

Yves.
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Old 2nd June 2007, 05:30 PM   #10
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by Yvesm
Bah !

Put one bypassed CCS in each cathode !
Even if I don't like bypassing caps at such critical place.

Yves.

Beat me to it, that's what I was about to ask about
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