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Old 28th December 2012, 07:43 PM   #1
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Default new amp 2a3 pp

hi i would be realize an interesting pp of 2a3 of about 12,5 watt in a2 class.
The driver would be a 6c45p poly cathode bypass, the phase splitter would be a lundhal 1:0,5+0,5, the output transformer would be a lundhal of about 4 to 6 kohms.
do you have suggestions? Can you suggest me a specific model of lunhal transformers for both functions not too expansive ( too see max 250 euros each)
comments of experts are very well welcome and appreciated. thanks for help
fixed byas of all tubes ( also 2a3 are cathode bypass if possible), Psu are CLC with 10 microfarad + 100 microfarad power stereo transformertransformer would be 200-300 VA and B+ 250 volts.
thanks for help.
by
Paolo ( worst rookie! :-( . )
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Old 28th December 2012, 11:02 PM   #2
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If you want to use a Lundahl for splitting, then get the S version which is for this. Otherwise you could use a differential pair for the input with a constant current sink under it.

If you use fixed bias you won't have a cathode bypass cap. A good way to do fixed bias would be to use one of Rod Coleman's regulators across something like a 47K resistor. There's a recent post on this, I think in the 4P1L SE thread.
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Old 29th December 2012, 09:24 AM   #3
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Hi Paolo,

As ouput transformer the Lundahl LL9202 wired fro 6,5k primary should fit.
As interstage I would rather recommend something with a slight step down. If you really want to push the 2A3s into A2 you want a real low impedance drive. Maybe a LL1692A wired 3,5 : 1 + 1.

I have written that before: I think the 6C45 and similar tubes are a very poor choice as a driver tube unless only little grid voltage swing is needed on the output tubes. It is a common mistake to try to squeeze as much as possible out of a single stage. Depending on the op point for the 2A3, each of them will need about 100V peak to peak grid swing for A1 already. Since both are out of phase that is 200V peak to peak which your driver has to deliver. A 6C45 barely can deliver that voltage. It will have no headroom and thus dominate the sound of the amp. For Class A2 you need even more voltage swing. If you are using a bigger step down, yet more voltage.

If you really want to go for PP Class A2, I would recommend a three stage concept. May be a 6AH4 as driver. It can deliver lots of voltage swing. For the gain add an input stage as apropriate.

A PP Class A2 is a challenging project!

Best regards

Thomas
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Old 29th December 2012, 09:46 AM   #4
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thanks to all guys,
I think a three stage with 6j5 input, plus a parallelled 6sn7as a driver plus lundhal
3,5: 1 plus 1 could be a sufficent choice. What do you think about?
thanks ,
by

P.S how much cost the splitter and the output trasformer?

cheers

I don't kno w6AH4 druiver...is it maybe microphonics? i hate also very low microphonics tube..-
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Old 29th December 2012, 09:54 AM   #5
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Hi!

I think a 6J5 driving 6SN7 would have way too much gain. That's why I proposed the 6AH4. I wrote an essay about the 6AH4 on my blog:

VinylSavor: Tube of the Month: The 6AH4

The list prices of these transformers are: LL9202: 220,- Euros each, LL1692A: 104,- each

Best regards

Thomas
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Old 29th December 2012, 10:00 AM   #6
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very thanks they bare good imo!! :-)
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Old 29th December 2012, 10:07 AM   #7
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hey guys wich are the points of -3dB for this output tube transformer and this phase splitter both lundhal?
thanks
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Old 29th December 2012, 11:49 AM   #8
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Hi!
I have the new tubes that I have in stock.
Input half 6N6P cathode bypassed. Driver paralleled 6BX7GT cathode bypassed.
The rest remain the same, excluding the PSU which the rectification is performed by two vacum diode paralleled.
Please indicate me also the choke for CLC and the power transformer of about 200-300VA always lundhall.

Thanks.
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Old 29th December 2012, 01:43 PM   #9
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Hi!

Quote:
Originally Posted by inertial View Post
hey guys wich are the points of -3dB for this output tube transformer and this phase splitter both lundhal?
thanks
You can check the datasheets on Lundahls website:

Lundahl Transformers - Transformer datasheets

There are frequency responses given for some sample configurations.

Thomas
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Old 29th December 2012, 01:45 PM   #10
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Hi!

Quote:
Originally Posted by inertial View Post
Please indicate me also the choke for CLC and the power transformer of about 200-300VA always lundhall.
You should first define the circuit and total current draw and voltages only then you can make a choice for chokes and power transformer. Also depends of course if you are building monoblocks or stereo with a common supply.

I'd expect such a Class A2 (or will it be AB2?) To be quite demanding in terms of power supply. Probably a choke input filter will yield a more stable voltage. But that would require a LCLC filter

Thomas
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