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Old 9th January 2011, 12:48 PM   #1
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Default #40-LL1660-4*2A3

I'm planing a 2A3 PPP am..
the operation point would be 300V with -62V on the grid (fixed bias)
its just like the RCA manual.
to drive the otput stage I have to aply 124V grid to grid to it.
the turn ratio of the LL1660 as a phase spliter is 2.25:2+2
so if I'm correct I have to aplly to it 69.75Vp-p and if my driver tube is #40 wich have an ampification factor of 30 so the input voltage is 2.325Vp-p so the RMS voltage is 0.82Vrms
I have some qustions:

1. am I correct whith the calculation?
2. how is the #40 tube?
3. how can I operate each 2A3 with it's own fixed bias with the IT transformer?
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Old 9th January 2011, 02:32 PM   #2
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Hi!

The 40 is a beautiful tube, but not at all usable as a transformercoupled driver. It has an extremely high plate resistance.

I have experimented with the 40 in a phonostage. It can practically drive nothing, it needs an extremely light load also in terms of capacitance to avoid any trble roll off.

Choke loaded you would need kiloHenries. With a choke laod the winding capacitance of the choke might already cause a roll off, CCS loaded it might work.

As a driver for a power tube, completely unusable I'm afraid.

Best regards

Thomas
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Old 9th January 2011, 04:36 PM   #3
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Ziv,

You are not quite correct with the calculation, apart from the fact that Thomas is right with the 40 being totally unsuited for inductive loading/coupling.
But let's assume you have a tube with an amplification factor of 30 which is well suited for inductive loading.
When you apply 0.82 VRMS to the grid the tube will amplify up to about it's mu of 30, so we have 24,6 VRMS at the anode.
The two secondary windings of the interstage transformer will each have 21,8 VRMS; this is not enough to drive the output tubes to full power in pure class A; we need 43,8 VRMS to do that.
Actually your calculation is a factor 2 wrong.

For biasing the PP output stage individually the interstage transformer should not have the "centertap" terminals connected together.
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Old 10th January 2011, 04:03 PM   #4
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thank you very much for your reply!
so if i use 6SL7 i get 1Vrms input sensitivity.


Quote:
Originally Posted by pieter t View Post
Ziv,

For biasing the PP output stage individually the interstage transformer should not have the "centertap" terminals connected together.
from this configuration i don't get individual bias for each 2A3,but i get bias for each side of the PP, while each side contain two tubes.Is there any other way to do this?
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Old 10th January 2011, 04:13 PM   #5
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Ziv,

Yes, with a 6SL7 you will get there as far as voltage gain is concerned.
However for driving two pairs of power tubes through an interstage transformer you need a competent driver stage; a 6SL7 cannot do this (it has also way too high Rp for inductive load).
My suggestion is to look at Lynn Olsen's designs of direct heated PP amps with interstage coupling (Amity, Karna); you will find first class info there.

For individually biasing parallel output tubes through the IT you would need a separate secondary winding on the IT for each output tube.
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Old 10th January 2011, 04:20 PM   #6
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The link to Lynn Olsen's designs is Nutshell High Fidelity.
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Old 10th January 2011, 04:26 PM   #7
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I cannot imagine that there will be any directly heated triodes capable of the job, I think Linn Olsen used something like a 5687 in the end. I have used 5687 as an interstage driver of a 6AS7 and it worked. The ECC99 is another good candidate.

Shoog
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Old 10th January 2011, 05:12 PM   #8
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Back in the old days they used the 56 tube (= 76) for driving an IT.
In fact this is not recommended because of high plate impedance.
I have some 76 tubes, is it useful to try them in a 76 => interstage transformer => 6B4 or 2A3 PP amp ?
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Old 10th January 2011, 05:19 PM   #9
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Shoog,

Take a look at Lynn's site then.
He uses 5687's and the likes for the input gain stage, and direct heated driver tubes (45's) in his "no compromise" Karna design.
In the Aurora, yes a 5687 or ECC99 can and is used as driver.
However all of these tubes have a mu of 20 or less so an additional gain stage is needed.
It is just what design considerations you have.
For a good design my opinion is that it makes sense to calculate "backwards": what is needed to drive the output stage (voltage, current), then how does the driver stage look (voltage gain), and in the end how much more voltage gain is needed in the input stage for a chosen input sensitivity. That, among other things, makes a good three stage design.
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Old 10th January 2011, 05:32 PM   #10
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Hi Jaap,

In your schematic the interstage transformer is also functioning as phase splitter.
No matter how good the IT is, you will not be happy when you measure bandwidth of the two phases, and also they will measure very different.
Maybe it will sound OK, but for modern quality designs you must look for a better driver stage as a certain amount of headroom is required for quality designs.
You can use the 76 as an input gain stage (even with an inductive load), but to drive the output stage take for instance a 6L6 with an IT/phase splitter.
In one of the Sound Practices magazines you will find a very good design that looks much like it (Feral Eye).
Hey, don't I owe you some transformers? Maybe the start of a nice project?
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