Since 1993 Copyright Notice
Lundahl Interstage Transformers
How to choose the transformer.
These are all wound like 1+1+1+1 : n+n. (n=Ratio). To get a first idea for a choice, do not compare everything with each other. Better is, to make those following steps, and you soon find the right transformer.
- Do you need a transformer, with a SMALL step ratio? (For inter stage mainly). It is found in TABLE A. (just below here)
- Or, Do you need a transformer, with a LARGE step ratio? (To achieve low output impedance). This is found in Table B
- A special one is the 300B / 2A3 interstage. So a 300B or 2A3, driving an interstage transformer, in Table C
- You sure will know if you have SE or PP at the Input and the Output. This is found the columns. .
- Each table (A and B) has shows different transformers just having another windings ratio, choose from there
Example: Suppose you need SE to SE, 1:1, that would be LL1671, ALT-S. Still, when you have it at the bench, you could also try 1:2 (Alt-T). That is just a matter of changing a few solder jumpers.
That way, you will find the transformer, and the connection scheme (The 'ALT' Version). WIth that you can look up in the selection table how the transformer is connected in detail. Finally, regard this text, and the EE27 board only good advise. If you want to go by the data sheets, and hand wire the transformer, you can as well do that of course. In the end the result is the same.
| TABLE A: Small gain, of small step down. For inter stage or line out. Use with low gain tubes. | |||||||||
PP to PP-Inter Stage |
SE to Output |
SE to SE-Inter Stage |
PP to Output |
SE to SE-InterStage |
SE to PP-Inter stage |
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Alt.M'' (2.25+2.25 :2+2)
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Alt.Q (4.5 :1)
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Alt.T (2: 4.5)
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Alt.N (2.25+2.25 :1)
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Alt.S (4 : 4.5)
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Alt.V 2.25 : 2+2)
|
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Alt.M (2+2 : 2+2)
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Alt.Q (4 : 1)
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Alt.T (1 : 2)
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Alt.N (2 +2 : )
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Alt.S (1 : 1)
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Alt.V (1 : 1+1)
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Alt.M (1.75 + 1.75 : 2+2)
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Alt.Q (3.5 : 1)
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Alt.T (2 : 3.5)
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Alt.N (1.75 + 1.75 : 1 )
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Alt.S (4 : 3.5)
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Alt.V (1.75 : 2 + 2 )
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| TABLE B: High step down ratio. This gives low output impedance. Use with high gain tubes. | |||||||||
Show configs |
PP to Output |
PP to Output |
PP to Output |
PP to Output |
SE to Output |
SE to Output |
SE to Output |
SE to Output |
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Alt.M (9+9 :4)
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Alt.N (9+9 :2)
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Alt.O (9+9 :1)
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Alt.P (18:4)
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Alt.Q (18:2)
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Alt.R (18:1)
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Alt.M (8+8 :8)
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Alt.N (8+8 :4)
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Alt.O (8+8 :2)
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Alt.P (8:4)
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Alt.Q (8:2)
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Alt.R (8:1)
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Alt.M (2.8+2.8 :4)
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Alt.N (2.8+2.8 :2)
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Alt.O (2.8+2.8 :1)
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Alt.P (5.6:4)
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Alt.Q (5.6:2)
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Alt.R (5.6:1)
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| LL2766 | ALT.A (79:2) | ALT.B (79:3) | ALT.C (79:4) | ALT.D (79:6) | ALT.A (79:2) | ALT.B (79:3) | ALT.C (79:4) | ALT.D (79:6) | |
| TABLE C 300B-2A3 inter stage. Specially made to drive it with 300B or 2A3. Can also run with AD1. | |||||||||
Alt. A
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Dedicated to be used as an inter stage, using 300B, 2A3 or 45, AD1, PX4, PX25, etc as a driver, by the SAKUMA principle. This elegant way to reduce distortion, uses no feedback. Published by Ratheiser in 1950, refined by Mr. Sakuma from Japan, who devoted half of his life to this. (More about it here) | ||||||||
For interconnecting the many windings, you can hand wire the transformer by the data sheet. This works of course, provided you make no mistake, and provided you do not want to change or compare configurations. At self costs, we offer this nice gold plated PCB, which can do any required configuration, by a matrix programming method. We have all data sheet configurations prepared. Just solder this little board on the transformer, add solder drops for the configuration - and it is already working.
The Lundahl data sheets give many examples of configuration, we found a few more, mostly advised by our customers. As Per Lundahl always said, new configurations can be, and will be found.
As an option, you can connect termination resistors directly on the board, for applications where the data sheet advises so. The solder connections for these are between Z1 and L2, and bewteen Z2 and L2.