The Solder Slinger Bros’ Bonding Imperative


This is a concrete example of how the Internet is revolutionizing the audio arts. Lundahl Transformer never would have crossed my path if it wasn’t for the Internet. This company is extremely serious company that makes no concessions to fads and its president Per, will not make anything that doesn’t meet his high standards. So from across the seas come heavy little packages filled with magical hunks of iron that create an expanded aural matrix of beauty.

When I was discussing, with my thermionic techno-shaman bro’, Lynn Olsen the mysterious experience I was having using plate chokes and interstage transformers he sent me the schematics of his now favorite preamp and power amp that is "loaded with iron", and he too commented that something must be happening on the electron level in the magnetic field of these "transformers". Lynn speculated that perhaps these magnetic fields are a form of LSD for electrons, and that explains their "para-normal" effect.

So again I offer you this caveat...If your speakers are capable of revealing the nuances of directly heated triodes the use of plate chokes and interstage transformers creates a escalated sense of beauty that we are searching for...and I can’t think of a simpler way to achieve that high goal.

So here is my current formula:

Use the Lundahl 1621 as a plate choke "load" on the first stage of a 6SN7 which is operating at B+ of 70 volts. ( If your voltage rail is 300 volts or higher then you can use resistors to drop the voltage before the plate choke) This first stage is direct coupled to the second stage. This second stage uses the Lundahl 1660 as a single-ended interstage transformer, with a B+ of 300 volts (on the plate of the second stage of the 6SN7). These two Lundahl parts have been chosen to work within the current ratings of this tube. In this case the plate choke is taking place of the plate resistor as the load for the first stage, and the IT is taking the place of the coupling capacitor. You are substituting almost $180 worth of parts for less then $20 worth of parts, per amplifier...and it is worth it in spades.

It is possible to use a different value plate with higher inductance...which will make it larger and more expensive...and in this case because the 6SN7 is only drawing about 10 ma per section it is possible to get up to 100 hy in a reasonable size.

I have a Bertolucci plate choke that is rated at 100 hy at 60 ma and it is the size of a small house, and would require a huge chassis to accommodate it.

This is not a conventional circuit configuration and I asked around if anyone had any experience with this configuration. This is what is so cool about tube circuits...there are endless new possibilities.

In this circuit configuration there are no capacitors in the signal path, and other than the caps and resistors on the cathodes of the 6SN7 for bias, that is all there is to this ultra simple circuit...and why its sounds and tastes so right.

Your 300B amplifier probably has a "normal" 6SN7 "front end" which consists of voltage amplifier first stage which is capacitor coupled to the second half of the 6SN7, which in turn is resistor/capacitor coupled to the grid of your output tube. If this is true and if your 6SN7 is operating at high enough voltage, converting to a Lundahl plate choke and IT is simply a matter of finding the room in or on your chassis...and only you know if you have the room. A skilled solder slinger can "refit" your circuit in less than a few hours.

There is another benefit to this circuit....there are so many different "flavors" of 6SN7.

What is the down side? Perhaps we should be experimenting with a tube that has a lower output impedance for the drive stage...and I will do that shortly...but there is something about the tone of the 6SN7 which explains why "better" tubes are not as popular.

Some of you may be wondering why I am not just using a single WE 437 or some other tubes, like the Nobu Shishido answer is (1) I wanted to demonstrate that it is possible with an the most common input tube to extend the performance boundary of any single-ended directed heated triode circuit....this is a mod that you can do to your amp if you have a 6SN&7 input stage (2) I am experimenting with lots of different "iron" enriched circuits and this one will act as a beginning reference.

Remember that there are benefits in only using the plate choke or the Interstage transformer. If you can’t afford both, you can use just the interstage transformer, or you can use the plate choke on the either sections of our 6SN7...with the greatest benefit on the second stage if you are not using a IT.

These Lundahl transformers can be connected in a variety of ways for a variety of uses and the instructions that can be downloaded off the Lundahl web site shows just how flexible these devices are. And you should also be aware that these are "open frame" designs.

I use this "front end" to drive the grids of a wide variety of tubes from KR 300BXLs to AV8 with superb results.

And let me remind you once again that what I am achieving is a much more authentic sense of harmonic wholeness; a musical integration of space/time, and rhythm that I have not been able to achieve any other way...and remember I am using regulated voltage rails.

And now for a warning: I know this will change soon because I am putting deep pressure on Per Lundahl to make it easier for audiomaniacs to use his products...but there is no one at Lundahl who can give you advice on tube circuits. You must know exactly what you want from the iron you are buying and that means knowing what kind of impedances, inductance, current, etc. The Triode Guild Chat room will be able to help you shortly. The information I just gave you applies to the 6SN7 tube, and if you are going to use a different driver tube you will probably need a different set of iron....but information on that will soon be available.

Cost of model 1621: $64.67 each

Cost of model 1660: $68.98 each

Approximate shipping cost by air for all four pieces is $42.00

Lundahl accepts VISA


I will be experimenting with the 6EM7, which has a lower output impedance and draws more current and the very expensive KR 10s, which is like a 6SN7 on steroids. The 6EM7 was a tube that Julius Futterman used as a regulator tube, and is now being used in an output tube in an OTL headphone amp that I will be telling you about soon. Ron Welborn also uses this tube in his Apollo amp.

Please be patient and I will post a schematic of the above input circuit.

Check Lynn Olsen’s web site to discover his "iron madness" :



I recognize that many of you have not yet gained the courage or dedication to learn how to solder or play with electricity safely, but that doesn’t mean that you can’t bond with a local solder slinger.

I know this deflates the "glam boys" of audio, but connecting a plate choke or interstage transformer is about as complicated as lifting the toilet seat when you pee, and is a good reason why you should use the web to find a local solder slinger who can help you advance your amplifiers refinement...if you can’t.

Most single-ended amplifiers have only one B+ rail which is used for both the output stage and input stage (which is not optimal by the way) and it usually is 415 volts for a 300B and 250 for a 2A3. The benefit of use plate chokes and interstage transformers is that you can use higher voltage and that can translate into a lower distortion and more dynamic sound. Adjusting your B+ voltage down is simply a matter of calculating the right resistance...which includes the resistance inherent in these devices.



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