Lundahl Transformers - Tube amplifier transformers

Lundahl Transformer
Manufacturer of Ultra High Quality Audio Transformers

Tube amplifier transformers

Introduction
Input (and DC-free line output) applications
Plate (anode) chokes, interstage and line output
Push-Pull and SE
Chokes for Power Supply
Mains transformers
Miscellanious products
Some Pictures
Storage

Choosing a Lundahl Choke


These Lundahl C-core products are also based on the double Coil technology, which gives options that are not possible with single coils. The coils can be put in series or in parallel, resulting in two different valuesl. Also, each coil can be separately used, giving higer Commnon Mode Rejection than normal (Single Coil) chokes. So, this gives three applications for each choke type. Here is an example for the 1673/10. This product has two coils of each 5Henry, 200mA 1) Both coils in series: Gives 10Henry / 200mA 2) Both coils in parallel: Gives 2,5 Henry / 400mA 3) Double coil connection: Gives 10Henry / 200mA at higher CMR. In the double coil configuration, one coil is in the positive lead to the second capacitor, and one on the ground lead to the second capacitor. This will give lowest field radiation coming from the choke. Such good results can not be achived with lower cost E-Core chokes, and the Lundahl double coil chokes are a major step into the direction of a really hum-free amplifier. The datasheets gives a good diagram of how to connect a double coil choke, using it's full features and get best noise rejection from it. (check for instance here, all at the bottem of the last page) Of course, it is still possible to use them as a single coil also.

Overview of types


1 ) LL1638	Wound with thick wire. Gives best performance for High Current applications. Available with different air gaps, which results in different current / henry.
2) LL1673	Same as above, but wound with thin wire. Gives best performance for High Henry applications.
3) LL1685	Same as 1673, but wound on the next smaller core. Lower in price, but less Henry.
Note:	At "medium" current, many combinations can be made, but some chokes will perform better than others. In the following table, the best combinations are given, with the most Henry for the money, at a several currents.


LUNDAHL	POWER CHOKES, BEST COMBINATIONS	Recommended DC-mA Important: Read Note below!	Henry
Hint:	for lower current, use LL1667 /25mA	25	168
LL1685 /17H	Coils in Series. LL1685= Compact construction.	100	17
LL1673/20H	Coils in Series. LL1673= High Henry type.	100	20
LL1685/13H	Coils in Series. LL1685= Compact construction.	130	13
LL1673 /15H	Coils in Series. LL1673= High Henry type.	140	15
LL1685/10H	Coils in Series. LL1685= Compact construction.	160	10
LL1673 /10H	Coils in Series. LL1673= High Henry type.	200	10
LL1685/13H	Coils in Parallel. LL1685= Compact construction.	260	3,25
LL1673 /15H	Coils in Parallel. LL1673= High Henry type.	280	3,75
LL1685 /10H	Coils in Parallel. LL1685= Compact construction.	320	2,5
LL1638 /4H	Coils in Series. LL1638= High Current Type.	400	4
LL1638 /4H	Coils in Parallel. LL1638= High Current Type.	800	1
* Note	DC-mA is specified very conservatively by Lundahl, at FULL specified ripple voltage over the choke. This will only happen when you work without first capacitor (is possible with Lundahl chokes) When working with a first capacitor, the DC-mA current can be exceeded with ca 10..15% !

SOME DESIGN RULES FOR A HUM-FREE TUBE RECTIFIER CIRCUIT

In this circuit diagram the primary Capacitor is C1, the secondary Capacitor is C2.

With High Voltage power supplies, the best results are always coming from a large choke. This has many times more effect than using large capacitors.
The BIG misunderstanding from people that made low voltage power supplies before. Here, using BIG capacitors, like 10.000...50.000uF will improve the result, because with Low Voltage designs, the energy storage is only in the primary capacitor. Not so with tube rectifiers! These are high impedance, and the energy storage is mainly in the choke. With tube rectification, you need to take the a small primary capacitor. (see also the next note). There is a maximum value for that in the tube datasheet. It is for instance 4uF for the 274A. The maximum is the highest value, and you should feel free to take a lower value also. The higher the value, the larger the charge current through the primary capacitor. The wires will radiate an AC magnetic field, and this gives hum into the pre-amp. (and you'll never find where this hum is coming from. The path is through air....)
Some applications leave the primary capacitor completely away, and use only the energy storage in the choke. This will improve the efficiency of the power supply, and gives more current. This is a method, not recommended with low cost chokes, because it will make the plate choke hum (mechanically). With the LUNDAHL chokes this is possible, and no hum will result from it.
Push pull amplifiers are very unsensitive to power suppy hum. They can work completely without hum, even with 30Volt AC on the DC voltage, but.... only as long as the rest of the construction is well done.
SE amplifiers are very sensitive, but if there are any hum problems it is normally not the power supply. Iimproving the power supply first capacitors can will help, because it reduces all AC voltages in the whole amplifier. Still you are curing the problem at the wrong place, and never get a full cure because of that. More often it is bad wiring, or some other mistakes somewhere. The recommened capacitor values in the circuit diagrams are normally more than enough.
With Mercury rectifiers people often say they "hum", or sound in some way "unquited". That is true, but it comes from bad designs. Good designs do not hum at all. These rectifiers have an "ignition" before they get in the conductive state. Because of that, they have a lower internal impedance, and can work with surpizingly large primary capacitors. But... this is the industry method, and not what we want for HiFi. Again.... what these rectifiers show in their datasheets are maximum capacitor values for industry applications, and no "must use" values for HiFi! Take the same low primary capacitor value as for non-mecury rectifiers, and you will have no hum.
Here is the final conclusion: If you want to oversize the power supply, the best effect comes from:

a) The CHOKE
b) The SECONDARY Capacitor
c) there is no c;)