Lundahl --- interstage

FAQ on interstage and line output transformers :

See diagrams for most common used tube circuits :


What impedance is the transformer ?
Is it better to parallel tubes ?
What is the best transformer for 600 ohm output ? I want to drive my headphones and line output .
Which tube should I use regarding Rp ?

Tube transformers (line output , inter stage ect )

Model : primary ratios secondary ratios

LL1630 7,2+7,2 1+1
LL1635 1+1 1+1
LL1660 1+1+1+1 2,25+2,25
LL1621 1+1 1+1

Formulas :

X = impedance ratio
N1 = primary turn ratio
N2 = secondary turn ratio

X= N1 * N1 / N2 * N2

Primary impedance or plate load=secondary impedance or speakerload * impedance ratio( X )

Lets study the LL1660/18mA :

Primary source impedance with secondaries open ( no load ) = 6K and 80H primary inductance
Max primary voltage at 30Hz = 240v rms or 338v peak
Rdc Static impedance : all primaries in series = 1110 ohms
Rdc secondary : in series = 1250 ohm

In optimum use of this transformer : primary as primary
All primaries in series ratio = 4 ( max inductance )
With the secondaries we can play , however we have only 2 options

Option 1 : secondaries in series X=16/25 = 0,64
Option 2 : secondaries in parallel X=16/5 = 3,2

In optimum use of this transformer : secondary as primary
All primaries in series ratio = 5 ( max inductance )
With the secondaries we can play , however we have only 2 options

Option 3 : secondaries in series X=25/16 = 1,56
Option 4 : 2 windings in series and parallel to other 2 X=25/4 =6,25
Option 5 : secondaries in parallel X=25/1 = 25

Basically what we see here is the following : ( circuit A or D )

If you want to load the secondaries example headphones or other fixed load or you just want a low
Impedance output you need the highest X factor available .

At a load of 600 ohms that would be option 5 with X=25

The tube sees primary inductance reactance in parallel with load times X

So load = 600 ohms * 25 = 15 K ohm
An inductance of 80H at 30 Hz = 15 K ohm

So tube load = 7,5K ohm at 30Hz this will improve with frequency

Example of Tubes you can use in circuit A :
All tubes with Rp lower then 7,5K or parallel them so that you lower Rp
Keep anode current within transformer ratings ( 10% more is ok )
Unloaded interstage or direct grid drive (circuit A,B&C)

What interest us :
Aa what frequency 80H will = 6K
F = 6000/6,3 . 80H) = 11,9 Hz ( lowest frequency )

Bb the recommended upper primary impedance = 6K , as soon as we load the secondarys this drops
Why is it the recommended impedance , because at 12 Hz we have -6 dB and 6 Kohm

In this particular case : ( no load on secondary )
Zprimary ohm frequency Hz
5000 10
10000 20
20000 40
40000 80
80000 160
and so on ,

if this is used for SE tube circuits then you will notice the benefit of not loading the secondaries ,
the impedance rises as frequency goes up and the tube really likes to see a very high plate load
as it doesn't like to put out current this will suit us fine.
The only point of interest is at low frequency's , as the primary impedance steadily will rise , but the question is what is the lowest frequency of interest ?
At very high frequency's it's a total different story because you get the parasite capacitance and leak inductance and this forms a resonant circuit , which you can kill by trial and error :
Input a square wave and on the secondarys hook up your scope and put a resistor ( 10K ) in series with a cap ( 47 to 560pF ) , hook this on the out put and try different caps out till the ringing goes away .
And this is also a gentle low pass filter limiting the tranny's high frequency response .

Off course if you play with this tranny :
The std version , can be connected for 36 mA and for 72 mA as well
But for 36 mA , we get a ratio of 2:5 and primary inductance decreases by a factor 4
And for 72 mA , ratio 1:5 and primary inductance by a factor 16 ( from initial value)

For 36mA version :
At 10Hz 1256 ohms primary impedance ( very low )
At 40Hz 5026 ohms

So this basically is suited for 40 Hz to 25 kHz

The 72mA version is absolutely not recommend because the primaries do not have identical
Primary dc resistance , they probably have identical turns (diameter bigger = more circumference )

Please note the LL1660 is available in 10mA ,18mA, 30mA ,40mA & 50ma
And new models LL1671 for the higher current versions

However as current capability increases , inductance will decrease and this affect low frequency response a lot
But much less if the current capability is achieved through air gapping instead of paralleling primary 's .

LL1621 : 1+1 to 1+ 1

Possible uses :

anode load
mini PP outputtrans former X=4
grid choke
interstage between 2 tubes
pre amp line outputdrive transformer ( balanced or unbalanced )

Here according to the specs we have a PP version and a SE 6mA and 20 mA std version
The PP is not of interest all tough please note the high 300H primary inductance if used as SE version
( example tolerating positive grid swing in class A1 or A2 designs )
so what is the primary impedance :

frequency : model 130H ( 6mA) model 30H ( 20mA )
10 Hz 8,2K 1,9K
20 Hz 16,3K 3,8K
40 Hz 32,6K 7,5K
80 Hz 65K 15K

this is without loading of secondary , please note that there is ample margin with Lundahl transformers
and they will not saturate very fast( these usually accept 40% more current then specified , but always check to be sure )

please note very high static secondary resistance so secondary load has to be very low

example with load of 10K circuit A or D 6mA model at 30Hz
primary in series secondaries parallel X=4
tube sees load of 40K // 24K = 15K

LL1630 :

Here we have a beautifull step down specimen ( line output , headphone driver ect )
Please note the very low static resistance of secondaries 14 ohm which makes this transformer
Very suitable for driving low impedances

Rdc each primary = 480 ohm
Ratio : 7,2 + 7,2 to 1+ 1

Possible uses : circuit A or D
Option 1 : primary in series and secondaries in series X=207/4 = 52
Option 2:primary in series and secondaries parallel X =207/1 = 207

So what is the primary impedance (in SE use )? ( unloaded = secondaries open )

frequency : model 130H ( 5mA) model 32,5H ( 10mA = 2 primaries parallell )
10 Hz 8,2K 2K
20 Hz 16,3K 4,1K
40 Hz 32,6K 8,2k
80 Hz 65K 16,4K

for the 5ma version
circuit A or D : loaded with 600 ohms at 40Hz with transformer reactance included 5mA version

option 1 : tube sees load of 15K and highest output voltage possible
option 2 : tube sees load of 26K and lowest output voltage possible

for the 10ma version or 20mA version note that inductance decreases and so does the load the tubes sees
at low frequencys resulting in more difficult bass response ( use tube with low Rp )
please also note 40% overload margin for dc currents so actual capability :
the 5 mA can handle 7mA and the 10mA can handle 14 mA

note : these qualities are paid with a price : max outputvoltage much lower then LL1660
however 40V rms at 600 ohm load = power of 2,6 watts
LL1660 can put out 90V rms or 600ohm load = 13,5 watt

LL1635 :

This is almost a copy of LL1621 the difference is in the primaries ,
The difference is very small and I cant figure out what the advantage or disadvantage is compared to LL1621
More sections in primary ?
Better frequency response ?
Here at least the value for good square wave response is given at 68pF

Conclusion :

so if you are planning to design anything with it , check out internal impedance of tube and choose the right transformer for you , take into account current at desired operating point and choose transformer with at least 2 to 3 times primary resistance load compared to tube internal resistance .( and remember tubes don't like to be loaded so higher impedande = lower load = better and more linear operating point of tube , the mu will stay constant and reach its maximum value )

good luck and have fun and please let us know when you compare tubes for sound what you're findings are

benny glass


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