How to choose a transformers for the 845 Triode

I have to answer this question so many times by email. I have put this together here, a short application information.

In general:

People think sometimes transformers are directly related to a particular tube, but this is not so. Questions come like this: "Hello, do you have a transformer for the 845 tube ?"

The situation1:

The 845 tube has a plate impedance which is actually depending on the anode voltage and current. Reason is, the 845 can work from 830 to 1200 Volts, but for practical reasons in HiFi, the voltage is chosen as lower than 1200 Volts. Then, companies that build 845 Amplifiers try to offer what people say they "must have", which is 25 Watt output power, or they do not want to buy it. So they put the 25 Watt on paper, but that would be with at distortion levels higher as you think.

The situation2:

Most commercial 845 Amps run at 1000...1100 Volts, and at lowest distortion this will produce approximately 24...25 Watt a 5% distortion, at the tube itself. This distortion drops rapidly, when you reduce output power. So at normal hearing levels, requiring a few Watt only, distortion becomes very low, as the 845 is a very linear tube by itself.

Output impedance of the 845 tube:

This value is sometimes believed to be a fixed number but it varies significantly by the working point you choose. Many of the very expensive transformers offer the 11k impedance, because we all "need" 30 Watt, and the for practical reasons, people use a 1000 Volts power supply. That would be a big mistake of course, as the required impedance drops to only half the value, just by reduce the plate voltage from 1200 to 1000 Volts!

Please do not take impedance recommendations exactly. These are choosen with wide tolerance, a compromise between output power, output impedance, and distortion. So when you read a number 3k4 in the table, it doesn't mean only 3k4 is right, and 3k5 is wrong. You can easily have 25% deviation, and you would not even hear the difference.

845 Specifications
Tube specifications only,
transformer loss and distortion not included
DC Plate Voltage
750
1000
1000
1000
1200
Volts

DC Grid Voltage

-98
-145
-145
-145
-195
Volts
DC Plate Current
95
70
85
90
80
mA
Plate Dissipation without signal
71
70
85
90
96
Watt
Plate Impedance 1700 1700 1700 1700 1700 Ohms
Transformer Ra
3k4
9k
6k5
6k
11k
Ohms
Output @ 5% Distortion
15
20
22
24
30
Watt
LL1691-70mA
ok
ok
LL1688-80mA wired at 9k2
ok
ok
LL9202-85mA wired at 6k5    
ok
   

datasheets at www.4tubes.com

How to choose a Lundahl output transformer, by tube type

Transformer choice:

When you do not ground the secondary, you may have better ultrasonic performance of the transformers, but this electrically dangerous to do, and not allowed. Some people say they can hear ultrasonic, but that is only in the internet. I would not follow such advise. I have never seen somebody demonstrating that.

LL1691. One secondary impedance only. This transformer has linear performance up to 20kHz. Thick wirde results in very low loss, only 0.25dB.

LL1688-70mA. Three primary impedances and three secondary impedances. This transformer will have a small frequency roll off close to 20kHz, which is the trade off for the more complex windings to get all the impedances. This effect is small and inaudible. (Unless you have bat ears). It is a very nice tramsformer when you go slightly above 1000V, as required impedane rises quickly then. Thick wirde results in very low loss, only 0.25dB.

LL1688-PP. This a a push pull transformer. Due to some natural feedback effect of the push-pull principle, this transformer has linear performance up to 20kHz.

LL9202 Very good match for 1000V, 85mA. A perfect ttransformer for medium budget. Loss is 0.8dB.