# How to wind a 5K audio tube amplifier transformer?

• gary350
In summary, the transformer needs a 5k primary and 8 ohm secondary transformer. The online impedance transformer calculator says, 5k to 8 ohms in 25:1 ratio. The online information says, audio transformers should be 60w x 5 = 300 watts over sized 5 times so peak pulses are not cut off. The suggestions are to wind 500 turns on the EI core, assemble transformer then test it. If 500 turns measures 7 Henrys then mu formula will show that to be 4690 impedance. I need the mu formula. I need help determining how many turns are on the 5K primary. Suggestions are to change the 500 turn primary several times and test it each time then draw a graph to determine how many turns

#### gary350

60 watt tube amp needs a 5k primary and 8 ohm secondary transformer. I have wound voltage transformers on my lathe many times. Audio transformer is no different. Online impedance transformer calculator says, 5k to 8 ohms in 25:1 ratio. Online info says, audio transformer should be 60w x 5 = 300 watts over sized 5 times so peak pulses are not cut off.

Online says, Don't try to calculate. Inductance will depend on many parameters. Some of these parameters are easy to determine (cross-section, magnetic path length), but others not so (mu, stacking factor, effective gap). Furthermore, inductance depends much on frequency and AC voltage amplitude. So, wind a test winding and measure it under your specific conditions.

Suggestions are to wind 500 turns on the EI core, assemble transformer then test it. If 500 turns measures 7 Henrys then mu formula will show that to be 4690 impedance. I need need the mu formula. I need help determining how many turns are on the 5K primary. Suggestions are to change the 500 turn primary several times and test it each time then draw a graph to determine how many turns are needed to = 5K impedance, then built it and test it to see if it is 5K..

Suggestions?

Last edited:
If the transformer requires a 5k primary, then the inductive reactance at the lowest frequency needs to be several times greater than 5k. Tube amplifiers are limited by the B supply voltage and current swing (emission) and so it sounds as if 60W is the peak power. I am not sure there is a need to design for peaks exceeding this. Can you say please if it is a triode or pentode/tetrode output and is it Class B, as these things make a difference.

Last edited:
tech99 said:
If the transformer requires a 5k primary, then the inductive reactance at the lowest frequency needs to be several times greater than 5k. Tube amplifiers are limited by the B supply voltage and current swing (emission) and so it sounds as if 60W is the peak power. I am not sure there is a need to design for peaks exceeding this. Can you say please if it is a triode or pentode/tetrode output and is it Class B, as these things make a difference.

I read low frequency to target is 20 Hz. Base notes will produce high peaks in the output transformer.

You might be interested in the amplifier described at this url; it is lower power than yours but the design considerations for the transformer are discussed. I believe these amplifiers both operate in class AB1. http://www.r-type.org/articles/art-117.htm

dlgoff
gary350 said:
I have wound voltage transformers on my lathe many times.
gary350 said:
I read low frequency to target is 20 Hz. Base notes will produce high peaks in the output transformer.
Your schematic shows two trahsformers that are operating above SELV. What safety precautions are you taking in your transformer winding and enclosure design? Are you well-versed in UL safety standards (single insulation, double insulation, etc.)?

berkeman said:
Your schematic shows two trahsformers that are operating above SELV. What safety precautions are you taking in your transformer winding and enclosure design? Are you well-versed in UL safety standards (single insulation, double insulation, etc.)?
My first job after college I worked in a transformer company, Thordarson Meissner. Now days transformers have built in safety fuses so you don't accidently burn your house down.

gary350 said:
Now days transformers have built in safety fuses so you don't accidently burn your house down.

That's not what I was asking about. What is the difference between a single-insulation transformer and a double-insulation transformer? Why is that distinction important for SELV applications?

tech99 said:
You might be interested in the amplifier described at this url; it is lower power than yours but the design considerations for the transformer are discussed. I believe these amplifiers both operate in class AB1. http://www.r-type.org/articles/art-117.htm

This link is amazing. Wow that is old. I remember those tubes when I was in high school. I see a problem. My 700 watt EI core is probably too small for a 3860 turn primary coil. Primary wire is .0199 diameter, I need to do math to see if 3860 turns will fit but I don't thinks so. Core holes are .625" x 1.875". Book says 46 turns per inch = 86 turns per layer. There is room for 31 layers, not including laminate and insulation. 25 layers might fit, 25 x 86 = 2150 turns & no room for a secondary winding. LOL. Nope my EI core won't work. Tomorrow I need to check see what other EI cores I have, I might have 1 the correct size or larger. Part of the fun of this project was going to be, build my own transformer.

I have also read the section in the Radiotron Designers' Handbook, which suggests a lower primary reactance than I previously said. The speaker in its enclosure will probably have a peak of impedance at a bass resonance, traditionally about 50 Hz. If used with pentodes, which have high plate impedance, the gain will rise at this frequency, maybe x4. If used with triodes, the plate impedance is typically only half the load, so the gain does not increase very much at resonance. For the ultra linear-connected beam tetrodes you are using, the plate impedance will be intermediate between pentode and triode. In order to counter the peak in gain, Radiotron say the reactance can be equal to the peak impedance. Alternatively it may be twice this figure, which provide some bass boost. The amplifier will unfortunately see a reactive load, which will restrict the maximum power a little at the bass resonance.
Regarding electrical safety, in my country, UK, I would connect chassis to the mains ground and place the fuse in the live, rather than the neutral, conductor. There is a lot of discussion about where to place fuses to protect the B+ supply with this sort of equipment.

gary350 said:
My first job after college I worked in a transformer company, Thordarson Meissner. Now days transformers have built in safety fuses so you don't accidently burn your house down.
No, transformers have no such thing "built in". The circuitry and components around them can help, but the transformer is just wires and a bobbin and a core (and maybe some tape...).

berkeman said:
That's not what I was asking about. What is the difference between a single-insulation transformer and a double-insulation transformer? Why is that distinction important for SELV applications?
Please answer my explicit questions, Gary. I don't want to close this thread as a dangerous discussion if you are well-versed in how to make a high-voltage transformer assembly and enclosure safe.

dlgoff