I am trying to do some simulations for my recent 2A3 project. It was planned to experiment with choke loaded drivers and selected for the Lundahl LL1668 25mA 100H anode choke. This may be used with D3a, 6e5p, ECC99 and alike. For selection purposes, I ran simulations with the above mentioned tubes and a corresponding anode inductance in LTspice. As the parallel capacitance of the choke is missing in the datasheet, I tried to measure it with a impedance analyser.
The measured value was in the range of 45pF for both windings in series. Do you think this value is correct? Did any of you measured comparable chokes to validate my measurement?
The measured value was in the range of 45pF for both windings in series. Do you think this value is correct? Did any of you measured comparable chokes to validate my measurement?
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It's a bit high. If this is correct, one windings has 90pF capacitance.
LL1660 primary (4:4.5, 130H) has about 30pF.
I have custom winded 640H choke, with -high- 57pF.
100H and 45pF is resonant at 2,373hz.
That does not seem unreasonable to me.
Most interstage transformers and output transformers I tested
were resonant somewhere between about 1kHz to 3kHz.
Are you using a plate choke and then RC coupling to the grid?
That does not seem unreasonable to me.
Most interstage transformers and output transformers I tested
were resonant somewhere between about 1kHz to 3kHz.
Are you using a plate choke and then RC coupling to the grid?
> Do you think this value is correct?
I'm sure it is not real wrong.
Small wound-iron lumps tend to have 100pFd, larger one will have hundreds of pFd. This is for when the maker is unconcerned about C. It tends to go as the surface area of the winding. So larger with physical size, and almost no relation to prime inductance.
However Lundahl knows what YOU are paying for. He surely considered minimizing C. It is a hard fight: Capacitance is everywhere. Getting down to 45pFd (as euro21 cites) would be good work, what Lundahl sells. Lower is possible if L could be sacrificed. As the computed self-resonance is well above middle audio frequencies, and extrapolates to a C-cutoff like 200KHz, this is a fine design. The values do not have to be very exact.
I'm sure it is not real wrong.
Small wound-iron lumps tend to have 100pFd, larger one will have hundreds of pFd. This is for when the maker is unconcerned about C. It tends to go as the surface area of the winding. So larger with physical size, and almost no relation to prime inductance.
However Lundahl knows what YOU are paying for. He surely considered minimizing C. It is a hard fight: Capacitance is everywhere. Getting down to 45pFd (as euro21 cites) would be good work, what Lundahl sells. Lower is possible if L could be sacrificed. As the computed self-resonance is well above middle audio frequencies, and extrapolates to a C-cutoff like 200KHz, this is a fine design. The values do not have to be very exact.
More turns equals more inductance.
More turns equals more surface area.
More surface area equals more capacitance, yes?
Small wire size is limited by current carrying capability (small does not carry much current). Bigger wires and the same number of turns equals more area, and more capacitance.
But there are many tricks, laying down in opposite ends and directions, proprietary techniques, etc.
I suppose that when I stated that there typically was resonance in the transformer at about 1 to 3 kHz that the amplifier would have that characteristic. Horrors!
No, it does not affect the amplifier mid frequency response at or near resonance.
That resonance is swamped out by the plate resistance (at least it is on my amplifiers which use triodes, triode wired pentodes, or triode wired beam power tubes).
Feedback is not necessary in that case for swamping out that resonance.
I suspect that in some cases using pentode wired, or beam power wired tubes might possibly need some feedback to swamp that resonance, because the plate resistance is so high (but probably not because the loaded secondary does swamp that resonance too).
But it more likely needs feedback to counteract the effect of the lower impedance at low frequencies because of the inductance, and also lower impedance at high frequencies due to the distributed capacitance.
The test of resonance is done with high impedance drive to the primary, and the secondary is unloaded. Where the voltage is maximum, that is resonance.
The inductance is also tested, this time driven from a medium impedance, and with no load on the secondary.
Once you have measured inductance and resonance, it is a simple calculation to find the distributed capacitance of the primary.
Next the secondary is shorted, and the leakage reactance is tested.
Now, the ratio of input to output voltage is tested, using a low impedance drive to the primary, and the secondary is unloaded.
Then the primary impedance is calculated.
After that, the secondary is loaded, and the insertion loss is tested.
It actually is a little more complicated than this, you have items like DCR of the primary and secondary for example. But those have been measured too.
Different drive tube configurations require different solutions to get optimal results from transformers. But done properly, they can all work well.
More turns equals more surface area.
More surface area equals more capacitance, yes?
Small wire size is limited by current carrying capability (small does not carry much current). Bigger wires and the same number of turns equals more area, and more capacitance.
But there are many tricks, laying down in opposite ends and directions, proprietary techniques, etc.
I suppose that when I stated that there typically was resonance in the transformer at about 1 to 3 kHz that the amplifier would have that characteristic. Horrors!
No, it does not affect the amplifier mid frequency response at or near resonance.
That resonance is swamped out by the plate resistance (at least it is on my amplifiers which use triodes, triode wired pentodes, or triode wired beam power tubes).
Feedback is not necessary in that case for swamping out that resonance.
I suspect that in some cases using pentode wired, or beam power wired tubes might possibly need some feedback to swamp that resonance, because the plate resistance is so high (but probably not because the loaded secondary does swamp that resonance too).
But it more likely needs feedback to counteract the effect of the lower impedance at low frequencies because of the inductance, and also lower impedance at high frequencies due to the distributed capacitance.
The test of resonance is done with high impedance drive to the primary, and the secondary is unloaded. Where the voltage is maximum, that is resonance.
The inductance is also tested, this time driven from a medium impedance, and with no load on the secondary.
Once you have measured inductance and resonance, it is a simple calculation to find the distributed capacitance of the primary.
Next the secondary is shorted, and the leakage reactance is tested.
Now, the ratio of input to output voltage is tested, using a low impedance drive to the primary, and the secondary is unloaded.
Then the primary impedance is calculated.
After that, the secondary is loaded, and the insertion loss is tested.
It actually is a little more complicated than this, you have items like DCR of the primary and secondary for example. But those have been measured too.
Different drive tube configurations require different solutions to get optimal results from transformers. But done properly, they can all work well.
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