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[email protected] patrick@turneraudio.com.au is offline
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Default Replace plate load resistor with plate choke

On Thursday, March 9, 2017 at 5:15:58 PM UTC+11, Jimmy wrote:
HellO!
I've read posting on other forum and it draws my attention.

IS the DCR of the plate choke should be equal to the resistive value of the plate load resistor if considering replacing the resistor with the plate choke?

Example: if the current plate load resistor is 1000 ohm, should I use plate choke with DCR 1000 ohm??

Unfortunately, you have not mentioned the tube type, or any of the operating B+, Eadc, Vac conditions, so none of us can answer your question properly..
Please specify ALL working conditions in future.

But let us suppose you have an anode resistor of say 10k0 which has 20mAdc to feed an EL84 in triode mode, where Ea = 250Vdc ( Vdc between anode and cathode ) The 10k will have 200Vdc across it, and there may be Rk = 515r, with Ek = +10.3Vdc, and B+ will then be 10.3V+250V+200V = 460.3Vdc.

Do you understand this fully so far?

Let us assume you want the bandwidth to be 20Hz to 20kHz.
Now this would be easily achieved by the EL84 triode with 10k0 load only, and no shunt C or L anywhere.

Let us assume you wish to replace the 10k0 with a choke.
The choke's dc wire resistance can be any value providing the heat generated in the winding wire does not cause the choke to overheat. So a typical value of Rw might be 470r, and so 20mAdc gives Pw = Isquared x R = 0.188W..
But the Vdc across L = 9.4Vdc, so the Ea across tube will increase to exceed +400Vdc, and the Idc will increase, and soon there may be a CLOUD OF SMOKE from your bench, and then you might blame us for the wrong advice.

So, DO NOT just remove the 10k0 and instal a choke; you NEED to understand a whole lot more about WHY you have asked us about a choke replacing the R.

The safe way to replace the 10k0 with a choke - assuming we want better Vac operation with less THD than with the R, is to keep the Ea nearly the same with choke as for without it.

You need to understand the properties of the tube involved, say EL84 in triode, with Iadc = 20mAdc.

Its Ra will be about 2k2. if you insist on bandwidth down to 20Hz, it means the F response is -3dB at 20Hz because Ra = L reactance XL = 2,200r.
This means the L must be at least 17.5 Henry. This leads us to wonder about core saturation and and air gap size and a whole lot of other L, C and R properties of the choke and so we can safely say there could be up to 7 items to be used in several equations to arrive at a design for the choke which will lead to the desired outcome of better result compared to having a 10k0.