Bottlehead Forum
General Category => Technical topics => Topic started by: xcortes on October 16, 2009, 07:49:48 AM
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I reduced the parafeed caps size on my tweeter and midrange amps to implement a first order high pass filter. When asked about how to size the parafeed cap PJ said:
"Just scale the capacitance by the impedance ratio of the transformer (turns ratio squared, which is about 336 at the 8 ohm tap) to find the value that it would act like in a speaker crossover. For example, to get the effect of a 3.36uF cap, use 0.01uF as the parafeed value"
I wanted to keep the crossover equivalent to bruce edgar's crossover (what I had before) which was1uF at the 8 ohm tap so I used 1/336 = 0.003uF or 3,000pF cap
For the mids the stock used a 40 uF cap so I'm using a 40/336 0.12 uF cap for an 8 ohm driver on the 8 ohm tap.
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Here's the formula for normal parafeed cap size:
C = 2*L / (R*R)
C is the capacitance in farads, L is the plate choke inductance in henries, and R is the load impedance seen by the plate (i.e. transformer primary impedance) in ohms. If you use kOhms the C is in microfarads.
This value gives the best bass extension while keeping the impedance seen by the tube's plate relatively constant and resistive to the lowest possible frequency. The value is not especially critical, anything from half to twice this value will work pretty well. For speakers with especially deep bass there may be some interaction with the speaker impedance fluctuations, and experimenting within this range of values can sometimes prove fruitful.
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Here's the formula for normal parafeed cap size:
C = 2*L / (R*R)
C is the capacitance in farads, L is the plate choke inductance in henries, and R is the load impedance in ohms. If you use kOhms the C is in microfarads.
This value gives the best bass extension while keeping the impedance seen by the tube's plate relatively constant and resistive to the lowest possible frequency. The value is not especially critical, anything from half to twice this value will work pretty well. For speakers with especially deep bass there may be some interaction with the speaker impedance fluctuations, and experimenting within this range of values can sometimes prove fruitful.
PJ, does the R in the formula refer to the primary impedance of the OT, secondary impedance of the OT or the speaker attached to the secondary? I'm guessing that the parafeed capacitor is there to block the DC as well as cancel the X sub L of the load seen by the plate.
Thanks,
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PJ, does the R in the formula refer to the primary impedance, secondary impedance of the output transformer or the speaker attached to the secondary? ...
Thanks for catching that, Grainger - I meant the primary impedance. I've edited my post.
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Paul,
What would the formula look like if we substitute the "L" (plate choke) with a CCS?
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Here's the formula for normal parafeed cap size:
C = 2*L / (R*R)
C is the capacitance in farads, L is the plate choke inductance in Henries, and R is the load impedance seen by the plate (i.e. transformer primary impedance) in ohms. If you use k Ohms the C is in microfarads.
This value gives the best bass extension while keeping the impedance seen by the tube's plate relatively constant and resistive to the lowest possible frequency. The value is not especially critical, anything from half to twice this value will work pretty well. For speakers with especially deep bass there may be some interaction with the speaker impedance fluctuations, and experimenting within this range of values can sometimes prove fruitful.
Edited in Red:
Paul, I just went to check my calculations. I have a TFA-204 with a BCP-15. Looking at Mikey's site the TFA-204 has a primary impedance of 3000 and the BCP-15 has an Inductance of "Good."
That isn't on my calculator. I'm using a 10uF Obbligato Polypropylene Film/Oil cap. And it sounds good. I guess I'll just assume it is right.
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Sorry to say, there is no formula I know of that works reliably for the case of current sources. The situation is even worse for preamps with effectively unloaded secondaries. The simplified theory gives L/(R*R) as before, with R being the tube's plate resistance (if the transformer is unloaded) and L being the OPT inductance. This gives enormous capacitors, and the inductance will vary hugely - more than a 10 to 1 range - depending on the frequency and voltage at which it is measured.
This simplified theory ignores the effective resistance in parallel with the OPT inductance. This resistance is a combination of wire resistance, eddy currents in the laminations which appear as resistance, and magnetic hysteresis which is nonlinear but absorbs energy so you can think of it as a kind of resistance.
In practice, these uncertain loss resistances are the critical parameter, so the only reliable approach I know of is to experiment with different capacitor values and make measurements. You'll have to measure the frequency response at a variety of signal levels to be confident of your choice.
Grainger, I hope you are not using a BCP-16 grid choke as a plate choke - it can't handle **any** DC current, and its inductance varies widely as mentioned above for parafeed OPTs. Are you using a BCP-15 perhaps (40 henries)?
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PJ, Yes, I have the BCP-15. Otherwise it would have burned out years ago. I should have written some of this down when I received. Instead I polished the brass end bells, lacquered them and put them in the amp.
Thanks for the correction. The post has been edited.