Tag: input capacitance

Input capacitance

Drive Mr. Miller, drive…

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Input capacitance test rig

Whilst designing triode driver stages for a 300B SE amplifier, I looked into the input capacitance of high-mu triode (or triode-strapped pentodes) that are suitable candidates here. We know Miller capacitance is the big drawback of the triode driver stage, so I kind of liked the idea of testing how much this capacitance could be for the lovely Russian triode-strapped pentodes like 6e5p, 6e6p, 6j11p, 6j5p, 6j52p, etc.

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You better driver yourself

Time ago I asked Rod Coleman about the driving requirements of my 01a preamp whilst investigating the addition of a source follower stage to the preamp. Let’s have a look at a DHT stage loaded with a gyrator (or could well be a choke or whatever you like) driving a power amplifier.

 

Have you asked yourself whether your pre-amp is capable of driving your amp? How much burden does the cable parasitic capacitance add to the mix?

As an example we will use my current setup. A 45 SE amplifier with a 6j5 driver stage. We can approximate input impedance formed by:

Ci=(Cp+(\mu+1)\cdot Cag)

Where Ci is the amplifier’s input capacitance formed by the Miller effect of the valve’s input capacitance (Cag) and the additional parasitic capacitance of socket, wires and so forth. In this practical example where the input valve is an 6J5GT:

Cp=50pF,\mu=20, Cag=4pF

Ci=(50pF+(20+1)\cdot 4pF)) \rightarrow Ci \approx 140pF
If we now add the cable capacitance which could easily be 100pF per metre and at least 2 metres run from my pre-amp to the amp then:
Ct= Ci+Ccable \rightarrow Ct=130pF+(100pF \cdot 2) = 330pF
So with an input resistance of 100kΩ and a capacitance of about 330pF let’s have a look at the current requirements to drive this load. The cable current @ 50kHz should be:
Xc=\tfrac{1}{2\cdot \pi \cdot f\cdot C}=\tfrac{1}{2\cdot \pi \cdot 50kHz\cdot 330pF} \approx 9650 \Omega
Preamps output peak voltage is around 10V, So the current demanded by the cables would be:
Ip=\frac {Vp}{Zc}=\frac{10V}{9650\Omega }\approx 1mA
So if we want the preamp valve to source 1mA to the load, we need x10 current driving capability to be on the safe side. Clearly with an 26 (or 01a) we are a little on the low side as the bias current won’t be more than 4-6mA.
So there are clearly facts here to support the addition of a source/cathode-follower stage to the amp in addition to the improvement on the bass response due to a lower output impedance. Something we will look at some other time….