capacitor increase?

[geoff 1965]

Hello,
is it normal for a coupling capacitors value to increase when in circuit?
the coupler going to the volume pot from pin 6 "plate" and the tonestack coupler in my amp both measure just over 10nf out but when in circuit they both measure around 14.5nf.

[JMPGuitars]

You can't usually accurately measure a component value in-circuit without a special meter. If you measure in-circuit with a standard meter, you get the value of the position in the circuit, not necessarily the component itself.

[geoff 1965]

thank's Josh, i did'nt know that,i was thinking the caps might be faulty!

[JMPGuitars]

No problem! That's the PIA part of testing components in circuit; If you want an accurate value, you usually have to lift one end.

My guess in your case is that you probably have another capacitor or some variable capacitance in parallel with whatever cap you're trying to measure.

[geoff 1965]

yeah Josh,all the other couplers and electrolytics are the same capacitance in circuit as out so that was throwing me.the 2 mentioned are basically in series with the volume pots resistance between them and the pot wiper goes to the 10nf into the phase inverter so maybe that's where the increase is coming from? normally 2 caps in series will half the capacitance and double the voltage but that's with no resistance between them.

[JMPGuitars]

I have no idea what circuit you're actually referencing, but I would guess if you adjust the pot you'll see a change in the in-circuit cap value. You probably already know that the value of caps in parallel is equal to the sum of their individual values.

[geoff 1965]

it's the lite2b circuit and yes adjusting the volume alters the capacitance,minimum volume=10nf and max volume=14nf.
so the more signal going to the phase inverter coupler the more the series couplers capacitance increases.

[JMPGuitars]

How does adjusting the tone knob affect the value with the volume pot maxed?

[geoff 1965]

opposite Josh, with volume on max and tone on min = 14.5nf and tone on max = 11.4nf, so that looks good! i can see how interactive this tonestack is now! adjusting the volume also affects the tone.
note; this goes back to the earlier discussion we had on the 18 watt normal channel tonestack,so anyone building the lite2 that does'nt like the "interactive" tonestack could use the very early marshall layout with the tone wiper going to ground.