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~ Testing for Capacitor Leakage in and out of circuit ~

One common problem or if not yet a problem 'factor' that affects the performance of old valve amplifiers is coupling capacitor leakage ~ It is often responsible for expensive valve or output transformer failure and gives many untouched old valve amplifiers a 'weather dependant' sound up to the point of failure

Capacitor Leakage can be expressed as a measure of the insulation resistance or the d.c. current a capacitor passes but either way it is only meaningful if the measurement is made at the rated voltage or at least at the operating voltage and temperature ~ Coupling capacitors should be rated at least to the HT supply ~ see below

Correspondence over the years suggests that some people get confused with coupling and decoupling using capacitors and sometimes inductors ~ Transformer coupling appears self explanatory ~ The section of QUAD 22 schematic below will I hope explain coupling and decoupling capacitors and effects of leakage on all valve circuits


The d.c. voltages shown in yellow above are normal with good valves and capacitors

C3 is a coupling capacitor which prevents 60V appearing at the tape output socket T.O. and the grid of V1 (more information here) Coupling capacitor C13 prevents 100V at V3A anode from affecting the bias of V3B while coupling the a.c. signal between the 2 stages ~ C16 prevents 200V (actually less due to R41 and R42) reaching the amplifier output

In most valve amplifiers the valve grids sit at 0V 'ground potential' but there are exceptions one of which is the QUAD II KT66 grids which sit at about 200mV and my amplifier/buffer ~ In the circuit above the valve grids are held at 0V by R12 RV1A and R26 ~ If say C13 had a leakage resistance of 1.5MΩ then the voltage at the grid of V3B pin 7 would be ?

Not 50V but far too much for V3Bs grid ~ Allowing for the 225V supply having a high impedance and the high 470kΩ anode load of V3A (R23) the voltage at V3B pin 7 would try to be about 43V but the grid is now forward biased and draws 10s of mA preventing V3B operating as an amplifier let alone a linear one

Even if C13s leakage resistance was a high 15MΩ the circuit would not perform correctly ~ Note C13 in later builds of the QUAD22 is a 350V rated polystyrene with insulation resistance >2000MΩ ~ its low value of 820pF gives a 20Hz high pass filter overall and you should find no need to change this capacitor or R23

C34 is a later addition input coupling capacitor to prevent d.c. reaching V3A grid ~ In the past good valve equipment ensured the high HT voltage never reached the output but transistor equipment had electrolytic output coupling capacitors and although the internal supplies were low they could via leakage pass d.c. to the high impedance input of a following valve amplifier

Coupling capacitor C16 positioned where it is would suggest that the capacitors C18 to C30 in the tone stage could be rated at only a few volts as there should be no d.c. across any of them ~ Actually as shown with the tone switch in operation there will be 160V across C18 and C19 although you may not be able to measure it

C18 and C19 are in parallel and have a combined value of 11.2nF which is in series with the 47nF of C16 ~ the total capacitance subject to the normal 200V at V3B anode is about 9nF ~ Using the formula to determine the d.c. voltage across capacitors in series we get 9nF/11.2nF x 200V = 160V which is why I use 160V polystyrene capacitors

Capacitor C8 is a de-coupling capacitor which 'couples' any a.c. including HT ripple from the screen grid of V1 to ground 0V ~ C5 across the cathode bias resistor for V1 (R14) de-couples a.c. cathode current to ground which would otherwise produce an a.c. voltage across R14 that would reduce V1s gain due to 'local negative feedback'

C7 C12 and C15 de-couple any a.c. that may appear on the HT lines which includes circulating signal currents and ripple from the rectified supply ~ good de-coupling is more important as stages become more sensitive ~ Ideally decoupling capacitors should return a.c. back to 0V ground close to the cathode of the stage being 'de-coupled'

C10 couples the output of the tone control section back to V3A as negative feedback forming an active Baxandall tone control or flat a gain amplifier depending on the position of the filter switch S3 ~ It also partially de–couples the cathode resistor of V3A (R21) to ground via half of the balance control (not shown) varying the left/right gain

~ Determining Capacitor Leakage in circuit ~

If a valve is removed or say the heaters fail the anode coupling capacitor will be subjected to the full HT voltage and so coupling capacitors should ideally be rated for the highest likely HT voltage ~ In the circuit above with all valves removed or before they heat up all the anode coupling capacitors could have at least 330V across them

It is ~ or should be ~ safe to remove some or all valves in many well designed amplifiers ~ Push Pull valve amplifiers used by the BBC had to work and be stable with one output valve removed ~ With an old original condition valve amp you may find that removing valves causes the coupling capacitors to fail so be careful

In the schematic above we know the 350V rated polystyrene C13 should be okay when V3 is removed but what if it was not ? ~ The 225V HT would be nearer to 260V provided C7 C12 and C15 were not leaky ~ which they will be unless new parts ~ and the rest of the amplifier is okay ~ The voltage across R26 can be used to determine C13s leakage current

Without V3 fitted say we measure 2V across R26 this can only be due to Leakage in C13 or across the valve bass ~ Assuming the valve base is good and our meter has 10MΩ input resistance we measure the voltage at V3 pin 1

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about C16 which if original is a Hunts AM108 350V rated metalised paper capacitor ~ It is likely that with S3 in the cancel position and >300V across C16 that there will be a measurable voltage at the output

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