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Old 22nd September 2007, 10:04 PM   #1
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Default What constitutes a leaky Coupling capacitor?

Hello,

I've changed the coupling capacitors in a KT88 SE amp that Iíve been building. The KT88's are configured as triodes with 200k grid resistors.

I installed two new Sequa paper and aluminium foil coupling caps yesterday. The sound is glorious but the voltage on the KT88 grid for each channel is 0.2v and .007v. When I swapped the caps over the voltages were 0.7v and 0.1v.

I would really value some advice....is the grid voltage too high, shouldn't it be closer to 0v? Are the caps leaky?

Thank you
Andrew Parsons
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Old 22nd September 2007, 11:47 PM   #2
KaDe is offline KaDe  Germany
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another reason may be grid-leakage (reverse current) from your KT88īs, swap them likewise
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Old 23rd September 2007, 12:28 AM   #3
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What's the grid voltage at,if you lift one end of the coupling caps?
If it's lower,that could tell you that the caps are a tad bit leaky.
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Old 23rd September 2007, 04:11 AM   #4
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Thanks for the ideas! Here's what I've tried:

1) I disconnected the coupling caps from the KT88 and measured a grid voltage of 14mv and 18mv on each channel respectively.

2) I replaced the coupling caps with 0.1uF orange drops and measured a grid voltage of about 19mv on each channel.

3) I then put the 0.047uF 600v Sequa coupling caps back in and measured a grid voltage of 128mv and 245mv on each channel respectively.

4) Next I changed the orientation of the Sequa caps in the circuit and measured a grid voltage of about 74mv and 291mv on each channel respectively.

For scenarios 3 and 4 above, I noticed that the initial leakage voltage was slightly higher within the first minute of operation before dropping by around 10% to the values recorded above.

The Sequa caps sound really good and the measured frequency response is great (-1dB 27Hz to 27KHz with only CFB). The question is whether 291mV of leakage is significant.

Kind Regards
Andrew Parsons
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Old 23rd September 2007, 05:17 AM   #5
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I've had some really obscure service problems on old test equipment due to leaky coupling caps, so I have little tolerance for it. Specifically, an increase in distortion in a waveform generator. I'd remove the caps and apply some significant portion of the rated voltage, then measure the leakage current with a meter. Any decent film cap will be nearly unmeasurable. They should be very low and at least match.
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Old 23rd September 2007, 06:33 AM   #6
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Hi Conrad,

Further to my last post, I borrowed my Dad's 500v insulation tester. The orange drops measured better than 100M ohm. The better Sequa measured 100M ohms and the other 50M ohms.

Cheers
Andrew
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