strange hum problem myJCA100

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kaoslord
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Joined: Mon Dec 09, 2013 4:20 am

strange hum problem myJCA100

Post by kaoslord »

Hi All,

i noticed that the hum level in the OD channel of my jca100 (a slo clone)
is huge compared to the clean channel that is VERY quiet.

i wanted to troubleshoot this and I thought it could be a leaking cap in the OD channel
So i first removed the C24 (2nd stage coupling): now there should be no signal coming from the OD gain

but - surprisingly - if i turn up the gain i can hear the guitar ! it is very low volume but it's there anyway.

how can it be ? there is NO cap ! :phair: :hmm:
conducting board ? interference between 2nd tube and OT ?

thank you
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newholland
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Re: strange hum problem myJCA100

Post by newholland »

how old are the tubes?
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Tortuga
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Re: strange hum problem myJCA100

Post by Tortuga »

I think I've read that the two channels are not completely isolated. If that cap does disconnect OD channel from signal, you could be hearing bleed-through from crunch channel. I wonder if turning the crunch volume down has any impact on what you're hearing?
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MikeO
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Re: strange hum problem myJCA100

Post by MikeO »

GRIMESPACE wrote:I think I've read that the two channels are not completely isolated. If that cap does disconnect OD channel from signal, you could be hearing bleed-through from crunch channel. I wonder if turning the crunch volume down has any impact on what you're hearing?


This. I've read the same thing a few different places from people who are well versed in that sort of thing.
Amps: 5150 (w/bias mod), 5150 II, PRS MT-15, FJA PV XXX, Framus Dragon, FJA PV Penta, PV Rockmaster-->Marshall 9100, PV Ultra 120
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shep
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Re: strange hum problem myJCA100

Post by shep »

The crunch channel is deliberately feeding through to the lead channel when the lead channel is engaged. The lead channel is a mix of both channels. As for the hum. The 50watt I have, had considerably more hum than my jca22. That is because of the heater traces on the board. Unless there is something wrong with your amp, it is common to get a lot of heater hum out of the high wattage jca amps.

if you notice, both gain knobs should affect the tone while on the lead channel. that is true for the jca50, but not the jca22. i have the jca22 and jca50 and have confirmed this through testing the relay connections on both amps. i have not confirmed this on the jca100 but would bet that it is the same as the jca50.

Solutions: noise gate, dc heater supply, or hand wire the heaters and cut the traces.

I hand wired them and made an artificial center tap with two 100 ohm resistors.
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