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Old 26th June 2012, 06:43 PM   #1
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Default PP kt88 amp problem.

I recently got myself a music angel xd-800mkIII amp, but it seems to be a problem with one channel as the 1st preamp (12at7/ecc88) is highly microphonic,
and sometimes puts out a noise that can remind you of a half opened soda or else leaking,

tho this is just in 1 of the channels, the noise comes and goes as it pleases. the other channel is working as it should be. some distortion can also be heard playing at louder levels.

I checked for cold soldier joints but i could not seem to find any. what can be done to fix this?

also i adjusted the kt88s bias to 40ma per tube.

neither grounding it properly or swapping tubes worked
the volume knob doesn't seem to make any difference at all.
what can cause a tube circuit to become microphonic and make strange pop noises when poked.
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Old 26th June 2012, 09:15 PM   #2
12E1 is offline 12E1  United Kingdom
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If you moved the tube between channels and the problem stayed with the same channel then I'd first suspect the socket, then the solder joints to the socket, then any components that may be hard wired to the socket. Are the socket contacts heavily tarnished, for example?
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Old 26th June 2012, 09:54 PM   #3
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the sockets are fine, and i checked the soldier joints and for any lose parts as well.
i tried with different tubes but that doesnt fix the problem. but the ecc85 tubes seem to work better than the 12AT7s.
and yes i changed the heater pins since the 12AT7s are sentertaped between pin 4 and 5 with pin 9.
ecc85 uses a screen between the twin triodes as pin 9. grounding pin 9 makes things worse strangly.

it could be a leaky cap or the lack of grid-stoppers in the first stage,
but shouldn't the problem be with both channels if that were the case

Last edited by ruben alstad; 26th June 2012 at 10:02 PM.
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Old 27th June 2012, 04:00 AM   #4
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Do a very carefull check of the actual components used. On at least one Music Angel I owned (PP EL34) components of wrong value had been fitted, or at least componets of perhaps the right values had been loaded in the wrong place. Get a schematic and compare the build against that schematic. If you can't get a schematic at least check that both channels have the same value components (they were'nt on that unit of mine).
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Old 27th June 2012, 09:45 AM   #5
DF96 is offline DF96  England
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Could be RF oscillation so check for grid stoppers.
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Old 27th June 2012, 02:37 PM   #6
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Originally Posted by DF96 View Post
Could be RF oscillation so check for grid stoppers.
added some missing 10k grid stoppers to the 12at7s and it seems to solve the problem.
its much less microphonic aswell. i wonder why they removed the grid stoppers in the first place.
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Old 27th June 2012, 03:11 PM   #7
DF96 is offline DF96  England
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Some don't bother with grid stoppers. With good layout and some luck you might not need them. Saves a few pence on a commercial item.

Also remember that some Chinese factories routinely omit any components which the customer might not notice are missing. An amp with an oscillating valve would probably fail an EMC test, but EMC compliance is often just a paper exercise (OK if done properly by competent people) or not done at all. A box of CE stickers is much cheaper than an EMC consultant!
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Old 27th June 2012, 03:21 PM   #8
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Hi Ruben

I have had similar problems with an xd-800 in the past. Without replacing the valves I got rid of the problem after the 3rd or 4th cleaning of the driver valve pins.

my problem was varied, clicking and popping, leaves rustling, microphony. Yours may be a different problem but the pin clean certainly sorted mine.

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