Valve Amplifier fault - nasty buzz.

pcmangler · 2008-11-15T14:40:39+00:00

Hi,My Fender guitar amp has developed a fault and I'm pretty good withelectronics, safety (discharging caps etc) and soldering, but I don'thave the circuit knowledge to work out which components are faulty.Could someone here offer any help please ?The symptoms are that it's making a horrible buzz on the drive & moredrive channels, as well as letting some sound through on the cleanchannel when the volume is at zero.Here's the full schematic.... [url]And here's just the preamp section: [URL=""]Here's a Wav file of the noise it's making... [URL=""]Amp_Buzz.wav[/URL]Be careful when listening to it, it starts off very quietly thengradually gets loud - so don't turn up your speakers because you can'thear it very well when it starts !The increase in oscillation frequency is caused by me turning the [B]Drive[/B] knob from 12 down to 0. Turning the [B]Bass[/B] knob from12 to 0 also has a similar effect. Adjusting any of the othercontrols does not cause a change in frequency - only in volume / toneof the buzz.This is happening *only* on the Drive channel, and gets nastier withthe [B]More Drive[/B] button pushed.The recording was done with nothing plugged into the amp.With the amp on the clean channel, you can hear a little output fromthe speaker with the [B]Volume[/B] set to zero - which it never usedto do. Apart from this anomaly, the amp behaves fine on the cleanchannel and I used it for practice this week.I've tried swapping positions of the preamp valves and also adifferent set of power amp valves - no difference. I've also triedresoldering R78 & R79, which apparently have a tendency to develop dryjoints, and all the big electrolytic caps.I'm more than happy to give test point readings to anyone who canhelp... the schematic gives TP voltages so hopefully that might be ofuse.Anyone ? Many thanks in advance - I really can't afford to take thisanywhere to be repaired.

Nov 15, 2008 24 Replies

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N_Cook 17 years ago

caps.

just

only

supplies,

when

it's

Of course with this sort of problem it could be just fiddling inside disturbed something , even just wiring looms and the onferlying problem is still lurking there. Unless something definitely found wrong with one of those caps

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Ron Johnson 17 years ago

Oh I first read that as Human Beef!

Ron

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Meat Plow 17 years ago

Hopefully it was at the very least Feline or Canine.

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Meat Plow 17 years ago

We are relying on the OP being honest here. Personally I would question this based solely upon the neener neener reply. I'm a firm believer in "if the shoe doesn't fit" in these cases and would have expected to find the problem in something other than the mains smoothers if upon nothing else than my 30+ years repairing guitar amps.

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Arfa Daily 17 years ago

Likewise, based on a similar 30+ years of repairing said equipment.

Arfa

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