Guitar preamp problem (intermittent)

This will probably sound too vague but here goes..............:

My son has a Peavey RockMaster preamp in his rig and is experiencing an intermittent problem, what happens is the pre starts to make what can best be described as a "fluffing" sound accompanied by a noticable loss in gain and any signal is distorted in a nasty way.

I had considered replacing the 12AX7 tube but I'm really clutching at straws so any suggestions as to what may cause this phenonema would be greatly appreciated.

Have you eliminated the power amp as a possibility?

You'll want to do some quick tests once the preamp is doing this - does the "fluffing" seem to be aggravated by vibration? Does it suddenly stop or change if you give the case a firm whack?

If so this would suggest a mechanical failure, meaning a loose connection or component with a broken solder joint. It could also mean the tube has an issue, or the socket itself. Having a brand new spare 12AX7 around is never a bad idea, and whatever the result of the whacking test try out the new tube, if for no other purpose than to eliminate the tube as the problem. If the new tube doesn't change anything, and you have another proven 12AX7 around, try it as well.

Be careful when opening any piece of gear, and properly discharge filter capacitors before attempting any repairs.

You should also examine any/every circuit board connector inside the unit. Are there burn marks on any of them? I've had some molex power connectors turn out to be the cause of a console doing some "fluffing" sounds. A failing connection on your DC rails can cause this sound, and discolored plastic connectors are an indicator of too much heat (resistance!) in the connector, which will degrade the connection.
No harm done if you clean every one of these - or better yet eliminate them by hardwiring them out (only if you find the burn marks described). Be sure to discharge the filter caps first though.

Play it safe and happy hunting!

Frank

Could also be a bad capacitor someplace in the chain. Is the cathode resistor on the tube bypassed? If so, that's a prime candidate.

Peace,
Paul

Thanks for the suggestions. Since posting the question, the problem hasn't manifested itself but the minute it does, we'll carry out a "thump" test and see what happens

Is that the models that has 4 fx loops ?
if they are serial , you may want to clean them and check contacts
if everything goes through them and you lose one ?
is it on all cahnnels or just certain ones ?

good luck

You can easily bypass the whole fxloop part by a few simple mods on the pcb. This the first thing you should do on this preamp, imo.

I've modded mine too and one of the nicest things to do, is to add a switch which activates the EQ of the clean channel when the amp is in crunch/lead mode. It gives you more sound possibilities and certainly more access to more classic rock sounds.

Maarten

okgb,

Yes it does have the 4 loops but I'm pretty sure they don't get used as my son's habit is to set up as "guitar---->pedals----->preamp".

Tubefreak,

I may have to ask for more details on those mods but I'll wait until my son has his rig back home in a couple of days. It is usually left at the singer's studio as there's still more than enough gear here, I'm actually tracking a number of songs for the band at the moment and my Rivera is being used in preference to the Peavey/Marshall power amp combination.

Chris

even if you don't use the fx loops , the contacts on the jacks could become flakey , so get some contact cleaner and " exercise " a
1/4 plug while shooting some spray in there ,
And if thats not the problem at least you've eliminated it .

regards Greg

Greg,

I have some Caig DeOxit here which I can use. Now that the unit is here, I'll pull it out of the rack case and have a closer look.

Tube,

Would you be able to give me some detailed info on those mods you performed.

Cheers guys,

Chris