The Steel Guitar Forum

My good friend and steel guitar mentor, Mr. Ole Bendix of Copenhagen Denmark, has a problem with his amp, and since he's not a Forumite, I've offered my help !!

The amp is a Peavey Session 400 Ltd. about 6-7 years old, and he has experienced this problem 3 times in all, this past weekend being the most recent:

The amp was turned on for 3-4 hours during a festival-gig, but it wasn't being played all the time, and when played, it was at a somewhat reduced volume, compared to his usual all-night club-gigs. Suddenly the amp started humming, quite softly at first, but then increasingly louder until he turned it of after a couple of minutes. The hum was independant of whether anything was plugged into the amp or not, and it was also independant of volume/gain settings.

This particular gig was outdoors, and with a very long power-cable supplying power to the stage, so it must be expected, that the line voltage was below our usual 220 Volts, and this has also been the case in the 2 other instances of this fault !!

Also he has played this amp at numerous gigs at higher volume all night, so there seems to be a relationship with the low line voltage here.

Any ideas ???

Also he has a specific question:
The "Shift" potmeter has been changed at one time, and he's unsure if it's the correct value. Can anybody tell us what this pot is supposed to be ??

Ole

Ole Dantoft.......

This problem isn't re-stricted to just "Peavey" amps.....

Any "Hi-Powered" audio ampifier will exibit this "Hum" when deprived of correct electrical power and voltage.

The reason is :
It takes an "X" amount of voltage to completely charge up the filter capacitors in an amp. If they arn't allowed to operate at the intended voltage of the device, the system is continually trying to charge the caps AND bias out the amp at the same time. Now,as the caps cannot get a full charge, they cannot perform the duties of filtering the AC ripple out of the DC conponnet. This in turn,is heard as a loud hum in the speaker. It sounds bad but the amp isn't at fault here. If the line voltage is low,one cannot expect the amp to operate as it would with a good power source.

If the amp is rated for 120 volt operation,then 105 volt AC will be below the needed voltage. If it's 240 volt system, then 220 is the lower limit for this amp.

The whole thing is like trying to run your hi-powered car on lighter fluid. It may work, but not good.

The mid-shift pot on my Peavey Session 400 Ltd is 10k. This is a factory unit.

Hope this helps.

Bill

Bill,
Thanks for chiming in on this one, and I'm sorry for not having responded earlier, I've been away from the PC for a couple of days !

Well, my initial thoughts were exactly like you describe the problem, but the amp doesn't just hum on top of the sound, it quits playing alltogether, and just hums, and the hums gets worse and worse if you leave it on.

Does the Peavey amp have a switch-mode supply ?? This could maybe explain the behaviour.

Thanks for the info on the shift-pot !!

Ole

I would suspect the filter capacitors in the power supply. If they get weak, or go bad, you will get the hum. Also if the mains voltage is a little low it will either not have enough voltage or will too low and the amp will only hum.

Check the power supply voltages, with normal mains voltage and if they are low - even a couple of volts - it's a good indication the filter caps are probably bad.

If you don't have a schematic, contact Mike Brown at Peavey and order one.

I think Jack nailed it for you.....

Low line-voltage and bad caps will do that !!

I think Jack has hit it in part,I am a power eng. for large US company.I have designed power amps (motor control)from 120 watts to 15Kw with peaks of 45Kw. for the last 37 years.Whenever we design amps to run on 240 VAC 50 hz, we generally use 105 deg/C parts and bump the Mfd rating up by about 30% from what would be used on 60 hz. This is to comensate for larger current ripple that they will see at 50 hz.Personally I think the caps have dried out and need to be replaced.The current ripple from the lower update time (50 vrs 60 hz) creates greater heat in the caps, heat being the killer for any capacitor.As long as the line input diodes can handle the inrush current, there is no reason that the mfd value could no be bumped up 30-40%.

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Jack, Bill, Curt,

Thanks a lot !! I too believe this may be it, in fact I'm quite sure !!

I'll contact Mike Brown for that schematic right away.

Thanks guys !

Ole