Stupid valve question!

Supergrover · Post by **Supergrover** » Sat Dec 31, 2005 11:42 pm

Hi, hope everyone has a fantastic new year!

I posted a while back regarding a hum comming from my AD30HTC and it was suggested that i tap the valves with a pencil to detect a fault, (i know the next question must seem really stupid, sorry in advance)is it safe to remove the outer case from the smaller valves (i think the pre amps?) or will i be toasted!?

As always im very greatful for your help/suggestions, one day someone will ask a question i can actually answer!

Post by **bclaire** » Sat Dec 31, 2005 11:55 pm

No you have to remove the metal covers on the preamp tubes as they are probably the ones to blame...

Make sure the amp is cool though....

fiveightandten · Post by **fiveightandten** » Sun Jan 01, 2006 3:23 pm

You don't <i>have</i> to remove the metal covers to do the tap test. But if you want to take the tubes out, you need to take them off...of course.

If you hit a bad one, you'll know it, weather or not the tube shield is on it. Heck, you'll probably be able to hear it just tapping on the chassis, or the faceplate of the amp, if the volume is up.

Don't forget, you do need the amp to be on, and the volume up a little (not too loud!), or you won't hear anything. You'll know it when you hit a bad one. you'll get high pitched ringing, and possibly pops or crackles.

But that being said, hum can come come a lot of different sources...and it may not be a bad tube.

-Nick

VintageJon · Post by **VintageJon** » Mon Jan 02, 2006 3:40 am

Does it hum with nothing plugged into Input?

-Jon

(edited for bloody spelling...)

Supergrover · Post by **Supergrover** » Mon Jan 02, 2006 5:49 pm

Im thinking that it is not a valve as i have done the tap test as suggested but nothing happened. The noise does happen when there is nothing plugged into the amp, however if i have a lead plugged into the input and then remove it the sound breaks up a bit more, i was thinking it could be a wire loose or dodgy connection on the amp input? However this noise does not happen everytime i play.......

irish_admiral · Post by **irish_admiral** » Mon Jan 02, 2006 8:49 pm

Does it happen on both channels? Have you tried tapping the power valves?

bgarrett_uk · Post by **bgarrett_uk** » Mon Jan 02, 2006 9:23 pm

Think you need to track down the hum a bit. By hum do you mean mains hum (60Hz Us // 50Hz UK)?

Summarising some of the suggestions above.

1) Does it hum with nothing plugged into amp? You said yes...

2) If yes, does it change volume with knobs... test vol and gain separately to reverb. Quite often hum does come from the reverb tank and the interconnecting wires - check with the reverb knob.

3) If hum goes up and down (with nothing plugged in) with amp vol/gain then probably in pre-amp stage. If goes up and down with reverb knob then probably connections to tank and/or driver valve.

4) If it only hums when you plug something in then more likely your leads and/or screening on your guitar - try a lead with no guitar first.

5) If only hums with guitar plugged in does it die down when you touch your strings and or bridge?

Hum can also come from the power supply if one of the power supply smoothing capacitors is kaput. Though this would be surprising on a new amp.

Cheers
Brian

Supergrover · Post by **Supergrover** » Fri Jan 06, 2006 10:26 am

I know all the leads are fine as i have used them with other amps and there has been no problem, and it hums when nothing is plugged into the input. The hum does increase if i turn up the volume, but sometimes i have removed the guitar lead from the input and it has stoped the hum for a few seconds, but it then resumes. As its the AD 30HTC is has no reverb to effect things.

This is a mind bender! Im stil thinking it could be something to do with the wiring behind the input on the amp?

Stupid valve question!

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