›

Changing valve fixed scratchy pots. Why?

What's Hot
GrumpyrockerGrumpyrocker Frets: 4198
in Amps
The pots on my Laney Cub 12R combo was getting a little scratchy. The reverb, gain, volume and tone pots were all scratchy. So I assumed (despite my complete lack of knowledge) that it might not be just a case of contact cleaner on the pots.

So I changed V1. Didn't fix anything. Changed V2 for a new ECC83 - and that did the trick. Scratchiness now gone.

So why would that be? Was the V2 valve leaking in some way? Was that voltage leakage dangerous?

And as an entirely separate question, do most of you leave the safety cages off the back of your valve amps?

0reaction image LOL 0reaction image Wow! 0reaction image Wisdom

Comments

  • ICBMICBM Frets: 74497
    A leaking valve can cause pots to go scratchy if it applies a DC voltage to them, yes. The voltage is not usually dangerous, either to you or the amp - which is a good thing, because one of the more common cases is the first gain stage leaking in a traditional valve amp without a blocking cap on the input jack, so the voltage appears on the guitar cable! This makes the guitar controls go scratchy, and can make true-bypass pedals pop very badly.

    I might leave the safety cages off the backs of my own amps, but never on any for a customer. Usually there is a risk of valve damage if they're exposed.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein

    0reaction image LOL 0reaction image Wow! 1reaction image Wisdom
  • ecc83ecc83 Frets: 1747

    All valves produce a "grid current" but in pre amp valves it is so tiny that it causes no problems and grids can be directly connected to pots without causing noise. (even so I don't like the practice!)

    A fault can develop however, perhaps a tiny piece of crap has attached to the grid and increasing the current. Now you will get a crackly pot.

    Excessive grid current can often be seen in power valves and in a fixed biased amp will tend to "beat" the bias causing the valve to draw more anode current than it should for a given negative voltage. This current tends to reduce rapidly over an hour or so, especially if the amp is driven hard. Good idea to check bias after an hour's thrashing!

    Safety cages for valves are a legal requirement for the manufacturer. What bods do after is their business!


    Dave.

    0reaction image LOL 0reaction image Wow! 1reaction image Wisdom
  • GrumpyrockerGrumpyrocker Frets: 4198
    Thanks for the replies chaps.

    0reaction image LOL 0reaction image Wow! 0reaction image Wisdom
Sign In or Register to comment.