1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  
    TalkBass.com has been uniting the low end since 1998.  Join us! :)

Ampeg svt cl -15db input

Discussion in 'Amps and Cabs [BG]' started by jimella, Jan 27, 2014.

  1. jimella


    Nov 19, 2007
    Hi, i'm looking for some advice on my ampeg svt cl.

    Over the last few months i've noticed that i'm running the gain/master a tad higher than usual to get the same volume, even at home i've noticed a slight difference. Could this be the output tubes starting to wear out?

    I've also just noticed that there is no difference in volume between the normal and padded (-15db) inputs, not sure if this is a separate issue or related? the difference was always noticeable although can't remember the last time i tried it!

    Any info would be much appreciated.

  2. I would suspect a failing preamp tube before the output tubes. The fact the padded and unpadded are the same indicates a problem at the first stage in the preamp.
  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I was thinking the same thing. The first pre-amp tube would be swapped out and see if it makes a difference. The -15db is a simple pad. Normally you would notice a difference between the two inputs. If you can't notice a difference in gain, the first tube isn't doing much.

    There are a number of different signs of a tube wearing out. There are some details of what to listen for here.
  4. jimella


    Nov 19, 2007
    Thanks for the replies, i'll try swapping the first preamp tube and see what happens.

    That info on tubes is very helpful, not had to replace the output tubes yet but know what to look for now if i ever do.
  5. jimella


    Nov 19, 2007
    Just tried swapping the first preamp tube and still no noticeable difference in volume between the normal and padded input.
    Also tried swapping the second preamp tube and same result.

    I'm wondering if this is the reason i am having to turn the amp up a tad higher than usual to get the same volume, its probably not possible - but could the -15db padded input be affecting the normal input and causing a slight loss in volume (-15db)?

    Thanks for the replies so far
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The pad is simply a 100K resistor connected to the -15db jack. Not much can go wrong there but you never know if there is an issue with the circuit without looking at it. I think that what you've tried by changing the tubes is reasonable. You could try reseating the connectors to ensure that the contacts are good.

    Other than that, it might be best to have a tech look at it. They could measure the input signal level and trace it through the circuit to determine what is going on.

    Have you tried another instrument and cable with this amp to ensure that the problem doesn't lie there?
  7. jimella


    Nov 19, 2007
    Just tried another instrument and cable, no difference.. re-discovered my old p-bass though, forgot how good is sounds!
    Tried comparing both inputs with the gain turned right up, no difference.

    The amp still sounds good to my ears, just a bit puzzled why there is no volume drop using the padded input, i'm sure there used to be!

    I've never had it serviced before so now might be a good time to check it out.
  8. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Clean your input jack THOROUGHLY.

    That MAY fix it.
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    A service would include cleaning all the jacks, including the pre out and power amp in, the tube socket contacts, checking that the power supply capacitors are performing up to spec, checking the tubes and power tube bias, etc.

    It is worth cleaning the jacks as Stuart said. Do all of the jacks, not just the ones at the input. A good product to use is Deoxit D100L but it is expensive. Products like this act to remove oxidation which can build up on the jacks and results in bad contacts. If you don't have a product like this, repeatedly plugging and unplugging a ¼" plug may help as would a Q-Tip and isopropyl alcohol (drug store) or vodka. This won't remove the oxidation but will clean any dirt or greasy gunk that is built up.
  10. jimella


    Nov 19, 2007
    I'll give this a try, although the input jack looks to be clean when looking inside with a torch, i never get any crackles or weird noises when fiddling with the jack when its inserted, i'll give it a try though.
    Do i need to remove the input jack to do this?
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The contact area is very small, not something that you will see. There are also shunts (switches) built into the jack. If you can, these should be cleaned as well. Your signal goes through a shunt on the power amp in jack on its way from the pre to the power amp. If there is oxidization, there is a degradation of your signal. Regular maintenance helps keep these contacts clean.

    No need to remove the jack. You can do a better job if you remove the chassis but just do the best you can. If you look from the inside of the chassis and insert a ¼" plug, you can see shunts open and where the contact is.

    For reference, here is the input schematic of your amp. The shunts are labeled TS and GS. The tip of your ¼" plug contacts the tip, labeled T. You want to try to clean all three contacts. The power amp in jack has the same shunts and that is what the signal flows through in going from the preamp to the power amp pathway when no plug is inserted in the jack.

    Skinhorse likes this.
  12. jimella


    Nov 19, 2007
    Thanks so much, gonna buy some contact cleaner and give it a go.