Bias an Ampeg SVT VR with 3 pair of tubes?

Discussion in 'Amps and Cabs [BG]' started by visualdistortion, Mar 14, 2019.


  1. visualdistortion

    visualdistortion

    Aug 24, 2014
    Hi, a friend give me his Amepg SVT VR and 3 new pair of 6550 tubes. Unfortunaly, I don't know the tubes layout on the amp to get the matching tubes on Push and Pull side. I send an email to Ampeg, but I don't get an answer. Some people tell that the tubes are like that:
    1 2 3
    1 2 3
    But I also find different informations.
    Thanks a lot for your help :)
     
  2. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Each triangle is a triplet set.
     
    Wasnex likes this.
  3. jeff62

    jeff62 Supporting Member

    Oct 24, 2005
    Central FL
    :). But is it:

    1 1 2
    1 2 2

    Or

    1 2 2
    1 1 2

    ?

    Edit: looking from back of amp or front of amp?
     
  4. Wasnex

    Wasnex

    Dec 25, 2011
    From this page SVT-VR | Tube Board
    [​IMG]

    The solder side of the board is shown at the page also if your interested.
     
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  5. visualdistortion

    visualdistortion

    Aug 24, 2014
    Thank you, I already looked at this website but normally the tubes works by pair, a push and pull side and I don't really find with pictures what need to be together.
     
  6. Wasnex

    Wasnex

    Dec 25, 2011
    The info you need is in the image. There are three tubes on each side of the output transformer. Instead of push pull pairs, you have push pull triplets. You match the tubes in triplets on each side of the transformer; not in pairs across the transformer. The triangles in the image I provided indicate how the matched triplets should be grouped.

    It's not necessary to match the tubes across the transformer because the SVT has two bias circuits to allow you to balance the current across the output transformer. Each bias circuit controls one triplet of tubes. If the SVT did not have two bias controls, you would need a matched sextet instead of two matched triplets.
     
    Last edited: Mar 14, 2019
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  7. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Wasnex is correct.
     
    Wasnex likes this.
  8. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Key point here: each triplet of tubes (one for "push", one for "pull") has a separate bias control, the three tubes in the triplet all get the same bias, so they need to be matched. As @Wasnex stated, the tubes within each triplet need to match, but the two triplets don't have to match each other.

    The OP has three matched pairs. The two tubes of each pair match each other, but may not match any tube of the other pairs.

    @visualdistortion - do the packages for the matched pairs (or the tubes themselves) have any numbers written on them to represent the characteristics that have been matched? You may be able to tell how closely the pairs match each other. If they are wildly different, you may not be able to get two matched triplets.
     
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  9. visualdistortion

    visualdistortion

    Aug 24, 2014
    This is what I got
    Thanks guys!!!
     

    Attached Files:

  10. Wasnex

    Wasnex

    Dec 25, 2011
    That looks like a lot of variation. Typically what I have seen is when you put a hot tube and a cold tube on one side of the transformer, the hot tube hogs the current. In other words, the hot tube runs hotter and the cold tube runs colder.

    You could try running (41, 41, 45) (45, 56, 56). Chances are the first 45 will run really hot and the second 45 will run really cold. An amp tech would most likely measure the individual tube current to make sure they are all in a safe range. They would also shuffle the tubes around in the amp to try and even the current as much as possible. Explanation: Due to component variation, some tube sockets tend to run hotter, while others run colder.

    Before you take the old tubes out, set the bias cold (minimum tube current). If the amp is working fine now, leave it alone. Bias Procedure is on page 15 of the attached manual.

    If one of the tubes is running too hot, it's far more likely it will malfunction and damage surrounding circuitry. Consider ordering some matched tubes or sending the amp to a tech. FYI, if you know the tube vendor who supplied the tubes you have, they should be able to supply singles that match the characteristics of the matched pairs you have. In other words, buy 3 tubes with the proper characteristics and you will have three matched triplets. Unfortunately tube testing/grading practices are not standardized, so you have to buy the tubes from the same vendor who supplied the tubes you have or they are unlikely to match.
     

    Attached Files:

    Last edited: Mar 15, 2019
    bigbottomend likes this.
  11. visualdistortion

    visualdistortion

    Aug 24, 2014
    Thanks a lot for all your fantastic informations, I will try tomorrow and tell you what I saw, I've also the Orange Tube Tester, I will check what it says.
     
    Wasnex likes this.
  12. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Sep 26, 2021

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