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Is a Matched Sextet for SVT necessary?

Discussion in 'Amps and Cabs [BG]' started by hibbs, Mar 31, 2009.


  1. hibbs

    hibbs

    Aug 14, 2005
    Cambridge, ON
    Bam, nailed it in the title.

    I am looking to retube to ol' Brick-CL and I was wondering if anyone has done any "brand mixing" when it comes to power tubes in their SVT. I am thinking of a JJ/EH/Winged C combination with a pair of matched 6550's for each brand.

    Talk me in/out of this.

    Thanks for the input!
     
  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    A matched sextet is overkill. SVT's use power tubes grouped in threes. So you need two triplet sets. Won't hurt to use a matched sextet, though. Not a big fan of mixing and matching power tubes, though. I suppose you can do it as long as they match but you really don't get any benefit out of it if you ask me.
     
  3. hibbs

    hibbs

    Aug 14, 2005
    Cambridge, ON
    Not really for the benefit, just to see what difference in sound it would really make. Thanks for the info on the groups of three. I wasn't sure if it was 3 groups of 2 or 2 groups of three. I will make sure to get them all matched accordingly.

    Thanks for the help
     
  4. bovine mind

    bovine mind

    Jun 17, 2008
    San Francisco
    i'm suffering some volume and distortion issues with my mismatch, two sovtek and four magnavox. my tech suggests against it with SVT's and so far thinks my problems are the result of it.
     
  5. Hi.

    When I had my CL for a short period of time back in 90's it had a pair of sovteks and a quartett of EH's IIRC. When replaced by a matched sextett the head sounded much better.

    It's hard to imagine what You're trying to accomplish by using 3 pairs of tubes, but if they're matched closely enough so the bias setting is in the ballpark for every tube, it will work.
    It will sound worse and most likely weaker than a matched set, but it will work.

    BTW, individual bias pot for every tube rules ;).

    Regards
    Sam
     
  6. if you have 3 pairs (or 2 pairs and a quad) you can make sure that you have one tube from each pair on either side of Push-pull power section. how those sides correspond to the tube sockets in the amp i dont know.

    having said that i have 2 old holden/wasp amps running 4 6550's. they both have different tubes in them (what they had in them when i got them) and they sound great, but i havent shelled out for a matched set (actually i did, and 2 were redplating when the other 2 werent. i used the 2 that didnt and 2 from the original mixed set.
    i swapped the tubes around and the issue was infact the tubes, not the circuitry on the sockets, so dont tell me.

    on the amp i will be making soonish (8 6550/kt88's) i will have a bias control for each pair so i can just use any pairs.
     
  7. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    SVT's don't work that way. They work in groups of 3. Most amps are AB AB AB, but an SVT works AAA BBB. Each group of three is separately biased, rather than working in pairs.
     
  8. which amps are AB AB AB? i thought most normally just biased each side of the wave form seperatly.
     
  9. hibbs

    hibbs

    Aug 14, 2005
    Cambridge, ON
    Sweet deal, thanks for all the info. Where would one go about getting an SVT modded to have individually biased power tubes. Is it a mod that would have to be done to the amp?
     
  10. hibbs

    hibbs

    Aug 14, 2005
    Cambridge, ON
    While we're on the topic, am I correct in assuming that KT88's require no modification to the amp to be used in place of the standard 6550's? Those JJs look awesome
     
  11. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I don't know...I was just using that as an example of the way most people think the push-pull thing works with power tubes in SVT's....in pairs rather than triplets.
     
  12. You will certainly need to re-bias the amp to suit KT88s.

    OP, you do not HAVE to have a matched sextet of tubes, there was no such thing as matching back when the amp was designed, but it IS a good thing. Matched triplets are useless to anyone. Matched pairs are also pretty pointless in an SVT or any other multi output tube amp for that matter. The reason for matching is so that all of the tubes a doing the same amount of work. Matching all six tubes goes a long way to accomplishing this aim. Matching in pairs, as long as one tube from each of the pairs is on opposite sides of the OPT balances the current on each side of the OPT, BUT one pair will do more work that the next. The only way to use "pairs" is to modify the bias supply to make each pair under the control of a separate bias pot. This way you can balance the current flow through each pair.

    Paul
     
  13. i see, there is a separate bias pot for each side of the PP set. makes sense
     
  14. bovine mind

    bovine mind

    Jun 17, 2008
    San Francisco

    will a matched pair damage an SVT-CL over time?
     
  15. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The advice on groups of three matched (matched triplets) is correct... that IS how the SVT operates. There is little benefit in matched sextets, but it doesn't hurt.

    And, there definitely WERE matched tubes in 1969, and before....... matched sets were commonly used in hifi amps.

    I explained it recently elsewhere, but the three tubes on each 'side" of teh SVT operate as one 'composite" tube, driven from a single input, with one bias voltage for all three, and one overall plate connection to the transformer, one sense resistor in the cathode circuit.

    Therefore, to make them operate together, without one "hogging the current", or alternatively failing to carry it's proper share, they should be identical in characteristics.

    The most usual way to get that, or close to that ideal, is the usual "matching" procedure, which assures that they are all within a certain range, at least when new. Almost no matching that I am aware of is comprehensive, over the entire range of plate and grid voltages, but it generally is sufficient to make sure they are within a small range at one setpoint.

    The two groups are balanced by the "balance control" (surprise), which can compensate for differing gains between groups. matched groups avoids that issue, but isn't absolutely required.

    There is NO balance control to adjust the bias and gains of individual tubes, which is why the matching of at least the three in a group is needed.
     
  16. hibbs

    hibbs

    Aug 14, 2005
    Cambridge, ON
    Thanks for the input Jerrold, just what I was looking for.
     

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