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6550 power tube current distribution in Ampeg SVT-AV/CL @ 50, 100, 200W/4 Ohm

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  • 6550 power tube current distribution in Ampeg SVT-AV/CL @ 50, 100, 200W/4 Ohm

    I have been doing preventative maintenance on the Ampeg SVT-CL, SVT-AV & SVT-VR bass amps in the rental inventory where my shop is located, restoring them to solid operational status, where some hadn’t been into the shop for a few years. Within those amps (having the traditional loose hardware, lots of solder joint fractures, noisy tubes, noisy pots, broken jacks, etc), I again come upon some that have one or two 6550 power tubes whose plate current is significantly off with respect to the remainder of output tubes. With 24mA being typical @ idle, +/- 3mA, those tube that are sitting at 14mA or 29mA, I’ll first try juggling the tubes to see if I can re-balance them.

    Usually it’s a loosing battle, so I’ll often dig into my collection of pulls accumulated over the years, trying to stay with the same mfgr’s tubes installed (Sovtek, JJ/Tesla, Svetlana). Today’s battle was with an SVT-AV that hadn’t been serviced since May 2010. After restoring all the mechanical issues, I put the amp back together, but without the hold-down clamps.

    As it was a working amp, I numbered the power tubes as a starting point, then powered it up, and measured the plate/screen current thru each of the 10 ohm cathode resistors, with the amp standing up vertically on it’s Pwr XFMR side, aided by a support block under the XFMR. The power tubes were Sovtek 6550we’s.

    Tube..Current
    V1.....14.3mA
    V2.....28.5mA
    V3.....22.7mA
    V4.....23.1mA
    V5.....20.2mA
    V6.....24.3mA

    After juggling the tubes around and a little re-adjustment of the bias pots, I still wasn’t able to deal with V1 and V2. So I tried all the other Sovtek 6550we’s I had on hand, but didn’t find anything even close. I found a Sovtek branded by Groove Tube/Fender, and a Svetlana 6550 Winged C that balanced out, after some further bias adjustment:

    Tube.....Current...Brand
    V1........22.6mA...Groove Tube
    V2........23.0mA...Svetlana
    V3........24.6mA...Sovtek
    V4........24.8mA...Sovtek
    V5........25.8mA...Sovtek
    V6........23.4mA...Sovtek

    I let this idle for about an hour, came back to see what I had, and then made DC and AC current measurements at idle, 50W, 100W & 200W into 4 ohms, to see what the current balance looked like under 400Hz Sine Wave drive. Also had the scope looking at the output as well as across the cathode resistors.

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    First DMM measurement photo shows idle current for V1 power tube @ 120VAC/60Hz/235W AC mains volts/power. Then, the DMM shows DC current and AC current for V1 at 200W/4 ohm output power. The numbers in the chart below are the Fluke 8060A readings across the 10 ohm 1% cathode resistors of the output stage. The scope images shows the Amber 3501a Monitor Output and the voltage across the cathode resistors for V1 & V5. Power Analyzer shows AC Mains current/voltage/power draw for 200W/4 ohm load. Resistive load bank next to the power analyzer.

    -----------------------------50W/4 ohm------------100W/4 ohm---------200W/4 ohm
    Tube....DCI, Idle.. Brand.........DC I..........ACI........... DCI........... ACI.........DCI............ACI
    V1.......23.1mA....GT/Fender..55.1mA.......62.2mA......77.7mA......92.5mA....1 12mA........138mA
    V2.......23.2mA....Svetlana.... 65.1mA...... 75.0mA.......90.8mA.....108mA.....128mA........155mA
    V3.......24.9mA....Sovtek.......62.0mA......70.1mA...... 85.8mA...... 101mA.....120mA........145mA
    V4.......23.9mA....Sovtek.......60.1mA......69.6mA...... 83.7mA...... 101mA.....118mA........145mA
    V5.......25.5mA....Sovtek.......57.0mA......64.5mA...... 80.0mA...... 95.6mA....112mA........137mA
    V6.......22.5mA....Sovtek.......58.7mA......68.0mA...... 81.9mA...... 99.1mA....116mA........144mA

    The SVT-CL or -AV actually makes a decent test fixture for measuring 6550 or KT-88 power tubes in pairs. I'll usually set the bias controls up for 24mA per tube using V2 & V5 tube positions. The Bias LED's won't work, but the amp will run. Then, installing the other tubes, one pair at a time being tested can be grouped based on the voltage reading across the cathode resistor (10 ohm 3W 1%).

    As you can see, the current distribution under drive varies a lot greater than at idle. These tubes installed (the Sovtek's) are at least 4 yrs old. I haven't run the same data on a new sextet, but will get to that soon.
    Logic is an organized way of going wrong with confidence

  • #2
    so the difference between the "hottest" and "coldest" tube at idle is 10%, and under maximum drive it's 14%
    not THAT much of a difference

    Comment


    • #3
      After moving the tubes around to even out the differences, and re-adjusting the bias voltage to get there, it's not that big of a difference. Where it started, with the coldest tube running 14.3mA and the hottest at 28.5mA, that was nearly 99%.

      In the distribution after balancing the two other used tubes, it was interesting to see the Svetlana tube running around 9% above average (of the four Sovtek's) under drive. Might be the Svetlana hadn't run as many hours as the Sovtek's. One of those and the Groove Tube behaved much the same.

      I thought I saw a comment of the Ampeg SVT-CL/AV's Window Comparator for the Bias LED's GRN range to be around 8mA. It's more like 14.7mA, computing the two voltages of the window comparator's voltage reference string (147mV on the Low side (NO LED LIT), 294mV on the High Side (RED LED LIT). That's 100% change, so it is tolerant of a wide difference of current in the two triads of tubes in the output stage. With 14.7mA being at the low threshold, 22.1mA is the center of the range. I'm usually setting them up in the 22-24mA area, and move tubes around to get as good of a balance as I can within the sextet. Maybe it's not worth all the trouble, but it seems like it would be better to get the load sharing as even as possible....thinking in the long run, it may extend the overall life of the sextet. I've never tracked them that far, but usually have the idle current values recorded in my service notes for later reference.
      Logic is an organized way of going wrong with confidence

      Comment


      • #4
        On one hand, I'm surprised that the comparator window is that broad, given the number of good pulls I've had that would not work with the five good ones in an amp.

        On the other hand, I believe that so far, if our Orange tube tester says that a tube matches the ones in the amp, all will be well. And sometimes it can be a number off and still work. (IMHO, the Orange's algorithms do not place enough weight on bias current, as the bias current of "matching" tubes can be as much as 25% off, and tubes that are a "number out of match" can have identical bias currents.)

        Comment


        • #5
          Next SVT CL I get open, I'll have a look at the comparator window again. I had both written down measured window voltages in 2010, and had also computed the string based on the schematic values. Doesn't say there haven't been revisions to that circuit. The amp I just serviced had two tubes installed that were near the edge of the window, and it still liit up GRN for both upper and lower sets.

          I wasn't aware of the Orange VT1000 Tube Tester....just looked to see what it was. Nice and simple, takes care of a lot of functions. I'll add that to my database on tube testing. Never did get around to turning an old Fender The Twin (in our boneyard) into a test set for matching tubes and such. A job for laterman.
          Logic is an organized way of going wrong with confidence

          Comment


          • #6
            The Orange is a useful tool, but not perfect (per my previous comments). Here's some results (Ignore the JP number) (first number is Orange rating, second is bias current e.g. 8/35) (Hope this isn't too jumbled up):

            Amp Model JP# Orange Rating/Current Draw (or Orange Rating/Did it Work in SVT when Subbed for #2 ?)
            Twin 3 8/36.5 8/31.3 8/30.7 8/37.8 All 8s, not very well matched
            Twin 15 9/18 9/20 10/22 9/18 I did not leave the bias so low of course
            Twin 15 10/38 10/28.2 10/32 10/35.7 All 10s, not very well matched
            Twin 42 10/36 10/37 10/37.3 10/38 Good match
            Twin 10/38.6 10/34.9 10/34.7 10/36.3 Pretty good match
            Twin 9/31.5 9/33 10/39.6 9/35.4 A bit of a spread on the 9s


            59 Bassman ? 11/40.2 9/22 9 and 11 almost 20ma apart
            59 Bassman ? 9/36.1 9/34.9 Good match
            JCM800 12/33 12/26.8 12/23.2 12/26.5 All 12s, not very well matched
            Plexi 1 10/32 10/29.7 11/33.3 11/33.3 11s matched, a 10 was good with the 11s, a 10 was only a fair match to the other 10
            Plexi 2 13/24 14/31 13/31 14/35 14s were a fair match, a 14 and a 13 matched, the other 13 was not even close to any of them

            Comment


            • #7
              After I looked at the manual for the Orange VT1000, I didn't see where you're getting your current values. The Matching number is, of course, one of the LED's that lights up. Orange doesn't seem to want to tell you any specifics, though. Is there an LED or LCD numeric display on this box? I couldn't tell from the photos. I'd guess the Matching number takes into account the transconductance of the tube along with the plate current, maybe also the mu. No idea what their internal plate and screen voltages are.

              At first, overlooking the 'matching number', I would have been inclined to re-sort some of your tubes, then saw similar currents, but associated with different matching numbers, after which you're looking at that usual bell curve in sorting thru tubes on hand to batch them into sets. Still, a handy device for pre-sorting and matching tubes.

              When I don't have cathode (or plate) resistors in circuit, I'll use a Compu-Bias Bias Probe meter on the lower plate voltage ratings, and a fixture to use the probes on my 4-Ch HP 3467A Logging DMM on the higher voltages, it having scaling networks in the fixture, as the Compu-Bias can't run on 600V+ supplies.
              Logic is an organized way of going wrong with confidence

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