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1987 Ampeg SVT LE tubes

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  • 1987 Ampeg SVT LE tubes

    Comes in as "everything works fine, just hasn't been serviced in 20 years". Once I got the darn heavy thing opened, I found everything covered in a layer of thick dust, including the 6 Sylvania 6550As. I checked the bias points on the panel, and got 75mV on one side and 24mV on the other. (Disclaimer, I understand this is voltage representing cathode current). Sounded like a tube or two wasn't conducting, maybe. Then I set something on the cab and the speaker went "thunk", so I tapped around to see what it might be. Tap, tap, SPARKS! And a blown up 10A power fuse, broke it right open. One of the 6550s shorted.

    Great, it was "working fine" until it came to me, now it needs tubes. The side that read 24mV before now reads 15mV, so it looks like another tube on that side is also bad, but this is where I cease further probing until I talk to the owner. My policy with big amps like the SVT with their 725 plate volts, with modern day manufacturing in mind, is that I do not sell power tubes for them. A 6 pack of 6550A from TAD is about $275. There are other online vendors that sell them for less, but I'm not sure I trust them. TAD at least has a good rep for testing and matching their "premium" sets. If sell them to my customer, and one blows up next month, he is going to be looking at me to make that right, and at those prices I can't afford that. So, I require the customer to purchase them himself from the vendor of his choosing, thus seeing the price, and I will install and bias when he gets them to me.

    Well, I just spoke to him, and no worries. He was happy to buy his own tubes, and said "that's cheap" when I quoted him a price. He said the last time it was done 20 years ago they charged him $600, but that was with NOS Sylvanias. When I mentioned it also might be time for a cap job, he said "do it", before I even gave him a price. "anything you thinks needs done, do it. I will pay cash. So there you have it, quite a repair bill. Now I have to go put the cap parts list together and source it, the part I don't get paid for.
    It's weird, because it WAS working fine.....

  • #2
    So, looking at this schematic, where does one get the 600v 30uF cap after the big 15 watt resistor? nevetslab?

    https://ampeg.com/support/files/Sche...e%20Manual.pdf
    Last edited by Randall; 09-08-2019, 09:29 PM.
    It's weird, because it WAS working fine.....

    Comment


    • #3
      Use 33uF 600V. They are available at various sources. (fliptops, justradios, etc.)
      P.S. mF is milli-farads. Years ago I wouldn't have mentioned it but mF is a real thing now. 30mF is 30,000uF.
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


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      • #4
        Also, what is 'LE' ? It is 1987 and uses the vintage 70's schematic?
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #5
          My first guess would be Limited Edition. Then much later they made the Legacy hand wired versions, but I think that was more like 2007 than 1987.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            I don't know about everyone else but I would love to see some gut shot pics of this amp. Would be interesting to see how it is constructed. I am assuming that it is supposed to be a modern version of the vintage svt.
            When the going gets weird... The weird turn pro!

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            • #7
              http://https://reverb.com/price-guid...ion-1987-black
              So this is the 87 limited edition. Did they only make 500?
              Last edited by DrGonz78; 09-08-2019, 12:01 PM.
              When the going gets weird... The weird turn pro!

              Comment


              • #8
                Yes, it is the Limited Edition, black version, only 500 made. This is number 42. Pics as requested.

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                It's weird, because it WAS working fine.....

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                • #9
                  There was a guy freaking out on another forum because he couldn't get the actual schematic from Ampeg, and apparently there are some minor differences from the vintage schematic.
                  I think he was also doing a re-cap, so take care about going by what is already there, rather than the vintage schematic.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #10
                    The 30uF 600v is the only one that I can't read the value from, so I have to go by the schematic there. Also, this schematic is provided under SVTHD service manual from Ampeg site, so I don't think there is much difference other than one of the driver tubes being changed to 12AX7 with it's component changes.
                    Last edited by Randall; 09-08-2019, 09:56 PM.
                    It's weird, because it WAS working fine.....

                    Comment


                    • #11
                      It is really not critical at all, 30uf will be fine even if it was originally something else.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #12
                        I just ordered the 33uF 600v axial, and 20/20/500v smaller 1" can (for a single 40uF can) from Just Radios at half the price of Fliptops, which was at $54 before shipping.
                        It's weird, because it WAS working fine.....

                        Comment


                        • #13
                          We have #187 of 500 of this 1987 SVT LE in our rental inventory. I haven't had to do any serious maintenance on it thus far (apart from replacing the screen resistsors with 220 ohm 1W, removing the parallel diodes, and putting in a fresh set of 6550's and rebiasing, after juggling the power tubes for best current balance @ idle), looks the same as the one you have there. I wish Ampeg hadn't made use of those mult-section cap-cans, as putting in more reliable replacements is a real PITA for this form factor. That one cap in the top view of the chassis threw me for a moment, until I saw the previous tech had used the clamp to mount a snap-in single section cap.
                          Logic is an organized way of going wrong with confidence

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                          • #14
                            " That one cap in the top view of the chassis threw me for a moment, until I saw the previous tech had used the clamp to mount a snap-in single section cap. "

                            Yeah, me too. He used a pretty clever shim between the cap and clamp, looks like part of a pill bottle.

                            " I haven't had to do any serious maintenance on it thus far (apart from replacing the screen resistsors with 220 ohm 1W, removing the parallel diodes, "

                            May I ask why you did this?
                            It's weird, because it WAS working fine.....

                            Comment


                            • #15
                              Ampeg Tech Info Bulletin # TIB003 June 2001 Screen resistor/diode networks

                              Originally posted by Randall View Post
                              " " I haven't had to do any serious maintenance on it thus far (apart from replacing the screen resistors with 220 ohm 1W, removing the parallel diodes, "

                              May I ask why you did this?
                              Technical Information Bulletin (TIB0003, June 2001) -- Power tube PCB modifications.pdf

                              Following Ampeg's recommendation per TIB003 attached.
                              Logic is an organized way of going wrong with confidence

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