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Ampeg SVT-CL problem

Discussion in 'Amps and Cabs [BG]' started by Chris 'Wighat' Jordan, Jan 13, 2012.

  1. I play a Reverend Brad Houser 5 string into an Ampeg SVT-CL/Ampeg SVT-410-HLF cab, gig 4 hours twice a week. Had the amp for several years and, although I am not a tech, I have had to learn to work on it a bit.

    This is what happened:

    Amp went in to fault mode during gig (blinking red/green light on front panel). Powered up fine but when turning on the standby switch, made a pop and went to fault mode. Noticed one tube in particular was making a bright blue flash at the same time as pop. Replaced that tube (there was a small crack where the retainers ride on the glass) with an old mismatch I had. Amp worked after that, although unable to bias (one light red only, no green). I pulled the power tube board and checked the screen grid resistors as they have blown before. One was out and I replaced it (with a 220 ohm, 1/4 watt resistor). When turned on the power switch, I got the red light as usual but when I turned on the standby switch, the whole amp shut down and it won't power up at all now.

    I am confident in the work I performed but don't know how to proceed.
  2. RickenBoogie


    Jul 22, 2007
    Dallas, TX
    You need a qualified amp technician. I too, am skilled with electronics, even designed and built some pretty fancy stuff-BUT, would never consider working on a 300 watt all-tube amplifier. Take that puppy in to someone with the right tools, test equipment, and know-how.
  3. If taking it to a tech was an option for me, I would have already done it. I live on a resort island off the Texas Gulf Coast and there is not even a qualified tube guy nearby on the mainland.
  4. Meaning no offence and caring about your well being let me say this: If you don't know how to proceed you should not be inside an amplifier containing the voltages that this one does. you need to trouble shoot with a live chassis and one slip could quite possibly be your last. Please take the amp to someone who knows high voltage tube electronics.
  5. BogeyBass


    Sep 14, 2010
    you probably have a bad pair of tubes.

    1/4 watt seems a little low for a screen resistor. Should be at least 1 watt. Upgrading them to 1 or 2watt resistors will solve your resistor blowing problem.

    I cant find a schematic with the CL screen value, vintage svt's used a 22 ohm resistor.
    so hopefully you didnt misread the resistor code as 220 instead of 22.
  6. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Schematic says 22, parts list 220, I think the latter is right.

    Do yours have a diode across them? If so are those diodes good?

    The diode should prevent any significant voltages across the resistors, accounting for the small wattage. They are really for "damping" oscillations.
  7. BogeyBass


    Sep 14, 2010
    yah I can only find a 3 pdfs for the CL they are all missing the power board.
    From what I can tell the CL came out in 93 and their was a bunch of revisions done in 94.

    Mainly replacement of a series of 1N3070 diodes which are 200 volt diodes. Are all replaced with IN4007 diodes which are 700v.
    Also a diode clamping circuit is added and a extra capacitor in the bias detection circuit.

    not sure if this is old news for you, but it would be worth it to check to see if your amp has been revised. since they all all related to the bias.

    or if the flash overs that occured may have taken out some of the lower voltage diodes. Not sure of the value of the diodes in the power amp that Jerrold was speaking of.
    It would be good idea to check datasheets for those diodes and upgrade them as well to a higher breakdown voltage.
    Its just important to find one with the same forward voltage drop. since this is the voltage needed for proper circuit operation.
    Similar to the IN3070/ 4007 change. Both diodes have a 1volt forward voltage drop for proper circuit operation. But the 4007 is 700 volts instead of 200 which would save it in case of a flash over or any other type of whoopsi doo.
  8. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I was almost right.....

    TIB0003 (from 2001) indicates removing the 22 ohm resistors AND the 1N4007 diodes, and replacing with 220 ohm 1/2W "flameproof" resistors.

    The "flameproof" resistors just open if over-powered, and don't flare up with a flame..... Potentially a fuse-type resistor would be fine there also.

    Stated reason is to improve tube life
  9. BogeyBass


    Sep 14, 2010
    yah I was surprised to hear they changed it to 220ohm

    The original 22 ohm ran the voltage on the screens to high.
    A 6550/kt88 doesnt really like more than 500/530 volts.

    But the SVT can blast em at 590volts or higher, a good upgrade for a old tech would be to remove the 22ohm 1watt resistors and replace it with a 180ohm 2watt resistor. This would lower the screen voltage and yes that would improve tube life. Alot actually you can overvolt the hell out of the plates, but screens you cannot.

    Anyhoo so the 220ohm resistor is a good idea. As far as the 1/2 watt flameproof resistor. the only good part of that is good is "Flameproof"

    the average screen on a kt88 pulls 33ma of current at full power and that 220ohm resistor is probable dropping it to 540volts on the screen. So your looking at about 1.7 watts of dissipation.

    so why you would use a 1/2 watt I dont know. unless you want to see it flame up. It needs to be a 2watt resistor, because the vintage amps with 1watts would blow those too. a 2 watt might not even fit well on that board so at least try to squeeze 1watt flameproofs in there

    otherwise besides sounding like a troll asshat,sorry, he had flashovers before failure so you might need to replace a tube socket or at least check for carbon tracking in the sockets and surrounding board. and the flash over might have taking out some of the wimpy low voltage caps in the bias detection section.
  10. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    The original 22 ohm screen values were a joke. Put some 1K 5 watts on there.
  11. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Well, the screens are not powered from the plate voltage........ rather they get a lower voltage, as per application data.....

    Sylvania lists 100W per pair at 600V plate, 300V screen, with 4 mA typical screen current no signal, and 41mA at max signal. In 220 ohms, 41 mA is considerably less than 0.5 W, and that is a peak value anyway.

    Overdesign is no problem, of course, but I think some folks may be hyperventilating here.
  12. Thanks to everyone who offered advice. I regret my belated attention to the matter. Our internet has been down for several days.

    Yes, there are diodes on the power tube board with the resistors. I did not see any carbon tracking on or around the tube sockets. The flash I saw was not horrific and seemed to be contained within one of the 6550's.

    I will check the diodes and recheck the resistors. As my memory serves, TIB0003 lists models subject to the diode removal/resistor upgrade and my amp, which I bought new in 2005 is not listed. I assume that means it has the 700V diodes, yes? The resistors are all 220ohm. Is my amp a candidate for removing the diodes and installing 2w resistors?

    I find it odd that my amp shutdown completely after replacing one blown resistor. What is likely causing my amp to not turn on at this point? Can tube, diode or resistor be causing this? Is there a diagnostic avenue I can pursue to insure I'm not barking up the wrong tree?
  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    It's not on there because the original TIB was before that date......

    I have nothing that says there was a change back...... but I was busy with other new projects and didn't pay attention to any later changes.
  14. My amp has 220ohm factory resistors but also has diodes. Should I remove the diodes when I upgrade all the resistors?

    In reviewing the TIB, I realized that it would be very easy to plug the multi-pin header a little off and not notice because of how tight it is at that stage of reassembly. I'm sure that would explain my amp coming on and then shutting down. I kinda hope that's it but also hope, if it is, that nothing is damaged as a result.

    Will check it out tomorrow.
  15. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    With the 220 ohm resistors, I am not sure why the diodes are there.... Even at the 4 mA idle screen draw, the diodes will likely be conducting (depending on exact screen draw), so it's just like the resistors are not present.

    So I think you should remove the diodes to have the unit at the status of the TIB.

    I can check with the engineer who did the project.

    What's the 5th letter in the serial#?
  16. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I checked with George McKale, and neither of us knows anything about having the 220 ohm PLUS the diodes.

    We say the diodes shouldn't be in if the resistors are the 220 ohm value.
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    On the current schematics, there is a note beside the diodes that reads DNP which stands for Do Not Place. They include the diodes on the schematic for historical reasons I suppose.

    The presence of those diodes never made a lot of sense to me. I have seen applications where they put a resistor in series with the diode. This is not quite the same as what is done with the SVT. For what it is worth, here is one explanation of what this configuration does.
  18. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I am not at ALL sure I agree with their explanation....... AC power????? Say WHAT?/??

    The tube ALWAYS has the same polarity...... plate is positive, screen is positive, etc, etc. There is no normal reversal of the screen grid polarity......... That's why there are two tubes as a push-pull.... and why in a class-A amp, the signal is a "variation" in the normal current flow, not a reversal.

    if the current COULD reverse, the screen or plate would have to EMIT electrons, which does not occur to any significant degree (it might if the plate or screen gets red hot).

    The "DEGREE" of "positiveness", the plate voltage, DOES vary, that is the "audio signal"

    The basic function of the screen grid is to present a fairly constant electric field to the cathode, reasonably independent of the plate voltage. That improves power output.

    Pentodes are more like "current sources" than triodes, the plate current varies less as the plate voltage changes with the signal.

    In a triode, the electric field that "powers" the plate current varies more directly with plate voltage, so current tends to be less just when you want more. That is why triodes generally tend to put out less power in similar conditions.
  19. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I figured that the AC power that they were referring to comes from power supply ripple and this is an attempt to control that at the screen.
  20. UPDATE:

    I was mistaken re the diodes. The power board does not have the diodes and DOES have the empty spots for them. No problems with the board in question- rechecked all cables/wires and resistors. Put it all back together and it powered up at home. Some tubes are glowing blue before it goes into fault mode (red/green blink on standby light). I took the amp down to the club where I am playing tonight where I had a couple of spare tubes and, oddly, it wouldn't power up again.

    I have never had tubes suddenly die such a violent death to explain this, but that is all I can think of. Yet I am at a loss to explain why the amp didn't power up at the club. I, of course, checked to make sure I had power at the outlet and that the EIC cable was good.

    Is there any potential for some undetected problem damaging a new set of tubes if I bought some? Anyone have any opinions on what's going on here?

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