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Ampeg SVT diode question

Discussion in 'Amps and Cabs [BG]' started by officespace2, May 26, 2011.

  1. I have a 74 Blackline SVT that I am restoring. I am moderately tech savy, but not an expert. I noticed that one of the screen resistors (22ohms) and the paralell diode associated with it were friend in the power pcb section. My questions are two fold:

    1) Should I replace all the 1 Watt 22 Ohm resistor and associated diodes with a 1KOhm 5 watt resistorys as suggested here:
    Electrical Audio • View topic - Diodes across the screen resistors in SVT.

    2) Alternatively, if I keep the original values and retain the use of the diode, what is the original value of these diodes? They are found on the board as D15,D16,D17,D18,D19,&D20.

    Any help would be greatly appreciated. I will try to post a picture of what I am talking about shortly.
  2. 22ohm is really low, 470ohm should be the minimum really to stop the screens cooking. 1K is more ideal though.

    just replace the diodes with 1n4007 or uf4007.
  3. JGR

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

    Jun 29, 2006
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    Yes, the 22s need to go. I don't think you need to keep the diodes. Put in 1.5ks
  4. Thanks for all of your feedback. I'm trying to take it all in and find the best course of action. Would it be reasonable to put in 5 Watt 1.5k flameproof resistors with the 1n4007 diodes next to them?
  5. WingKL


    May 12, 2007
    If you are going to replace the screen resistor with a 1000 ohm 5W then don't put in the diode or you're not going to get the supposed sonic benefits of having just the resistor.
  6. seamonkey


    Aug 6, 2004
  7. I don't know why the decision was made to include the diodes, the arguments to get rid of them and replace with a higher value resistor seem plausible. However:

    That is the single most successful tube bass amp in the history of ever, and I have never thought of it as a particularly trouble prone design. History seems to have judged that the designers of that thing knew what they were doing.
    bobyoung53 likes this.
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The original spec for the diode is 1A, 1000 peak inverse volts. As stated, a 1N4007 can be used. The resistor is 22 ohms, 1/2W 10%.

    There is some variation in the use of these components within the SVT models. The earliest SVT's, based on the 6146B power tubes, did not have the diodes. They were introduced when the design was converted to 6550's. The current SVT-VR uses a 220 Ohm, 1/2W, 5%, flameproof resistor. Although the schematic shows a 1N4007 diode, there is a note "DNP" which stands for do not place. They aren't installed. Perhaps Ampeg feels that they are not necessary with the current power tubes that they are using.

    The wattage of the amp decreased a bit when they switched from the 6146B to 6550 tubes. I believe that the diodes are used to get every last watt out of the 6550 tubes and perhaps why they added them in the early 70's. I'm not sure why they selected 22 Ohms for the resistor.
  9. Seans


    Jul 4, 2008
    Herefordshire, UK
    I did think of doing something on my Blueline, preventative maintenance etc, but I am thinking if I can find the original spec versions then this is what I would prefer to use.
  10. With tubes in parallel and with low or zero value screen resistors one tube can hog the screen supply. In my mind a 1KΩ 5W wire wound resistor acts to balance the screen grid currents. I believe the tubes will last longer and have less chance of burned screens.

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