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1970 Ampeg Portaflex questions

Discussion in 'Amps and Cabs [BG]' started by gbyxbee, Feb 28, 2016.


  1. gbyxbee

    gbyxbee

    Feb 28, 2016
    Just replaced the output transformer and have come up with some things that I could use help with that I hope someone here might be able to answer ...
    1) What is the acceptable line voltage spec for this amp? ( especially the high end)
    2) What should the bias voltage be thru D1/R36 IN STANDBY?
    3) Is the fuse a Slo-Blo?
    I ask these questions because the line voltage in my home is between 125 and 126. This puts my V4/V5 plate voltages at 492 and the V4/V5 grid voltages at 488. This troubles me in particular because I'm very close to the voltage rating of the 40/40/40 tricap (500). I have no Variac to lower my line voltage to a more reasonable 120 to see what those, and other critical bias voltages, would be. I figure the power transformer has a 1:6.2 ratio, so if line were 5 volts lower, I might reduce these voltages by about 30? By the way, my filament voltage is at 7.
    The bias voltage is at about -53, but when I go into STANDBY, it goes to -80 ... is that okay by design? Again, this is with the 125 line. All other bias voltages are reduced by about 60%.
    The amp sounds fine otherwise, but I don't have a trained ear really.
    Can anybody help?
     
  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Don't worry about the bias in standby. It is normal for the bias voltage to be higher instandby, the tube is cutoff. The high voltage is not applied, the bias is in a safe place.

    Your voltages are high. This will stress everything in the amp from the powertransformer to the caps and the tubes. Consider building a bucking transformer or getting a variac. Follow the wiki liks at the top of the page to the portaflex wiki technical page for more information. There are line voltages on the schematics in the wiki. they were 117 in the late 60's and 120 vac in the early 70's.

    For a 117vac amp their spec was 105 to 125, so around -10% and +7%. But just because you may be at the upper limit of the spec, it isn't good to operate it there. As you said, you are over the limit of the cap can. This will shorten it's live.

    The fuse is a standard fuse.
     
  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
  4. gbyxbee

    gbyxbee

    Feb 28, 2016
    Thank you for your quick reply! Never heard about a bucking transformer until your mention. Have you knowledge of the APC LE1200 voltage regulator/surge suppressor? I'm not in the mood right now to have to pull out the soldering iron, etc. to build something if I can get something already built. By the way, my line voltage has reached 126 today!
     
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I'm not familiar withthe APC product. Some of these products generate noise. It has to be able to cope withe loads presented by an amp.

    a variac works well. You have to ensure that it is set properly and that the dial is not turned. Cranking an amp up to 140vac is not good.
     
  6. gbyxbee

    gbyxbee

    Feb 28, 2016
    Thanks again for your input Sir!
     
    beans-on-toast likes this.

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