Ad Widget

Collapse

Announcement

Collapse
No announcement yet.

Peavey Classic VTX very low B+

Collapse
X
Collapse
 
  • Page of 1
  • Filter
  • Time
  • Show
Clear All
new posts
Previous template Next
  • #1

    Peavey Classic VTX very low B+

    I just picked up a Classic VTX very very cheaply, owner said it had a "hum". It does indeed have a minor hum which comes and goes, 120 Hz. Bass notes are extremely distorted, some distortion on higher notes.

    I'm getting B+ at 326 VDC where it should be about 525 (yes I'm on High power, low power it is less than 200), even without the tubes in. The PT is reading 185-0-185 on the HV AC. The PT is the correct Peavey part number (it's actually the same PT that's in the Triumph 60). The two 100 mfd main caps show no sign of discomfort or bulging, and they are not getting warm to the touch. Both plate readings were about the same, so I'm not inclined to believe there's a problem with the OT drawing excessive current. (The low voltage to the preamp is OK, 26-0-26 before the zeners)

    Conceptually I'm, wondering WTF is dropping 200 volts off the B+ regardless of whether the tubes are in or out. I have not had time to put a scope on the power supply yet...just curious if somebody has messed with Peavey's high/low power supply enough to offer a quick explanation...
    Last edited by nashvillebill; 08-04-2010, 04:19 AM.
    Tags: None
  • #2
    It hums and the B+ is low. Go back and measure that 326VDC. Now flip the meter to AC volts and read the same spot. Got a ton of AC there?

    Caps don;t have to look different when they are bad. Imagine an empty cap. WOuldn;t do anything useful, would be "bad" by any measure, would look fine. A dried out cap won;t do any filtering. That leaves tons of ripple, and the DC will read low.

    You can disconnect the OT and find out if it is dragging the B+ down. It probably is not.

    And the circuit must be intact. Did you pull that 1/2A fuse out and check it with an ohm meter? In high voltage mode, that 0.01uf C1 can pass enough AC to put voltage in the filters.

    I am not looking at the amp, so I could easily be wrong, but my reaction is not what might be dragging the B+ down 200v, I wory more that the amp can't make 500v in the first place.
    Education is what you're left with after you have forgotten what you have learned.

    Comment

    • #3
      Yep, I think i've got a couple of new filter caps i can put in tonight, along with new diodes if necessary. Might disconnect C1 to see if I get proper voltage on high power--not even sure why they have C1 in there. Thanks!

      Comment

      • #4
        Won't hurt to try that, but I doubt C1 is a problem. I brought it up as it might explain voltages still present with an open circuit around it.
        Education is what you're left with after you have forgotten what you have learned.

        Comment

        • #5
          I replaced the filter caps, didn't have to do anything else. B+ is up where it ought to be, and the amp is now working properly! Thanks Enzo!

          I had bought one of these amps new back in the early 80's but sold it maybe 12 years or so ago. I had forgotten how good they sound.

          Comment

          Previous template Next
          Working...
          X