Tubes Asylum

RE: Runaway 7581A

63.247.31.12

In Reply to: RE: Runaway 7581A posted by Thermionic27609 on July 03, 2009 at 05:28:40

T, when I asked about the bias you set the amp to, I was trying to figure out why the 7581A's were bad, but the 6L6GC you replaced were ok.
If one tube is starting to go ballistic first, it's not uncommon for its push-pull partner (and since there are only two tubes, all power tubes :-) .... to shows signs of too much current. This is due to the excess draw by the first tube. This one is prolly the bad tube.
The 7851A can test fine and when in the amp & powered up, be no good. If the 6L6GC you replaced in the amp work fine at volume for a good length of time, then prolly need to look for new pair of 7581A's. If you want to keep that tone.
You may want to eyeball the resistors on the power tube sockets for damage or out-of-tolerance values.

Edits: 07/03/09

Follow Ups:

RE: Runaway 7581A - Thermionic27609 08:06:07 07/03/09 (4)
- I usually change those resistors - FenderLover 16:05:24 07/03/09 (3)
  - pink ceramic based STR-415 (Sylvania specials) - FenderLover 04:21:20 07/04/09 (2)
    - RE: pink ceramic based STR-415 (Sylvania specials) - Thermionic27609 07:16:40 07/07/09 (1)
      - Plate voltages - FenderLover 04:52:28 07/10/09 (0)

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