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What kind of tubes can I use?

Discussion in 'Amps and Cabs [BG]' started by Greg R, Oct 18, 2003.

  1. Greg R

    Greg R

    Mar 11, 2003
    Avonmore, ON
    I picked up an old "Rogue" tube amp a friend was going to toss out. He thought it was a low power piece of garbage. I took it home and played through a 2x12 and noticed the tubes had all gone microphonic, as soon as you turned up the gain or the treble and bass pots you could hear it through the speakers. I changed out the preamp tubes with new 12AX7's and replaced the 2 6K6GT's with 6CA7's. I know they aren't the same tubes but the amp works and is loud as hell. Nothing blew up but I'm wondering what kind of damage I'm going to do to the amp if I was to leave it like this long term. I do get a lot of hum out of the amp but it gets quieter as you turn up the volume even without masking the noise with instrument signal.

    Any advice would be appreciated.
  2. I don't know about the long-term damage, but definately get the correct tubes. Go to Triode Elec http://store.yahoo.com/triodeel/index.html

    or Antique Radio Supply www.tubesandmore.com

    or Angela Instruments www.angela.com

    If you still have hum, most likely it's from a filter capacitor. The electrolytic filter caps (on all of the old amps)get old and start letting a hum come through. If you're good with a soldering iron, you can get new caps from any of the previous plsces, Antique Radio has the multi-section caps found in many old amps. These will be big silver cans marked with values like 40 mfd/ 450 Volts. Always unplug the amp and make sure the cap is de-energized, if you don't feel comfortable with the job take it to a tech. Warning: there's high voltages in there that can kill you if you poke around with the power on.
  3. Greg R

    Greg R

    Mar 11, 2003
    Avonmore, ON
    Yeah those caps actually have APR1968 stamped on the bottoms and must be dry as hell. I am assuming April '68, but maybe that's too simple. Luckily I can get this stuff through work. I've played the old "let's disharge the capacitor on your hand" game before... not funny at all.

    Both types of tube are 6.3v, but the 6CA7's are pulling an extra amp each so I don't think the rectifier tube is going to like it much. I should state that the only tubes changed from the original type are the power outputs. I'd like to mod the amp to use the higher-powered 6CA7's... anybody know what to change?
  4. The 1.4 amps per tube as oppesed to the .5 amp per doesn't have anything to do with the rectifier tube. You're referring to heater current supplied directly from the power transformer.

    You'll need a new separate filament transformer rated to handle the 6CA7's current draw, of 3 amps at 6.3v. You'll need to isolate their filament lines to just the new PT and connect the new power tranny in parallel with the old PT's primary wall voltage input. The hum you're getting could also be a function of the increased filament current creating stronger internal fields which could be getting picked up on the preamp lines, but my first guess would be filters.
  5. BassGreaser


    Aug 22, 2002
    Austin, TX
    here is my stupid question for the night....how can you tell when your tubes have gone microphonic?
  6. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Turn on amp...let warm up thoroughly..

    tap on each tube in turn with a pencil or some such thing.

    if the tube causes a "boing" or rattle when you tap it, its microphonic.

    If its just a little, you might think it sounds funny. if its a lot it will take off and sing if you turn up the gain/volume.
  7. Greg R

    Greg R

    Mar 11, 2003
    Avonmore, ON
    Psycho... thanks for the tip!

    I've put the original power tubes back in and just changed out preamps with new ones I had kicking around. Sounds great!
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