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new tubes for my 1970s ampeg V9-SVT

Discussion in 'Amps and Cabs [BG]' started by ojthesimpson, Aug 5, 2003.


  1. ojthesimpson

    ojthesimpson

    Jul 21, 2003
    Draper, UT
    this is a 300 watt at 4oms tube amp. I've yet to put new tubes in it since I got it. I have a friend who can do the work for me. Now all I need to know is what are some premo tubes i could put in it. I want a nice and deep grity growel out of it. Something that can still resemble something pretty when need be but for the most part really mean sounding. I have a Nemesis 8x10 cab and a MIA Fender Jazz Delux.
     
  2. What makes you think it needs new tubes?
     
  3. lneal

    lneal

    Apr 12, 2002
    Lee County, Alabama
    You beat me to it PBG!
     
  4. ojthesimpson

    ojthesimpson

    Jul 21, 2003
    Draper, UT
    the gain is really weak and it's losing some of it's balls. You don't think the tubes are worn out?
     
  5. Might be the tubes, might be any number of other things. Could be something as simple as a loose connection or as serious as a bad power transformer. Let a qualified tech make the call as to whether the amp needs tubes or not. As for biasing it, if you're worried about the cost, there are built in test points and instructions for biasing the amp painted on the chassis. You'll you need a dummy load, a tone generator, a voltmeter, and an oscilliscope.

    FWIW, good power tubes ought to last 30+ years; preamp tubes should last the life of the amp.
     
  6. Rickenbackerman

    Rickenbackerman

    Apr 17, 2001
    Laurel MD
    Man, some nice timing on this thread. I just pulled the Sovtek KT88's out of my '74 last night to throw my extra set of (30 year old) GE's in there. Old tubes are the bee's knees.

    BTW, Psycho - I used my bias probe to record current measurements and then swapped the tubes around to get what I thought was the best balance between sides.

    I ended up with 18, 24, 22, and 19, 23, 22 mA.

    Mutual transconductance for all tubes was between 8000 and 8750 micromhos.

    Are these close enough to be considered a matched sextet? They did come from an SVT in the first place...
     
  7. As long as none of them are cherryplating, and you don't hear anything weird, they're cool. You could have a perfectly matched set of power tubes and an unbalanced driver tube would still cause them to run unevenly.

    A lot of guitar amp guys hate using a scope to check for crossover distortion when setting bias, but for a bass amp, it's tbe best way to make sure the amp is putting out balanced A/B power. Put the output on a dummy load and take a reading off the load to check for a clean power reading at a 40 Hz sine wave. If one side of the wave is drastically larger in amplitude than the other, you have a problem. If not, it's fine.

    You can also check for crossover offset by connecting the amp to a speaker with no input signal. If the speakers move and hold that position when standby is switched off, you've got one side running harder than the other.
     
  8. Rickenbackerman

    Rickenbackerman

    Apr 17, 2001
    Laurel MD
    Thanks, Psycho. The wave looked pretty good to me when setting the phase inverter balance, but then again my breadboarded tone generator doesn't exactly produce the cleanest sinewave in the first place... It also produces a 500Hz tone - is that cool or should it really be 40Hz?

    I haven't played through it since swapping tubes; I'll do that tonight. I'll check for no-signal speaker movement (same as DC offset, right?) and I'll also watch the tubes when playing.

    Judging by the way it biased and balanced up, I think everything is cool. I just want to be careful with those ($$$) GE's in there. The only thing that seems funny is I'm only measuring about 33 volts into a 4-ohm load (which gives me about 270 watts), but that could be my funky 40 year old meter. I was using my DMM on the PI balance at the time.

    Rob
     
  9. 500Hz will work; lower freqs are just easier to see. If the tubes are going to have trouble pushing current, lower frequencies will make it show up faster.

    Yup, you're talking about DC offset. "DC" or no speaker movenment should be occuring right at the crossover point of both sides. If one is too high, the difference bewteen the amplitudes will produce motion in the cone, either positive or negative.

    The GE's will give you less power than the KT88's that were in there, but will sound louder due to their midrange. You could try biasing them hotter for more disappation and more power, but that's your call.
     
  10. Rickenbackerman

    Rickenbackerman

    Apr 17, 2001
    Laurel MD
    Awesome. Thanks! I'll post an update tomorrow.
     
  11. Rickenbackerman

    Rickenbackerman

    Apr 17, 2001
    Laurel MD
    Welp, I plugged into it last night - and it still doesn't sound as good as my '73. I think it's the speakers. I use the '73 with my '76 tilt cab with the weber recones. This one (a '74) I'm using with a '71 coffin cab with late 70's speakers (the same style eminence squarebacks as in the '76), but 5 of the speakers are pretty tired. The other three are practically NOS.

    I think the webers are making for a huge difference in sound between the two rigs. Man, if I knew half as much about *anything* as that guy knows about speakers...

    He told me a while ago he was working on a CTS alnico SVT clone and that it should be available by the end of the summer. I hope so, cuz I'll take eight, please.

    Rob