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70's Ampegs... V4 vs. V4-B

Discussion in 'Amps and Cabs [BG]' started by Basonsubatomia, Nov 18, 2002.

  1. Basonsubatomia


    Nov 8, 2002
    Hey all,

    I'm looking at getting a 70's Ampeg amp of tubey goodness, and I was wondering if anyone had any sort of experience with playing on a V4 versus a V4-B... is the V4 suitable for bass?

  2. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    There was a thread about it not too long ago.
    IIRC the only major difference was that the V-4 has a reverb. Apart from that they're pretty much the same.
  3. Check earlier thread, I have a V-4 and the reverb is the only significant difference, same amp and preamp....it'll work great for bass.
  4. There's a little bit of difference at the first preamp tube. The bias resistor is different, and the coupling cap is a 0.01uF instead of a 0.1uF. The V4 will still work good for bass, but to be identical those couple of parts would have to be changed.

    Somebody in the earlier thread said I was wrong and there was no difference between the 2 amps except for the Reverb, but I checked the schematics, and that first cap is smaller by a factor of 10.

  5. Yep, there is a slight difference in the stage between the first half of the 12AX7 and the second, that's because they added an ultra low switch (which cuts some of the mids out to make the lows seem deeper, according to the Ampeg sales brochure). But that's not really significant IMHO....:cool:

    Next step for me will be seeing how much power I can coax out of it....with 7027's I measured 100 watts into 4 ohms when it started to show visible signs of clipping on the scope. This is the early 70's version, not the last version which claimed 120 watts, the last version was a completely different amp.
  6. Yep, I think I got 104 or so Watts with 7027a's back in like 1993 when I last retubed it and checked bias. I had always thought about going to 6550's, but those dang 7027a's last forever so I never had to!

    I've also got a B25B which is a 50 Watt version that uses a pair of 7027a's, so I've built up a decent supply of them. I never use the V4B anymore, and I only use the B25B maybe a few hours every few months, so those tubes should last another 10 years or maybe more.

    Those amps have got THE sound though. Every time I play the B25B through an old Peavey 1-15 cab with a single Altec 421A in it, all the other players are like, "Aww man, that's how a bass is supposed to sound." Fat, smooth, round, bouncy, growling, with just enough bite and edge to define each note.

    I think the secret is the input to that first tube - note that there's no resistance between the input jack and the grid, and the input impedance is 5.6Meg! It makes the Tone control on a Jazz act completely different. Even when the Tone control on the bass is all the way down, there's still tons of treble in the signal. It's weird, and cool. I plan on putting that input circuit along with the 390K plate load on a Fender Bassman to see if it has the same effect. I bet it will.

  7. I agree that the tone secret is the direct coupling to the control grid, nothing in between the pickup and the grid!

    I think instead of 6550's I'll go with KT88's because the screen voltage (535V) is outside the limits of the 6550 but OK for Kt88's, plus the KT88's can dissipate more at idle. Also I see the recommended plate to plate impedance for the KT88's is about what the recommended is for the 7027's (a couple of charts I have show different numbers for the 6550's)

    Although many people say to change the screen stopper resistors to 1K instead of the 470, I believe I will keep the 470. I believe Weber said in his book that he just routinely makes 'em 1K anytime he mods an amp, but why? if there's no oscillations, why mess with the tone any further? Maybe his thinking is to reduce the screen voltage for longer tube life...but if I change that I'll do so at the dropping resistor off the B+...

    I'm going to add a new filament transformer, I see I can get one from Angela for $30 that'll drive all the heaters, leaving the original PT to handle all of the high voltage supply. And since the KT88's are so dang big I'll add a cooling fan. I'm hoping to get 150 watts with the KT88's.
  8. That should work. The extra filament load was my main concern about putting in 6550's or KT88's. Adding that extra filament tranny will put you in the clear. Good luck and let me know how it turns out.

  9. Rickenbackerman


    Apr 17, 2001
    Laurel MD
    Just curious, what's the plate voltage on a V-4?
  10. Something like 540V or so.

  11. My schematic shows 545 plate, 535 screen. That's about what I've got on mine, depending on incoming line voltage.

    Bias voltage shows -62 by the way.

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