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6550 or KT88 for VT22

Discussion in 'Amps and Cabs [BG]' started by Persuader, Dec 15, 2002.

  1. Persuader


    Feb 7, 2001
    Which tube would be best for a vt22,old-made in linden? I will be playing bass thru it and 4x12 v4 cab...vintage tone wanted.

    The 7027 conversion had been started by my friend who gave me the amp;he changed a few resistors for use with 6550's.

    How are the kt88's...worth it?

  2. I was considering kt-88's for my V4 (same amp as your VT22) but the tube sockets are too close to the power transformer. The 6550's would fit barely, though.

    I think the Kt-88's would give more power but they measure 2 inches in diameter. Measure your spacing carefully!

    Also consider the extra heater current draw. The 7027's draw 0.9 amps each, the 6550's or KT-88's draw 1.6 amps each. Although Lord Valve says the power transformer can "probably" handle the extra current, I personally would want to consider putting in a separate filament transformer. I would use the new tranny to power all the heaters, freeing up the existing PT for the extra current the plates will be drawing.

    And don't forget a cooling fan. We're talking lots of heat. :)
  3. Persuader


    Feb 7, 2001
    BTW;i have 8 7027's kickin around from the old amps.
    Is it even worth the mod,as i liked the sound of the magnavox V4 with stock tubes?

    Also;there is a 12au7 in the 12dw7 spot,what should be done about that?

  4. Several references say the 12DW7 can be replaced by the 12AU7 but as PBG says there is a difference in gain. If it bothers you, mail order a brand new one from Triode Electronics. Angela Instruments stocks some older ones but personally I wouldn't pay the extra price for NOS. You might check Parts Express too, that's where I got my last 7027's from.

    Just hope you don't need the 6K11 compactron!

    I decided to stay with the 7027's (after much hemming and hawing). It's probably more important to change out all the electrolytic filter capacitors and probably the coupling capacitors, IMHO. And maybe the negative feedback cap (C14) if you get some crackle, that was a ceramic disc on mine.

    You will probably need to readjust the bias back to the original if the amp was re-biased for 6550's.

    Mine sounds great now. I've decided if I need more power I'll just tack on a second power amp, the V4/VT22 has two preamp outs so I could power another amp and still go into the PA. Try doing that with most amps out there today!

    Good luck, many people don't realize the potential of this beast.
  5. Lord Valve

    Lord Valve

    Dec 13, 2002
    Denver, CO.
    I'll address all the comments on this thread in this reply:

    1) KT-88s, as someone noted, are rather large. If you don't want to use what you *should* be using (NOS JAN Philips 7581A) the obvious pick is the Sovtek 6550WD, which uses a plastic base. If you use anything with a large-format metal base, you'll need to remove the beartrap clamps, and this is a pain in the ass on Ampegs, since they are riveted in place.

    2) 12DW7s are neither rare nor expensive. They are currently made by at least two manufacturers that I'm aware of: JJ/Tesla (ECC832) and EI, which makes two versions, one "regular" and one in the Elite Gold series, which, as far as I can tell, is the "regular" version with gold-plated pins, a really snazzy-looking box, and a higher price tag. A 12DW7 is a dual triode, with one hi-mu section (same gain as a 12AX7) and one low-mu section (same gain as a 12AU7).

    3) I have tons of 6K11s, they're not all that hard to find.

    4) A 7027 is just a 6L6 with a couple of the elements brought out to "extra" pins that would not normally be found on a 6L6. Unless the sockets are wired to take advantage of this (and Ampeg sockets are not so wired) then any *good* 6L6 will sub for one. The 7581A is probably the best 6L6 ever made, and will survive handily at the voltages encountered in a V-4.

    5) Filament current is not a consideration on any V-4 that I'm aware of; in fact, on some of the later schematics, they list 6550s as plug-n-play with no mods at all.

    6) If you're re-tubing a V-4, consider asking your tech to add an adjustable bias circuit. This only uses around $2 worth of parts, and any halfway-competent tech can do it in less than 15 minutes.
    It greatly increases the universe from which you can select power tubes.

    7) I do not ordinarily replace the electrolytics in a V-4 other than the cathode bypass caps in the preamp stages, since the caps used in these amps were very high quality. On occasion, I may install *additional* HV filters if the output waveform is showing any ripple on peaks/troughs during power testing. Replacing an electrolytic that isn't defective is just a wate of money.

    Lord Valve

  6. Persuader


    Feb 7, 2001
    The VT 22 went to my tech yesterday and he pretty much quoted Lord Valve;tube retainers must be drilled out for 6550,caps don't need replacing unless voltages are off,and since i have 8 7027's to chose from,just keep the amp with stock tubes.

    Thank You all for the advice!

    This restoration should turn out really well.
    I'll let you know how it sounds.