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increasing headroom on an old Ampeg V4

Discussion in 'Amps and Cabs [BG]' started by nil, Jul 11, 2001.

  1. Are there *easy* mods to a '7? Ampeg V4 to increase clean headroom before distortion?

    I get a useable range up to around 10o'clock before the distortion starts to become too noticable. Yeah yeah I know this is normal... ;)

    It's still running it's original Magnavox 7027s, most preamp tubes are still Magnavox, with a couple of others (RCA, etc).

    Would changing to 6L6GBs (or another tube) give any benefit? How about swapping to lower-gain preamp (12AU7, 12AT7) tubes?
  2. If you are wanting to make it louder before clipping sets in, you are out of luck. The volume level where distortion starts is pretty much governed by the power amp output rating (100 Watts). It's not worth upgrading the power stage to get any more volume (new bigger power tranny, new bigger output tranny, new tube types, basically just build it into an SVT). New tube types won't do much for you, they might make it worse.

    The biggest thing you can do is get a different speaker setup, so that 10 o'clock volume setting gives you more sound output because of more efficient speakers. What are you playing it through?

  3. Rock City

    Rock City

    Apr 8, 2001
    Sorry, but I don't agree with THROBBINNUT!. I have been using a V-4B(same power amp as V-4) for years now. I have modded it to 6550 power tubes and high grade NOS preamp tubes. The amp not only puts out 136 watts now, but it has more USABLE clean headroom. Beware of the 6L6 conversion! Although Ampeg reccommended this mod in the seventies, the 6L6 tubes of that day would handle WAY more juice from the power supply. Todays 6L6 will not handle it and you'll be asking for a fire!(Although this makes for a cool effect on stage, it's not very safe!!!!)
  4. 100 Watts boosted to 136 Watts is an increase of 1.3 dB. Supposedly not even noticeable to the human ear. GibsonGeek, I do not doubt that you can tell a difference, I just don't think it's a big enough difference to mess with. If the original 100 Watts isn't cutting it, 136 ain't gonna be a night and day difference, if even noticeable. I personally have never had a problem with my v4B using 7027A's and putting out its 108 original Watts. Matter of fact, I always opt for my B25B with just 55 Watts from a pair of 7027A's.

    Another thing, I am assuming that nil's amp is in good shape and already putting out 100 Watts. The 7027A tubes are probably still fine after 30 years, but the electrolytics are probably drier than ..... something that's really dry. He may benefit from a tune up and new electrolytics in the amp.

  5. Mod it out to take KT88's ;)
  6. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Without increasing the power supply you're not going to be able to mod ANY amp into much more wattage.

    I actually own two amps, one 400 watt t e other 250 watt, from the same manufacturer that share the identical power amp circuit (I have the schematics). The ONLY difference is the supply rails!

    Then with a tube amp you'll also need to change the output transformer. The end result will be difficult and expensive, easier to just buy an SVT head.
  7. Cool, thanks for the responses!

    I'm playing through an SVT410HLN, which (to me) sounds the bomb. Killer tone though, couldn't ask for anything more there. I'd like to add another SVT410 to give some more punch, but i'll have to wait until i'm famous or rich or both! :) Oh yeah, and have access to a vehicle that can handle two quads!

    I guess I was expecting a little more overall volume...mainly based on experiences with my current tube amps (35w, 50w and 200w varieties). Also it's my first amp with 7027s and "decent" preamp tubes (my others use all sorts - 6L6GB/C, KT88, and cheapo 12Ax/u/t7's) so don't really know how these compare.

    What's required to mod to 6550's?
  8. Pretty much just add a pot to make the bias adjustable, and throw in the 6550's, adjust the bias for optimum (if you know what you're doing, go for it, if you don't know what I'm talking about, stay out of there or you could be killed). I'd measure the B+ before I did the swap, then set the bias with the 6550's so that the B+ was the same. This would basically mean the transformer wasn't working any harder delivering the B+ current (it will be working a little bit harder anyway due to the increased heater current in the 6550's, but I don't think I've ever heard of anyone killing a power tranny due to the swap.) Of course measure plate current to insure all is within ratings of plate dissipation (42 Watts for US 6550A's). Then it may be possible to decrease idle current a little for optimum sound with minimal dissipation. This will yield max power since you'll be going farther into class AB operation, approaching class B, and the power supply voltage will go up slightly (watch those electrolytics.)

    Speaking of electrolytics, are they original? You may get more bang for your buck by replacing them, and if you don't, changing to 6550's may not give you all that you could get if the power supply was fresh.

    The only other thing to check is that there are no funky wiring changes at the power tube sockets. The 7027A had some extra pinouts that the 6550 does not, but the 6550 had its shield connected to pin 1.

    If my 7027A's ever die, I'll probably do the same thing with the 6550's. I just won't be doing it looking for improved performance. Let me know how it goes.

  9. Electrolytics look original...since they're an easy swap i'll probably tackle them first.

    I'll probably give the 6550 a go:
    a) when I can find some 6550s cheap-ish here
    b) when my V4B arrives just in case the V4 dies from it
    Bias, plate voltages etc I understand *to a point* - but i'll get my techie to do it. Maybe I should invest in a good tube amp book? :)

    How do you "drain" (sorry, mental block, this word will do! ;) ) the caps before unsoldering them? Can they simply be left for a certain period of time before unsoldering, or do they require something else?
  10. If you leave the Standby in the "On" position after the amp is turned off, the caps will all drain through the power supply resistors that keep the voltage equal across the stacked caps. It should take about 20 minutes or so.

    The caps in those things are the old multi-section cans. Expensive. And one of them has it's outside metal elevated to about +250V, that's why it is isolated from chassis and covered with a golden cardboard tube. You may want to add in the equivalent discrete caps over on the side of the chassis connected to tag strips. That's what I did on my B25B amp to keep from having to buy expensive multi-section caps.

    Study and memorize the schematic. There will be a test tomorrow. :D



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