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Increase headroom in bassman 50W

Discussion in 'Amps and Cabs [BG]' started by PetrusLarsson, Nov 27, 2013.

  1. PetrusLarsson


    Nov 27, 2013
    Just bought a silverface bassman 50W 1968 with the 2x12 cab. It must be one of the earliest silverfaces, possibly blackface circuit?
    I'm very happy with the sound, it's just what i'm looking for. This is also my first tube amp and first "real" amp.

    My only concern is the volume! Yes, I have realistic expectations on a 50W amp, and yes, I know that the old fender cabs are not considered great. With that said, can someone give me any tips for getting the most out of this setup volume-wise? I have no problems with a little grit/overdrive, but now it's farting out all over the E-string with volume at 3,5! Guessing that the speakers are responsible for this fart sound?

    First step is to change the drivers to eminence beta 12. Thoughts on this? What can I expect?

    I guess that a tune-up of the amp only makes marginal difference. The tubes are svetlanas, don't know about the capacitor condition. Maybe I shouldn't expect miracles from a capacitor change and bias adjustment?

    What about jumping the channels together? I use the normal channel, it seems more focused. Any inputs on tone settings or channel jumping on a bassman? Or other ideas or tips?

  2. CL400Peavey

    CL400Peavey Supporting Member

    Nov 7, 2011
    Grand Rapids Michigan
    Dont mess with your current drivers. Get a new cab altogether. I suggest a proper bass 215.
  3. 1958Bassman


    Oct 20, 2007
    How do you set the controls? If the Bass is jacked up, you won't have much headroom. If it's not jacked up, try a different tube in V1. If it has a 12AX7, try a 12AY7 or a 5751.

    If the caps go bad while it's powered up, bad things happen, so you may want to have them checked out. This can't be done with a multi-meter, either- this requires an ESR meter (Equivalent Series Resistance).
  4. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Recap and louder cab is the way. Lots of old amps are really quiet with dying caps, 50w valve amp is brutally loud with a decent cab.

    50w selmer with active 5 string, not really opened up so its still clean: https://soundcloud.com/oli-foxen/selmer-demo

    All the warrior pope tracks on that account are with 50w guitar head along with metal drummer, not subtle.
  5. zacmac


    Dec 10, 2012
    What he said but first have a tech replace the filter caps before playing or you are playing Russian roulette with your transformers!

    Those sound great with a JBL E140 or E145 speaker in a closed cabinet. just remember also that you will need a 4 ohm load.
  6. jnewmark

    jnewmark Just wanna play the groove. Supporting Member

    Aug 31, 2006
    Stax 1966
    Play guitar.
    +1 on the check up and 4 ohm load. I always used a 215 with the Bassman head back in the day. If anything, I would replace that cab with a more effecient one.
  7. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Swapping preamp valve types doesn't just change gain level, different plate impedances do other things, depending on the circuit/position in it, such as shifting eq centres.
  8. Steve Dallman

    Steve Dallman Supporting Member

    In the late 60's and early 70's I struggled with a 50 watt Bassman. Despite using efficient speakers (EV SRO's) it was never enough, even for practice with the band. My first 100 watt amp, a Norlin/Gibson SG was far above the Bassman headroom wise.

    After becoming an amp tech, I got quite a few Fenders in to see if I could improve headroom. My standard means were to rewire the tube socket to use 6550's, and greatly increase the B+ filter capacitance. After the mods, the amps could use 6L6's with a simple bias adjust. In 100 watt amps I would add an auxiliary filament transformer, but didn't find it necessary in a 50 watt amp.

    The biggest "problem" was the height of the 6550's. In a 50 watt Bassman, the chassis would have to be pulled to change the tubes.

    Increasing the B+ capacitance alone should help...some. FWIW, I had written to Fender in the early 70's about the headroom issue, and was told, the Bassman is only realistically suitable for lower volume jazz and country bands, not for rock groups.

    I rewired a Bassman 100 that I got from a prison. It had been stripped over the years, using it's parts to keep other Fenders working. It was basically an empty eyelet board, chassis, with most of the sockets. I rewired it for 6550's, increased the B+ capacitance 5 times, wired up two new preamps, the bass channel being an Ampeg B-15 type, and a few other tricks. I have another stock 100 and it is a marginal bass amp, headroom wise.

    The modded one is a great bass head. I got plenty of volume and headroom from it. Because the voltages aren't changed, and the 6550's aren't driven very hard from the stock PI, there really isn't any more wattage, but the headroom is no longer an issue. Tight, loud, deep and plenty for the bands I played it in, I love it.

    The customers who had the modded 50's I did reported the amps were inadequate before the mods, but were enough after.
  9. PetrusLarsson


    Nov 27, 2013
    Thanks for the replies!

    Usally go with about 4-5 bass and volume at around 3. Tried 7-8 on bass to get more low end out, but then I need to lower the volume a notch. The end result is about the same. It is kind of a competition between the bass and volume knob.

    Of course replacing the cab would be a big improvement. But I really like the look and the vintage vibe of the original cab together with the amp, so replacing it would not be an option, at least not right now.

    Interesting comments on capacitors and different tubes. Modding it to use 6550s is a bit more complicated than rewiring the sockets and adjusting bias, I understand?
    And about putting a lower gain tube in the preamp, wouldn't it just shift the "scale" on the volume knob and not improve the actual headroom?

    A filter recap probably couldn't hurt! Have to check the prices on that. Maybe I can do it myself, those old amps seem to have plenty of room to work in and easy access to the components. Is there any way to see if the caps are stock or newer ones?
  10. CL400Peavey

    CL400Peavey Supporting Member

    Nov 7, 2011
    Grand Rapids Michigan
    Why not get a vintage looking cab?

    This is one of those new-fangled-modern cabs. A little vintage grill cloth and your retro vibe is right there.

  11. chadds


    Mar 18, 2000
    Beg to differ. For guitar, yes. 15w can be a lot.
    An 85w Showman walked all over the Bassman. A 100w Traynor YBA-1A eclipsed both. I played a 200w Acoustic at a gig along with another bassist who had two 50w Bassman heads and two 2x12 cabs and he couldn't be heard by anyone. I've used two of those Bassman with two cabs and couldn't keep up with a 40w Super Reverb.

    (All of these amps were new or less than 5 years old so cap jobs weren't due.)

    Though it's not my fave amp this is the reason the SVT exists.
  12. Joebarnes


    Oct 4, 2011
    Surrey, BC
    Much good advice has been presented. Easiest solution (aside from taking it into a tech for a checkup, as that is always a good move with old tube amps) is of course more speakers. I understand the nostalgia of wanting the old 2x12 cab, but to compensate for less wattage, you need more speaker area. Try that head with an old 2x15 or an 8x10 and see if that floats your boat a little better.
  13. PetrusLarsson


    Nov 27, 2013
    Well, you've got a point there! Haven't been shopping for cabs before so I guess I have a jungle to explore..

    A 2x15 maybe is the most appropriate option as you said. How heavy and bulky is a modern 2x15, compared to my fender cab?
  14. I agree with the concepts of 1) get it recapped and completely serviced, and 2) get a more efficient bass cab.

    Still, though, at the end of the day, 50 watts ain't really that much for bass. It might be great in some circumstances but IMO it's not enough, even though it's a tube amp and all that.

    I personally would not recommend trying to hotrod it with 6550's. This is a vintage amp, even though it's not pre-CBS it's still fairly desirable-- if it hasn't been molested or hacked up. If it doesn't float your boat after it's been serviced, sell it in its unmolested condition and get a more powerful amp instead.
  15. Then you'll be forever frustrated, sad to say. The problem is almost certainly not your amp. If you took that head and plopped it on top of an 8x10 you'd be in for a real surprise.
    Selecting replacement drivers is not as simple as it seems, and even if you do it, you simply don't have enough power on tap to push a pair of modern 12's to gig volume. You really need more speaker area.
  16. Johnk_10 built a modern 215 (has a thread here) it weighs around 50 lbs.
  17. PetrusLarsson


    Nov 27, 2013
    Do you think so? :)

    At this point I feel convinced that I should replace the cab! Thanks for all the wise inputs.

    And I was never into modding the amp, just asking out of curiosity. It is a fine piece of history and I would never "molest" it by making it into something it's not. Just trying to get the most out of it!

    Will probably take it to a tech, and start looking for cabs ;)

    Guessing that I want 4 ohms? Does that make it easier or harder finding a good cab?
  18. CL400Peavey

    CL400Peavey Supporting Member

    Nov 7, 2011
    Grand Rapids Michigan
    My greenboy Dually is 70 pounds with a tweeter and full steel grill. So maybe 65ish with just grill cloth. Oh, its will also trounce an 810 in volume and sensitivity.
  19. godblender

    godblender Banned Supporting Member

    Mar 11, 2009
    If I were you I'd be looking at Eminence Kappalite 3015 or Faital 15pr400 15" drivers. Drop two of them in a properly tuned box and you'll love the sound/volume that you can achieve. You can build yourself or have an excellent, professionally built cab by any number of the authorized Fearful builders or Don at Low Down Sound...probably around 55-60 lbs. depending on the lumber you use.
  20. MIMike


    Jan 1, 2013
    West MI
    Petrus, I see you just joined the forum today. There is a ton of reading available on this site regarding amps, and especially cabs. It would be well worth your time to look through these and get up to date on modern cabinets and drivers. You don't need 8 drivers or 100lb cabinets to really move air these days (although that still does work).
    The more you read the more you will learn, the more you learn the better informed you will be, and the more informed you are the better your decision will be on what cab and what configuration will best fit your amp and needs.
    You've already got some great advice, there is more out there to glean from this community.