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Converting Tube Guitar amp to Bass amp?

Discussion in 'Amps and Cabs [BG]' started by FunkHead, Jan 14, 2013.

  1. FunkHead

    FunkHead Supporting Member

    Mar 10, 2007
    Stewartsville, NJ
    Is this a waste of time? I would like to put some decent 12" Bass drivers in a Peavey Classic 50 and use this amp to record with. This is the older Model with 2 6L6 Tubes and S/S Pre-amp.

    I'm looking for more scientific answers rather than Emotion driven responses.

    indieandrew11 likes this.
  2. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    That's an open back, no? I would look at running the amp section into a bass cabinet you're a fan of rather than worry about reinforcing the cabinet and creating a proper bass enclosure out of it but that's me. :)
  3. Me too. All you need is a proper bass cab.
  4. FunkHead

    FunkHead Supporting Member

    Mar 10, 2007
    Stewartsville, NJ
    Okay Thanks,

    No Mods or Crossovers required?
  5. IPYF


    Mar 31, 2011
    What kind of recorded tone are you looking for?

    Why not just use the classic as it is at a moderate volume?
    Does it have pre and post gain? If the goal is to get it to roar at you you could probably achieve that without stressing the guitar speaker too much.

    Depends on your mic too. No point forking for a bass cab if your mic isn't going to appreciate the difference adequately.
  6. No need to mod the amp. The tone controls may wind up in a different spot than a bass head is all.
  7. 1958Bassman


    Oct 20, 2007
    The Classic 50 will work, but it may not be as full-sounding in the low end as a regular bass amp because the input section may be designed for guitar, not bass (the cathode bypass cap is a smaller value on a guitar amp). If it's a faithful reproduction of a Tweed Bassman, it will do better with the low frequencies than many guitar amps. Use speakers with the correct impedance and it will make the amp happy. Another thing that's different on a guitar amp is the output transformer- on a bass amp, it can have a major effect on the low AND high frequency response but in this case, it should be close enough to work well.
  8. Hi.

    Definitely not a waste of time, especially if it's "just" for recording purposes. The problems arise when things are being pushed to the limits.

    I thought that anything scientific would be banned or at least frowned upon on TB amp subforum discussions ;).

    What basically determines any tube amps "worthiness" -for me anyway- in low frequency reproduction is the lower resonant frequency of the output transformer. That is a function of many variables, but the cross-sectional area of the core is one of the most important variables. For every doubling of the area, the frequency drops an octave. Within reason of course.
    The price also at least doubles and the weight usually more than doubles.
    There are ways to work around the restrictions of the cheap EI cores by using different laminations and/or materials, but usually the tone suffers, as we are accustomed to hear EI core OT's behaviour when driven hard.

    Luckily there's ways to "cheat".

    The easiest is to turn down
    The OT's inability to cleanly convert power is the key here, so if the power level is reduced, the low frequency reproduction doesn't suffer (that much ;)). BUT, the tone does usually suffer. The grit and compression we (well, I at least) seek is gone.

    The next easiest is to restrict the power going to the OT, so it'll only need to process 1/2 or less of the rated power across the frequency spectrum.

    Some do that with Yellow Jackets and use EL84's in place of EL34's, 6L6's etc.
    Some lower the B+ and use 6V6's instead of 6L6's.

    2 pair PP A/B amp owners have it easy. They just pull one pair (and re-bias the circuit) and use the appropriate impedance tap for just one pair.

    The last one is my favourite, but obviously not possible in Your case.

    One option is also to restrict the signal level before the OT, but that takes quite a bit more modding to sound good.

    A bit of a warning though.
    Ever wondered how can a high quality (or otherwise sought after) 100W tube amp OT alone cost more than a cheap 100W tube amp???

    When talking about extending the low end capabilities of a tube amp intended primarily for guitar use, one has to keep in mind that many guitar amp manufacturers cut corners more than would be necessary and even advertise "weak" OT's as a distortion and character building means. When in reality they have just found the very cheapest OT that people would buy as long as the marketing is done boldly enough.
    Those will sound like crap with any imaginable modification.
    IME at least anyway.

    Since the tone is what matters, and only You can detemine what tone is it that you're after, just experiment. But do Yourself a favour and do that with a real bass cab. Just replacing the existing drivers with bass drivers will only make You think the experiment is a failure.

  9. FunkHead

    FunkHead Supporting Member

    Mar 10, 2007
    Stewartsville, NJ
    The Classic has 4 different inputs and gain controls etc. I'll post a Pic of the Front of it today.

    I'd be happy using the Classic as is but 1 of the 12's is blown(Works if I lightly push on the cone but it's dead otherwise.)

    I was thinking just put 2 12's in it that were for Bass. I'm trying to get the Tone like 3-Steps by Lynrd Skynrd. More or less just playing around and enjoying the Tube's.

    I just have a cheap off Brand Mic. But I would like to get something good.

    Thanks B-String. I would need at least 1 12" to replace a blown one. And then Close the Back in.

    I have plenty of good Plywood so it would be an easy project.

    Do you think I would need to add a Port in the front??
  10. Might be too small to port. Stuff the inside with fill. Bracing will be needed and of course the amp head can NOT be enclosed.
  11. FunkHead

    FunkHead Supporting Member

    Mar 10, 2007
    Stewartsville, NJ
    Thanks for the great info.

    I started out thinking it would be real cool to upgrade the Speakers(One's Blown anyway) and have a Nice little portable Tube amp for lower volumes. I guess It's still possible but I would need to enclose the back which I could easily do. Then maybe some sort of Reflex Port in the front which I would do if needed and provided that it sounded pretty good already.

    This amp is never really going to be worth all that much so I figured why not "Play" around with it as a Bass amp.
  12. I wouldn't spend good money on an unproven experiment. I'd do like everyone suggests and plug it into a bass cab to see what it sounds like. It's never going to sound any better with your frankencab. How come you haven't tried yoiur avatars already? You got some irrational fear of underpowering perchance?

    Odds are at low volume it will sound fine, more tubey as you turn up, maybe fizzing out at higher volume, but this is a recording project so you get to choese the level where it sounds best.

    Spend money on recording gear!
    Sonic Bong likes this.
  13. dspellman


    Feb 16, 2012
    The issue is the cabinet; the amp/preamp will work just fine.

    Decent 12" bass drivers want a cabinet designed for bass. Take care of that and you're golden. If you're porting, do the research and know what you're doing; simply drilling a hole in a closed cabinet ain't that.
  14. seamonkey


    Aug 6, 2004
    For recording, DI from your bass to the board. It's been done this way for ages.

    Then if you want to include an amp - "re-amp" the direct track.
    You can tweak all day on mixing the direct track with the "re-amp" and keep the bass nice and solid as the DI track won't thin out from the transformer or speaker limitations.
  15. FunkHead

    FunkHead Supporting Member

    Mar 10, 2007
    Stewartsville, NJ
    I'll bring it up tonight and plug into My Avatars(1 at a time per 4 ohms each). I bet it will sound pretty good but probably underpowered a little bit. I will definitely be getting some recording gear once I figure out what to get first. :)
  16. xk49w

    xk49w Supporting Member

    If I had a jones to convert a Peavey Classic 50 to bass and to use the stock cab I'd close it in as much as possible, cover up those 12 holes and put a single 10 in it. Then it might have enough internal volume so that the right 10 could develop some bottom end.
  17. FunkHead

    FunkHead Supporting Member

    Mar 10, 2007
    Stewartsville, NJ
    I ended up getting both 12" speakers to work. There was 2 problems. #1 the Polarity was reversed and #2 the Speaker cable was plugged into the 8 Ohm output instead of the 4 OHM. They are 8 Ohms 12820PA Eminence wired in Parallel. The Amp sounded great but I only played My Les Paul through it so Far. This Amp and Les Paul were part of a Purchase that included an Ibanez CN200 which I already Sold. So now I have < $400 into the LP & Classic 50.

  18. burkorobe


    Jan 15, 2013
    The tone controls may wind up in a different spot than a bass head is all.[​IMG][​IMG]
  19. Ah, this is NOT a "Classic 50". This is a Classic VT.

    Totally different amp. The Classic VT uses a preamp frontend and tube power amp with 6L6GC's. The Classic 50 uses 4 EL84's in the power amp and a tube preamp section. (I would not recommend a Classic 50 for bass)

    Peavey made a number of amps over the decades with the word 'Classic" in the name somewhere so it's critical to properly identify them.

    Back to the original question,other than the cab being open as others have noted, the tone controls are going to be voiced a little more for guitar thanbass, but still should be usable. The output transformer is possibly not going to ahve quite as good bass response as a true bass-purposed amp, but Peavey used some pretty good sized iron back then so odds are it will be good enough.
  20. BAce


    Jul 31, 2012
    Also keep in mind that if you close off the back of the amp to get it to sound good with a bass, that the heat from the tubes will have nowhere to go except back into the chassis. I destroyed a good amp in my teenage years like this. That was long long ago in a galaxy far far away. Your best option is to build/beg/borrow or steal an inexpensive cabinet with bass amp specific drivers and power it with the speaker out jack, IMHO. At our jam space I use a Peavey Austin 400 amp, and it's certainly not a bass amp. Heck it's not even my amp. But it is plugged into an old subwoofer cabinet loaded with a Black Widow 1502, and even that really isn't a good "bass" speaker (crossover thingies removed--it's just a big freakin box now) Sounds quite convincing. Stupid loud too:smug: