converting a guitar amp to a Hifi amp...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by spaceboy, Nov 9, 2004.

  1. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
    well, that's the plan. it's an old (got a service sticker inside that says 1979, but looks way older than that) FAL Kestrel solid state amp. It's got two identical channels, each with a volume, bass and treble pot and a Normal and Bright input, then a master Volume, Depth and Presence control. I guess it must be a bass or keyboard amp or something... friends of a friend found it on a skip, but it still works! just about...

    It's got a gorgeous clean sound, but i need a decent Hifi, so am I right in thinking it wouldn't be too hard to convert it? I mean, even if I just end up with a working (ie. fix the crackly/cutting out power switch/master volume) guitar amp, it'll still do... but I'd LIKE to change whatever needs changing to make it better for Hifi use.

    I'll remove the spring reverb, but should I remove the preamp section(s) altogether? I wouldn't have thought I'd need them... it'd be useful to have tone control there, but i imagine it'd be easiest to just avoid any possible guitar-orienteated colouration and get rid of it, so I've just got a Hifi power amp... right so far?
     
  2. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    IMO...

    It's a hell of a lot easier and probably cheaper to just go down to your nearest secondhand store and buy a small 1980s hi-fi amp - something like a a Sanyo, Pioneer, Panasonic, Sony or whatever in the 25w/channel range. They go for £25-£50, and it will be a proper stereo amp with a preamp designed for normal hi-fi level signals. Most of them sound quite good too.

    What you've got is a mono amp of unknown (but if I remember, unremarkable) tonal quality which will need a fair amount of work to make it useful. I wouldn't bother. If it works, keep it as a spare amp - you never know when it might come in handy - but I wouldn't spend any more time and effort on it than the parts inside are worth... which is next to nothing.

    I used to come across these fairly regularly in junk shops ten years or more ago, and no-one wanted them even then.
     
  3. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
    you think...?

    i dunno, i figured it'd just be a case of removing stuff, and maybe spending a few quid on different value components or something... is this true?

    ...cos I've got all that power there... just waiting to be used, it seems a waste not to when it's just the components around it that are spoiling it... but then I didn't think of the stereo aspect.

    bugger.

    and the tonal aspect. i just thought about the values and stuff needing changing, but didn't think about the overall quality of it...

    ok, nevermind, i guess I'll have to find something else to be my "first amp project"...

    maybe a kit amp.

    but they cost money.

    and i'm a bastard student.

    arses.

    thanks for the help though! silly me, not thinking things through as per usual.
     
  4. aeolian

    aeolian Member

    Messages:
    6,083
    Joined:
    Apr 5, 2004
    Location:
    Santa Cruz, CA.
    The 70-80's Sansui, Kenwood, etc... stereo amps were in a distortion spec race and were adding massive amounts of negative feedback which resulted in really strident sounding gear. This amp would probably sound a lot warmer, but probably blurryer and less resolution.

    Our beloved guitar amps are decended from cinema amps, so there's precedent for using an early generation transistor guitar amp to listen to music through. Back to the days of glorious mono, just like some Japanese tube junkies. The tone circuits are probably similar to cheap stereo design, and compared to a modern set up, you probably have a "fully discrete" circuit. Just think of how you can wow your buddies with this old school set up. Hey, I almost dumped my Levinson/Classe set up for a single ended EL84 Audio Note a few years back. It just played music really well. Might need to pad down the input a bit, but have fun. The worst you could do is blow up the probably unprotected output transistors and then you'd be back to the paperweight you had before. :)
     
  5. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
    hmmm.... and now i'm all confused!

    but i wouldn't like to be without stereo. and there's no way to run a stereo signal through a singal power stage right?
     
  6. drbob1

    drbob1 Silver Supporting Member

    Messages:
    22,187
    Joined:
    Aug 9, 2004
    You're only going to get mono output, although most stereo recordings are set up so this works ok. I have to agree with another poster, decent stereo amps are not super expensive at pawn shops-$100 will get you something that's pretty respectable. And $250 will buy you an all tube, decently maintained Scott or Fisher stereo amp from the 60s. They really do sound great, far better than just about any SS design.
     
  7. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
  8. aeolian

    aeolian Member

    Messages:
    6,083
    Joined:
    Apr 5, 2004
    Location:
    Santa Cruz, CA.
    There you go. And those weren't too heavy on the NFB. Should sound pretty nice. A little contact cleaner and you'll be rockin the house. Careful about getting contact lubricant. And getting it on the string that runs around the tuner dial. That tuner is a variable capacitor and just needs to be clean between the plates. Tuner cleaner/lubricant is for older TV sets that had multiple contacts for the different channels.
     
  9. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Yes, those are quite nice. I've still got a little Pioneer 30wpc amp (from a few years after that I think) as the power amp for my computer sound/home studio setup. Sounds good.

    That Fal Kestrel... I'm almost surprised there are any left, and not at all that it was found in a skip! :) I can remember having a look inside a dead one (which was also quite normal) once and thinking that it was a pile of junk. I haven't seen one for a few years though. The funny thing is that I think I might even have played a gig about 15 years ago where the PA was the 100W 4-channel version - it was something like that anyway - it had no volume at all. My '58 Fender Princeton (all 4 watts of it) was loud enough to keep up with it, unmiked outdoors :eek: :). Luckily the entire audience was about ten people ;).

    They were kind of like Torque amps are today - they look like they should be OK from the outside, but actually they're just too cheaply built to really be any good.
     
  10. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
    so, Contact cleaner, NOT Tuner cleaner?

    cheers :)

    And i really should fix the kestrel up sometime.. I really thought it had a nice clean tone - for what it was, which was a very warm, round tone. the problem with it is that it crackles and cuts out a lot, mainly to do with the master volume pot and the power switch... the pot should be piss easy to replace, but i tried getting that damned switch out a while ago to have a look at it - and the bugger wouldn't budge! couldn't see any way to get it out short of some drastic brute force... guess i've got nothing to lose though. could still be a back up/practice amp with a pedal or two - or even a make-shift pa like we used to use it for ^_^

    anyway, thanks guys!
     
  11. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
    yay, the beauty arrived!

    now to the Pawn shop for speakers! ^_^ lovely pawn shop.

    a couple of things i'd like to get straight -

    you think this one is a 30 watter too? doesn't seem to say anywhere...

    not being used to solid state amps, when it says 6-16 ohms on the back, that means that it'll work with ANY value in that range... but doesn't it give higher output with a lower impedance or something...? or is that something that only some amps do..

    cheers
     
  12. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    I'm not certain about that model, but it can't be any more than 30W per side if the consumption is 130W.

    Yes, it can use any speakers between 6 and 16 ohms. You will lose a little power with higher impedances - but don't worry about it, almost all hi-fi speakers are 8-ohm anyway. The difference between 6 and 8 will be negligible.


    Is the switch on the Kestrel one of those square rocker switches BTW? I think the later ones had those... if so, you need to push in the grips on the inside, then push the switch out through the front - it's a bit tricky.
     
  13. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
    excellent, as I thought. cheers!

    yeh - that's exactly the switch! little bastard. just couldn't get the grip things to go in enough to let more than one side of it out... maybe a mini vice to hold em in and a hammer to get it out... will have to investigate it again sometime. maybe just find someone with more finger strength...
     
  14. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,062
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Well,

    There you guys go....

    ...and here I was thinking of just getting
    a standard guitar cable and plugging one end into the phones jack of the CD player and the other end into the guitar jack of the amp.

    Then dial it in until it sounds good.

    Yeah I know, but it is all relative folks.

    It ain't no high fallootin' audio-phoole,
    cork-sniffin' stereo;

    BUT

    it works and you can listen to some
    discs or play along with them (depending on the amp).
     
  15. fullerplast

    fullerplast Senior Member

    Messages:
    6,792
    Joined:
    May 5, 2003
    Matter of fact, it ain't no "stereo" at all....which channel do you prefer to listen to, left or right?:rolleyes:
     
  16. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    This isn't always a good idea. If you put a mono plug into a stereo socket, you'll short the right channel (ring) to ground. Sometimes the headphone driver IC does not like this. Usually they're short protected, but not always...

    You're better with a stereo cable split into two mono jacks at the other end - you can plug them both into the amp at the same time. You'll now be able to hear both halves of the stereo - mixed into mono - and you won't risk frying the driver IC.

    Sorry to be boring... :)
     
  17. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
    well, my beautiful new Hifi amp is a piece of junk... the left channel doesn't work - it emmits the signal very quietly and very fuzzily, and at the same volume no matter where i turn the volume knob... definately not the speakers or speaker cable - checked.

    got a refund (minus postage), so it's not all bad - but it would cost upwards of £50 to get fixed apparently, so no point in that when i can try ebay and get one for 30-40... but, do you think I could fix it myself? any idea what the problem might be? for fixing stuff like this is the general procedure to get a voltmeter (have to get one sometime... maybe i could borrow one from the engineering faculty...) and probe around the problem area (the left channel output), working your way back til you find where the courrent/voltage stops/starts and see if there's anything obviously wrong there? something like that?...
     
  18. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Unlikely, without a lot of experience and a schematic. Solid-state is nowhere near as easy to troubleshoot as tube - stages are often DC-coupled, which means faults on one can show up on another... and you may find some of the transistors are obsolete and hard to get. And if it's some non-standard IC that's died, forget it!

    If you were offered the refund, take it! It's a little cheeky of the seller not to refund the postage though. If he sent you a dead amp, why should you be out of pocket for that?

    You're right that it isn't worth spending £50 to fix it.

    BTW, I did have a quick check in my loft to make sure I hadn't got anything like this still lying around that you could have... but no.
     
  19. spaceboy

    spaceboy Member

    Messages:
    256
    Joined:
    Jun 10, 2004
    Location:
    Dundee, Scotland
    heh, cheers for the look around! guess it's off to the scrap heap with it then... and back to ebay to look for another one! *sigh* silly evilbay - but nevermind

    cheers!
     
  20. drbob1

    drbob1 Silver Supporting Member

    Messages:
    22,187
    Joined:
    Aug 9, 2004
    I strongly recommend looking for a Fischer or Scott stereo tube head from the early 60s. Great sounding, pretty foolproof. Get one guaranteed to be working, or better yet, recently serviced and you shouldn't have to pay more than $300 or so.
     

Share This Page