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OH, Mighty vintage tube AMP guru, enlighten me

Discussion in 'Amp Central Station' started by andre4999, Sep 16, 2010.

  1. andre4999

    andre4999 TDPRI Member

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    Due to unfortunate circumstances I have acquired a lot of vintage amps.
    I am good with solder and understand how things work electronically.

    Have watched webers video and read a book or two on tube amps as well as worked on a few when i can get direct replacements.

    MY skill is terribly lacking however in finding good "substitutes" for caps when I can't read or match up a vintage cap directly.
    AND if I should use mica/mylar/polypro etc...as well as the voltage requirements for caps/resistors.

    Would like to keep things relatively correct of course.

    No problem with tube substitutions...seems there is plenty online for that.

    ANY??? Books/advice other than the proverbial "experience...don't know how bad I wish I had that!" on what is a good substitute for what (what values come near enough and what the difference in bass/treble/distort/heat etc... would be?) ... always go higher??

    I know this is a mouthful and a huge hill to climb all at once, but when I get ready to order via mouser I just 2nd guess and 3rd guess then tuck my tail between my legs and wait....

    I haven't seemed to find any literature on replacing vintage parts with equal enough values and so forth.

    Really appreciate all and any wisdom imparted.
     
  2. tbradshaw

    tbradshaw Tele-Meister

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    Sorry, no guru here. It sounds like we're at a similar level of education on the subject. I have not come across any reference material like you are requesting...not to say it's not out there but you're talking about a huge world of vintage amps with a near-infiinte variety of design quirks. My suggestion would be to ask about specific amps/circuits at a forum dedicated to that amp/circuit.

    Also, have you tracked down schematics for the amps you're dealing with? I'm pretty sure it's one of Weber's books that has a huge library of schematics with it.
     
  3. dog fart

    dog fart Friend of Leo's

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    No help here either. I just had to compliment you on your sig file. Also, I live west of Winston-Salem and I'm always looking for music stores in the area. Do you have any favorites? Feel free to PM me if you don't want to derail this thread.
     
  4. Wally

    Wally Telefied Ad Free Member

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    Andre, as you noted...that is a lot of territory to cover. Also, one gets in to personal preferences and thedebates that come along with that, right?
    Electrolytic filter caps....I like Spragues, but haven't had any trouble with other less expensive caps. IN the first stage, more capacitance results in firmer and bigger low end.
    ONE has to pay attention to the capacitance here if the rectification is done in a tube becasue there are limits for each type of rectifier tube as to how much capacitance they can safely handle. You want to assure that the voltage ability is above the circuit's demands.
    Bypass(cathode) caps.....larger caps result in more gain and can muddy the low end if one goes too large in the preamp section. I tend to stay with stock values here.
    Tone caps....larger values pass more lows. Tone shaping can be done here. Different folks like different caps in the tone section.....if you can hear a difference then you can make tour choices, right? Soemtimtes the amp they are working with is a determining factor. What soemone likes in a tweed they might not like in a BF Fender....that sort of thing. I use Silver Micas for the treble caps....much smoother sonics to my ear.
    Bias cap.....you don't want to go overboard with the capacitance here. There are problems with maintaining proper voltages if the capacitance is too large....'time constant' is the phrase used in this situation....it refers to the ability of the cap to 'recharge' and provide voltage as needed. 100mfd at 100 volts is the in this application, imio.
    I am no tube amp guru, and these are very basic thoughts. There are a huge number of books out there. I like Dave Funk's Tube Amp Book. IT is especially helpful in that it has the best FEnder schematics that I have seen in book form. There is a lot of insight on how these amps do what they do. Hoepfully, others will give you some other choices to look at.
    Google Aiken Amplification and read his technical sections. Good luck with it all....
     
  5. SnorkelMonkey

    SnorkelMonkey Tele-Holic

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    No guru here but I'm a trained tech and can offer you up some advice based on my experiences.

    Advice:

    If you're really serious about it go to school for training in electronics. Even a 6 month course at your local community college will enlighten you about the basics of electronics, its terminology and the mathematics involved. I see guys on DIY amp sites that might have been self taught by some "amp gurus" (yeah right... :rolleyes:) DVD or book series that don't understand the very basics of electronics and follow that "amp gurus" basic mistakes because they do not know better. As they say you can't put the cart before the horse. ;)

    Tools:

    Besides the very basics like dikes, pliers, etc.. You need a good soldering iron, multimeter, signal generator, and an oscilloscope as your four main tools of the trade. A multimeter will only get you so far. It's a very valuable tool and indispensable in this line of work but an oscilloscope can help troubleshoot issues in minutes that otherwise might have taken hours. My advice would be to learn how to use them well.

    Older Tektronix scopes can be found used for affordable prices. You don't need all the fancy bells and whistles when working on tube amps and most MI/pro-audio equipment so an older reliable scope will work out just fine. You'll need to feed a signal to view these waveforms on the scope. Again with audio work you don't need the bells and whistles just something that can produce a stable sine-wave and you're golden unless you are doing distortion analysis which requires a signal generator that produces very low (almost non existent) distortion. Generally expensive and not needed for basic day to day work in the trenches.

    Books:

    I always recommend starting out with (and in order)...

    1. Dave Hunter's The Guitar Amp Handbook. It takes you through a MI tube amp section by section breaking it down and describing basic terminology.

    2. Merlin Blencowe's Designing Valve Preamps for Guitar and Bass. A more technical book but still accessible. Sticks on the topic and the basics of what you need to know.

    3. Richard Kuehnel's The Fender Bassman 5F6-A. A legendary amp in its own right, many guitar amps owe their lineage to this basic design but Kuehnel takes it further from a clinical perspective and treats it more as a breakdown of basic tube amp technology in general.

    4. Richard Kuehnel's Guitar Amplifier Preamps, and Guitar Amplifier Power Amps series. These books provide a more technical dissection of guitar amp circuits. Here you'll really learn what does what and why, the p's and q's of tube amp design. Lot's of math formulas, again taking that class at community college would have helped by now. It would have started to come in handy by book choice #2. ;)

    5. Kevin O'Connor's TUT series particularly 1,2,3. Lots of info on modifications done intelligently and well thought out. There's also valuable discussion involving and an emphasis on proper lead dress and basic layout design.

    After that I would recommend reference material like Shrader's Electronic Communication (earlier the edition the better for valve reference). Radiotron Designer's Hanbook 3rd and 4th edition (I like the 3rd better, smaller easier to read on you lap, but the 4th is an expanded edition). Howard Tremaine's Audio Cyclopedia (this is one of my all time favorite geek audio electronics reference materials btw). RCA Receiving Tube Manual (absolute must, chock full of tube data). Robert A. Pease's Troubleshooting Analog Circuits (just as the title says). The Tube Amp Book 4.1ed, Aspen Pittman (loads of amp schematics in hard copy form. Later editions on CD and bad quality. The technical information is hit and miss, if you're just starting out I would disregard it altogether and use it just for the schematics).

    Final word of advice? Don't electrocute yourself. The graveyards are full of people who unfortunately went that route. ;)
     
  6. Ronsonic

    Ronsonic Tele-Afflicted

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    I'll take this one on. The nearest "standard" value to what you are replacing is close enough. There are no 0.05uF caps anymore, the amp won't mind if you use .047. It gets a little trickier when there are close values on either side, but still not a problem. Component tolerance back in the day was pretty sloppy so anything within 10% will be as close as the original was. Electrolytics had terrible tolerances, like +80 / -50%. The usual rule for general repairs is anything greater than the original spec and not more than twice. We like to keep it tighter than that so go with the nearest standard value. A slight rounding down like 47 for a 50 is just fine. Back in the day a 10% resistor was considered a precision part. So don't sweat it much.

    HTH.
     
  7. andre4999

    andre4999 TDPRI Member

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    thx all

    OI, my acct. setup had me receiving no replies..duhh
    well I appreciate all the info, some helpful comments for sure, especially the last!

    Right, if I was an electronic engineer/math guy I could deduce what would be decent substitutes....or if I used common sense? I do this all the time at my job on commercial hvac, but in that application exact replacements are often critical for long life and safety. Here in the amp world I reckon safety an issue as well, but most results are probably gonna be fidelity details.

    As with everything else in life, live/learn/do....and do over. Somehow I thought there'd be a way around this for old amps:eek:
    BUt in reality, wouldn't it be GREAT to have someone write a book on what modern components would work with vintage dead ones....then everyone and their uncle would be rippin stuff apart themselves....ok, maybe thats why gerald webers video has so many disclaimers!

    thanks all
    very much
     
  8. andre4999

    andre4999 TDPRI Member

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    and thx much snorkel, i understand what things do, just not how much they should do them! i think your book recommendations are spot-on as I hadn't even heard of 2 of them...am ordering'em now!
     
  9. Coop47

    Coop47 Poster Extraordinaire

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  10. DonMack

    DonMack TDPRI Member

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    Seems like you have two specific issues: determining what value cap to use and what type of cap.

    To determine the value of the cap you should always refer to a schematic of the amp in question. I use this site all the time: http://www.schematicheaven.com/

    Someone earlier mentioned that it is not critical to find the EXACT replacement cap value - I completely agree. The next closest capacitance value will be just fine - you will not be able to detect a difference in tone.

    Regarding what type of cap, there has been much written already in this thread, but to recap (pardon the pun!) electrolytic caps are used in power supplies and as cathode bias caps for preamp tubes and some power tubes (you'll see on the schematic that any cap that has a + sign associated with it is an electrolytic).

    Voltage values are really important with electrolytics and not so much with any others. As I just mentioned electrolytics are usually in the power supply or high voltage area of the amp and so should have ratings higher than the highest voltage you would see in your amp.

    Bias electrolytics can be lower voltage 50 - 100 is plenty safe, but confirm with the schematic.

    I've talked a lot about electrolytics because that is the type of cap that will fail over time. Coupling caps and tone control related caps, not being electrolytics, should be fine unless specific damage has been done to them.

    Or, you may want to change some of these tone shaping caps to alter the response of the amp.

    Regarding, what type of non-electrolytics to use, you might consider ceramic discs for tone controls as they are compact and often the tone control related components are mounted on or close to the tone pots with not a lot of room to maneuver.

    Other types that you can use are:

    • Metal-Foil Polypropylene
    • Metalized Polyester Film
    • Metalized Polypropylene Film
    • Polystyrene Film
    • Dipped Silver Mica

    Any or all of these are fine. I don't bother with silver mica because they cost more and, to my ears, they don't sound at all different.

    Having said that, I am a proponent of buying readily available, good quality caps, but not esoteric, high-end components - the improvement in tone that may be created by using these sorts of components is negligible at best in a lo-fi device like a guitar amp.

    Heh, some call me a heretic, but in my experience you really don't have to worry too much about how the cap was constructed - any of the above types will work just fine.

    Hope that helps!
     
  11. fezz parka

    fezz parka ---------------------------

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    I'd only replace coupling caps if they're leaking DC.

    Check out some of these chapters from the Jack Darr book.
     
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