Valve Amp Maintenance

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BarnezyBarnezy Frets: 2174
Can anyone advise on what’s required to maintain a valve amp? How often should the valves be swapped? Are there any risks to leaving it until a valve fails? What are the big ticket items in maintaining a valve amp? What causes a transformer to fail? Anything else a newbie to valve amps should be aware of? 
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  • crunchmancrunchman Frets: 11413

    In my experience, valves normally tend to start to sound bad before they fail.  Either they go dull and muddy sounding, or you start to get frying pan spluttering sounds from them.

    Preamp valves are easy to replace, so you can do that yourself easily enough.  With all the health and safety these days, they can sometimes be a bit of a pain to get at on modern amps.  You might need to be careful what valves you put in certain positions in amps with a  cathode follower circuit.  Some modern valves can't cope with the current required.  @ICBM will know a lot more about that than me.

    Poweramp valves might need to be biased depending on the amp.  Things like Vox AC30s and Fender Tweed Deluxes, and amps that are derived from them, are cathode biased so you can just put new valves in.  If the amp does need biasing, you will need a multimeter.  Some modern amps have low voltage test points for the bias, with a 1 Ohm resistor in the cathode of the power valves, so it's relatively safe.  If you don't have that, then it gets more complex, and you might end up needing to take measurements of plate voltages in the region of 400V DC.

    On some amps, you do need to open them up for biasing.   If you need to open up an amp at all, you need to be very careful.  Remember that the power supply caps can store charge for a long time even after the amp is switched off, so you will need a method for discharging them if you want to do any work on the amp - even with it off.  Sometimes you might need to take measurements with the amp on.  Basic rules are always to stand/sit on an insulated surface, and never ever put both hands inside the amp.  If you are measuring voltages to ground, then put a croc clip on the ground with the amp off, then turn it on.

    To start with, I'd just figure out how to change valves, and bias it if necessary.  Hopefully, you will never need anything beyond that.  If you do then it's normally time for a tech anyway.

    There are sites dedicated to building and modding amps that will have a lot more information than this.

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  • MayneheadMaynehead Frets: 1782
    There are serviceable parts in a valve amp with a range of service lifespans:

    Short: power valves. These wear out the fastest. Depending on usage, they tend to last 2-3 years. As they wear, the low bass and high treble frequencies fall off, leaving you with a thin, boxy tone.

    Medium: preamp valves. The preamp valves can last a long time and do not need to be regularly replaced, unless there is a problem. Usual symptoms of preamp valve problems include microphonics (hum, ringing), crackles and pops, and weak signal.

    Long: filter caps, pots. These should be replaced every 20-30 years or so, as they start to leak or dry up. Usual symptom is a constant hum/buzz.

    Other parts (including transformers) should not need to be replaced on a regular basis unless they suffer at fault. The main cause of transformer damage is turning on the amp with no load (i.e. not connected to a speaker), or connecting to a speaker with a large impedance mismatch, so this is something to be vigilant of when using a valve amp.
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  • ICBMICBM Frets: 71960

    Barnezy said:
    Can anyone advise on what’s required to maintain a valve amp? How often should the valves be swapped? Are there any risks to leaving it until a valve fails? What are the big ticket items in maintaining a valve amp? What causes a transformer to fail? Anything else a newbie to valve amps should be aware of? 
    Mostly, unless the amp isn't working right, don't do anything. It's largely a myth that valves need replacing 'regularly', unless you're really using the amp a lot. Valves can damage the amp when they fail - if the amp isn't very well-designed, mostly - but this can happen at any time and not just if the valve is old, so there's no extra risk in not changing them until they're worn out. 

    Power valves can last many years and preamp valves decades.

    Transformers are not as fragile as commonly thought (some brands aside...), and usually don't fail unless the amp has been run at high power with no load connected.

    Maynehead said:
    There are serviceable parts in a valve amp with a range of service lifespans:

    Short: power valves. These wear out the fastest. Depending on usage, they tend to last 2-3 years. As they wear, the low bass and high treble frequencies fall off, leaving you with a thin, boxy tone.

    Medium: preamp valves. The preamp valves can last a long time and do not need to be regularly replaced, unless there is a problem. Usual symptoms of preamp valve problems include microphonics (hum, ringing), crackles and pops, and weak signal.

    Long: filter caps, pots. These should be replaced every 20-30 years or so, as they start to leak or dry up. Usual symptom is a constant hum/buzz.

    Other parts (including transformers) should not need to be replaced on a regular basis unless they suffer at fault. The main cause of transformer damage is turning on the amp with no load (i.e. not connected to a speaker), or connecting to a speaker with a large impedance mismatch, so this is something to be vigilant of when using a valve amp.
    I would mostly agree with that, except that pots do not need to be changed every 20-30 years - if they're good quality in the first place they'll last essentially forever and just need occasional cleaning. 20 years is probably a bit pessimistic for filter caps, especially modern ones which seem to be longer-lasting. 40-50 years is about the usual limit, but 30 years is a reasonable time to change them.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • crunchmancrunchman Frets: 11413
    ICBM said:

    Power valves can last many years and preamp valves decades.

    That does depend on the amp.  Don't buy a Peavey Classic 30.  I used to have to change the power valves every year at the minimum.
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  • BarnezyBarnezy Frets: 2174
    Thanks guys.

    It seems it gets a bit more complex when bias is needed. The Amp I’m buying is a Princeton Reverb RI 65. Is this a straight forward amp to maintain? 
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  • crunchmancrunchman Frets: 11413
    Barnezy said:
    Thanks guys.

    It seems it gets a bit more complex when bias is needed. The Amp I’m buying is a Princeton Reverb RI 65. Is this a straight forward amp to maintain? 


    I've got the 68 version but I haven't needed to change any valves yet.  Preamp valves are easy.

    Power valves will need biasing when they are changed.  Google tells me there is a 1 Ohm resistor in the cathode of the power valves so you can take measurements in the mV range, not hundreds of volts.  You will need to take the back off to measure.  I would expect power valves in a Princeton to last a good while, so you might not need to do that for years.

    It would be worth getting yourself a couple of spare preamp valves.  One 12AX7 (ECC83) and one 12AT7 (ECC81) should be enough.

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  • MayneheadMaynehead Frets: 1782
    ICBM said:
    I would mostly agree with that, except that pots do not need to be changed every 20-30 years - if they're good quality in the first place they'll last essentially forever and just need occasional cleaning. 20 years is probably a bit pessimistic for filter caps, especially modern ones which seem to be longer-lasting. 40-50 years is about the usual limit, but 30 years is a reasonable time to change them.

    You are correct of course, I was actually talking about the filter caps, and completely forgot to mention about the pots. They only need to be changed if they've been worn out, and cleaning does not clear up any scratchiness when turning. Mainly this is confined to a couple of the pots that sees the most use.
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  • Shouldn't really need any maintenance.

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  • AlmachAlmach Frets: 105
    Don't let the blue magic smoke escape
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  • My advice is to get it serviced when you buy if it's used and then every five years or so unless you suspect something wrong
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  • crunchman said:
    ICBM said:

    Power valves can last many years and preamp valves decades.

    That does depend on the amp.  Don't buy a Peavey Classic 30.  I used to have to change the power valves every year at the minimum.
    Mine needed one replacement EL84 due to failure in about 7 years of regular hard gigging.

    It also had a replacement output transformer and a replacement metal power switch as the plastic one was shit.
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  • martinwmartinw Frets: 2149
    edited February 2019 tFB Trader
    I would never advise anyone that an amp needs servicing on any sort of regular basis (perhaps with the exception of pros who earn money from their kit).
    However it's surprising how many times I find things to do on an amp in that has "no issues, just needs a check over".
    There's often something quite minor that may well prevent a failure down the line...loose jack or pot nuts etc. Of course, you can do that sort of thing yourself, but some people don't want to.
    Other times, less common, you find something more serious.
    Common sense is required; if you play at home, the amp isn't used too much and not thrown in a car boot twice a week, there's less of an issue. If it fails, it's not a problem, just watch telly instead.
    If you've done 5 years of gigs and rehearsals with the amp, there's certainly justification for at least getting a safety check, and those are the amps I'm likely to find loose handles, sloppy jacks, crackly pots etc.
    I also get brought some broken amps that failed mid gig or rehearsal where that could have been prevented with maintenance. Not many, but some.
    So it's a judgement thing, No definitive right answer. IMO!
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  • DmlkDmlk Frets: 15
    How are you getting on with the princeton 65 ri? Did you have it serviced or not when you bought it? I am just curious.
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  • BarnezyBarnezy Frets: 2174
    Dmlk said:
    How are you getting on with the princeton 65 ri? Did you have it serviced or not when you bought it? I am just curious.
    Really liking it. You can get a nice sound a home levels. Looking forward to band practice and cranking it up a bit.

    The guy I bought it from was very genuine and had taken it to a top London tech only a few months ago to get the Reverb tank changed, checked over and a new speaker installed. Plus it’s only a couple of years old anyway and only seen home use, so no need to service it just yet. He was selling it as he couldn’t get the sound he wanted from it, hence the speaker upgrade, and finally bought another amp hence that sale of this one. 
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