Hum and fuse blow help

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Rich210Rich210 Frets: 289
Hi guys and girls,

I was playing bass through the head of my musicman sixtyfive in a Aguilar db115 and when I turned it up the fuse blew. 

I replaced the fuse, lowered the volume but instant hum and then blew another fuse.

Both of the tubes light up and no obvious damage to the vacuum. And they're newish tubes.

I'm running JJ kt88's and revised accordingly. The plate resistor is 220k, most reconnections is to swap this out for 100k but as it's only 65watts I thought it would be fine as per the spec sheet of the tubes..

Any theoretical wonderings appreciated!

Best

Rich
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  • ICBMICBM Frets: 49476
    New-ish or not, the chances are it’s a blown power valve.

    "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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  • Rich210Rich210 Frets: 289
    ICBM said:
    New-ish or not, the chances are it’s a blown power valve.
    Thanks pall, could you hazard a guess of why it might have blown other than a defective tube?

    Best

    Richard
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  • ecc83ecc83 Frets: 1044
    I am confused by "Plate (proper name Anode) resistor 220k" ? and KT88. Do you mean the grid leak?  If so I would have thought 100k  a bit low for the PI driving it although low GLRs are otherwise a good idea.

    When I had 88s die The Purple Death they cooked the 10k grid stoppers. Did not change value but went very brown!
    I don't think KT88s from any new source are as tough as the originals?

    Dave.
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  • ICBMICBM Frets: 49476
    edited December 2019
    It’s actually a very unusual amp design that operates with very high voltages (700V HT) and can be very hard on the valves. The resistors in question are the grid stopper resistors and should be 220R (not K).

    A common mistake is to bias them way too hot, they need to be set using a scope rather than by power dissipation, just above the point of crossover distortion.

    https://drtube.com/schematics/musicman/gp3a.gif


    "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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  • ecc83ecc83 Frets: 1044
    edited December 2019
    Seven hundred volts! I am sure Balckstar will not mind me telling you that the S1 200 had the transformer revamped to drop the HT to around 650V iirc on the Anodes. This was purely because it proved impossible to find KT88s that could take the rated Va of the orignals.  Well, not at least at a price that was practical for a guitar amp!
    The valves are biased to 25mA  so Pa is very safe 16W or so. There are of course 4 valves.
    There is also an an anode fast fuse and an interlock to stop the beam plates melting if it blows!

    ...In'ell! Just noticed the transistors in the 88 cathode circuits! I bet they have gone home to the great Silicon graveyard in the sky.
    Pretty daft idea if you ask me? Especially with KT88s in guitar amp.

    Dave.
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  • ICBMICBM Frets: 49476
    Indeed, personally I wouldn’t put modern KT88s in one either.

    The best valves for these are old-production US 6550s.

    "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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  • Rich210Rich210 Frets: 289
    Thanks for your input as ever,

    The amp is 2100-65 chassis that should have 6CA7's

    http://www.thetubestore.com/lib/thetubestore/schematics/MusicMan/Musicman-2100-65-Schematic.pdf


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  • Rich210Rich210 Frets: 289
    Thanks for the feedback, I'll take her into Matamp on the way to work and let them give it a once over. 

    I was hoping it might just have been a case of swapping the tubes checking the bias and replacing a resistor. 

    It's a really good bass amp, probably end up with some E34L's in there a see how it goes.
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  • ICBMICBM Frets: 49476
    Ah, OK - but even so the same things about HT voltage (even higher, 725V) and bias apply. There are no 220K resistors in this one either, the grid stoppers are 1K5.

    If you can’t find any old-production KT88s at a sensible price you would probably be better with old-production EL34s - despite the lower power dissipation, they’re rated for 800V.

    "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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  • Rich210Rich210 Frets: 289
    Well I had new production JJ kt88's that were rated at 800v. I think JJ EL34s are up to 2000v which seems to good to be true but there you go. It was running on some siemans el34 and I switched them over to the 112rp65 version during the service. 

    The plates actually run at 780 volts on the 210 sixty five....
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  • Rich210Rich210 Frets: 289
    @ICBM ; so out of interest what is R55 on the schematic? Its 220r resistor, I got mixed up before. 
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  • ICBMICBM Frets: 49476
    I wouldn’t believe the voltage ratings on any modern-production valve, to be honest.

    Servicing MusicMan amps in the 90s was interesting... pretty much any new valves would just blow immediately. They have at least got slightly better since then.

    "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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  • Rich210Rich210 Frets: 289
    ICBM said:
    I wouldn’t believe the voltage ratings on any modern-production valve, to be honest.

    Servicing MusicMan amps in the 90s was interesting... pretty much any new valves would just blow immediately. They have at least got slightly better since then.

    So what would you say is the best new tube out there to put in it?

    I'll keep an eye out for NOS stuff!


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  • ICBMICBM Frets: 49476
    Rich210 said:
    @ICBM ; so out of interest what is R55 on the schematic? Its 220r resistor, I got mixed up before. 
    It’s very hard to read that schematic but it looks like it’s the grid stopper for the top power valve, and is 1K5. I can’t see any 220s in this one, which is why I thought it was the other circuit.

    "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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  • Rich210 said:
     I think JJ EL34s are up to 2000v which seems to good to be true but there you go.

    No, can't be true.  I reckon the valve base wouldn't take it, it would arc over.  Just looked at the datasheet, pretty scant compared to valve datasheets of old.  They show "Ua0" = 2000V.  I'm guessing that's a maximum voltage with no anode current flowing.
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  • Rich210Rich210 Frets: 289
    @ICBM the clear schematic is on the third page I think. 
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  • ICBMICBM Frets: 49476
    No wonder I’m baffled, that’s a completely different amp to the one on page 2!

    Yes, R55 is the grid stopper and is 220R. I would definitely not lower the value.

    "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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  • Rich210Rich210 Frets: 289
    @ICBM ; ok cool! So when you read about KT88 mods, there's a resistor change from 220r to 100r what's that about or is it not relevant to musicman amps?

    Some people have used kt88's in the rd100 which is another interesting amp design in that it produces 100watts while having only 2 6L6GCs. 

    Another thing, do you think it's just shitty tubes? This is actually the second set of KT88s it's destroyed. The first blew when I clicked the deep button as my mate was playing quite loud. It actually cracked the bottle. These tubes were returned and replaced as defective. 


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  • ICBMICBM Frets: 49476
    Rich210 said:
    @ICBM ; ok cool! So when you read about KT88 mods, there's a resistor change from 220r to 100r what's that about or is it not relevant to musicman amps?
    No idea, I've never put KT88s in a Musicman. I have used 6550s in the 6L6 models, but always old-production GE 6550As.

    Rich210 said:

    Some people have used kt88's in the rd100 which is another interesting amp design in that it produces 100watts while having only 2 6L6GCs.
    That's because they run them at very high voltages and biased almost to Class B. That's what I meant about many people biasing them far too hot - if you try to do it like a conventional Class AB amp, you'll blow the valves.

    Rich210 said:

    Another thing, do you think it's just shitty tubes? This is actually the second set of KT88s it's destroyed. The first blew when I clicked the deep button as my mate was playing quite loud. It actually cracked the bottle. These tubes were returned and replaced as defective.
    Quite likely, as Dave said from his work on the Blackstars.

    It's unfortunate that these amps were designed for the quality of valves that was available in the US in the 1970s and really pushed the technology to the limit.

    "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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  • Rich210Rich210 Frets: 289
    ecc83 said:
    I am confused by "Plate (proper name Anode) resistor 220k" ? and KT88. Do you mean the grid leak?  If so I would have thought 100k  a bit low for the PI driving it although low GLRs are otherwise a good idea.

    When I had 88s die The Purple Death they cooked the 10k grid stoppers. Did not change value but went very brown!
    I don't think KT88s from any new source are as tough as the originals?

    Dave.

    Hi Dave,

    I was getting a bit confused to be honest. I was talking about a 220r grid leak, but I got muddled when reading KT88 specs and trying to put two and two together!

    I'm learning that I probably shouldn't have swapped the EL34's out for KT88's even with a sensitive rebiasing. I was advised to run them at 780volts to avoid impedance issues (the amps are switchable from 720 to 350 on spec, mine runs 780???), but it looks like where one preventative action was made a risky action happened in light of less than military grade modern tubes. At some point I'll take it to someone around Manchester that knows a lot more than I do.

    I'm probably gonna ditch the KT88 journey in the sixty five, as the impedance miss match with the output transformer might be an issue. The impedance of the output transformer is 9.5k. And it's clearly impossible to get hold of decent tubes that don't cost the same as the amp itself is worth! So I'm looking out for reasonably priced EL34s that'll do the biz...I've heard that JJs output tubes are getting sketchy, even the E34L which was a tube recognised being able to cope with these designs.

    @ICBM I can see why these amps go for not a lot these days, clearly there are issues with finding tubes that can keep up with the design. I love the tone that these produce! Maybe they're only good for running on the 350volts setting.

    Thanks again for both your input


    Richard



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  • ecc83ecc83 Frets: 1044
    Apart from all the other problems remember the damage electricity can do to your body goes up as voltage squared!

    780 volts is a hell of a lot more dangerous if you cop it than 350 even 500. In fact unless someone was around to give you CPU I doubt you would survive.

    I dare say all us old techs have had a jolt ot two from tellys and amps, even the mains but 700V+ is whole other ball game.

    As for lots of power from EL34s? The old Mully spec shows 100W from two with Va at 800V. Don't think anyone has tried that for guitar amp and I doubt there are any valves made today that would live!

    Be 'KING careful!

    Dave.
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  • Rich210Rich210 Frets: 289
    @ecc83 ;

    Hi Dave, 

    Thanks for the tips, I've spied some on ebay that I'm going for and a couple of siemens el34s that were originally in it, and working in a different amp that's running just under 700V, like I said before the E34L historically seemed to be well thought of on musicman amps, but I've heard bad things on JJ's quality of late, so I'm just gonna keep my eyes out for decent deals. 

    I'm not gonna touch this thing plugged in until I know what I'm doing...I'll be sending it to the tech I use in Manchester but I have doubts as to whether they have bench room at the moment. Do you know any guys up my way?

    Thanks for doing some research and advice on my behalf! Really appreciate the input. 

    Best

    Richard
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  • ICBMICBM Frets: 49476
    One tech I know of who is definitely OK with this sort of stuff is John Chambers at Champ Electronics in Nottingham - don't know if that's close enough.

    http://www.chambonino.com/construct/const9.html

    Yes, that does say 2,500V on the plates :).

    "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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  • Rich210Rich210 Frets: 289
    ICBM said:
    One tech I know of who is definitely OK with this sort of stuff is John Chambers at Champ Electronics in Nottingham - don't know if that's close enough.

    http://www.chambonino.com/construct/const9.html

    Yes, that does say 2,500V on the plates :).

    Cheers pall, I'm going to hold out for my local tech for a few days cos he was the last guy that touched it plus I might be able to get a refund on the tubes. Nottingham is a bit of a schlep atm, but if it's not resolved in the new year I'll give him a ring. 
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  • ecc83ecc83 Frets: 1044

    Dave Hunter Notts again . We email each other from t2t. He is almost always slammed with work mind!


    Dave.

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  • Rich210Rich210 Frets: 289
    @ecc83 ;

    Cheers Dave, currently on the hunt for some 'they don't build them like they used to' valves
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  • hywelghywelg Frets: 3234
    ecc83 said:

    Dave Hunter Notts again . We email each other from t2t. He is almost always slammed with work mind!


    Dave.

    He's on here. @DJH83004 ;
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  • normula1normula1 Frets: 400
    ICBM said:
    One tech I know of who is definitely OK with this sort of stuff is John Chambers at Champ Electronics in Nottingham - don't know if that's close enough.

    http://www.chambonino.com/construct/const9.html

    Yes, that does say 2,500V on the plates :).

    Lovely guy. I spent a couple of hours with him a few years ago when he rewound a transformer for me.
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  • Rich210Rich210 Frets: 289
    Thanks @normula1 ; and @hywelg, I've found some safe troubleshooting techniques I'm going to have a go at myself. Before I take it to a tech. @ICBM I can run the amp at 450v so if it's just the tubes that have gone, I'm going to swap them out and rebias it to spec with EL34's I've got, and hazard using some E34L while I keep and eye out for some affordable wing c's or siemens el34s on eBay. Hopefully that'll tie me over till the new year!
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  • Rich210Rich210 Frets: 289
    @ICBM @ecc83 I had some spare KT88 tubes so I stuck them in and turned it on...she works so no amp damage. 

    I'm just going to run her on the low setting ie so the plate voltage is 450.

    Are there any key reasons I should stop using the kt88's in the short term? I recall being told about impedance issues. 

    Cheers guys

    Richard


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