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
Comments
"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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Best
Richard
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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The amp is 2100-65 chassis that should have 6CA7's
http://www.thetubestore.com/lib/thetubestore/schematics/MusicMan/Musicman-2100-65-Schematic.pdf
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.
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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The plates actually run at 780 volts on the 210 sixty five....
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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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!
"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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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.
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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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.
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.
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
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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