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400+ running on less tubes?

K-Funk

K-Funk

Sep 24, 2007
1,182
21
Auburn Hills, MI
#1
Just curious, but if a tube goes out at a gig and I don't have a spare, can I just pull it and its match out and keep going, or will this hurt the amp? is there any long-term detriment for running it for extended periods with missing tubes? thanks.
 
#4
fenderhutz said:
Wow with the Mesa auto biasing I have no clue.
Click to expand...

For the sake of avoiding a confusing misinterpretation (at the least ;)), let's not use the term "auto biasing" WRT Mesa amps.
They don't bias automatically; but, rather, are a non-adjustable fixed bias scheme.

I'd just hate to see someone new to the instrument figure "cool, it biases automatically" and suffer consequences from that.

Then again, perhaps I just jumped and you already knew that. ;)
 
#6
see.. I know with the SVT and it's "compete tube protection" thing, a resistor is supposed to blow when a tube shorts out, which lets the amp continue running at 5/6 power without a problem (or so says The Tube Amp Book)

What about the 400+? does something protect it if a tube dies? and if something does happen, can I take the tube (and it's match) out and keep playing (and if so, for how long before it causes trouble?)
 
#8
I've read a few posts by 400+ owners successfully pulling a non-functioning pair for fairly brief periods of time.
I dont know if anyone is gonna know exactly how long you can get away with it. That's kinda like asking "how long can I get away with driving 60 miles an hour through school zones. (maybe quite a while... maybe not)

I believe there's some references to the effects of power tube pulling in this thread. (Psycho Bass Guy and Jerold's posts)
http://www.talkbass.com/forum/showthread.php?t=238662
 
#9
K-Funk said:
Just curious, but if a tube goes out at a gig and I don't have a spare, can I just pull it and its match out and keep going, or will this hurt the amp? is there any long-term detriment for running it for extended periods with missing tubes? thanks.
Click to expand...

If you lose a single power tube during a gig, chances are good you won't even notice it. Besides, how do you intend to determine which tube to pull if you think something may be wrong?

fenderhutz said:
Wow with the Mesa auto biasing I have no clue.
Click to expand...

Dave M already covered it, but since alot of people on TB only skim the threads, it bears repeating that Mesa amps do NOT "auto-bias;" they simply have preset bias voltage (that is often wrong, BTW)

K-Funk said:
see.. I know with the SVT and it's "compete tube protection" thing, a resistor is supposed to blow when a tube shorts out, which lets the amp continue running at 5/6 power without a problem (or so says The Tube Amp Book)
Click to expand...

The screen grid resistors act as fuses in the event of a power tube short. Instead of a shorted tube drawing more and more current until the mains fuse blows, the single resistor will burn up and turn the tube "off." It isn't healthy for the amp in the long term to run with an unbalanced power section, but it's worse for your speakers.

What about the 400+? does something protect it if a tube dies? and if something does happen, can I take the tube (and it's match) out and keep playing (and if so, for how long before it causes trouble?)
Click to expand...

Tubes just don't generally up and die. As I said before, if you lost a single 6L6 in a Bass 400+, you probably wouldn't even know. Why are you worried about it?
 
#10
Psycho Bass Guy said:
Tubes just don't generally up and die. As I said before, if you lost a single 6L6 in a Bass 400+, you probably wouldn't even know. Why are you worried about it?
Click to expand...

not necessarily worried, I just want to know as much about my amp as possible. I have almost no knowledge on the subject of amp function and am really interesting in preventing issues in the future, caused either by the amp or my sheer lack of experience.
 
#11
guitarists do it all the time with 100 watt heads to tame the volume and/or overdrive the power section. but the trick is that you have to half the impedance setting on the amp to keep things properly matched - so if you were playing with the amp set to 4 ohms, you'd need to set it to 2. BUT, this is assuming you have 4 power tubes and are pulling 2 of them. if you have a sextet, not sure what you would do, probably not a good idea. with 12 tubes, i would imagine you need to pull six of them to get the impedance correct. at that point, you'd be better off just having a backup amp.


JR
 
#12
Psycho Bass Guy said:
The screen grid resistors act as fuses in the event of a power tube short. Instead of a shorted tube drawing more and more current until the mains fuse blows, the single resistor will burn up and turn the tube "off." It isn't healthy for the amp in the long term to run with an unbalanced power section, but it's worse for your speakers.
Click to expand...

does the 400+ have screen grid resistors, or does the main fuse just blow?
 

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