Blown fuse on DRRI - voltage question

[jamrine]

I need to replace the fuse on my DRRI, and on the chassis it calls for a 2A, 125V fuse.  The only fuses I can find at Radio Shack and the auto parts store are 2A 250V.  I read on the web that I shouod be ok with the 250V rating, but wanted to get a double check from any experienced folks here.  Will I be ok with the higher voltage rating as long as the aperage is correct?

Also, here is me crossing my fingers that the fuse is the only problem.

[øøøøøøø]

It's the amperage that's important. 

The issue is why the fuse blew.  Fuses do not blow for no reason.  I've never heard of a fuse blowing unless either it was the wrong-rated fuse, there was a power surge (unlikely), or there's something the matter.

The good news is, the "something wrong" is most likely a tube.

Preamp tubes going down will almost never cause a fuse to blow, so that narrows it down to the power and rectifier tubes.

When you put your new fuse in, before powering up, remove those three tubes.

Power the amp on, wait a sec.  Does the fuse hold?  Yes?  Okay you're probably good in the 'expensive stuff' department. 

Now put in JUST the rectifier tube and give it a sec.  Does the fuse hold?  OK good.  If the fuse blows, replace the rectifier tube.

If the fuse holds, power the amp down and insert the power tubes.  Power the amp on (standby first! and then mains) and watch for problems.  If the fuse still holds, wait for the problem to surface again (might be something intermittent somewhere).  If the problem doesn't re-emerge, maybe it was just a fluke.

[jamrine]

Ok, inconsistencies from normal operation which were in effect when I was playing it:

First, and I think totally unrelated, was that there was a fan going while we were playing.  I think it was on the same circuit, but I can;t remember from when I replaced the outlets in that room.  I was getting a crackling noise when I initially turned the amp on.  I started running through standard novice procedure for troubleshooting - plugging guitar directly in without pdeals, etc.  Nothing seems to stop the crackling till we turned off the fan.  Then it sounded normal.  Just weird interference there.

Then, something I normally don;t do, but tried last night, and maybe a total no-no, I plugged in my ABY pedal going like this:

                              ---> loop pedal ----> non-trem channel on amp
guitar ---> ABY OD pedal -------> wah -----> trem channel on amp

I wanted to run the loops into the non-trem channel and then play over that on the trem channel.  The looping got kind of thickly layered, and then I was playing over that, both volumes at about 3-4 range.  I thought perhaps I had tripped the fuse by being a dunce and running into the first input on both channels.  Maybe that is a bad idea (I don't know enough about it).  I am hoping if it wasn;t that, it is, like you said, a tube issue.  All the power tubes wre lit when it was crackling and I was looking them over, and all the others have covers, but were warm, so they seem to have all been powered, but I don;t know how much that really tells me.

Thanks for your help.  I was hoping you would chime in.  I have noticed from your posts that you are clearly experienced with the Fender tube amps.

-Jamey

[øøøøøøø]

nothing you were doing was out of the ordinary or should've tripped the fuse.

The crackling... I assume it was loud, ugly crackling?  Did the volume control affect it?

Sounds like a power tube issue especially if the volume control didn't affect it.  The stoppage of crackling when you turned the fan off was probably coincidental.

It is perhaps possible that your thickly-layered mayhem put just enough extra strain on the power tubes to cause an already-failing one to go over the edge.

[jamrine]

Ok, thanks for the insight.  I will take the power tubes to my tech buddy for testing.

As for the running stuff into the two different channels - that, then, is kosher?

[øøøøøøø]

Fender designed that amplifier to have up to four separate instruments running into it. 

So I'd say with two, you're totally fine

[jamrine]

Ok, the new fuse allowed the amp to power up, but one power tube is not glowing as bright as the other.  I will go ahead and change these out.  A couple of questions:

1) Do I need to replace both?  Should I?

2) Do I need to have these biased?  I am not totally sure I understand what that means, but I gather it all depends on changing something about the tubes as delivered.  Is this necessary for this amp?

[øøøøøøø]

1) tubes glowing differently doesn't mean much.  Unless one is glowing red or something, or one isn't glowing at all, or one is way way bright like a plate/filament short. 
2) It's probably your power tubes anyway, and yes, most of the time you'll change them as a pair, unless they're brand spanking new and the production run is consistent enough (it never is with current-production tubes) to just replace the faulty one.  Replace both.
3)Yes, you should have the amp biased up for the new tubes.  There's a little trimpot inside the amp.  You can either buy a Bias Probe or Bias Rite from Weber or whomever and do it yourself (it's EASY) or take it to a tech and have them install the tubes/bias.

Biasing an amp is simply varying how much bias current is allowed to get to the tube.  This affects how many watts of power each tube must dissipate.  There is a 'safe' range where you won't blow the tube up, and it depends on plate voltage and class of amp operation.  Typically you want to bias for a maximum 70% of the tube's rated dissipation at idle.  For 6V6s in a Deluxe Reverb, that's about 19-20 mA of bias current.

Oh yeah, I should say that not biasing your amp for the new tubes could result in two things:

1) shortening the tube life/red-plating the tubes/blowing more fuses in extreme cases
2) poor sound, typified by "crossover distortion" (a waveform that distorts in the middle, at the zero-crossings, instead of being clipped at the peaks.  Sounds terrible.  No, really... sounds really really bad).

Biasing is a range.  It's possible that you could just drop the new tubes in and they'd be within a safe range.  But biasing is so easy and/or cheap, I wouldn't recommend it.

[jamrine]

Yep, two new tubes fixed everything.  One was toast, the other nearly dead.  My tech went over everything else and found no other issues.  $56, and it's just like new.

[øøøøøøø]

glad to hear a success story.  Having the new tubes installed by your tech means they are properly biased, as well.  Worth it.

[jamrine]

Yeah, I forgot to mention the biasing, but of course he did that.

Thanks for all your input.  It is great to have a knowledgeable person to share their experience with stuff like this.