randall rm100 tube help

[belias]

hi, I found this site on the randall forums. Seems a little slow over there, so I'll ask here too.
I'm experiencing my first ever problem with tube failure. I noticed a "tube failure" light on tube #2, which is odd because I haven't really used it that much yet. I borrowed a voltage meter from work and followed the directions in the manual. My tubes are the Ruby 6L6's that came in it.
With the DC section of the meter set on 200M, my readings are: T1)50 T2).2 T3)33.5 T4)43.2. To me, these numbers seem out of whack.
Another thing i've noticed is that the "top" light on tube #1 seems dimmer than the other three. By top light, I mean that each tube seems to have two illuminating points, one towards the top, the other towards the middle to bottom, if ya get me?
One other note: when I bought it roughly over a year ago, the whole system blew out shortly after I plugged it in for the first time. The local repair center fixed it quickly...I think they said they replaced the main board or something, but didn't know exactly what caused it.
It sounds fine now, but with this tube thing, I don't want to use it and risk damage.
Any help is greatly appreciated.

[Enzo]

If your "tube failure light" came on, the first thing I'd suspect would be... yes, a tube failure.

[belias]

so let's do this

uh, yea. so the randall manual recommends 28-35mV for these tubes. I guess the questions i have are 1) are the readings on the other three tubes being altered because of the loss of one? 2) Are these the correct values with the meter at 200m DCV? D3) I noticed they're purchased in pairs-should I replace all four? or maybe just the first two (with the same kind)? 4)How do they come out? Twist, pull, what?
I'm not interested in going to a tech for something that I'm inevitably going to have to do again, maybe on a moments notice. If you're going to own a tube amp, you should learn how to change the tubes yourself, aye?

[black_labb]

tubes dont absolutely need to be rebiased (or matched for that matter) its a way to ensure the amp sounds as close to possible to the way it did before. the values recomended by the manual would be mA measuring current, not mV measuring voltage. those voltage valued you measured are the bias (im pretty sure atleast) and they seem a bit out, as they should all be closer, but that can be due to the tube. this may be well due to tube 2 not running, so get that replaced and then see if it fixed the bias voltage.

[belias]

I feel you, but these cats did it in mV
http://www.randallamplifiers.com/abo...M100manual.pdf
page 10-11...(unless I'm reading it wrong?)

so, basically, I unscrew the two little screws on each side of the tube and it comes out with ease? they recommend replacing the pair...so #2 and #1? And then I should replace the (#2)fuse as well. Then match the bios up as needed. Sound right?

[Alex R]

That amp has bias test points that give mV readings across a resistor, like modern Marshalls. Current through the tube is what is being registered in those mV figures so it's fine to follow the manual and think about mV when setting the bias, like the manual says.

The meter range only matters if you can't read what it says.

Yes those numbers are a bit high but not scarily so. They will change a bit with the new tube in, but I think you'll end up adjusting them down a bit.

[Alex R]

Our posts crossed...

Generally tubes pull straight out, don't think there are screws to undo.

The other tubes are functioning ok. If you have individual bias adjustments for each tube then I'd try just replacing one at first, see how it goes.

[black_labb]

That amp has bias test points that give mV readings across a resistor, like modern Marshalls. Current through the tube is what is being registered in those mV figures so it's fine to follow the manual and think about mV when setting the bias, like the manual says.

The meter range only matters if you can't read what it says.

Yes those numbers are a bit high but not scarily so. They will change a bit with the new tube in, but I think you'll end up adjusting them down a bit.

sorry for giving bad advice. this means you can set the bias yourself which is nice (once you get a new tube)

[belias]

alright cool...i guess I'll head out on my journey for tubes later in the morning.

Generally tubes pull straight out, don't think there are screws to undo.

they all seem to be in there pretty tight, though. I don't want to be overly forcefull and break them (especially with my hand). They have (what look like) round metal fastening devices around them, attached to the frame, with the two small screws. Maybe I can get a pic up somewhere.
btw, isn't it great how the manual gets you all pumped up to do this! "In the past you go to a tech, but now any idiot can do it" lol, we'll see. I'll probably get electrocuted or blow this thing up, lol. If you don't hear from me by tonight, look for a strange occurrence on the news.

[Alex R]

Hope your widow isn't litigious.

If the retainers have a screw that tightens around the base as well as a couple that go into the chassis, then unscrew the tightener. But if they are the much more common type of 'bear trap' retainer that just grabs the base of the tube with two semicircles that curve up and dig in, all you need to do is to push the semicircles down towards the chassis with your fingers to break their grip - bend them down a bit, that's ok - then pull out the tube.

[belias]

all you need to do is to push the semicircles down towards the chassis with your fingers to break their grip

DeNiro: "(pointing) you. you. you, my friend, are a genius."

That worked...thanks man. does it matter how they go back in? there are more holes than pins?

According to guitar center's web site, they carry these at the store by my house..anyone want to take bets?

[black_labb]

DeNiro: "(pointing) you. you. you, my friend, are a genius."

That worked...thanks man. does it matter how they go back in? there are more holes than pins?

According to guitar center's web site, they carry these at the store by my house..anyone want to take bets?

most music shops have tubes. there is a certain direction for putting the tube in. there is a keyed piece in the middle of the tube which fits into the right spot in the socket. have a look at how it goes in before you try to force it in.

[Enzo]

6L6 is one of THE most common power tubes there are, if Guitar Center has any tubes at all, they will have these. They don't have to be Ruby, though I think the Ruby 6L6GCSTRM is a good sturdy tube.

If you look at the 6L6 you pulled out, there is a plastic peg in the center of the circle of pins. Note there is a ridge sticking out along one side of this peg. Now look at the empty socket on teh amp. See the hole in the center for this peg to fit in? There is also a groove cut into the side of this hole. That groove is for the ridge on the tube peg. THAT is how they line up correctly. MAke sure your groove and ridge line up.

[belias]

Hi guys. Just following up. It took a while for the tubes to come as I wound up purchasing them online.
So there's a good lesson to be learned here, and probably an obvious one: The problem wasn't the tube, just the fuse. After I installed the new tube, the indicator light was still on. I moved the fuse over one spot and than that light went on instead. I should have been smart enough to try that originally. Oh well, the spare tubes will come in handy sooner or later.
So, question about fuses: The amp specifies a "250 mA Fast Blo" fuse. At radio shack I picked up a couple different kinds as they didn't have that specific one:
fast acting .5 amp 250v gma
fast acting .315 amp 250v gma
fast acting 2.5 amp 250v gma
Will any of these work? They didn't have anything 1/4 amp (as I'm assuming 250mA might be?)

[Alex R]

Well they'll all work but won't offer the same degree of protection against tube damage and (ultimately, and not very likely ) fire as will the recommended value. They are safety devices. The lowest value you have is the .315, that's not too much higher than the .25 that's recommended. In this particular situation I'd stick the 315 mA in and then take a look at mouser.com or mcm or somewhere similar for the 250 mA type, or rswww.com if you're in europe.

Sometimes fuses fail mysteriously but there is usually a reason. Tubes can short intermittently. Keep an eye out for another fuse failure.

Those individual tube fuses in your amp are quite a luxury feature, most amps don't have them, and they are useful. It is well worth getting hold of some of those 250s! (yes 250mA is 1/4 amp))