First practice: amp died, help analysing problem?

[Slashpepper]

Right, so I bought a second hand RM100 off ebay and it seemed to be working okay at home (quietly), took it to my first big band practice to turn it up and it worked for about 2 hours but then cut out and the fuse lights for the two outside tubes lit up.

They're apparently brand new tubes so I didn't think they'd blow and they still glowed... surely if the tube blew it wouldn't glow at all anymore?

I had a quick look at the time but just had another look now (just got back in) and it looks like the Ohms switch is on the wrong setting, 16 Ohms on the amp instead of 8 Ohms which the cab is. I'm sure I checked it at the time and it may have got switched on the journey back, but if they were mis-matched, is that something that may have blown the fuses but not the tube?

I'll get my hands on some new fuses soon but I don't particularly want to replace it and not find out until the next practice that it's gonna do the same thing after a while.

Also, would that have changed the tone at all? (Load mismatch) Not sure if it was my guitar but playing through the Ultra module it was really difficult to have it loud enough to hear myself without squealing and feeding back horribly. Had to turn the gain right down, but there were hardly any mids even with the mids and treble maxed out (which didn't help the squealing, tried all sorts of levels)

It's one of the early Ultra's with the black stripe.

I did not have a good practice I must say... So annoyed.

Thanks guys.

 

[kc2eeb]

The tube most certainly can be blown and still light up.
The fuses that blow and light up the leds are on the cathode circuit
not the heaters (the parts that glow)
Don't use those tubes, or at least until you can check them.
A 16 to 8 ohm mismatch is usually not enough to blow fuses or even make the output transformers unhappy.
So, set the impedance switch for the speakers you're using and try new,
tested tubes.

 

[JKD]

Did you check the bias was correct for the tubes you were using - before you were playing and after the amp warmed up a bit?

Be sure and do this after you've replaced the tubes/fuses.

Older RM100s had under-spec'd 250mA fuses in these slots too...you can increase the value but I forget what to...375ma I think...might have to search.

 

[JKMV12]

for the fuses i read somewhere that 500mA works and that is what i have been using. But i believe Bruce recommended 400mA.

 

[Slashpepper]

So for one possibly try higher specced fuses. It definitely says 250mA fast blo fuse only on the back. The modules in it are from 2002, so I'd imagine it was built around a similar time (not sure how you can tell?)

How can you test if the tube is okay or not? The tubes weren't put in by me and my volt meter is broken so I'd assumed they'd done it right. Perhaps not the best idea, but they're E34L's so I would have thought if anything they'd be biased cold, but who knows.

Good to know about them still being able to glow when blown. Useful info thanks. Wasn't sure how they were set up.

 

[Slashpepper]

This may also be a stupid question... but how do you replace the tube fuses, I can't seem to get them out in the way that I'd expect.

Edit: Ignore me, I wasn't paying attention. The fuse for tube no. 4 is realy blackened inside the glass :-s

 

[Julia]

EL34s and EL34Ls are some of the worst for reliability. I went big glass. Ruby 6550C bias at 39 mV.

Order some 400mA Littlefuse on Amazon.com. That's what Bruce recommended, not 500 mA.

 

[Slashpepper]

Apparently we don't have Littlefuse brand on Amazon.co.uk, but there are quite a few 400mA Quick Blow fuses, I'll give them a go.

I'm still reeling slightly from the initial purchase, so I'll stick with E34L's for the moment (so I only have to buy 2 at the moment, still have lots of other things I need to buy :-s But I'll look into alternatives at some point)

 

[kc2eeb]

If the 250ma fuse blackened the glass when it blew, it makes it more likely that the tube is really defective. If you put the 400ma fuse in and it blows,
the tube is no good. Don't then try another 400ma fuse or worse a 500 ma fuse. Most times, when a fuse blows that is a little under rated for the current, it will just open without a lot of blackening of the glass or you may even still be able to see part of the fuse wire inside. Most tubes you can use in the RM100 will not draw over 250ma cathode current. (A KT88 nominal
Cathode DC current at 800 V is 230ma.)
So, the 400ma give a good margin for peak surge current.
Hope this is helpful.

 

[Julia]

New tubes can be bad right out of the box. Same with NOS tubes.

 

[Slashpepper]

If only everything new was guarunteed to work!

The ebay seller (being a proper store) has offered to pay for it to be fixed anyway, so I'll just take it to the local technician. At least then I'll know everything works as it should then and not have to worry.

This is still really helpful for the future, thanks guys. Gonna have to start trying out different pre-amp tubes as well, I have a feeling they could all be nearing 10 years old...

 

[chunktone]

When I got my RM100 it needed repair. I brought it in to the local Randall repair shop. When I went to pick it up, they told me the plate voltage was over 600 volts, and recommended putting in different power tubes. It had el34's, and they said alot of current production el34's cannot handle this much voltage. If your amp is sending as much voltage to the tubes as mine does, it could be the tubes are not able to handle it. I was told to switch to 6l6's or 6550/kt88's.

 

[bruce egnater]

If they measured 600 volts, it sounds like the voltage setting on the AC inlet may be set to 100V. Check this ASAP !! This is a bad thing.

 

[chunktone]

If they measured 600 volts, it sounds like the voltage setting on the AC inlet may be set to 100V. Check this ASAP !! This is a bad thing.

Hey Bruce!
I checked the input and it is set for 120v.

 

[Slashpepper]

It's still sat in the repair shop, so not sure what's up with it yet. 600V? My my. Still crossing my fingers for just a bad pair of tubes!

 

[chunktone]

It's still sat in the repair shop, so not sure what's up with it yet. 600V? My my. Still crossing my fingers for just a bad pair of tubes!

Ya, that would be the easiest fix. Hope it works out for ya.

 

[bruce egnater]

You should tell the repair to check for that because it may not be something they think of. The 600 volt measurement will have nothing to do with "bad tubes". Don't wait to tell them. Are you in the US?

 

[Slashpepper]

I think we're confusing two different things; I've never measured 600V on my amp, that was Chunktone...

And I'm in the UK.

 

[Slashpepper]

Gah, got the amp back from the repair shop and it still doesn't work... So annoyed. Will be taking it back again... Just makes a strange rustling type sound, like really distorted guitar but, weird. Im guessing a loose wire somewhere? I tried different cables etc and both effects loops just in case to try and narrow it down and the speakers seem okay.

Do you have to wire up an effects loop even if you're not using them? I saw a post earlier that said theirs worked without but I thought you weren't meant to leave them empty...

Was really excited to get to play it again too :-(

 

[Slashpepper]

Urgh, update: It's the cab that's gone... So presumably that blew and then that's what broke the tubes.

So annoyed.

I noticed sound was only vaguelly coming from one speaker so tested it with a little portable amp and it made the same noise as the Randall.

Anyone have any hints on repairing the speaker cab? I'll take it apart in a bit and see if there's any loose wiring that's obvious. I don't mind taking apart these because they won't kill me if I touch the wrong bit!