RM100 Blows power fuses often

[JSuttie]

Greetings, all, from Durban, South Africa.

I've had my RM100 since Dec. 2009, and it's been blowing power fuses under certain conditions for ages.
When it's been standing for a day or two, it starts up without fault. If it's turned off and on again within what seems to be 8 - 12 hours, the power fuse pops. It's never popped one while running, switching over from standby to play, etc., only on initial startup.
I bought 2A fuses as a precaution, as I couldn't find 2.5A fuses and I won't risk going over factory spec.
I always kept the amp very accurately biased to 38mV with the standard 6L6 tubes it came with. I'm meticulous with things like this, and I've always tried to keep the amp in the best condition possible.

The biggest part of the problem for me is that I don't know of any tube amp techs out here, so "take it to your local amp guy" isn't an option for me. The agents for Randall also don't have competent support for tube amps out here, so they can't do much either.

So now I'm here. I discovered your forum this weekend and I really don't know why I didn't look for it sooner - this looks like the most epic collection of MTS info out there!

Anyway, if there's anyone out there who may know what's wrong with my amp, I'm really keen to hear your thoughts.
I'm fairly confident I can fix it myself, as I've designed and built guitars from scratch, wired up many industrial machines, and worked for a fuel injection specialist finding and fixing circuits for a while.
Never opened a tube amp though, and my policy is to check with those who know better than I whenever it's possible.

Thanks for reading,
Josh Suttie

 

[JSuttie]

I forgot to mention - here we run on 220V @ 50Hz in theory, but a check with a multimeter over a few hours usually reveals a true output of anything from 215V to 239V, so I set the amp to 240V to be safe. I'm planning on running my amps off a pure sine wave inverter in the future though.
Yeah, we have one of the least reliable power companies in the world...

 

[webrthomson]

Hey and welcome.

The RM100 manual states 28-35 as the accepted range for 6L6's, see here:

http://geartunes.s3.amazonaws.com/gear/owners manual/RM100manual.pdf

Also I would set your mains voltage correctly as there is a range that it is supposed to work for. I don't know what that is for South Africa but in the UK its 240VAC + 6% and - 10% which makes the actual range from 216VAC to 254VAC.

Now I've never see it at the extreme of either end but my house supply is normally about 247VAC.

At a guess if you are biased hot and supplying too much voltage I'd guess the fuses would give out now and then

 

[JSuttie]

Thanks man. I'll re-check the bias when I have more fuses. I must have made a mistake above - I made sure the bias was within the range listed for 6L6 in the manual. Our power is always up and down because of illegal connections on the power grid, poorly maintained / completely neglected power plants, and something called "Load shedding".

Do you know of anyone on here regulating/rectifying their power inputs?

 

[Jaded Faith]

I'm not in front of an RM100, but what's the AC Mains fuse called for on the rear of your unit? If it's 2.5A and you're using 2A or a fast blow, that's the issue. The reason it's blowing on initial fire up after being off for a length of time is what's called "inrush current" as the filter caps are receiving their initial charge up. You can view this using an ammeter. My money is on a fuse issue. If not, it's either a filter cap or rectifier.

 

[JSuttie]

It's meant to be a 2.5A fast blow according to the manual, but I only had the option of 2A or 3A, so naturally, I went lower to be safe. I've just got a bunch of the correct fuses and it started up fine. I had one tube go (I think the cat must have bumped it or something because it's cracked in half) so I pulled it along with its fuse and the fuse for its partner.

The amp is busy running right now and I'm about to bias it for the first time in ages. It's been standing since last year.

Thanks for that - that seems like the most logical thing I've heard yet. Some local guitarists have strange ideas about tube amps...

 

[Jaded Faith]

No problem happy to help. I just demonstrated the concept to a young fellow yesterday in my workshop with his 5150 III. He was pretty impressed to see that the initial surgery went up as high as 2.7A before settling at 1.5A during normal operation a second or two later.

 

[JSuttie]

OK, so I fired it up for the first time in ages this weekend, and everything was fine. I checked the bias, which I actually had set to 33.4mA, not 38. I re biased down to 31mA to be safe, as we've had all kinds of wild power fluctuations recently.

The fuses seem to have been the issue. Either that or it was the tube that was cracked. I tried turning it off and on again within about a minute, and when this would have caused the fuse to blow very violently in the past, it just started up calmly.

I like the fact that I can crank the volume a bit more now it's running 50W only, It sounds really sweet!

 

[webrthomson]

Good news - glad that got sorted for you!

If you are running two tubes I *think* you need to change the impedance into the cab - can't remember the exact details hopefully Rob from JF will jump in!

 

[JSuttie]

Yes, it gets halved. I kept the cable in the 8 ohm sockets and flicked the switch over to 16 ohms. Glad I found out about that - it's not mentioned anywhere in the manual.

 

[Jaded Faith]

I'm not sure if I am misreading your post. You are correct that the impedance is halved if pulling two tubes. If you have an 8 ohm cab, the head should be set to 4 ohms not 16.

 

[JSuttie]

Oh ****. Of course. Can running it like that have damaged anything? I didn't put it too loud because I was playing at night, and I didn't play for too long either...

 

[Jaded Faith]

You could have, but it's also likely you would have noticed it already. Just set it correctly and play a while at lower volumes. If you don't hear anything obviously wrong you dodged a bullet.

Also FWIW, there is little to no reason to pull two tubes. Running at 50W is insignificantly quieter than at 100W and all you really lose is a touch of headroom.

 

[JSuttie]

Really? It seems a little quieter, but maybe that's got to do with the impedance. I didn't hear anything out of place when I was playing, but I'll give it a play with the switch on 4 ohms a bit later.
The reason for running 50W was that one of my tubes was broken when I took the cover off the amp last Sunday. I pulled the broken one and the fuse for its partner. I'll only be able to get a replacement pair next month.

Thanks again for your advice! You probably saved my amp from taking actual damage.

Slightly off topic - what's the story with putting a patch cable in the effects loop? I saw it being mentioned a few times, but I don't see why it would make a difference.

 

[Jaded Faith]

Running a jumper cable in the parallel loop and using the effects knob on the front will run the signal through the tube that buffers the loop circuitry. It will add some hype to the signal but is something you want to use sparingly at louder volumes. You may find it enjoyable for low-volume playing.

 

[JSuttie]

Oh cool. I'll definitely give it a try then. Most of the time, I'm playing at night when the family's asleep, so I usually don't get to crank the volume too often.