Time for new power tubes?

Hi all,

Last night my Lynchbox was sounding spongy... and then I started getting microphonic squealing and kind of a clicking sound when I was running at high gain. And this happened regardless of which module I was running through. Does this sound like it might be time for new power tubes?

I've always had a repair guy replace my power tubes with other amps but I understand it's supposedly pretty easy with the RM100. Do I need any special equipment to rebias? I'm using EL34s now, I think for this go round I'll probably stick with them, but I'm interested in trying some of the other configurations that others have talked about... KT88s for instance.

What do you guys think?

Peter

It's really easy to re-bias - all you need is a multimeter. I'm pretty sure basic instructions are in the Randall/Egnater owners manual, if not it should be pretty easy to find in searching this forum. I run two Mesa Boogie 6L6's and two JAN Philips 6L6WGC's (like 5881's), and couldnt be happier. Just grab a few sets of different tubes and experiment, since it's so easy to swap and bias on these heads. It literally takes longer to wait for the tubes to cool down enough to touch and pull out, than it does to install and bias a set.

Have fun!

Thanks so much! You gave me the courage to go out and get a multimeter and try it! And duh, I didn't realize when a power tube blew in this amp, an LED lights up in the back and a fuse blows, indicating which tube it is.

However I have run into a bit of an issue. For the second power tube, I'm not getting any reading at all on the multimeter. I thought maybe the new tube was bad, so I swapped it out for one of the originals -- and this one was one of the ones that was ok. However, still no reading!

Tried out the amp anyway and it sound great. However, I don't want to use it a lot without it being correctly biased.

Another issue that I ran into was the fuse on the back was labeled 400mA. However in the manual it says don't use anything but 250 mA! I figure the marking on the back is reliable. What say you guys?

I recall a couple posts on the forum stating the Bruce approved the 400mA fuses for the tubes. I keep 250mA in mine just in case... In any case, is the tube that your having trouble biasing lighting/heating up? If so, it's prolly just a misaligned bias test point, and just kinda squirm the multimeter tip around a bit in the hole :shock: . A couple of mine can be tricky too, so that's prolly all it is. If the tube isnt lighting/heating up, youve got something else going on. So, try the above first, and report back...

It lights up fine... plays fine... just can't get any reading on it. Wiggled the lead around as much as I felt prudent. Didn't want to get violent with it ;-) Still nothing.

Allright, well it sounds like the bias test 'test point' has either shifted from it's proper position, is corroded, dirty or filled with a foreign material, or has disconnected from the board (broken). I'd try to look in there with a flashlight to see if you can ID the problem. Clean with a little contact cleaner, and/or make sure there isn't something else hanging around in there. You should be able to see the metal contact. If you cant find it, or something seems off, you'll have to pull the chassis as the bias 'test point' and trim ring thingy are only accessible from inside.

Do you remember if you had to turn the bias 'pot' very far to get the bias right on the tube's pair (the other inside or other outside tube, depending on where this one is)? If you didn't have to turn it very much, the other one should be pretty close, but that's pure conjecture. If you're concerned about it, you can always pull the tube's pair, and run @ 50W until you get it sorted out. You can safely run an RM100 with only just the outer, or inner pair of tubes, making for a 50W amp (how I ran mine for some time, as I tend to prefer 50W tone anyhow).

Hope this helps.

Wow, that does help! I think I'm ok for the time being since the original tube is in that socket. It must've been biased at some point! But I may try the 50 watt idea... this tube is one of the inner pair.

Thanks so much for your help!

Peter