Rebel 20 biasing

I have several RM MTS amps so I understand how those work but, I just picked up a Rebel 20 last night. I was wondering if I open it up, is checking the bias just as easy on these? Are the contact points labeled and all that?

The bias test points are marked and the recommended bias voltage is printed on the PCB legend.

That was a better answer than I expected! Thank you.

Where do you plug the black lead from the volt meter?

WIRED":3alityhj said:

Where do you plug the black lead from the volt meter?

Click to expand...

You could just stick the black lead into the screw hole on the chassis,it's just a ground.

Really dumb question but I'm asking anyway...when you're biasing a Rebel 20 or 30, you do it with the power on, speaker(s) plugged in, guitar plugged in right? Or can you bias it while on standby? Or while it's off?

Turn it on, no guitar. Not on standby, let it warm up and take your readings and adjust with the trimpot if necessary. Be careful, there are lethal voltages in there. If you're not sure, let a tech do it.

First of all mad props & thanks to Danelectro & bbrodiefor the graphic and tips. That being said, the picture has the test points backwards. The 6V6 testpoint is actually the one that Danelectro has labeled "EL84", however it's kind of hard to screw up anyways because Bruce in his mighty wisdom has them labeled on the PCB as well as the correct voltages to bias to.
Now that I've got all that out of the way, I must say that biasing my Rebel 20 was a cakewalk. Went to the shack and picked up a digital multimeter for 20 bucks. Owned my Rebel 20 for about a year. Haven't swapped any tubes, still has stock JJ's in the power and soveteks in pre. Interestingly, when I checked the bias, BOTH the 84's and the 6V6's were a tad on the cool side...about about 36mv on the 84's and about 40 on the 6V6's. Makes me wonder if power tubes have a bit of a break-in period. Made the adjustment in a matter of moments and a great amp sounds even a little better. Thanks for this thread and all the help. Egnater rules!

Break in period and they drift as time goes on.

The drift is pretty pronounced in the beginning and tends to taper off as the tubes age. This is why I tend to bias amps twice for most of my customers... Once, when new tubes are installed, then again about a month later once they have had some play time, burn in, etc. After that, they seem to remain stable until the next tube swap.

Good tip, Check your reading with the existing tubes before installing the new ones. If you see a wide "out of range" reading, it could mean you have a component failure, and you wouldn't want to toast your brand new tube until this is corrected.

I use these test points mainly to get the amp in 'range'... then I put it on the oscilloscope with a sine wave to make sure I'm out of crossover distortion (all tubes are different)... typically, at that point, I will go just a bit hotter. I find the amp opens up more this way, and most people are willing to trade some tube life for better tone (especially clean tone). I understand the thought process behind cooler biasing, but, personally, I don't care if the tubes last forever if it sounds like sh*t... the tubes will last forever then, because I wouldn't play it.

It funny. Some of my amps are bias cold. Some are bias hot. I set it by sound and feel. What I want from that amp.

Here's a question. I went to bias a Rebel 20 tonight. I took initial reading with the old tubes, rather than risk the new set if something was wrong. The EL84/6BQ5 side read 9.9mV (40 is the recommendation) the 6V6 side read 13mV (45 is the spec)... Maxxing the bias trimmer only gave a reading of 20-22mV (far below spec)... Any idea what could be going on? My multimeter is a Fluke with fresh batteries.

That does sound far off.