Bias insanely high?

[Deformed]

Hi!

Just got my RM100 (with recto and XTC atm, clean is for wussies :twisted: ) and played it for a while. At first, it sounded crap - straight forward not any good. Waaaayy to muddy. This was with a ported 2x12" loaded with legend V12's. Then, I tried it with a ported, supersized 4x12 with v12-60's and some custom peavey black widows, and all of a sudden it was spectacular! Amazing oomph, nice and articulate but with bone-rattling, head-smashing low end, perfect grind and chug Kinda weird though, since my ENGL fireball sounds wicked with both the 4x12 and the 2x12 cab...

So, I decided to swap tubes. Never liked ruby, so I swapped the whole lot with JJ ecc83s in modules and amp, and 6l6 gc in the poweramp section. I flipped the amp on and plugged in my multimeter, and the bias for the output tubes was measured to about 75-80mV :shock: I reduced it to 35, and started playing. Now the amp sounds great with both cabinets

The thing that now bakes my noodles is: was it the tube itself that made the improvement, or could the entire change be because of the bias? If the bias was set to 75 with the el34's, that must have been way too much? And it really bugs me that I didn't measure the current before I swapped tubes

 

[Chad]

Yes, I would think a bias setting that high would greatly reduce the life of the tubes. If you're wondering what caused the tone change, the easiest thing would be to put all the original tubes back in and see how it sounds.

I've heard of bias being off many times on these amps. However, it seems usually that people are complaining that the tubes were biased too cold instead of too hot. I don't know if this happens at Randall or not.......could happen in shipment from vibration.

Whatever the case, Randall (if they're not already doing this) needs to put a WARNING hang tag on the amps that advises users/dealers to consult the manual and make sure the bias is properly set before first using the amp.

 

[Deformed]

Whatever the case, Randall (if they're not already doing this) needs to put a WARNING hang tag on the amps that advises users/dealers to consult the manual and make sure the bias is properly set before first using the amp.

Agreed, if it's the case with many of these amps. However, I think it's kinda weak that your newly purchased amp won't be playable until you've made some adjustments. Not everyone has a multimeter lying around.

Anyone know where these amps are put together? The cardboard box mine came in says "made in china", but that could be just the box and not the amp. It seems to have been put together in a hurry, at least the modules. The tubes were barely in their sockets, they were like halfway in and at an angle. Don't know about the tubes in the rest of the amp, I told the dealer to remove the tubes before he shipped it. It's a shame that such a great sounding and looking amp should be victim to these minor imperfections

 

[RD/MAN]

Deformed,

Any time that you change the power tubes they will need to be rebiased to that particular amp. So the EL34's would not have been set to same bias setting as your new 6L6's where when you first plugged them in.

Also, the different in tube types will give you a pretty big tone change. The EL34's have quite a bit more low mids, so they will feel softer and muddier, where the 6L6's have a much flatter voicing causing them to sound clearer and more defined.

We actually like the 6L6's better in the amp as well, that is why most of the modular amps now come with JJ 6L6 tubes.

Hope this explains it a bit better.

Thanks!

 

[Deformed]

Agreed, the jj 6l6 gc definitely kicked the amp up a notch. But, you're saying that if I had measured the bias current when the el34's were in, I wouldn't have seen the same values? The bias wasn't set too high from the factory, it was just that the 6l6 tubes affect the currents inside the amp differently, and therefore would provide higher readings?

That's actually good to hear

 

[RD/MAN]

Correct. Any time you put a new set of power tubes in an amp the bias will have to be adjusted to those particular tubes. However, the bias readings on the power tubes will float a bit, so you should checked it occasionally as it will mostly likely need to be adjusted. We recommend setting the bias on the 6L6GC's to around 40mVDC. Something close to that will give you a good tone and long life span out of the tubes.

Thanks!!

 

[scotty]

I bought a used RM100 with 6l6s and they biased at 80mVDC which I quickly changed.

 

[lucidness]

I noticed on my RM100C that after a few months of playing my bias was high, like 50ish. Its nice to be able to check and re-bias your tubes from the back panel as your tubes age.

cody