RT100H Power Tube Bias Questions

[cgergely]

I purchased a new Randall RT100H head a few weeks ago and I'm pushing a Marshall 1960a cab with it. I am playing around with the Bias on the power amp tubes and am a little bit confused about it.

My RT100H has only 2 bias test points so I assume that each test point is for a pair of tubes. Now I went to check the voltage on each test point and got a reading of 70 mv. From all the reading I have done 6L6GC tubes should be around 28 to 35 mv.

So why is my RT100H giving me such a high bias reading? Is it becuase the bias test point is showing the total voltage for the pair of tubes? 70mv divided by 2 equals 35 mv if im not mistaken? Is this correct?

Any help would be greatly appreciated

Thank you

Chris :?:

 

[FarT_Rhin0]

I have the same exact question but funny how I've been looking through the forum and no one has answered this yet.
I changed my stock Ruby 6L6GC tubes for a matching quad of JJ 6L6GC's and when I read the bias, it was at 106mv. my question is "is that the sum of both tubes current or the current going through each?"
I tried lowering the voltage many times, went from 95 to 90, then 80, even tried it as low as 44mv (that's as low as my bias pot would allow me to go) but now I'm back at 64mv, assuming that 2x32mv = 64! lol
there was no way I could reach the 23-35mv as stated in the manual.
so what am I doing wrong here? any help would be greatly appreciated.

 

[karpathion]

I just recieved a 503 last week. If you go by what the biasing instructions say, mine was biased way too high, too. I was reading 50mv on each tube.

If you go by the instructions, the 503 with 6l6s should be at 28mv-35mv. I dropped mine to 31mv.The amp opened up bigtime. It sounds nowhere near as compressed.

It does make you wonder though. Is the stock bias wrong? Or, is the manual wrong?

 

[FarT_Rhin0]

Well, I found another RT100 today (sealed and unopened) opened and checked its bias. the amp wasn't even biased. one pair would read 36mv the other 78mv! so much for factory bias.
mine wouldn't even go lower than 44mv (the pot wouldn't turn)

 

[karpathion]

Well, I found another RT100 today (sealed and unopened) opened and checked its bias. the amp wasn't even biased. one pair would read 36mv the other 78mv! so much for factory bias.
mine wouldn't even go lower than 44mv (the pot wouldn't turn)

From how I read the biasing instructions, I think you are right about the 64mv on each test point, for the rt100.
I've found a few other guys who have found the exact same thing in their RT series amps. The stock bias is just way different from what the instructions say they should be. Always a lot higher.

I guess the best advise we could give to anyone purchasing one of these amps is, Check the bias as soon as you get it.

I like the 503 I have to pieces, OD1 just does it for me. The only other problem I've had is a broken connection on the output of the reverb tank itself. It took about 10 minutes to get the tank out, solder the wire back, and put the tank back in.
I just wish I could have a little verification from Randall, that the stock bias has been wrong, or the instructions have a little glitch in them.