Mesa Boogie Power Tube Specifications

[Riki182]

Hello everyone,

I've done some searching on the internet and couldn't find any respectable official source to answer my question, so I'm writing here in the hope that someone can shed some light on it.

I know that Mesa Rectifiers do not have a bias adjustment switch, so the replacement tubes for the e.g. Dual Rectifier need to conform to Mesa power tube specifications so that the amp could run optimally.

Now, from some sources which I won't mention here, I obtained the information that for -40V grid voltage at 450V plate voltage, the plate current should be about 30-35mA according to Mesa specifications. Is this information reliable and where does it come from?

I've bought replacement tubes for my Dual Rectifier from TAD - the 6L6GCM-STR REDBASE. TAD provides documentation for those tubes and the documentation contains the following graph:

So, from this graph we can see that for -40V grid voltage at 450V plate voltage, the plate current is about 55mA. Judging from this, it seems that the 6L6GCM-STR REDBASE power tubes are not a very good replacement for the stock Dual Rectifier Mesa tubes.

Is my assessment accurate? Any information would be appreciated.

Many Thanks

 

[Fiveband]

Do you have a Bias Rite or similar meter? Testing in-amp with a few Mesa tubes and non-Mesa tubes is probably the best way to get the answer.
MaxiMatcher also allows testing at 325V and 400V plate, with -36, -48, and -60 bias volts. It will show plate current. You could make a simplified curve for each tube.
I'd bet you will see wide variations.

 

[Riki182]

My understanding after a bit of more research (which might be wrong but someone more knowledgeable could answer that) is that because of the fixed-bias design, the bias circuit restricts the current to a safe and controlled range (20-40mA for Mesa Rectifier Amps) because of the amp’s circuit design, including transformers, resistances, and operating points, which impose external constraints that restrict the actual current flowing through the tube. The graph shows the maximum theoretical plate current the tube can draw with no external constraints.