Mesa Tubes Bias help

[BrentSSL]

Okay so I bought a Mesa rectoverb head series 1 from my local Guitar Center recently. The guitar amp has been modded slightly it has an el34 6l6 aftermarket switch. It also has a jack labeled vintage modern on it which could be a foot switch according to Mesa. The guitar amp did not have Mesa tubes in it. However it shows no sign of a bias mod except for the switch. They say made in Russia el34 0306 and I think they are Svetlana electron tubes. When I took it apart I could not find a bias pot. Are those tubes within the Mesa fixed bias range? Or are they possibly just thrown in there to work? Currently everything works fine but I'm more of a fan of 6L6 tubes I would like to put a matched pair of Mesa tubes in there. Any help would be appreciated.

 

[kurtsstuff2003]

Well..Mesa does not make tubes and they just screen bulk tubes for different bias parameters and slap "Mesa" on them for their amps.
If you know how check and adjust bias (Mesa's do not have an adjustment as you know) so just check in this case. The switch is probably just going back and forth between 2 different bias resistors. It's probably fine if nothing is blowing up or glowing red etc,etc..

 

[311boogieman]

They were probably just thrown in.

I wouldn't worry about it too much. I have a Series 2 Rectoverb and I pop different kinds of 6L6s and EL34s in there all the time. If you want to be certain as to your next move you can email Boogie and ask them what they recommend as an acceptable bias range for 6L6s in your amp. Then you'll know. Then you can shop around and ask for tubes within that range.

 

[BrentSSL]

OMG thank you did not think of that I figured it would have been posted on their website somewhere.

 

[psychodave]

OMG thank you did not think of that I figured it would have been posted on their website somewhere.

The boogie board has a list of the colors. Typically there is around 7-10mA between color grades. Regarding 6L6’s (and I’m going on memory) Red is the coldest then yellow and green. On my tube testers @400 volts, reds are around 14-19mA. Yellows are 19-26mA, greens are around 30mA. I just translate this with other tube brands as needed.

 

[Snave]

Okay so I bought a Mesa rectoverb head series 1 from my local Guitar Center recently. The guitar amp has been modded slightly it has an el34 6l6 aftermarket switch. It also has a jack labeled vintage modern on it which could be a foot switch according to Mesa. The guitar amp did not have Mesa tubes in it. However it shows no sign of a bias mod except for the switch. They say made in Russia el34 0306 and I think they are Svetlana electron tubes. When I took it apart I could not find a bias pot. Are those tubes within the Mesa fixed bias range? Or are they possibly just thrown in there to work? Currently everything works fine but I'm more of a fan of 6L6 tubes I would like to put a matched pair of Mesa tubes in there. Any help would be appreciated.

Those sound like SED Winged C EL34s which are really good tubes. I suggest you buy a multimeter and octal bias probe (the Eurotubes one is good for $28). Then you can calculate the dissipation of whatever amp you want, even if it doesn't have bias points. Around 40-70% dissipation is acceptable.

Set your multimeter to 600V DC (around 11:30 on the pictured dial) to read plate voltage, and 200mA (3:30 on the dial) to read cathode current. You want to always set the multimeter to higher than the expected value. Divide output of the tube type (25 watts for EL34) by the measured plate voltage, then multiply by 1000. Divide your measured cathode current by this number to get dissipation.

OMG thank you did not think of that I figured it would have been posted on their website somewhere.

The boogie board has a list of the colors. Typically there is around 7-10mA between color grades. Regarding 6L6’s (and I’m going on memory) Red is the coldest then yellow and green. On my tube testers @400 volts, reds are around 14-19mA. Yellows are 19-26mA, greens are around 30mA. I just translate this with other tube brands as needed.

You're right about those three color codes. They go from coldest to hottest in this order:

Red
Yellow
Green
Gray
Blue
White

This aligns with my experience that a set of yellow 6L6 STR440 biased at 25ma (~40% dissipation) and a set of grays biased at 31ma (~50% dissipation) in a Dual Rectifier (bold/silicon setting).

 

[JMP2203]

what about Transconductance and Plate current in the colors codes? any info in that?

 

[Snave]

what about Transconductance and Plate current in the colors codes? any info in that?

Mesa aren't very forthcoming about the specifics of their tube matching process. Whatever metrics they're using, tubes are matched within "plus-or-minus 5%" of each other in each color code.

 

[BrentSSL]

A little update had the switch mod removed it was a mod that bypassed a resistor and jacked the voltage up for El34s. It was actually starting to burn some things out. A matched pair of Ruby's and one new preamp tube later the thing sounds very close to my rack mount dual.