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Power tubes in Mesa Mark III


brzeczyszczykiewicz

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I have Mesa Mark III which has two EL 34 and two 6L6 power tubes. I have questions for you:

 

1. Can I use EL34 tubs in all 4 sockets? I'm reading in "OPERATING INSTRUCTIONS" that "the inner tubes

must be 6L6's while the outer tubes must be EL-34's."

2. Can I remove two 6L6 inner tubes and play using only two outer EL 34 tubes in Class A mode?

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Something is wrong with one 6L6 tube. When I switch Standby this tube starts glow very bright blue (like bulb) and I hear laud humming. Now I see she is damaged (near base) and I wonder is there any solutions because I don't have any 6L6 tube. I have only four EL34. Main question is can I remove all 6L6 tubes or not. In Class A mode this amp uses only EL 34 tubs.

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The tubes use different biasing and tubes should be matched and biased in cases where there are bias adjustments.

Do not swap tube types around. I blew a power transformer doing that by accident once. Its not worth doing several hundred dollars worth of damage just because you're hard up for a tube. Go on line to Tube Depot and get yourself a matching set and replace them and don't run the amp till they are replaced.

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From the manual...

 

TUBE SWITCH

This is the "Class A/Simul-Class" switch or the "60/100" watt switch, depending on amplifier type. In the Simul-Class amplifiers the

power (before clip) is much less in the Class A position - mere 15 watts. But the unique way the system is wired allows you to achieve a

much greater degree of power tube distortion than with most other amplifiers ... and it is still very loud! In Class A, only the outside

(outer left & outer right) pair of tubes - the EL-34's will be on. The inner pair - which are 6L6's - will also be turned on when the switch is

in the Simul-Class position. In addition to this obvious difference, the biasing and control circuitry of the socket pairs are radically

different ... and this is the heart of the now famous (and patented) Simul-Class circuitry. It is worth noting that almost all of the major

players who have gone over to Boogie have chosen the Simul-Class power design!

NOTE: In the past, MESA/Boogie advertised the "interchangeable power tubes" feature of the Mark III Simul-Class. We told players

that they could use either EL-34's or 6L6's in the outer (Class A) power sockets. However, in recent years we've observed a much

greater reliability factor with the use of EL-34's in these outer sockets, with much fewer incidents of tube failure. Therefore, today we

strongly recommend using EL-34's in the outer sockets and 6L6's in the inner sockets of your Simul-Class Boogie.

 

It seems that you might safely operate the amp sans 6L6s if you you stay out of simulclass. In other words plain class A won't power the inner sockets. As to using four 34s, what WRGKMC said.

 

If it's that much of a concern to you, use the telephone.

 

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I have just received the answer from producer:

 

[h=3]If the Mark III is a Simul-Class then the customer can use either 6L6 across the board or two EL34’s on the outside tube sockets and 2 x 6L6’s on the inside tube sockets. You never want to use all EL34’s across the board. Also the amp really wants to see all the power tubes in place so removing two of the 4 power tubes in a Mark III is not a good idea.[/h]

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