Mark IV Question

[Jak0lantern01]

As I understand, a Mark IV can run both EL34's along with 6L6's simultaneously. I've read the manual, but there's no indication of having to change any settings or switches when using this combo. I do understand that the EL34's would have to be the outter power tubes. How is this possible without a re-bias?

 

[t0aj15]

Read pages 12-13 of the Mark IV owners manual. It can be downloaded at Mesa's site.

 

[Jak0lantern01]

I read that already, and just re-read it, but it doesn't answer the question. Perhaps I'll simplify: how does it work? Every amp, including Mesa's, are subject to requiring the correct bias to operate properly. On the recto's, there's a switch to select between 6L6 and EL34. So, how can one just install two EL34's on the fly with no problems? I do understand that it works, I'm just wondering how.

 

[JPIndustrie]

I'm not exactly sure, but I assume it has to do with different bias adjustments with the inside and outside pairs of the power tubes.

Bob from eurotubes says that the inside tube sockets are puposely biased colder than the outside sockets, which facilitate the use of 6L6's in the inside sockets, while running EL34's in the higher/hotter bias settings in the outside.

Hope that helsp.

 

[visualrocker69]

That's what simul-class power is all about! It's designed so that EL-34's just...WORK in the Class A slots without rebiasing.

 

[Boogiebabies]

The bias supply is set at -57V. It's run through four 220K resistors which would lower the negative voltage a bit. From there, the inner two 6L6 sockets have a 2.2K resistor to pin 5. This lowers the voltage to -51V. On the outer sockets to pin 5 there is a 2.2 Meg resistor. This lowers the negative voltage to -39V. Since the the voltage has already been converted to negative off the low voltage tap on the power transformer secondary with a reversed diode, the more resistance you put in series with it will lower the negative voltage. Each pair of tubes has a different value of resistor to allow them to run different negative voltages. On a side note, EL34's do not necessarily run hotter due to the lower negative voltage. They require a different amount than 6L6's to operate optimally.

 

[jamme61]

It's a compromise, I was told that the older boogie MKIV's had a different value resistor in there to run the el34's a little hotter, but boogie told me that most people run 6l6's in there so they made it biased more close to match 4 6l6's. I found on my MKIV the el34's were running really cold compared to the two 6l6's in the middle. I messed with the resistor to run the el34's hotter (the techs at boogie helped me with this and also some guys here, thanks again). Still in the end I should have left it alone. It's a compromise but it works.