Mark V adjustable bias mod

[luckneto]

Hi guys!

This is my first post, but I have been reading this board a long time ago….

I bought some NOS Sylvania 6L6GC and Siemens EL34 and I want to adjust the Mark V bias to optimal levels.

There is some adjustable bias mod for this amp?

Where can I get the schematics of this mod?

I searched the board and can not find I answer.

Thank you very much!

 

[IronSean]

It's certainly possible, and wouldn't be that hard to do.

The reason you probably haven't seen much talk about it though is that most of the Mark Vs around are still on their 5 year warranty and therefore not modded to maintain that warranty. Unless the switchable power modes also make it somewhat tricky.

 

[bandit2013]

From what I have found, any adjustable bias mod would only effect the 45W and 90W mode and not the 10W mode. The 10W mode runs two tubes in cathode bias format and the other pair remain connected to the grid bias and act as dummy loads (no signal to the grid).

The voltage divider network that is shared between the inner and outer tube is where the bias is actually managed. Inner pair are biased for Class A and the outer pair are biased for Class A/B. There are some similarities to the Mark IV and Mark III but with differences due to the 10W function and of course component changes.

It may be possible to find a 2009 schematic but the more up to date version is not available unless you are a Mesa tech. I have attempted to adjust the bias in mine only because I was killing tubes on the inner pair. Mark V was a red plate machine with the Mesa STR440 tubes. However, I was able to change the parallel resistor (82K) to adjust the bias enough to allow me to use the Mesa tubes and prevent instant red plate issues. Note I was killing tubes using variac mode and 45W setting on CH3. I no longer use the 45W mode but now it is safe to do so.

What I discovered with the 2012 model, the resistor is the same as that in the 2009 schematic. Do a search for mesa boogie schematics and a few sites will pop up. The one on "the tube store" site is the same as the one I had found a few years ago. R156, 82k sits next to the large power supply capacitors and is in parallel to a 33K resistor. If you plan on doing this mod, you may want to check the diode in the circuit as Mesa had a few parts during production that were not very good as there was a defect or issue with the manufacturer of the component. If you plan on placing a pot in there you should also a resistor in series with the pot so you do not allow for the negative voltage to get too low as this will definitely red plate your tubes. 1/2W rated will be safe enough if you can find one. A 1/4W may be okay.

Another reason you do not see much in bias mods for the amp is also related to the simul-class circuit. What will work for the 6L6 will not work for the EL34. Food for thought.

The obvious You will also need to use a bias probe on the inner tube sockets. The outer tubes will run on the cold side and there is no individual adjustment for that as the bias supply is global and the voltage divider on the power tube board remedies the bias for the outer pair based as well as the applied grid signal level. The key is the inner pair and if you know the current for a class A (note it is not a pure class A circuit as it is a push pull but at the same time it is not a Class A/B either). Available Power supply voltage based on the print is 448Vdc, screen voltage will be about 445Vdc.

One thing I need to get is a bias probe as I have no clue what is going on with my Mark V but so far I have yet to red plate a Mesa STR440 tube since I made a change from 82k to 91K.

 

[Wolfgar926]

but, you cant run el34 tubes on each end and 6l6 tubes in the middle. like a mark iii or mark iv. so, i dont see why the bias pot mod wouldnt work just fine. mark v you have to run all el34 or all 6l6 power tubes. i think if you just run 6l6 tubes all the time. theres no problem with installing a bias pot..the el34 sound like crap in the mark v anyway..lol

 

[notch269]

I am writing for the first time.
I installed a bias adjustment POT on my V.
This is August 2022 and it is working without any problems.

 

[domct203]

Welcome to the forum.

Please share how you accomplished this, I’m sure many MKV owners would be interested.

 

[notch269]

My MV was fortunately identical to the schematics that were available.
Schematics below.

I just modified this bias circuit as follows.

To be precise, I have also removed the LED and installed the check terminal.
With this modification, the bias voltage of my MV can now be adjusted from -65V to -28V.

The schematic shows the bias voltage as -51V, but the voltage before the modification was -60V for 6L6 and -44.75V for EL34.

If you are going to do this mod, you must first check that the schematic and your MV are the same.
And you must put on a protective resistor in case the adjusting volume shorts out.
And the adjustment volume may not have to be 50k ohms.

 

[mace]

Very interesting! So, do you still need a quad matched set of power tubes with this set up or can each pair of tubes be biased independently somehow?

 

[notch269]

It is best if you can prepare matched quad tubes, but two sets of matched pairs should work well enough.

The bias voltage cannot be adjusted for each pair.
And to do that we have to prepare a number of variable resistors.
The only thing we can do without changing the circuit is to add a bias balanced volume.

Note that this mod works well in both 45W and 90W in full power mode, but I haven't checked it in variac mode.
And I'm sorry my English is not good enough.

Thank you.