Mark V adjustable bias mod

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luckneto

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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!
 
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.
 
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.
 
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
 
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.
 

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

Bias.png


I just modified this bias circuit as follows.

Bias adj.png


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.
 
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?
 
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.
 
This discussion is old, but I noticed that my MarkV doesn’t work in 10W mode at all like the description above by Bandit2013. The bias meters show the inner tubes with full bias current in 45W mode and the outer tubes with reduced bias current. But in 10W mode I see the two tubes nearest the rectifier tube getting full current and the right two with reduced current. This makes sense because we are operating in class A and only pulling one direction with the tubes.

I also see my new Tung Sol EL34 tubes running too hot, and I will need to make the bias adjustable too! Thanks to everyone for leaving their experiences and ideas. I found this all very helpful.
 
I forgot I even posted on this.

I did not describe the 10W power mode in any detail. That is good, never really studied it all that much.

10W mode disconnects the bias voltage from the tubes in use. It gets reconnected with some resistors added into the circuit: cathode bias resistors, 360ohm, 7W along with 1.6k (25W) resistors on each plate of the pair that remain connected to the phase inverter. The two tubes closest to the rectifier tube are in Class A mode. As for the other tubes, one has the cathode disconnected and floating, no current through that tube, it is the other outer tube. The other inner tube, disconnected from the phase inverter circuit but still connected to the bias voltage rail. Mesa described it as being used as a ballast to compensate the OT winding. I assume that is what they mean by using it as a ballast. It will have a bias voltage but no signal to make it do anything. So yeah, the other center tube will be drawing idle current. I was unsure if both tubes were tied to the bias voltage, they are, however the outer most tube does not have a cathode connected so it is open circuit in 10W power mode. Still, with plate voltage on the tube, heater active, there could be some current flow but doubt it will contribute to the overall circuit.

I am sure it is described in detail under one of the 12 patents listed on the schematic. Schematic also indicated patents pending so there may be others not listed.
 
For what’s it worth, when I do pre-amp tube rolling I use 10W mode and remove the two power tubes on the right (when viewed from the rear). I am able to swap pre-amp tunes while in stand-by mode and then power up right away. The Mesa guy I deal with said this will not harm the amp for trouble shooting (running with just the left 2 power tubes installed in 10W mode).
 
Wow, thank you both!

Bandit, thank you for your analysis of the circuit and for pointing out the patents. That will help me understand better. The thing about 10W mode was minor, and I just wanted to register it In case people read this thread looking for help as I did.

And Mace, thanks for the method for swapping preamp tubes. I can do it that way on the Plexi because there standby mode disconnects B+. In theory it would even be ok with power tubes, though the bias is still on, but I still let them cool first. But the Mark V is so much more complex that I don’t know how things work yet.

I am fixing two amps right now and learning about things, but if I can get my Mark V to sound good in the end I will be very happy. What happened is that after these years of having the Mark V combo, the low voltage diodes went, and I bought an old reissue Plexi and a beautifully modded Bassman with 5880 tubes in the interim. I realized then that I had never played a real tube amp. The Mark V has never been what it should have been. So I replaced the diodes and tried the Tung Sol EL34s and that is a big improvement, though they run too hot. So if I can fix the bias and I guess replace all the preamp tubes I’m hoping I will have a great amp.

Have people tried using tube adapter sockets to install four EL84 tubes in the Mark V? Or maybe set it up for 6V6s? I might like it better as a noise machine, and if I can get it down to 30-50W max then I can use it with a 4x12 greenback cab and have nice speaker distortion. Those extra dimensions of power tube and speaker distortion might transform this amp into something special.
 
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Some of this is off topic but may be of interest.
As for running the adapters for EL84, it may fit in a combo but not a head. No space. Mark V will not work with either (assumed) you could inquire about this through email to Mesa as they should be able to support the product as it is still in production.

In my opinion, the Mark V90 sort of stepped off the Mark amp norms, good or bad. It just does not have the characteristic I would expect for a Mark amp.

You lucked out, you bought a Mark V90. If you wanted to run 6V6, the Mark IV in tweed power (same as variac) was possible.

Since they upgraded the power for the extended class A circuits to 45W, that places the center pair in the hot zone. I assume that is in Pentode configuration. Ch3 can switch to triode so it is no longer running at 45W, more like 22W.

I had compared the Mark V90 to the Mark VII as it is quite different amp in terms of its power section. Not only does the Mark VII sound amazing (it better for that price) it has some similar traits to the Mark V90. At full power it pushes a good peak at 140W rms. Just like the Mark V90 and also similar to the JP2C (100W class A/B). Not a slouch in power by any means. Totally different preamp design that is well balanced on all modes as well as a good match to the JP2C in the IIC+ and IV modes. The Mark V90 just does not get there, too many preamp tubes and attenuators to compensate for the extra gain stages.

Anyways, at 90W both Mark VII and V90 are about the same power level wise. Most of the post gain distortion is due to the center pair in extended Class A operation. Mark amps will utilize power tube saturation and distortion, not exactly a Dumble but close enough. At 45W, again both are at the same power level. On CH3 of the Mark V, change from pentode to triode and you end up with a drop in power level down to 22W to 25W, well that is what the Mark VII does, no more 10W mode but has a 25W power mode, runs the center pair extended class A in triode.

You almost get a bit more with the Mark V90 since you have tube tracking for CH1 and CH2, pentode/triode mode for 90W or 45W (this only affects the center pair). Then there is the 10W power mode.

To be honest, I would rather use the Mark VII over the Mark V90. It just sound better and I Can do more with it. However, the V90 may need some slight mods but still working out what should be changed. I did have to adjust the bias just so I could run Mesa STR440 tubes, before the change it would red plate them in no time. SED =C=6L6GC or Tung Sol 7581 were the only tubes that could be used without instant death.

The Mark IV in general could run a mix quad of 6L6 and EL34 like the Mark III simul-class. It too had a pentode/triode switch for the class A tubes. Also, there was no bias switch for tubes, so running a full set of EL34 was not possible.

You can view the Mark IV manual on Mesa's website if this interests you.


EL34 power mode, this has some good effect but will run the screens much higher in voltage. Gold Lion KT77 did well at full power. However, the manual states when using EL34 power tubes, you should keep the amp in Variac power for longer tube life. Since there is no voltage regulation or suitable voltage drop (typically the screens will have Zener diodes to act as voltage drops to reduce the screen voltage relative to the plate). So, the reason for variac power use is to reduce the overall screen voltage and keep it at a safe operating level. Most amps that are designed for use of EL34 like the Badlander or Triple Crown will have zener diodes in the screen circuit. Once you convert over to 6L6, it sounds like it is lacking power. It is all due to the screen circuit.

Also, on the circuit of the Mark V90, there are two zener diodes that get pulled into the screen circuits when using 10W power mode. After looking at the schematic for the Mark V90, there is a lot of things going on for each power mode. Not easy to keep track of what relay does what.
 
I have good news about the El84 adapter sockets in the MarkV 90. I have the combo and so no height restriction, and now that I understand the simulclass setup I realized I can run two EL84s on the inside sockets and two EL34s on the outside sockets, in variac mode. The EL84s will get biased hotter and used for class A, but i think my old Groove Tubes adapters are a bit conservative and the tubes don’t seem to be overheating. 10W mode still works this way and is even more textured, which I like best on channel 2. And now 45W AB mode uses the two EL84s and it sounds great for channel 3 in triode mode. In that configuration it gets to the level of power amp distortion I want. I have 90W mode on the clean channel in the middle voicing with the gain turned up to 3 o’clock or so and lower master volume so all three channels are the same volume. The EL84s sweeten the sound, well beyond what I was getting with the EL34s alone.

I also read in the manual that I can safely mismatch the speaker impedance for special effects, and I found that I like to run my 8 ohm speaker in 4 ohm mode. I want to try this with my 4x12 cabinet also. Once I started getting sounds I like today the amp feels like a huge and unlimited playground like it should have been to begin with. I think in the past I was just struggling to dial out the disagreeable brittle-sounding character, and it felt like that limited the number of sonic places I could go.

I just played for a few hours and I think this amp is a beautiful thing finally, after years of not really sounding good. I need to check with the bias current meter, but I think I won’t need to adjust the amp’s bias now, because the outer tubes are running cooler by design and the inner tubes are somewhat protected by the adapter sockets. I don’t know this for sure, but my ears and eyes are telling me that all four tubes are close to where they should be.

The foot switch pedal has never worked right and it seems to now that I replaced the diodes in the low v supply! That is a big improvement. I didn’t like the 1N4007s they used in the low voltage supply because thought its 1A rating might be low for the application. I also thought the unnecessarily high 1000V reverse limit forces undesirable trade offs in other parameters like the forward voltage drop vs current. And I already had 1N4003 with adequate voltage rating and much higher current rating, so I used those instead. Please check carefully before following what I did, and know that there are higher current choices that also have high voltage ratings. There are also reasons that a diode with a larger forward drop might be preferred, e.g. the knee frequency of the circuit with the filter caps, or a need to limit the peak current charging those caps. I haven’t checked all those things (yet). The data sheet for the diode just tells us that all 1N400X parts have a max 1V forward drop, so it is probably not part of the design.

For people with Mark V 90 heads, I believe that EL84 adapters are available with a right angle shape. I‘ve seen them in the past. Otherwise a shielded right angle converter with the desired geometry could be soldered up from copper pipe and fittings, and then the tube sockets glued into the fabricated adapter with epoxy.

I still hear a small amount of the high frequency character that turned me off with the 6L6GC quad the amp came with. Once I validate my current setup I’ll try replacing all the preamp tubes with the Tung Sol 12AX7 tubes that everyone recommends. I’ll also try various other tubes for v1. If that doesn’t do the trick then I’ll keep experimenting and modding until I get what I want out of this amp. I feel the Mark V is becoming the great amp I had thought it would be, and I‘m rediscovering the things about it that made me choose it in the first place.

Thanks to all the people who have contributed here! It really did help me find a way to make my amp work as it should.
 
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