Mark III MYSTERY MODS - What could they be...... Help!

I have a Mesa Mark III Black Dot here, and upon opening it up to do some preamp tweaks i noticed..... mysteries! :shock:

Nothing too crazy really, but in addition to already being aware of the bias pot added (before i purchased the amp) - i saw something quite different in the power section. Let's just say it doesn't match the schematic -



While being decently proficient at soldering, i can say that i am no expert technician. I'm doing some of the cap mods listed on here for taming some of the fizzy-ness in the preamp section (C27, C30, C516, etc)... but i'm not well versed with the operation of power sections (other than how to drain caps, etc). For the sake of gathering data and learning more, i decided to post this on here and see what some the Boogie Board experts had to say.

From what i've been able to dig up this amp looks like it's been converted over to Pentode operation, seeing as no pins are wired together like the triode designs are. Would somebody be able to confirm this for me...? I just wanted to make sure it's done right and the values look healthy, even if the soldering job doesn't quite....





Notice how two of the leads from the transformer have been "taped up" (look where the zip ties and heat shrink tubing are). In regular Mark III's i noticed that each of these leads would be wired straight to pin 3 of each tube. Instead, it seems that only one of the green and one of the orange are used, while the other one is wrapped/taped up..... with the pins instead being jumper'ed across to it's neighboring power tube. Does this look right? Like does it match what say a Green Stripe pentode setup would be?

Also - since the knob is here anyways, i'd like to get the bias dialed in with my tubes as best i can. I've done the 1-ohm resistor shunt before, but heard there was an easier way with mesa's... measuring between Pin 3 and the center tap of the OT. Where is this on my amp? Things are changed so much from the schematic i'd rather just hear from an expert where best to measure from, based on these pictures... like which color wire would i be measuring from?



Any/all advice would be much appreciated. Im new to this forum, but have owned Mesas (DC-5, Mark IIB Coliseum, Blue Stripe Mark III) for several years now.... I'm very excited to start expanding more in the knowledge/operation of their design. Thanks!

Anyone??

I'm still trying to figure out why those two OT taps are not being used (one green and one orange). I actually figured out the center tap thing; and took a resistance reading between CT and the taped off green tap - it's 56 ohms. This actually is a close match to the other orange tap, meaning the one already going to the other set of tubes - which is 57 ohms. The green one currently going to pin 3 on those tubes is like 68ohms....

Would it be a good idea to use the other green tap (56 ohm taped off one) instead, since it more closely matches the resistance of the second set of tubes?

looks like someone disconnected the Simulclass OT taps and chose to run it as a straight 100 watter with 6l6gc's

lovetoboogie said:

looks like someone disconnected the Simulclass OT taps and chose to run it as a straight 100 watter with 6l6gc's

Click to expand...

Ok, cool. That makes a lot of sense.... It had 4x6L6's in there when i bought it. I thought somebody just went for that choice since they can technically run either (6L6 or EL34). I might weigh the pros and cons of this now -

Since you mentioned 100watts, does that mean i gained 25w headroom over the original configuration (15w/75w)? Like is there any advantage to this feature, to where I'm better utilizing the features of a 6L6 tube (higher breakup point, less compression, etc)...?

Not knowing of the changes, i had swapped all the power tubes out... mostly due to not being very fond of the 6L6GT's that were in it. I went with the Ruby 6L6GCMSTR for the center 6L6's, and JJ KT77's for the outer tubes. I chose the KT77's since they they seemed like a nice midway point between EL34 and 6L6 sound.

This explains why they only sounded good when the volume was up high, and like total **** when the volume was low. I don't suppose those outer tubes are getting the current draw they typically look for, from being wired to the 6L6 taps. I'm guessing to properly utilize these KT77 outer tubes i either need to run those extra taps.... or... just change out some resistors on the grid points?

gepetto33 said:

lovetoboogie said:

looks like someone disconnected the Simulclass OT taps and chose to run it as a straight 100 watter with 6l6gc's

Click to expand...

Ok, cool. That makes a lot of sense.... It had 4x6L6's in there when i bought it. I thought somebody just went for that choice since they can technically run either (6L6 or EL34). I might weigh the pros and cons of this now -

Since you mentioned 100watts, does that mean i gained 25w headroom over the original configuration (15w/75w)? Like is there any advantage to this feature, to where I'm better utilizing the features of a 6L6 tube (higher breakup point, less compression, etc)...?

Not knowing of the changes, i had swapped all the power tubes out... mostly due to not being very fond of the 6L6GT's that were in it. I went with the Ruby 6L6GCMSTR for the center 6L6's, and JJ KT77's for the outer tubes. I chose the KT77's since they they seemed like a nice midway point between EL34 and 6L6 sound.

This explains why they only sounded good when the volume was up high, and like total **** when the volume was low. I don't suppose those outer tubes are getting the current draw they typically look for, from being wired to the 6L6 taps. I'm guessing to properly utilize these KT77 outer tubes i either need to run those extra taps.... or... just change out some resistors on the grid points?

Click to expand...

6l6 is around 20W , 6l6 GC 30w, max output power! so he meant that any 4 6l6 gc is a over 100 watter amp potentially

gepetto33 said:

lovetoboogie said:

looks like someone disconnected the Simulclass OT taps and chose to run it as a straight 100 watter with 6l6gc's

Click to expand...

Ok, cool. That makes a lot of sense.... It had 4x6L6's in there when i bought it. I thought somebody just went for that choice since they can technically run either (6L6 or EL34). I might weigh the pros and cons of this now -

Since you mentioned 100watts, does that mean i gained 25w headroom over the original configuration (15w/75w)? Like is there any advantage to this feature, to where I'm better utilizing the features of a 6L6 tube (higher breakup point, less compression, etc)...?

Not knowing of the changes, i had swapped all the power tubes out... mostly due to not being very fond of the 6L6GT's that were in it. I went with the Ruby 6L6GCMSTR for the center 6L6's, and JJ KT77's for the outer tubes. I chose the KT77's since they they seemed like a nice midway point between EL34 and 6L6 sound.

This explains why they only sounded good when the volume was up high, and like total **** when the volume was low. I don't suppose those outer tubes are getting the current draw they typically look for, from being wired to the 6L6 taps. I'm guessing to properly utilize these KT77 outer tubes i either need to run those extra taps.... or... just change out some resistors on the grid points?

Click to expand...

I didn't previously see this post, sorry...

Like i said, someone simply wired this amp up to get maximum horsepower from 4 6L6GC's. The screens and the grid have new resistors on them(470ohm and 2.2k)...

With that said, you SHOULD NOT run KT77's in this amp...the screens, control grid, and bias matrix have been modded from the original simulclass design.

Without knowing if they rewired the simul/class A switch to now operate as a 100/60w switch by grounding the cathodes for the inner pair of tubes, then you are really getting no results by using two(2) different types of tubes in the inner and outer sockets...hence also why it probably doesn't sound good.

Albeit, the mod could have been done a little cleaner, if you just use four(4) 6L6GC's I bet this amp would sound fantastic at any volume. FWIW, I have been getting great results for some time now with Mark series amps using the Tung-Sol 6L6GC-STR reissue power tubes...

lovetoboogie said:

gepetto33 said:

lovetoboogie said:

looks like someone disconnected the Simulclass OT taps and chose to run it as a straight 100 watter with 6l6gc's

Click to expand...

Ok, cool. That makes a lot of sense.... It had 4x6L6's in there when i bought it. I thought somebody just went for that choice since they can technically run either (6L6 or EL34). I might weigh the pros and cons of this now -

Since you mentioned 100watts, does that mean i gained 25w headroom over the original configuration (15w/75w)? Like is there any advantage to this feature, to where I'm better utilizing the features of a 6L6 tube (higher breakup point, less compression, etc)...?

Not knowing of the changes, i had swapped all the power tubes out... mostly due to not being very fond of the 6L6GT's that were in it. I went with the Ruby 6L6GCMSTR for the center 6L6's, and JJ KT77's for the outer tubes. I chose the KT77's since they they seemed like a nice midway point between EL34 and 6L6 sound.

This explains why they only sounded good when the volume was up high, and like total **** when the volume was low. I don't suppose those outer tubes are getting the current draw they typically look for, from being wired to the 6L6 taps. I'm guessing to properly utilize these KT77 outer tubes i either need to run those extra taps.... or... just change out some resistors on the grid points?

Click to expand...

I didn't previously see this post, sorry...

Like i said, someone simply wired this amp up to get maximum horsepower from 4 6L6GC's. The screens and the grid have new resistors on them(470ohm and 2.2k)...

With that said, you SHOULD NOT run KT77's in this amp...the screens, control grid, and bias matrix have been modded from the original simulclass design.

Without knowing if they rewired the simul/class A switch to now operate as a 100/60w switch by grounding the cathodes for the inner pair of tubes, then you are really getting no results by using two(2) different types of tubes in the inner and outer sockets...hence also why it probably doesn't sound good.

Albeit, the mod could have been done a little cleaner, if you just use four(4) 6L6GC's I bet this amp would sound fantastic at any volume. FWIW, I have been getting great results for some time now with Mark series amps using the Tung-Sol 6L6GC-STR reissue power tubes...

Click to expand...

Cheers, LTB! I actually didn't see this either, since it sent me no reply notification for some reason... weird.

The Simul/Class A switch is indeed wired to the cathodes of the two inner tubes, thus achieving the 100/60w effect you described.

So would you have any suggestions for leaving this wired in pentode, but still being able to use kt77/el-34's in the outer sockets? I'm assuming the simul/class A switch would need to be changed back to stock for that purpose either way.....