Factory Mods to the Mark series

[mark1406]

not to be too critical, but I’m a bit surprised that people here aren’t across the various factory mods to Mark series amps.

My poison is the Mark 1 amps as I own an SOB Boogie.

There have been a few incarnations of the Mark 1. One amp was the Kingsnake amp. I think they replaced the 10K midrange pot with a 100K pot, from 1-5 the pot acted similar to the 10K pot, (due to audio taper of pot) but from from 6-10, the tone stack is less effective in the circuit.

They also had a power switch lowering the B+ so 6V6’s could be used.

I have heard of reverb mods, and there are various versions of PI circuit.

Regards

Mark

 

[James]

There's the FX loop mod Mike B offers for the IIB and earlier (I'm not sure if he still does)

It adds an additional volume control (post FX loop, while the stock master acts as a send level) and the sound changes a bit. One of the best Mesas I've played was a MK 1 with this mod.

I believe the MK 1 reissue has something similar, but it's not always active - it requires using a jumper cable in the FX loop.

I've also seen one or two original MK 1s that were modded to Simul-Class spec in the 80s.

 

[mark1406]

That’s interesting that several Mark 1 amps have had post production factory mods.

This is the Boogie forum, there should be a lot of circuit info on the Mark 1 amps and MB should be cool with it.

Regards

Mark

 

[James]

I don't think there were many - the ones I've seen were done in the 80s. When I asked Mike B about the loop mod for the MK 1, he seemed a bit hesitant to modify an original due to collectability

Here are the instructions for the FX loop mod explaining how it works. It can get some different sounds depending on how the master 1 and post FX level are set. They seem to interact.

 

[bluebug]

Very interesting information as I have an effect loop installed on my Mark 1, and I always wondered if it was done by Mesa or not. Some of the components used for the FX loop mod look quite different from the ones used on the remaining of the amp, so at first I was thinking not Mesa, but I'm not quite sure anymore. Do you know more about that? I can certainly say that using my effect loop and playing with different settings of the effect loop pot does impact tone in an interesting way.

How did you get that information about interaction between the FX level and master 1, do you happen to have an amp with it at some point?

Fred

 

[mark1406]

Very interesting information as I have an effect loop installed on my Mark 1, and I always wondered if it was done by Mesa or not. Some of the components used for the FX loop mod look quite different from the ones used on the remaining of the amp, so at first I was thinking not Mesa, but I'm not quite sure anymore. Do you know more about that? I can certainly say that using my effect loop and playing with different settings of the effect loop pot does impact tone in an interesting way.

How did you get that information about interaction between the FX level and master 1, do you happen to have an amp with it at some point?

Fred

Hi Fred

I had a Fetron in my Mark 1 amp. As luck would have it, I lent it to a friends brother who returned it with a blown Fetron. I didn’t get annoyed about it as these things happen, but I was annoyed as he didn’t offer to contribute financially or tell me that it happened.

I then bought another Fetron from Mesa and installed it per the instructions. It didn’t work, I wrote back to them and was advised to put 100K plate resistors in and 1.5K cathode resistors.

With regard to your effects loop, I think it wasn’t done by Mesa Boogie as the caps aren’t orange drops. One would think they would have stacks of them about to build amps. Of course I could be wrong. The design also appears to be different from the reissue design but that doesn’t mean a lot.

Regards

Mark

 

[James]

Very interesting information as I have an effect loop installed on my Mark 1, and I always wondered if it was done by Mesa or not. Some of the components used for the FX loop mod look quite different from the ones used on the remaining of the amp, so at first I was thinking not Mesa, but I'm not quite sure anymore. Do you know more about that? I can certainly say that using my effect loop and playing with different settings of the effect loop pot does impact tone in an interesting way.

How did you get that information about interaction between the FX level and master 1, do you happen to have an amp with it at some point?

Fred

I saw yours - I don't think it was installed by Mesa. If I remember correctly, the one I mentioned didn't look quite as busy and had fewer components. Mike's initials were also on the inside.

Is the added pot always active, or does it only work when the send/return jacks are connected?

I have a IIA and IIB with that mod and know someone with the MK I. The MK I and IIA are slightly different, as they have an added 12AX7 - presumably because those amps didn't have an effects loop to begin with.

When I say interact, it's more that they affect the sound in different ways. For example, I've found that setting the post FX level on 10 and using the front master to control the overall output results in a brighter more aggressive sound.

Going back to the original question - I recall there was a presence control mod that the Kingsnake had. I could see it being useful, as I've found the original MK I presence control somewhat subtle

 

[bluebug]

I saw yours - I don't think it was installed by Mesa. If I remember correctly, the one I mentioned didn't look quite as busy and had fewer components. Mike's initials were also on the inside.

Is the added pot always active, or does it only work when the send/return jacks are connected?

I have a IIA and IIB with that mod and know someone with the MK I. The MK I and IIA are slightly different, as they have an added 12AX7 - presumably because those amps didn't have an effects loop to begin with.

When I say interact, it's more that they affect the sound in different ways. For example, I've found that setting the post FX level on 10 and using the front master to control the overall output results in a brighter more aggressive sound.

Going back to the original question - I recall there was a presence control mod that the Kingsnake had. I could see it being useful, as I've found the original MK I presence control somewhat subtle

Thanks for the information on the fx loop. In mine the pot does not do anything when nothing is connected to the FX loop, so I was a bit puzzled to read the fx loop mod document above, I thought they meant that the installation of the mod affected the tone at all times because always in circuit, but your comment means the pot works and has an effect all the time? If that is the case, it would indicate that on top of not being from Mesa, my fx loop is not copied from another Mesa fx loop install and is just another circuit. Definitely no initials inside.

About the sound interaction, it also has an effect form me on the sound even though it is not the Mesa fx loop. The interesting sounds are found with a lower setting of the FX pot, and higher master volume. The sound gets brighter and tighter (possibly because of less bass?) and gives an impression of tighter and higher gain.

Fred

 

[James]

Thanks for the information on the fx loop. In mine the pot does not do anything when nothing is connected to the FX loop, so I was a bit puzzled to read the fx loop mod document above, I thought they meant that the installation of the mod affected the tone at all times because always in circuit, but your comment means the pot works and has an effect all the time? If that is the case, it would indicate that on top of not being from Mesa, my fx loop is not copied from another Mesa fx loop install and is just another circuit. Definitely no initials inside.

About the sound interaction, it also has an effect form me on the sound even though it is not the Mesa fx loop. The interesting sounds are found with a lower setting of the FX pot, and higher master volume. The sound gets brighter and tighter (possibly because of less bass?) and gives an impression of tighter and higher gain.

Fred

Yes, the additional pot is active at all times. It’s worth mentioning that the MK 1 reissue appears to have a similar modification as standard, but it’s only active if you use a jumper cable in the send/return jacks. Otherwise only the front master functions.

That’s interesting. It sounds like a similar type of mod at least - it might be worth sending the pictures to Mesa for verification. If it's not factory, do you think you'll return it to stock?

I found this one to work the other way round. It gets brighter as the front master is turned down.

 

[Nick Athanasatos]

Hi..I wonder if there are many Mark IIC Amps left which have not been converted to C+ ?
Does anyone here have a Mark IIC that actually prefers it to a C+ ?Thanks

 

[user 41014]

I have a S IIC. I loaded a trad with redbacks at the time I got the C and they paired amazingly well and I have no interest in modding it. That's not the same as preferring but when your fingers tell you don't change a thing it's smart to "listen".
I do have a stable of II's with many B's and a C+, most coli's but a loop mod S IIB that is a little beast. I don't try to dial in a tone so it's about what the signal chain inspires. I'm lucky to really like many of the non EQ ones that many have no interest in.

 

[DavidMenke]

not to be too critical, but I’m a bit surprised that people here aren’t across the various factory mods to Mark series amps.

My poison is the Mark 1 amps as I own an SOB Boogie.

There have been a few incarnations of the Mark 1. One amp was the Kingsnake amp. I think they replaced the 10K midrange pot with a 100K pot, from 1-5 the pot acted similar to the 10K pot, (due to audio taper of pot) but from from 6-10, the tone stack is less effective in the circuit.

They also had a power switch lowering the B+ so 6V6’s could be used.

I have heard of reverb mods, and there are various versions of PI circuit.

Regards

Mark

By the way, you could have a reverb installed in the SOB if desires by Mesa. They add the control to the back panel. The reverb is slight, not drenching like a Fender. In my icon, is an SOB Long Chassis with Reverb. It started out as a head, and later on Mesa installed it in a cabinet with Reverb and add effects loop.

 

[mark1406]

By the way, you could have a reverb installed in the SOB if desires by Mesa. They add the control to the back panel. The reverb is slight, not drenching like a Fender. In my icon, is an SOB Long Chassis with Reverb. It started out as a head, and later on Mesa installed it in a cabinet with Reverb and add effects loop.

I would like to see pictures of the inside of the chassis. I’m wondering about the effects loop circuit and if the power supply needed another node.

The reverb circuit is okay, though I prefer the Fender reverb. It sounded deeper.
On my Mark 1 amp, the reverb tank was mounted on the back baffle of the cab opposite the speaker. The tank had a locking mechanism on it for transport. The speaker reacted with the reverb tank and I couldn’t turn the reverb control any higher than “3” before it went into feedback. The fix would have been to relocate the tank to a similar position as used in Fender amp.

Regards

Mark

 

[DavidMenke]

I would like to see pictures of the inside of the chassis. I’m wondering about the effects loop circuit and if the power supply needed another node.

The reverb circuit is okay, though I prefer the Fender reverb. It sounded deeper.
On my Mark 1 amp, the reverb tank was mounted on the back baffle of the cab opposite the speaker. The tank had a locking mechanism on it for transport. The speaker reacted with the reverb tank and I couldn’t turn the reverb control any higher than “3” before it went into feedback. The fix would have been to relocate the tank to a similar position as used in Fender amp.

Regards

Mark

The amp is in storage until August, the only picture I have is attached. This is a long chassis, and the reverb pan sits at the bottom of the cabinet like a Fender Amp.

 

[mark1406]

The amp is in storage until August, the only picture I have is attached. This is a long chassis, and the reverb pan sits at the bottom of the cabinet like a Fender Amp. View attachment 1335

I have the same holes in my SOB awaiting an effects loop. I have a few circuits in mind but I was wondering what Mesa Boogie considered a suitable circuit.

August is a long time to wait for a circuit diagram

Blue bug has a good circuit in his Mark 1 amp.

Valve Wizard loop

I have a reverb transformer for reverb but not the reverb pan, I don’t know if I want reverb in the amp.

Regards

Mark

 

[DavidMenke]

Mark,

Michael Bendinelli did the effects loop on my amp back in the 90's. It was wired the same as the Mark series. Do not have the schematic, but Mesa should be able to get that to you. Good Luck on your adventure. Looks like your Amp was modified from the picture you posted.​

 

[mark1406]

Mark,

Michael Bendinelli did the effects loop on my amp back in the 90's. It was wired the same as the Mark series. Do not have the schematic, but Mesa should be able to get that to you. Good Luck on your adventure. Looks like your Amp was modified from the picture you posted.​

Hi David

Thanks for your reply. I did contact Mesa Boogie and exchanged emails with Mike Bendinelli on the topic of effects loops and below in italics is the reply.

As you can see it’s more or less a description of the Mark 1 reissue effects loop. I’m sure both loops work well, I tend the prefer to use a cathode follower for the send section. Years ago I built a Dumbleator for a guy and I liked the colouration that was achieved by driving the cathode follower hard.

I would be really curious to see what has been implemented in your amp.

Regards

Mark

“I have installed a loop on a very few original MK1 Mesa amps. I inserted after the master, at the input to the phase inverter.

My usual would be an added 12AX7 for the return stage and passive padding of the signal from the original master of the amp providing the send.
I have no documents to provide though. And Mesa never did a detail on this either.
I guess you could use FET and do an active send buffer too if you wanted. I never have done that.”

Mark 1 reissue effects loop