Mark IIB gain boost pot needed.

[flamenco4]

:?:
Hello everyone. Long time lurker here. I have a problem with my MkIIb. Just got it back from Boogie today after a complete check-up and they said they disabled the gain boost pot as it was no longer available as the amp was too old and the pot was likely to cause a short. They did add a jumper underneath the chassis that I could pull out and get gain but it sounds terrible when used. Someone out there must still sell these or have a stash of them. Anyone with a line on these please let me know.



Tim in Napa
MkIIb
Studio 22+ combo
Two 1"x12" extension cabs.
C90
Vintage 30s
Vintage black shadow 50
Altec 417-8h
Celestion G10-20's

 

[Boogiebabies]

I'm thinking your amp was modified to have the pot for gain level. The stock IIB had a 5000pf cap and 33K resistor to ground added to the mid control circuit. It was a preset boost.

Do you have the old pot and pictures of where it was ?

 

[flamenco4]

Wow, thank you for your quick reply Boogiebabies

Here is what I got in the invoice from Mesa.

Written on invoice.

"Supply “short cord” to cover potentially troublesome gainboost pull switch. We have no replacement for this pot".

They told me that it may short out so they added a jumper plug underneath. There was already a jack for the gain boost foot switch before they worked on it.

Then on the plug which in just a 1/4" guitar plug it says this.

Written on plug.
To operate gainboost remove plug and pull pot out.

I can't open the cover on the plug to take a picture on the inner details.

Here are pictures of the pull switch from the front of the amp, switch from the inside, the plug, the jack.

Ok, so when the jack is in the sound is like a bass boost and treble killer. Doesn't matter if the boost is on or off. Boost does work with or without jack inserted. Without the jack sounds like a boogie amp, but there really is not much treble in the amp.


It seems all the photos I see on the web show the MKiib with the gain boost pull switch. I am not sure that this was a mod.

Link to pics.

http://www.flickr.com/photos/12918656@N00/sets/72157605046201356/

Thank you very much for your help.

 

[jngreene79]

It looks to me like a simple spst switch off of the mid pot. Not sure why a stereo jack is used b/c a mono jack would do what is necessary - ground a 33k and .005 paralleled cap. With the switch on the amp pulled out, the footswitch should turn the boost both on and off. The pot is a 10k spst. You can buy the pot here: http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=thispage&thispage=parts5.htm&ORDER_ID=554862975

It sounds like MB just didn't have any of these pot on hand. The switch is mechanical and they thought it was getting old? The "short cord" bit was mentioned because using a long cord might effectively change the sound of the boost (not likely, but it could increase the resistance to ground)

 

[Boogiebabies]

It seems like they wired the Gain Boost to function only with the footswitch and gave you the wrong plug. The setup is for a stereo cable where the tip grounds the reverb and the sleeve to ground activates the Gain Boost. In this case, you would need a two button footswitch like the Boogie REV/EQ. One shorts the tip and one shorts the sleeve. If they actually gave you a stereo plug, you would be able to rewire it to short the ring to ground. Regarding the major increase in Bass, I would look at the invoice to see if any caps were changed. It sounds more like a IIC gain boost in your description. If they added a 15uf cathode cap somewhere further down the preamp, it would give more if a bass boost and about a 6db increase in gain.

 

[gonzo]

sounds like they ruined it.

what a bummer...

if the gain boost was an intergral part of your sound, what do you do now?

 

[Boogiebabies]

Un-Modify.

 

[flamenco4]

Hi !
Wow, thanks for all your help guys.

Ok, I pulled the plug out and it is a mono type.

There is a rev/gain footswitch listed in this guy's website. I assume that is a stereo plug.

http://homepage.mac.com/mesaboogie/MarkIIB.html

It's not just that it is really bassy with the gain boost on, it sounds very gritty like a bad distortion pedal turned all the way up and only square waves are passing through.

I pulled up a schematic on the web and it shows a 1 meg pot goes in the master 1/gain boost position. So I ordered a 1 meg spst and a 10k spst pot from the Hoffman amps website. Thanks for send the link to me. I will send the amp back when they come in and have Mesa fix it back to factory specs.

I just got the invoice from Mesa today in the mail, here is a list of changed parts.

>Power cord ac
Cap elec 220uf 300v a, 2 each
Cap elec 47uf 100v ax, 2 each
Cap elec 30uf 500v ax, 4 each
Pot 450/s 1m 10% short 3 each
Pot 450/s 250k 10% short 1 each
6L6 tubes 1 each

Whatever the sound was like, even if it was fantastic the resale value went way down without a fuctioning pull switch. I will have it fixed as
soon as the parts come in and hopefully my Studio 22+ combo will be finished from it's checkup by that time.

Thanks everyone for helping out, Boogiebabies, gonzo, jngreene79. The beer is on me and plenty of refills for all.

Take care,
Tim