Lead gain pull pot, or the pull bright pot? The Pull bright, depending on what rev it is could have a cap that rolls off highs (low pass filter), when pulling the pot, should disengage the cap, that shunts to ground rolling off highs, if there is a tantalum electrolytic cap going to ground through the pull bright switch, its a cathode bypass and should add gain in brightness with a low value, .22 etc. a big 22uf would add gain and too much bass there. The Mark II's evolved into the IIC+ and later Marks with a .22uf Lead bright cap, the early II's would see a high roll off, low pass filter, and should disengage the cap with the bright pull switch pulled out, allowing the highs to come through, not being shunted to ground, in theory anyway
Lead gain pull pot, or the pull bright pot? The Pull bright, depending on what rev it is could have a cap that rolls off highs (low pass filter), when pulling the pot, should disengage the cap, that shunts to ground rolling off highs, if there is a tantalum electrolytic cap going to ground through the pull bright switch, its a cathode bypass and should add gain in brightness with a low value, .22 etc. a big 22uf would add gain and too much bass there. The Mark II's evolved into the IIC+ and later Marks with a .22uf Lead bright cap, (I have seen 2.2uf on the Mark IVB) the early II's would see a high roll off, low pass filter ( this low pass filter like a .01-.047 in some cases is also seen on some earlier pull boost pots in parallel with a 100k resistor across the pull pot terminals to roll off highs when pulled, it basically bypasses the tone stack and you get everything, gain, bass, mids and treble, and hiss/noise so that cap in that scenario rolls back the treble/hiss where with the bright switch it should disengage the cap with the bright pull switch pulled out, allowing the highs to come through, not being shunted to ground, in theory anyway on the earlier II's. The Lead Gain pot should only engage the Lead mode when pulled, so I'm thinking you mean the Lead Pull Bright, since you said there was a 22uf cap, too big, Im saying from experience it should be .22, no bigger than 2.2uf there, as you are experiencing all bass boost, its like what the Pull Deep does on V2B at the cathode bypass, adding bass (Deep). I hope that makes sense. It must have been installed or they transposed the value by accident, it has been known to happen
I added a picture - the .01 orange cap cap is the one being switched to ground when you pull out the Lead Master pull bright pot - that same Cap (.01) runs to pin 2 of V4- so it cuts frequency's to ground- depending on the size cap. IMHO- I put some test leads on there and increased the value of .01 cap- to hear the effect - the larger the value of the cap the more bass is rolled off - So on my amp there is a 100K resistor going from pin 6 of V3 to Pin 2 of V4 with the cap .01 going to ground - when the Lead master bright pot is pulled out - when Lead master bright pot is not engaged (pulled out) the .01 cap is not grounded or doing anything - I have to look into the 22uf - there's a 22uf on Every cathode in the amp except V2B- which is at 15uf. That 15uf gets a 22uf paralleled with it, when pulling the Gain Boost on Master 1 out.
That's understandable, with all the revisions, it's a fun exchange lol...
If you have a beeper/continuity tester, trace the connections from the switch to the attached drawing. The black wire from the Pull Bright switch should be going to ground, the other side of the switch should go to the un tapered side of the 226 black cap. That is how it should be wired according to the schematic. I need more details as to why it would have anything to do with V4, that's the Reverb tube.
As I mentioned, I know next to nothing about this but I'm puzzled as to why on one of the switch lugs , there's a lead going to the Lead Drive pot?
Isn't the pull function of that pot shutting to ground wherever that black lead goes?
I believe so, as per the schematic, it shows on the IIB, that wire should go to ground connecting the bright bypass cap. In the pic I just posted shows V3A as the reverb tube, V3B is the Lead drive section
Yea that’s not what’s happening here - I looked at the mkIIc schematic and it similar to what is in my amp
So the .01 cap (Orange drop) is going to the verb circuit. The 226 (22uf) might be the factory value which is too much, they were still learning what worked back then, and they ended up with .22-2.2uf for the Lead Pull Bright cap in the later IIC+ and on. I am having a tuff time reading that value on the schematic, but it could be what they put in the early IIB's.I believe so, as per the schematic, it shows on the IIB, that wire should go to ground connecting the bright bypass cap. In the pic I just posted shows V3A as the reverb tube, V3B is the Lead drive section
Your amp looks to be a IIB from all the info and pics, but I could be wrong, like I said they were making changes along the way so anything is possible, sorry I could not be of more help, all the best with it!
Lug 3 on the drive pot goes to ground, test with a meter. If you are still having problems I would contact Mesa Engineering and tell them your issues a tech might be able to help you further
Im no amp tech but why would someone wire up a pull switch just to connect two ground points? Are you absolutely sure that lug is ground? Put a continuity meter on it to be sure, with the pull bright switch pushed in. Put a lead on it and the other to chassis. On V3B, that fatter grey lead from pin 7, is that going to the middle lug of pot?
