Hi guys,
I've been reading quite a bit on the forum which has been a great resource so far but I have not yet been actively contributing. So: Here we go!
I have just bought a used Mesa Mark IIA Combo (1979, Export) with GEQ, Reverb and the EV12L speaker. Got it relatively cheap here in Germany (approx 650$) but it had some issues which I'm resolving one after the other. It already sounds incredibly nice and is freakin' loud.
Well, I'm quite interested in the technical aspects and have looked at the schematics of all the Mark II series amps and traced out the circuit of my particular amp. Doing this I noticed that the amp is essentially the actual IIA circuit but there are some resistor values that are slightly off the values given in the schematic. However, I'm pretty sure these resistors are no later modification but have been used already by the factory as they are all the same type as most of the other ones (those red ones, supposedly half-watt rating) and have the same amount of dirt etc on them.
Now I am wondering if this is something that is common to those old Mesa amps...? Did they just use values that were close to the ones needed when they ran out of supply? Or was it done on purpose? I looked up gutshots of other IIAs and couldn't find many. But in those pictures I found most values are more to spec than in my amp even though still not all of them match the schematic.
Some of the resistors I'm talking about are:
Tonestack slope resistor is 82k instead of 100k
plate R of V1B is 62k instead of 82k (also saw 100k on a picture online)
PI tail resistor 10k instead of 22k
resistor between V1B and Lead Drive pot 220k instead of 56k (transition towards MK IIB specs?)
Cathode R of V2B 220k instead of 150k
Reverb Pot is wired with Signal input on the wiper and signal output on pin 3. I suppose that's been done in order to not change the volume in Rhythm mode upon using the reverb pot (pot resistance to ground forms a voltage divider with that 2M2 resistor). Have seen this on pictures of other Mark IIs as well, but not on all.
Additionally there were some other changes which I can tell have been done later and likely not by Mesa. These included some modern blue metal film resistors and many ceramic caps across resistors (both to increase and attenuate treble content). I removed these already and replaced them with schematic-correct values.
So my question is if somebody can confirm that the MkIIA schematic is actually correct, so that I can convert my model back to stock specs? I would be highly interested in your experience regarding this aspect of the early Marks. If somebody would be able to provide some pictures of the guts of another MkIIA I would highly appreciate it. Would love to see if other amps are all to spec or if they have used different values back then.
I'm also wondering if the Reverb circuitry of the Mark IIB is an improvement compared to the Mark IIA...? I like the sound of the Reverb, but unfortunately the noise floor is increasing with increasing amount of Reverb. In the MkIIB circuit it looks like the Reverb pan is driven harder compared to the IIA, allowing to keep the Reverb pot lower and therefore lowering the noise. Is this true?
Well, I'd be happy to read a lot about your experiences! I've already read some of the famous Mark IIA topics here but couldn't really find information on what I am looking for.
Thanks a lot!
I've been reading quite a bit on the forum which has been a great resource so far but I have not yet been actively contributing. So: Here we go!
I have just bought a used Mesa Mark IIA Combo (1979, Export) with GEQ, Reverb and the EV12L speaker. Got it relatively cheap here in Germany (approx 650$) but it had some issues which I'm resolving one after the other. It already sounds incredibly nice and is freakin' loud.
Well, I'm quite interested in the technical aspects and have looked at the schematics of all the Mark II series amps and traced out the circuit of my particular amp. Doing this I noticed that the amp is essentially the actual IIA circuit but there are some resistor values that are slightly off the values given in the schematic. However, I'm pretty sure these resistors are no later modification but have been used already by the factory as they are all the same type as most of the other ones (those red ones, supposedly half-watt rating) and have the same amount of dirt etc on them.
Now I am wondering if this is something that is common to those old Mesa amps...? Did they just use values that were close to the ones needed when they ran out of supply? Or was it done on purpose? I looked up gutshots of other IIAs and couldn't find many. But in those pictures I found most values are more to spec than in my amp even though still not all of them match the schematic.
Some of the resistors I'm talking about are:
Tonestack slope resistor is 82k instead of 100k
plate R of V1B is 62k instead of 82k (also saw 100k on a picture online)
PI tail resistor 10k instead of 22k
resistor between V1B and Lead Drive pot 220k instead of 56k (transition towards MK IIB specs?)
Cathode R of V2B 220k instead of 150k
Reverb Pot is wired with Signal input on the wiper and signal output on pin 3. I suppose that's been done in order to not change the volume in Rhythm mode upon using the reverb pot (pot resistance to ground forms a voltage divider with that 2M2 resistor). Have seen this on pictures of other Mark IIs as well, but not on all.
Additionally there were some other changes which I can tell have been done later and likely not by Mesa. These included some modern blue metal film resistors and many ceramic caps across resistors (both to increase and attenuate treble content). I removed these already and replaced them with schematic-correct values.
So my question is if somebody can confirm that the MkIIA schematic is actually correct, so that I can convert my model back to stock specs? I would be highly interested in your experience regarding this aspect of the early Marks. If somebody would be able to provide some pictures of the guts of another MkIIA I would highly appreciate it. Would love to see if other amps are all to spec or if they have used different values back then.
I'm also wondering if the Reverb circuitry of the Mark IIB is an improvement compared to the Mark IIA...? I like the sound of the Reverb, but unfortunately the noise floor is increasing with increasing amount of Reverb. In the MkIIB circuit it looks like the Reverb pan is driven harder compared to the IIA, allowing to keep the Reverb pot lower and therefore lowering the noise. Is this true?
Well, I'd be happy to read a lot about your experiences! I've already read some of the famous Mark IIA topics here but couldn't really find information on what I am looking for.
Thanks a lot!