coliseum series ga300 mark 3 questions

[85voyager]

I am looking at buying a coliseum amp, but have some questions. The front panel looks like a regular mark 3, has the half power switch. The serial # is K59X and it says GA series 300 on the back. On the rear panel you can see where it said "presence" and "reverb", but they have been marked over and it looks like the jacks have been removed. There is a presence knob on the front of them amp. Also, it says it is Simul-Class, but I do not see a switch for it. I am assuming this amp does not have reverb? Also you can see that something was marked over under the fuse holder.

Anything else ya'll can tell me about this amp would be great - thanks!

 

[>Photi G<]

I don't see this "GA Series 300" you speak of. Oh, wait I see it now. That's the type of fuse you need to use. A 6 Amp Slow Blow. The reason that the other markings above it are crossed out is because Mesa used the same rear faceplates on all the Mark III's, so they listed all the fuse types, crossed out the rest, and only left the one that belonged to this amp visible.

The same goes for the Presence and Reverb. Coliseum Mark's have the Presence knob on the front panel, so a plug is placed in the hole in the back, and the "Presence" etching underneath it is crossed out. As for the Reverb, when this amp was ordered, the owner didn't want reverb, so it was never installed, a plug was placed, and the "Reverb" etching was crossed out.

The Simul-Class switch is on the front of the amp. On Coliseum Marks, it is labeled as Full/Half power. The full power setting is really Simul-Class Mode, and the half power setting is really Class A mode.

As far as other things go, is there anything else you want to know about the amp?

 

[85voyager]

Will Mesa install a reverb tank in it?

 

[>Photi G<]

They probably could, but it'll be expen$ive. Just get a good pedal to put in the loop.

 

[Boogiebabies]

The Simul-Class switch is on the front of the amp. On Coliseum Marks, it is labeled as Full/Half power. The full power setting is really Simul-Class Mode, and the half power setting is really Class A mode.

The half power switch does not cut out the four outer tubes. The only tubes wired to Class A are the center two.
The last Series 300 I saw used the 1/2 power switch to cut the voltage to the screens on all six tubes.

 

[>Photi G<]

The half power switch does not cut out the four outer tubes. The only tubes wired to Class A are the center two.
The last Series 300 I saw used the 1/2 power switch to cut the voltage to the screens on all six tubes.

I think that's the 180 Watt AB Coli's. The half power switch is wired as a Pentode/Triode switch for all 6 power tubes. I'm pretty sure that the Simul Class Coli turns off the 4 AB tubes and leaves the 2 A tubes running. I'd need to see the circuit differences though to be sure. I may be wrong. I think I actually read the screen voltage theory somewhere, and the pentode-triode theory somewhere else.

 

[Restless Rocks]

Up to S/N K-178 they were Class AB with a pentode/triode switch on all six 6L6's. What a monster!!

 

[Boogiebabies]

The half power switch does not cut out the four outer tubes. The only tubes wired to Class A are the center two.
The last Series 300 I saw used the 1/2 power switch to cut the voltage to the screens on all six tubes.

I think that's the 180 Watt AB Coli's. The half power switch is wired as a Pentode/Triode switch for all 6 power tubes. I'm pretty sure that the Simul Class Coli turns off the 4 AB tubes and leaves the 2 A tubes running. I'd need to see the circuit differences though to be sure. I may be wrong. I think I actually read the screen voltage theory somewhere, and the pentode-triode theory somewhere else.

If you cut the four AB tubes you would only have two Class A running 15-30 Watts. The Simul K series are only 150W so it does not add up to half power.
Knocking out the four other tubes would crucify the two Class A from the lost voltage draw. Cutting the screen voltage is a pentode/triode switch.

 

[JOEY B.]

The half power switch does not cut out the four outer tubes. The only tubes wired to Class A are the center two.
The last Series 300 I saw used the 1/2 power switch to cut the voltage to the screens on all six tubes.

I think that's the 180 Watt AB Coli's. The half power switch is wired as a Pentode/Triode switch for all 6 power tubes. I'm pretty sure that the Simul Class Coli turns off the 4 AB tubes and leaves the 2 A tubes running. I'd need to see the circuit differences though to be sure. I may be wrong. I think I actually read the screen voltage theory somewhere, and the pentode-triode theory somewhere else.

If you cut the four AB tubes you would only have two Class A running 15-30 Watts. The Simul K series are only 150W so it does not add up to half power.
Knocking out the four other tubes would crucify the two Class A from the lost voltage draw. Cutting the screen voltage is a pentode/triode switch.

Just to chime in, the Mark III 150 watters that I have owned dropped the screen voltage to the Class AB sockets when in half-power mode. The 6L6's were not turned completely off, but they were turned down a good bit. This made for an extemely punchy sound in rhythm 2 mode with the half power activated and EL-34's in the middle two sockets. I commented to another board member that it woukd "out-Marshall a Marshall" in this configuration. :wink:

 

[85voyager]

ok I bought it - will post more pics and a report when it gets here.