Masco MA 25P
I wrote this originally in another Masco thread but thought it might get lost so I'm repeating my question in its own thread. Thanks!
Hi Skip! I've got a Masco MA 25P that came my way for little $. It has a new 1/4 inch input jack and 8 ohm output jack as well as a new 3 prong power plug and was said to have been gone over by a local amp builder. For those who don't know the P means it has a removable phonograph. I got that too but wow it's old looking so I removed it. The amp has two 6L6 power tubes that I wouldn't touch for concern about the biasing. There appears to be 4 preamp tubes on the front left side of the chassis near the original and the new inputs. I removed the preamp tube closest to the input to bring down the volume. Someone mentioned the amp could be made more manageable by making this a self split amp done by grounding the input on the out of phase tube. I'm guessing that's one of the 6L6's. Is this a big proposition or something that could be done relatively easily and inexpensively? Thanks!
The big Masco is too clean on its own. Lots of volume but feedback comes in fairly early so that's why I removed one of the preamp tubes. Still not getting the tone from the amp I'd like but it makes a great second amp to line out my Kalamazoo and get a louder sound. Kind of a personal PA. On it's own I could use a Lone Wolf pedal in front to get a dirtier sound but don't own one at this time.
It's possible the term self-split isn't commonly used. The best definition I can find it's a push-pull output stage configuration by eliminating the phase inverter tube. The driven push-pull tube operates in Class A and the other has a cloned signal applied to it. Output power is thereby reduced.
As best I can tell some people are able to install a switch that goes back and forth from standard to self-split.
Smoke- Well........ People have been looking for ways to make a big amp into a small amp for a long time. Attenuators, master volumes, power soaks, power-scaling, etc. all "work" but I'd say most people would prefer the tone of a smaller amp!
I guess I'm saying that I would be surprised if you were ultimately satisfied with a mod to the phase inverter circuit.
Hello Smoke- It sounds to me like you have an old amp but you aren't happy with the sound! Making it so you dig the sound might be tricky, especially if you have to work on the amp yourself. I'm not quite sure what you mean by "self-split" but I can tell you that there are lots of ways to change the sound of an amp, just like there lots of ways to make bread.
A stock, serviced MA-25 or MA-25P is a cool amp, but they are very difficult to play loud without feedback. The ones I've done for harp players needed significant mods to the circuit to be harp-friendly. I wouldn't say it was expensive, but it sure wasn't easy!