Realistic MPA-20 SS PA amp, need advice on possible fun mods

[Pixel]

Hi,

I have had this little amp since 2017. Thanks to Cameron Jenkins in this forum I got a schematic. The thermistors were glued to the chassis but had come loose so I glued them to a chunk of aluminum with thermal adhesive and bolted that to the chassis. Otherwise I just cleaned up the pots, changed the PS capacitor from 2200uF to 3300uF and replaced the power supply diodes with UF4007s. That helped out with bass and I have just been goofing around with the amp since then.

Now I would like to make a few changes to the amp just for fun. I was thinking about building a Tillman preamp into the amp instead of using a boost pedal, but was wondering about tinkering with the MIC preamp already in the amp instead. Any advice on how to go about that or should I just leave it all alone and find something else to do?

The other thing I was wondering about is why is there is a 2M2 resistor between HOT and the chassis? The chassis is also connected to the power ground.



Thanks, Jeff

[g1]

The other thing I was wondering about is why is there is a 2M2 resistor between HOT and the chassis? The chassis is also connected to the power ground.

Where is that on the schematic?

[Pixel]

Where is that on the schematic?

It isn't on the schematic that Cameron drew but I found it on the back of the switch in the amp and on another schematic at hifiengine. https://www.hifiengine.com/manual_li...c/mpa-20.shtml

[nickb]

The arrangement of the output power transistors is interesting. It meant they could bolt them directly to the chassis to save money and best heat dissipation. These transformer coupled transistor output stages can sound quite nice when overdriven with a guitar. The preamp could do with replacing with something more suited to guitar. Were you looking at the Tillman FET buffer for a piezo type pickup?

The 2.2meg is present on the US version. It seems to be located on the neutral side, the power switch is in the live side, and so provides a path for any leakage currents via the power transformer to flow to earth IMHO. Best removed and rewired with a three pin plug and safety earth to the chassis.

[Pixel]

The arrangement of the output power transistors is interesting. It meant they could bolt them directly to the chassis to save money and best heat dissipation. These transformer coupled transistor output stages can sound quite nice when overdriven with a guitar. The preamp could do with replacing with something more suited to guitar. Were you looking at the Tillman FET buffer for a piezo type pickup?

The 2.2meg is present on the US version. It seems to be located on the neutral side, the power switch is in the live side, and so provides a path for any leakage currents via the power transformer to flow to earth IMHO. Best removed and rewired with a three pin plug and safety earth to the chassis.

I was thinking of the Tillman just to have a higher impedance input than what is there now. No piezos in the house yet, just humbuckers on a 70s Strat copy.

The schematic and reality are quite different with respect to how the mains plug is wired into the amp. The amp already has a factory 3 pin power plug. The 2.2meg goes directly from the hot on the back of the switch to the chassis. After the switch the hot goes to the fuse and then on to the power transformer. The neutral goes straight to the power transformer. The ground on the plug goes to the chassis. So basically as soon as you plug the amp in the hot and ground are connected via the 2.2meg regardless of whether the amp is turned on or not.

[g1]

That's kind of odd they would leave the resistor there with a three prong AC plug. The CSA version shown lower on schematic has 3 prong without the resistor.

[nickb]

Perhaps someone rewired it? It makes no sense going to hot. I'll wager that resistor couldn't stand a surge on the power. Is the safety earth solidly connected to the chassis?

The input impedance is little on the low side for guitar, but far from awful. Try the buffer and see what you think. Personally I think the right place for a buffer is at the guitar end of the cable. That way you are immune to the effects of the cable capacitance.

[Pixel]

Perhaps someone rewired it? It makes no sense going to hot. I'll wager that resistor couldn't stand a surge on the power. Is the safety earth solidly connected to the chassis?

The input impedance is little on the low side for guitar, but far from awful. Try the buffer and see what you think. Personally I think the right place for a buffer is at the guitar end of the cable. That way you are immune to the effects of the cable capacitance.

I am sure the power section was done up at the factory as it has the same white goop on all the terminals and the power cord itself is from the same manufacturer as the rest of the wires in the amp.

I'll get rid of the 2.2M and then leave any further modifications to the amp until after I can put a scope on it to see what is going on in the mic preamp. It is tempting though to just replace some of the parts in the mic preamp to turn it into a Tillman or Fetzervalve.

Thanks to all for weighing in on this.

[g1]

Perhaps someone rewired it? It makes no sense going to hot. I'll wager that resistor couldn't stand a surge on the power.

The old style North American 2 prong plugs were non-polarized. So it may or may not actually go to hot depending on which way you plug it in. So perhaps it is just being used for some kind of hum mitigation like the standard 'death cap' (if it hums, flip it over) ?

[Pixel]

The old style North American 2 prong plugs were non-polarized. So it may or may not actually go to hot depending on which way you plug it in. So perhaps it is just being used for some kind of hum mitigation like the standard 'death cap' (if it hums, flip it over) ?

This unit had a factory 3 prong plug AND a factory 2M2 resistor from hot to the chassis. I snipped the resistor out so now the unit matches what is in the schematic for the Canadian model.

I also went ahead and replaced all the electrolytics in the unit. Playing guitar through this now sounds a lot better and I don't need the boost pedal anymore.