Building my own 2203

[dax_the_ax]

I’m going to build a 100 watt MV from the Ground up. My good buddy David Hopkins is gonna help me mod it into a fire breathing monster!!! Just started on the board. I’m no expert. And it’s not finished. Any advice or critique would be much appreciated. For example: I’m using 2W metal film resistors instead of metal oxide. Not sure if that’s bad or good. Thanks. And I still have to change the diode arrangement, it is now as a 50 watter.

Thanks

Dax_The _Ax

 

[dax_the_ax]

And I had to move the NFB to the middle. The board was a project.

 

[XTRXTR]

Conservatively the first dropping resistor would be dissipating .6 watts, and the general rule is to have 4 times the operating wattage as the resistor rating. That would be 2.4 watts, its a beast of an amp too so I would go with 3 watt rating. Most DIY builders would use Metal Oxide as they take the heat better but 3 watt Metal Film should cover it.
I actually don't know if you can get 3 watt Metal Film however. So that's a discussion for others here. Depends on the schematic you use but the one I use Marshall '81 shows two 10K 1 watt in series but a drop of 468 to 330 that comes to .9 watts.

Wall voltages are higher in the USA now than when that was designed, something to consider. I mean it shows 365 volts at the choke on the 2204 but mine is 470V

 

[Chris-in-LA]

Advice? Run all your questions through David. But keep posting pics of your progress here. Looks good so far.

 

[thetragichero]

in the vein of measure twice cut once: layout everything and check several times before plugging in the iron. it's a lot easier to move stuff when you don't have to suck up a bunch of solder

 

[thetragichero]

oh also: leave the grid stoppers of the board, attach them directly to the tube pins. run shielded wire (only grounded at one end) for preamp tube grid connections. these both should help keep the noise level/oscillations down

 

[neikeel]

I would use MO on the B+ rail and MF in the tone/signal path if we are being modern and purist about this (even tho' I am not!)

 

[dax_the_ax]

Can you be a bit more specific? B+ Rail? Which ones are these?

 

[XTRXTR]

The B+ rail as it is sometimes called is the equivalent to HT VDC, the rectified high voltage where the voltage dropping resistors are located and are part of the resistor capacitor filter network the amp uses to filter out the 100/120 Hz ripple voltage for the entire amp. In the above schematics its the rail that is marked with A or A` where the two sheets of the schematic are to be connected.

MO metal oxide, MF Metal Film

 

[XTRXTR]

The signal paths are partially marked with B, and then splits after the phase inverter to D and E.