Masco MA-25 Power Supply

[Deric]

Cleaning up a Masco MA-25 that had been "converted" to a guitar amp.  Looks like it had been modded before that with an extra input via screw terminals and a Thermador GH-20 transformer to the phono input. 


Filter caps and power resistor had all been "modified".  First filter cap was rated 500v and had vented. 


First things first - need to get the power supply sorted.  I have 600v caps and plan on using 22u/600v for the plates and screens.  The 5000r (5k) screen resistor is missing.  I have 470k or 510k at 3W.  Is a 3W resistor adequate here? 


On the schematic the 5000r and a 10k form a voltage divider before the next stage.  Is a 3W resistor adequate here ?


Schematic in this thread... https://el34world.com/Forum/index.php?topic=21115.msg223742#msg223742


Thanks!

[sluckey]

The 5000r (5k) screen resistor is missing.  I have 470k or 510k at 3W.  Is a 3W resistor adequate here? 

You cannot replace a 5K resistor with a 470K or 510K resistor!

On the schematic the 5000r and a 10k form a voltage divider before the next stage.  Is a 3W resistor adequate here ?

No way! There will be approx 500VDC across that voltage divider, so 33mA flowing through the 5K and 10K resistors. The 5K will need to be 10 watts and the 10K will need to be 20W.

[Deric]

Good catch!!  I meant 4.7k or 5.1k  Doh!


Will order appropriate rated resistors.


The 200r shared cathode resistor for the power tubes appears to be 10W.  If I replace it -is 10w sufficient or should I up that too?


Thanks

[Deric]

Chassis mount resistor question...


On the old tubular chassis mount resistors - is there supposed to be an insulated spacer on the top and bottom?  I would think there should be...  The only one left in the amp is the power tube cathode resistor and it does not have one between the chassis and the resistor.  That end of the resistor is connected to ground so it doesn't really matter but...  ??  The 5k 10w resistor is still there but was disconnected.  It is not insulated front the chassis.  Are the insulated spacers available anywhere?

[DummyLoad]

depends on the resistor size, make, etc.

somewhere to start... 

https://www.mouser.com/Ohmite/Passive-Components/Resistors/Resistor-Hardware/_/N-7sfb3?P=1z0zldmZ1yzu922Z1yzu927Z1yzu920Z1yzu91zZ1yzu921Z1yzu8onZ1yzu91xZ1yzu92d

https://www.mouser.com/ProductDetail/Ohmite/7E-100?qs=sGAEpiMZZMsrgWFlZUYFRmHGff1AC3YM%2FQlhb8ADy6s%3D

--pete

[sluckey]

On the old tubular chassis mount resistors - is there supposed to be an insulated spacer on the top and bottom?

Yes. They are not used for electrical insulation though. They provide a cushion between the chassis and the fragile porcelain resistor body. This cushion allows you to tighten securely (not cranked down tight) without breaking the porcelain.

Should be able to find soft fiber washers at Lowe's. Don't use hard washers. Faucet washers work OK if you can't find soft fiber washers. Rubber tends to harden over time, especially when in contact with a hot resistor.

[Deric]

Thanks for the info.  Made a couple washers out of a chunk of fiber board I had laying around - worked great.


Amp is up and running and sounds great.


Thanks for the help!