1972 lead 50 circuit help

[Progear]

Hello all, I'm new to this forum.

Just purchased a 1972 marshall lead 50 and have some question on the circuit. Below is a pic to review.
Circled in blue, usually these are .022 , I have .1 , what's the difference ?
Circled in red, should have a 47pf cap, I have none, differance ?
Circled in green, a resister between V3 and power tube , why is that resistor there ?
Circled in black , was planning on changing to a 500pf silver mica, good idea ?
Circled in yellow and very hard to see, a series of diodes between two pins on both power tubes, why are the diodes there ?

The amp amp seems to have an replaced older power transformer.

Any comments and suggestion are welcome, I'am try to get the amp as stock as possible ..

Thank you
Progear
[/url][/img]

[xtian]

Ones I can answer:

blue: larger value will let more bass signal thru.

red: missing snubber cap can tame fizz or high freq oscillation.

yellow: probably flyback voltage protection. need clearer photo.

green: need clearer photo.

[martin manning]

Is there a paper label with a model number on the chassis? The 0.1uF Mustard PI couplers suggest that this is a bass or PA amp, or else they were installed in error... The input has the Lead configuration on V1's cathodes, so I'm guessing the 0.1's are somebody's mod. The marking is not like the 100n just to the left.

The resistor going from the from the PI socket to the power tube socket might be half of an artificial center tap for the filament circuit. If it goes from pins 4 and 5 on the PI to pins 8 and 1 on the power tube that is likely the case. I see another resistor on the other power tube socket that looks like it goes from pin 7 to pins 8 and 1 (that would be the other half), and both look like they could be 100 ohms. Does the replacement PT have a CT?

[Progear]

Ones I can answer:

blue: larger value will let more bass signal thru.

red: missing snubber cap can tame fizz or high freq oscillation.

yellow: probably flyback voltage protection. need clearer photo.

green: need clearer photo.

Xtian, thanks
Here are some additional photos for yellow and green hopefully you can provide additional info.

[IMG:768:1024]http://i1258.photobucket.com/albums/ii540/ssl4040ask/1972%20JTM50/imagejpg24_zpsd94b7ae7.jpg[/img]
[IMG:768:1024]http://i1258.photobucket.com/albums/ii540/ssl4040ask/1972%20JTM50/imagejpg22_zps8ae7135e.jpg[/img]

[Progear]

Is there a paper label with a model number on the chassis? The 0.1uF Mustard PI couplers suggest that this is a bass or PA amp, or else they were installed in error... The input has the Lead configuration on V1's cathodes, so I'm guessing the 0.1's are a mistake.

The resistor going from the from the PI socket to the power tube socket might be half of an artificial center tap for the filament circuit. If it goes from pins 4 and 5 on the PI to pins 8 and 1 on the power tube that is likely the case. I see another resistor on the other power tube socket that looks like it goes from pin 7 to pins 8 and 1 (that would be the other half), and both look like they could be 100 ohms. Does the replacement PT have a CT?

Thank you for the info, artificial tap makes sense, Is that good or bad ?

Here is a label pic and some additional photos.
[IMG768]http://i1258.photobucket.com/albums/ii540/ssl4040ask/1972%20JTM50/imagejpg17_zpsb07c9d49.jpg[/img]
[IMG768]http://i1258.photobucket.com/albums/ii540/ssl4040ask/1972%20JTM50/imagejpg7_zpsae8429cb.jpg[/img]
[IMG768]http://i1258.photobucket.com/albums/ii540/ssl4040ask/1972%20JTM50/imagejpg13_zps4792171a.jpg[/img]

[martin manning]

You need some kind of ground reference for the filament circuit or you will have a lot of hum. The additional pics leave no doubt that's what those two 100R resistors are for. I assume the replacement PT does not have a filament CT (but you should check). If not you have to keep them in. As expected, the diode strings are to protect against a no-load situation.

The label confirms it was built as a Lead model. If you want to restore it to factory spec I'd get rid of the diode strings and change the 0.1uF PI coupling caps back to 0.022uF. For a small(?) premium you could even get some period correct caps.

[Stevem]

The first two preamp tubes and the Marshall factorys way of laying all the preamp tube leads down on top of one another makes for a lot of heater voltage buzz getting into the tubes grid wires.
Be sure to pull up all the wires other than the black and red ones. Do this with the amp on and a guitar plugged in with the amps volume up and the guitars down and you will hear where the best wire position is for the least amount of buzz.