Help identifying function of circuit please

[whopperplate]

Also wondering, are the screen resistors essential for modern tubes, even in an amp not reaching 450v, like most people say modern tubes can't handle?

Thanks
Charlie

[Tazin]

With the way that the solder terimals are layed out on the Drake 1202-118, 1202-133, 1202-55 the wiring up of the PT to the rest of the amp is easiest if the PT is orientated with the heater connections facing the front of the amp....PT orientation dictated by design. Remember, were talking about a 50w amp here not a 100w. The 100w OT was rotated 90 degrees around the beginning of 1969.

The 1K/5w are pretty much essential for modern production tubes more or less due to poor construction or less robust design.

[whopperplate]

Got it, screen resistors for anything modern.

So am I correct in understanding that the PT orientation will NOT have a sonic effect either way?

Thank you
Charlie

[Tazin]

In the case of the Drake 1202-118 (and a like) I don't think there would be any sonic differences between the way they are normally mounted (heaters facing the front) or if orientated 180 degrees.

The "standup" Drake 1202-164 PT may offer less noise due to its orientated, but I don't think there would be any noticable tone differences if both PT's were the same specs.

[whopperplate]

Thank you for the education!

Here's a thread I found referencing this amp in detail.

viewtopic.php?f=3&t=40699&hilit=.68+chicklet

Thanks
Charlie

[Tazin]

I remember that thread. It's a good sounding amp for sure, but as you can tell from the discussion the amp had a "normal" circuit for this era. It's just one of the "good ones" when it comes to Marshall's.

[whopperplate]

Yes, I have no interest in anything but stock circuits. Pleximaster's guitar is actually very similar to my 70 custom les Paul and a can pull those kind of sounds out of my stock 12 series build no problem, albeit very loud. Replicating this circuit should be a good starting place to capture it at a slightly lower volume. Just slapped the iron on into the chassis
Only parts I do not have are .68 mustards, but I do have .68 chick lets to sub. I was going to use Daly 50/50 but they won't form. Subbing NOS LCR 50/50. Only thing close to 40/40 cans I know are the JJ 40/20/20/20 and CE.
I may have a LCR 40/20/20/20 that could sub if it forms. Crossing fingers.
Charlie

[whopperplate]

I notice that many amps from this era it seems are wired rectifier directly to the mains fuse, as opposed to the filter cap in the schematics. I imagine this way would be more prone to blowing fuses but otherwise would make no difference, no?

Thank you
Charlie

[Tazin]

It doesn't matter since there is a snubber cap on the H.T. fuse.

[whopperplate]

Is that the .05uf 1000v? What is exactly the purpose of this snubber?

Thanks
Charlie

[Tazin]

It suppresses the audible "pop" noise when the Standby switch is operated. And since it is a capacitor it also offers some degree of filtering.

[whopperplate]

Any reason a .05 600volt cap of a similar style wouldn't work in its place?

Thank you
Charlie Plate

[Tazin]

Since the H.T. voltage is only around 420vdc you could use a 600v cap. Marshall used the 1000v for a better safety factor. The 1000v rating may also have to do with safety in case of a rectifier failure and then your dealing with ac voltage.
I think a 600vdc cap is only rated for 250vac if memory serves me correctly. The secondaries on the 1202-118 is something like 315vac.

[whopperplate]

That's what I was wondering concerning the safety factor, thank you. isnt this suppose to be a certain type of capacitor that shorts instead of going open upon failure anyways for the same reasoning?

Thanks so much

Charlie

[whopperplate]

Would any of these suffice for the snubber?

http://www.ebay.com/itm/NEW-5-ACUSHNET- ... XCUDXJ5lZg

http://www.ebay.ca/itm/AEROVOX-CAPACITO ... 0988600189

http://www.ebay.com/itm/10x-05uF-1000V- ... QOY6V5vKuQ

Thank you
Charlie