Taming the beast that is the cascaded 50 watt plexi

[LOPPSTER76]

Hey all, new to the forumn but have been reading it for years. I recently built a 50 watt Plexi with an added LAR/MAR PPIMV. I also added a switchable cascaded preamp so i can flip back and retain the stock plexi sound however, I never changed the values for the different caps and resistors that are recommended for this mod. Understanding theres gobs of gain when it is engaged, if i dime volume I and II i get very nasty squeal so i roll back volume I to about 8 or just till the point where it goes away. My question is if this has anything to do with not changing the values on the board?

[Reeltarded]

Not so much!

You need to snub a little of the RF and audible highend in general. With a similar setup to your's (sort of) I have a 220k resistor on V1a grid and a 33k on V1b grid. They are the green wires that feed that first tube... yah.

I also went to 10k replacements for the stock values (5.6k) at the power section grids.

The resistors are soldered directly to the pins of the tube sockets with the shortest of leads. about --- <-- that long.

The 50s may be prone to a little more nonsense than the 100s, at least in the small box format. Typical! Good luck!

[neikeel]

Not so much!

You need to snub a little of the RF and audible highend in general. With a similar setup to your's (sort of) I have a 220k resistor on V1a grid and a 33k on V1b grid. They are the green wires that feed that first tube... yah.

I also went to 10k replacements for the stock values (5.6k) at the power section grids.

The resistors are soldered directly to the pins of the tube sockets with the shortest of leads. about --- <-- that long.

The 50s may be prone to a little more nonsense than the 100s, at least in the small box format. Typical! Good luck!

Not wishing to be controversial but I find the opposite, my switchable 50 is as quiet as a mouse other than the noises it is supposed to and mine is running the MV in vol II pot with no extra shielded wires and my grids resistors on the board. I find 100w a bit more picky.
How tidy is your wiring? Have you obeyed the rules of good lead dress??

[Reeltarded]

No harm!My 50s shhhhh like a 15mph breeze, but they are a little more savage than the 100s too.

And thanks for all the clues. I constantly copy from you and Roe! Keep talking "amongst yourselves".

[hdahs]

Are there any diagrams/layouts for the switchable cascading gain/plexi setup?

Just finished a 50 watt build using Metro's instructions, that sounds great stock, but I wouldn't mind the option if it isn't going to require a major rewire.

Thanks

Harold

[Reeltarded]

Hi Harold! Sorry I don't have one off the top of my head, but Steve does (SDM)

Google: SDM Marshall layouts. Comcast site has link to his great resources, and good luck!

[neikeel]

This works:
http://home.comcast.net/~jbjdav26/Neil% ... 202in1.pdf
You do not need to rip anything out for this mod.
You keep the channel 1 as a normal plexi. plug into hi 1 and push the gain pot in you are stock
The volume 2 control becomes your simple master volume (just max it for stock plexi)

To get 2204 tones pull out the push pull pot and the hi normal input becomes a 2204. In this mode the lo channel 2 can be used as lo 2204 if you wish.

Steve did an altenative layout which is better in a 1959 chassis.

I have a metro 50w exactly as my layout and it is as quiet as a mouse no oscillation or noise in either mode. More than a couple of people have tried to separate it from me

[hdahs]

Thanks guys

Neil, on the orange line between the the DPDT switch and the bright channel volume pot, there is a .0022uf cap inline. What purpose does that serve?

Also would you happen to know where to acquire the 1Meg Audio DPDT push pull switch for this mod. I dont see it at the usual sources (CE, Valvestorm, etc)

Thanks

Harold

[neikeel]

Thanks guys

Neil, on the orange line between the the DPDT switch and the bright channel volume pot, there is a .0022uf cap inline. What purpose does that serve?

It acts to cut some of the lows out of the first gain stage (just like the bright channel on a stock lead spec 50 or 100w Marshall). If you want a fatter sound then leave that of the switching out and keep the 0.022 in on its own ( a small cap in series with the larger cap acts like the smaller cap).

Also would you happen to know where to acquire the 1Meg Audio DPDT push pull switch for this mod. I dont see it at the usual sources (CE, Valvestorm, etc)

I get mine from Tube amp doctor in Germany (but I am sure if you ask Robert at Valvestorm nicely he might get some in stock )

Here is a slightly blurred pic of my initial version as a two holer:


I'll have to take a couple of pics of the three holer to show how neatly it all fits into the chassis.

[Shakkal]

Also would you happen to know where to acquire the 1Meg Audio DPDT push pull switch for this mod. I dont see it at the usual sources (CE, Valvestorm, etc)

You can also get them here (the small ones):

http://www.guitarpartsresource.com/elec ... llpots.htm" onclick="window.open(this.href);return false;

They also have full sized SPST pots should you ever need one, which I haven't found many elsewhere.

[hdahs]

Neil,

Do you also have the PPIMV installed on your 50 watter, as shown on the 2203/1959 diagram shown above? And if I may ask, why have both the basic master on the VOl II pot, and the PPIMV?

Thanks
Harold

[Reeltarded]

I have a couple amps with three masters, plus the gain controls for both stages of cascade.

The PPIMV sounds like crap at low volume if you like NFB. I like NFB. The simple master sounds like shit at low volume if you like lowend. I like lowend. The Jose master is where I stage how hard I am hitting the PI, so no splatty garbage when the PPIMV is at high attenuation.

Typical settings: 15% knocked off with the PPIMV, Jose @ 50-70%, simple master @ 70-100%, both gains dimed, all tone controls straight up, presence 80-100%. gain staging. The PPIMV should have been listed last, it is the actual volume master.

[Harold]

The Jose master is where I stage how hard I am hitting the PI, so no splatty garbage when the PPIMV is at high attenuation.

This one?



What Zener did you use?

[Reeltarded]

Exactly, but I am using .33uf as coupler instead of the .22.

This coupler has much effect over depth and what happens to the tone slope. I like every variation I have tried, but this one best.

You don't have to add the clipper, but I do have it on 3 switches. Master on, 3 variations on one switch, and 2 non-symetric selections on the third switch, all through a 25k pot to ground so you can dial it in.