18W tremolo PPIMV details

[Bieworm]

hi

I want to add a PPIMV on my 18W build. When looking online for schematics something is not that clear to me...

I use a 500k dual audio pot. The plan is connecting 2 shielded wires from the righthandside lugs (seen from the back of the pot) to the junction of the two .01uf caps + 470k resistors. I remove those resistors as now the pot replaces them. The ground goes from the lefthandside lugs to the ground turret where the other side of the two 470k resistors were connected.
Further.. I have to attach two other shielded wires from the wipers to lug 2 on both powertubes (EL84 / 6BQ5)
according to the schematic I should attach two 2M2 resistors between grond and wipers of both pot-gangs.

my questions are:

1. the shielding from the wires: I shrinktube/ seal-off the side where te wires are connected to the .01caps and the grids of the powertubes + connect the shielding to the ground-lug of the pot?

2. when the wipers from the pot are connected to the grids... do I have to remove wires + the two 8K2 resistors that lead to the conjunction of the .01 caps and the former 470k resistors?

3. shouldn't I use these 8k2 resistors on the pot instead of the 2M2 resitors?

Here are the standard schematic and the adapted version (changes highlighted)

18Wceriatone.jpg

18W PPIMV MOD.png

[geoff 1965]

i have'nt seen that version applied, but i initially had the dual 500K pot which replaces the 470K grids and that was good but not really suited to the 18W it cleaned the power section up too much,okay if your breakup is coming from the preamp. now i have the one in the downloads which is very good,check it out.

[Bieworm]

i have'nt seen that version applied, but i initially had the dual 500K pot which replaces the 470K grids and that was good but not really suited to the 18W it cleaned the power section up too much,okay if your breakup is coming from the preamp. now i have the one in the downloads which is very good,check it out.

Which one do you mean?

[geoff 1965]

storage/18wattMV.gif
if you use 820K grid leaks in parallel with the 1 meg pots it gets you closer to the 470K area, you'll need 2 extra turrets to seperate the 8K2's and coupling caps,also keep the same 8K2 and capacitor from each side going to the same pot.

[Bieworm]

storage/18wattMV.gif
if you use 820K grid leaks in parallel with the 1 meg pots it gets you closer to the 470K area, you'll need 2 extra turrets to seperate the 8K2's and coupling caps,also keep the same 8K2 and capacitor from each side going to the same pot.

What is the big difference in comparison 500K pots? Is the 30K going to be so different?
The lay-out I drew is a copy of:

20200213_185638.jpg

[colossal]

Bieworm,

I think you are a little confused on the 18W application of the PPIMV. The PPIMV is intended to replace the 470k/470k grid leak pair on the power tubes, making them variable with the dual gang pot. The drawing you are referencing shows 250kA-dual with 2M2 resistors in parallel. The 2M2 resistors pull down the pot value to 225k, which is close enough to the standard 220k grid leak values used in other amps, such as a Plexi or Vox AC30. The 2M2 resistors also serve as a bit of protection if one or both of the wipers fail on the pot. To apply this to an 18W amp, you want to use either a 500kA dual gang, or, a 1MA-dual with 1M in parallel to reach 500k. The 8k2 resistors are grid resistors for the power tubes themselves and should be either (ideally) mounted directly on the tube sockets, or, traditionally, on the turretboard.

[Bieworm]

Bieworm,

I think you are a little confused on the 18W application of the PPIMV. The PPIMV is intended to replace the 470k/470k grid leak pair on the power tubes, making them variable with the dual gang pot. The drawing you are referencing shows 250kA-dual with 2M2 resistors in parallel. The 2M2 resistors pull down the pot value to 225k, which is close enough to the standard 220k grid leak values used in other amps, such as a Plexi or Vox AC30. The 2M2 resistors also serve as a bit of protection if one or both of the wipers fail on the pot. To apply this to an 18W amp, you want to use either a 500kA dual gang, or, a 1MA-dual with 1M in parallel to reach 500k. The 8k2 resistors are grid resistors for the power tubes themselves and should be either (ideally) mounted directly on the tube sockets, or, traditionally, on the turretboard.

Thanx for pointing that out for me
So, I have only got a dual gang 500K pot and want to use that one . So on the mod-layout in the starting post, I leave out the 2M2 resistors of the pots if I understand correctly? But how do I do the wiring on the 8k2 resistors ..since I have to wire the pots to the gids of the power tubes. Could you help me on that one?
Let' s say I put the 8k2 resistors back to the stock position.. I wire the pots to the junction of the wire coming from the gids to the turrets where the 8k2 are attached? That should be it? Or is it better to wire directly from the pot to the grid on the tube sockets?
And where do I preferably attach the one end of the shielding of those 4 wires coming from the dual 500K pot?
Can I do something else to safeguard in case the wipers malfunction?

[geoff 1965]

you can't have the 8K2's in the stock position as they need to be seperate from the coupling caps,so if you don't have room for the extra turrets put them on the grid pin of the el84's socket like colossal suggested and wire from the pot wiper to them. if you bridge a piece of wire across the ground lugs of the pots you can connect your shield grounds to that.
the wipers only have the very small AC signal voltage on them so the main component regarding failure is the coupling caps on the phase plates,make sure you have good quality ones here.

[Bieworm]

you can't have the 8K2's in the stock position as they need to be seperate from the coupling caps,so if you don't have room for the extra turrets put them on the grid pin of the el84's socket like colossal suggested and wire from the pot wiper to them. if you bridge a piece of wire across the ground lugs of the pots you can connect your shield grounds to that.
the wipers only have the very small AC signal voltage on them so the main component regarding failure is the coupling caps on the phase plates,make sure you have good quality ones here.

Those coupling caps are SOZO caps, so I think I'm good there.
Here is the adapted layout... this is how it should be wired I think?

The attached pic is where I stand in my build. I was wondering if the wiring to the filaments is done correctly... they have to be very closely twisted, but that isn't easy at the tube sockets. So I hope this won't give me hum. What's your opinion?

[geoff 1965]

no that won't work! you need to seperate the coupler from the 8K2 i.e. the coupling cap from the phase plate goes to lug 1 of the pot then lug 2 "wiper" goes to the 8K2 into the grid.

[Bieworm]

no that won't work! you need to seperate the coupler from the 8K2 i.e. the coupling cap from the phase plate goes to lug 1 of the pot then lug 2 "wiper" goes to the 8K2 into the grid.

like this then? god it's confusing... but I'm learning

18W PPIMV MOD rev1.png

[Bieworm]

to all: thanks for being very patient and very helpful!!!

My aim is to use this amp at home + rehearshal and small gigs. So the MV can be applied when needed...

My other amps are a Vibro King and a Custom Vibrolux reverb. They are on the edge of being too loud foor rehearshal. Our music is kinda Tindersticks, Nick Cave,... being very subtle and intimate sometimes.
Reverb is going to be a problem with the 18W. I need verb and every pedal I've tried was disappointing (..owning a VK might be the origin of the woe, I'm really spoiled there)
But that's a problem further on down the road. Let's get this amp operating in the first place. For later reverb I'm tinkering on adding a small chassis or stompbox housing that will contain the wiring and valve + transformer for the reverb. I plan on placing it on the sideboard under the amp's chassis... (crazy?)

[JMPGuitars]

Reverb is going to be a problem with the 18W. I need verb and every pedal I've tried was disappointing

I've built plenty of 18 watt amps with reverb. Look for the "one tube reverb" or "single valve reverb" on google and you should find it.

[Bieworm]

Reverb is going to be a problem with the 18W. I need verb and every pedal I've tried was disappointing

I've built plenty of 18 watt amps with reverb. Look for the "one tube reverb" or "single valve reverb" on google and you should find it.

Thx for the tip!!
What is your opinion of my MV lay-out? Is the last layout going to work?

[zaphod_phil]

Reverb is going to be a problem with the 18W. I need verb and every pedal I've tried was disappointing

I've built plenty of 18 watt amps with reverb. Look for the "one tube reverb" or "single valve reverb" on google and you should find it.

JMP, you already know what I think about putting reverb into Marshall 18W type amps! I call it desecration.

[zaphod_phil]

What is your opinion of my MV lay-out? Is the last layout going to work?

Bieworm, there are plenty of posts on this general topic already. Since the classic Marshall18W gets nearly all of its distortion from the power stage, a post-PI MV is going to be pretty ineffective in any case. It will basically just clean up the tone as you turn down the MV. You really need an amp that has a lot more distortion in the preamp, and use a post-preamp MV instead - ie you would want to build an 18W TMB instead. Otherwise, a VVR or an attenuator between the amp output and speaker(s) are your best options.

[JMPGuitars]

JMP, you already know what I think about putting reverb into Marshall 18W type amps! I call it desecration.

Lol! I know all too well, my friend.

I still like it though, but I agree, it isn't needed.

[Bieworm]

What is your opinion of my MV lay-out? Is the last layout going to work?

Bieworm, there are plenty of posts on this general topic already. Since the classic Marshall18W gets nearly all of its distortion from the power stage, a post-PI MV is going to be pretty ineffective in any case. It will basically just clean up the tone as you turn down the MV. You really need an amp that has a lot more distortion in the preamp, and use a post-preamp MV instead - ie you would want to build an 18W TMB instead. Otherwise, a VVR or an attenuator between the amp output and speaker(s) are your best options.

Maybe I will be very disappointed;)
I can dime the MV anyway and get the powertube overdrive.
It may sound funny, but I'm not really after the distortion.. just a little beef and crunch. I play clean on the verge of breakup mostly. I hope to reach that with the 18W . The MV is probably just for homeplay
It will probably be just fine without the MV.. won't be using it a lot. I'm used to a VK, so volumewise it'll crunch at the volume I am used to play anyway.
The build is nearly finished I'm afraid. With the MV already in position. Just have to hook up the preamptubes and I am gonna light it up. Any warnings before startup except checking all the wiring? Going to do a voltage check first ofcourse

[geoff 1965]

yeah try it,it might suit you but that's why i mentioned earlier about the dual 500K being more suited to amps with preamp overdrive. ZP is right and it actually is more a post inverter drive than volume.you can easily swap to the 1 meg dual & grid leaks if you don't like the 500K, you get more breakup at lower volume with that version.
good luck

[Bieworm]

yeah try it,it might suit you but that's why i mentioned earlier about the dual 500K being more suited to amps with preamp overdrive. ZP is right and it actually is more a post inverter drive than volume.you can easily swap to the 1 meg dual & grid leaks if you don't like the 500K, you get more breakup at lower volume with that version.
good luck

You're right...you did say that.
I will see . if i don't like it I can always adapt to 1M dual pot. The wiring is there already