50watt Plexi cathode bypass and other voicing mods help

[Matt117]

Hey there,

I'm starting to monkey around in my 50watt plexi. This was my first build and am revisiting it after spending some time playing it, knowing how it sounds and what I'd like to change to voice it more to my liking!
Its a ceriatone plexi all access 50watt, so it has a lot of mod values on switches already.

My target right now is somewhere between the normal and bright channels voices.
I find the normal channel too bass heavy, and the bright channel lacking a bit of body. Being a plexi with the tone stack so late in the circuit, it does not have quite the affect I was hoping for in evening everything out to where I want it.

I know the obvious solution is JUMPER the channels! Well... it gets too gainy to quickly for me when doing this. I like having the sparkly clean tone, and then add pedals for more gain.
I like to keep the channel volumes at 2 or below, to keep it in the Marshall clean world, as well as play on the low input, the high inputs get to ragged (distorted) for me.

The normal channel has a 330/820 cathode cap/res, and the bright channel a .68/1k. So I had some thoughts of running a 25uF and 1.5k or so. Running some calculations on the calculators over at ampbooks.com, I see that the 330/820 and 25/1.5 result in almost to the decimal point same gain/db at 82hz and 10khz.

I have bright caps on switch, and currently use the bright channel without a bright cap -- but looking further into the circuit it looks like a 500pf still exists in parallel/bypassed? by the 470k mixer, just with the option of putting a 100pf or 1000pf in on the front panel switch?.. I've attached the layout incase I am misinterpreting this.. confused a bit here. Not quite sure how to track that layout, but looking at the schematic of a 1987/1986 going I see a 500pf there. Using the calculators again, I wonder what a 1000pf-1500pf would help split the difference. Anyone played with this value?

Other thought was going bass spec on the PI output caps or at least to 0.1uf from the 0.022uf. Might be too late in the circuit too.. Not sure.

Looking for advice from you all who are much more knowledgeable than I am.

Thanks!

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[bepone]

- you have many options, all swithes put in off, lets comment basic amp topology

- on normal channel you can put 1uF on V1 cathode instead of 330uF to remove bass/ or any other value which you will like
- on normal channel you can add 47pF over gain pot from middle to max viper to add presence on lower pot. turn position
- when you are using normal channel keep treble channel gain pot rotation close to max to add more presence to normal channel / you will find that normal is not so muddy at all
- on treble channel you can add 1uF i paralell on 0.68 (1.68) to add some bass / or any other value which you'll like
- you can lower PI entrance cap from 0.022 to 1n5 to remove excessive bass further
- you can put Sozo caps in both 22nF tone stack capacitors to tame excessive highs and mellow things
- tone stack 56k-270pF move to 33-39k-470pF to add brightness and remove lows
- master volume how it is made now is killing highs below 70% of rotation so i suggest to keep master as high as possible (80%) is ok to preserve tone

[statorvane]

Might want to try a 12AY7 in V1.

[pdf64]

Try removing the 470pF cap across the treble channel’s 470k mixer resistor (I hate the effect of that cap ).
Switch to shared V1 cathode.
Switch to old style tone stack, ie with 56k slope resistance. Turn the presence and master vol up full.
Try the treble channel’s bright cap at 100pF.
My thinking is to revert the 1987 back closer to a 5F6a.
Changing the 470k mixers to 270k should help too.
How does each channel sound now?

[Matt117]

Hey Thanks all for your replies! I will give those things a try!

- I'll fuss around with the normal channel a bit more and see if I can tweak it to more of my liking.
- The 12ay7 to lower gain is a good call too!
- I've been using the shared cathode setting - so 330 is on all the time, I could try bringing that down to 1uf, or 4.7uf or 25uf I have on hand.
- luckily I have 2 sozo 0.022uf kicking around so I'll toss those in the tone stack coupling caps
- I've been using the 33k/500pf tone stack slope values - sounds more open which I like.


Thanks all I'll play around a bit more to ensure I am not missing a combination I have in there already - then will also check out the mixer value -- 0 or 270k as mentioned.
Thanks for all the ideas!!

[Matt117]

- you have many options, all swithes put in off, lets comment basic amp topology

- on normal channel you can put 1uF on V1 cathode instead of 330uF to remove bass/ or any other value which you will like
- on normal channel you can add 47pF over gain pot from middle to max viper to add presence on lower pot. turn position
- when you are using normal channel keep treble channel gain pot rotation close to max to add more presence to normal channel / you will find that normal is not so muddy at all
- on treble channel you can add 1uF i parallel on 0.68 (1.68) to add some bass / or any other value which you'll like
- you can lower PI entrance cap from 0.022 to 1n5 to remove excessive bass further
- you can put Sozo caps in both 22nF tone stack capacitors to tame excessive highs and mellow things
- tone stack 56k-270pF move to 33-39k-470pF to add brightness and remove lows
- master volume how it is made now is killing highs below 70% of rotation so i suggest to keep master as high as possible (80%) is ok to preserve tone

So with the master up to 6 and above (probably the 70% you mentioned) it seems to get the power tubes working more, wish made it cleaner, and loose the "low volume dirty when set clean" problem. If anything.. its too loose with the gain up higher on (either channel). I think I will still change the cathode cap from 330uf to 1 or 25uf to see how that goes. As it probably still a bass related issue.
I had forgotten about that trick using the bright gain/volume up even when on the normal channel! I had done that in the past and just had completely slipped my mind. thanks for that reminder.

[bepone]

first find proper tone balance what you want and then we can lower the gain 10x this will be ok

[Matt117]

first find proper tone balance what you want and then we can lower the gain 10x this will be ok

-Put in a 25uF on the cathode, and swapped to sozo 0.022 on the tonestack.

I don't really notice much difference with this 25uF compared to the 330uF.
Which is what the calculator showed me anyways - so I wasn't expecting much.

The sozo, maybe a bit more laid back feeling.

My amp has 405vdc on the plates / ct - definitely lower than most I've read.

Another question for you, do you have any reference photos of what you'd consider great lead dressing in a plexi?
I am planning to continue to refine mine. I have been poking around with a chop stick, the V1a and v2a grids are really sensitive. Is it advised to run shielded cables from the grid resistors to the pin? I've done this in my dumble build.

I'll get some photos of mine on here later, which would be awesome to have some knowledgeable eyes on to help.
I was really nervous when I built this last year, about having underboard jumpers on the turrets and possibly falling out when changing parts (which I knew I wanted to do as part of hands on learning). So I ended up.. after having the board populated, placing jumpers on the top side of the board.
Lesson learned to do that first.
The other thing I've since seen is.. running them under board still. However, pulling the lead all the way through the turret and folded over the top. If I pull the board out at some point, I will do that change.

[bepone]

-Put in a 25uF on the cathode, and swapped to sozo 0.022 on the tonestack.

I don't really notice much difference with this 25uF compared to the 330uF.
Which is what the calculator showed me anyways - so I wasn't expecting much.

there is no difference between 25uF and 330uF in cathode. i recommended this:

- on normal channel you can put 1uF on V1 cathode instead of 330uF to remove bass/ or any other value which you will like
- on normal channel you can add 47pF over gain pot from middle to max viper to add presence on lower pot. turn position

1uF , not 25uF. put 1uF. or less, 0.68uF, in cathode bypass where you have 25uF now.

The sozo, maybe a bit more laid back feeling.

better for overdrive, but you will wait for that

My amp has 405vdc on the plates / ct - definitely lower than most I've read.

Another question for you, do you have any reference photos of what you'd consider great lead dressing in a plexi?
I am planning to continue to refine mine. I have been poking around with a chop stick, the V1a and v2a grids are really sensitive. Is it advised to run shielded cables from the grid resistors to the pin? I've done this in my dumble build.

about lead dress is too long story you need to find what works for you. you can shield yes, all grid g1 wires in preamp

I'll get some photos of mine on here later, which would be awesome to have some knowledgeable eyes on to help.
I was really nervous when I built this last year, about having underboard jumpers on the turrets and possibly falling out when changing parts (which I knew I wanted to do as part of hands on learning). So I ended up.. after having the board populated, placing jumpers on the top side of the board.
Lesson learned to do that first.
The other thing I've since seen is.. running them under board still. However, pulling the lead all the way through the turret and folded over the top. If I pull the board out at some point, I will do that change.

ok give some pics and we will see

[Matt117]

-Put in a 25uF on the cathode, and swapped to sozo 0.022 on the tonestack.

I don't really notice much difference with this 25uF compared to the 330uF.
Which is what the calculator showed me anyways - so I wasn't expecting much.

there is no difference between 25uF and 330uF in cathode. i recommended this:

- on normal channel you can put 1uF on V1 cathode instead of 330uF to remove bass/ or any other value which you will like
- on normal channel you can add 47pF over gain pot from middle to max viper to add presence on lower pot. turn position

1uF , not 25uF. put 1uF. or less, 0.68uF, in cathode bypass where you have 25uF now.

The sozo, maybe a bit more laid back feeling.

better for overdrive, but you will wait for that

My amp has 405vdc on the plates / ct - definitely lower than most I've read.

Another question for you, do you have any reference photos of what you'd consider great lead dressing in a plexi?
I am planning to continue to refine mine. I have been poking around with a chop stick, the V1a and v2a grids are really sensitive. Is it advised to run shielded cables from the grid resistors to the pin? I've done this in my dumble build.

about lead dress is too long story you need to find what works for you. you can shield yes, all grid g1 wires in preamp

I'll get some photos of mine on here later, which would be awesome to have some knowledgeable eyes on to help.
I was really nervous when I built this last year, about having underboard jumpers on the turrets and possibly falling out when changing parts (which I knew I wanted to do as part of hands on learning). So I ended up.. after having the board populated, placing jumpers on the top side of the board.
Lesson learned to do that first.
The other thing I've since seen is.. running them under board still. However, pulling the lead all the way through the turret and folded over the top. If I pull the board out at some point, I will do that change.

ok give some pics and we will see

Hey thanks. Yeah looks like I missed over some of your advice. I’ll dig around or order a 1uf.
I think for now I’ll leave the 47pf off the normal volume.

I think the sound in my head is some combination of vox meets Marshall. Fullness of Marshall with the chime/sparkle of a vox.
Which is what is driving these mods.

Here it is.. don’t roast me too bad. First build.. also I know for sure that v1 needs cleaning up.

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