Best place to mod preamp to reduce blocking distortion?

[leftu2]

I'm modding a peavey windsor fairly successfully and I need to reduce some more blocking distortion.
My question is what are the best places to mod the preamp, any preamp not just this one, to control blocking distortion
without messing up the harmonic content of the amp which is pretty good right now. I do want to be able to get more distortion out of the
amp at some point without going to diode clipping and I figure I should be able to get it with 4 gain stages if the preamp is tuned properly.

I changed the high pass filter that was formed at C2 and R13 between the gain control and V1b grid to a 470K grid stopper at C2
which helped quite a bit, should I make that a voltage divider instead?

-Mod the voltage divider values between stages?

-Mod the cathode resistor/cap and plate values?
   -Can I keep theses values and mod around them
    to keep the harmonic content close to the way
    they are now?

-Mod the grid stopper values?

I'm including the preamp schematic which shows the mods I've done in highlighted in yellow.

In stk trim this amp has 4 gain stages, plate driven tone stack and a cathodyne pi.
A few other mods that are not in the schematic I'm showing is, I have changed to pi to a ltp using
marshall values and changed the coupling caps from the pi to the pwr tubes to .022uf from the stk .01uf,
changed the neg fb resistor to 100k from the stk 33k and changed the presence cap from the stk .22uf to .1uf

I don't have a scope yet but ordered one today along with a bnc 1/4 cable for the signal generator so I can see whats going on
instead of guessing.

****After I posted my question last night I found out the grid stoppers for the power tubes were 1.5k, with a little research
I learned that increasing the value of those will help prevent blocking distortion. Though I don't know if that's only at higher volumes?
I'm going to change them to 5.6k as that's the marshall value
 

[AlNewman]

What's your goal?

[Merlin]

Your circuit already has high-resistance dividers between stages, I kinda doubt it has much problem with blocking distortion?

[leftu2]

Your circuit already has high-resistance dividers between stages, I kinda doubt it has much problem with blocking distortion?

The low notes are buzzy, even if the gain control is at 9-10 o'clock and the bass control at 1/2 way with the resonance control at 0.
Stk the coupling caps at V1a and V2b were .047uf. I put a .0022uf coupling cap in onV1a, and changed V2b to .022uf.
V1b and V2a had .022Uf coupling caps already. I think I have a lot of low end trimmed already with those cap values so I don't think that's my issue.

After I posted my question last night I found out the grid stoppers for the power tubes were 1.5k, with a little research
I learned that increasing the value of those will help prevent blocking distortion. Though I don't know if that's only at higher volumes?
I'm going to change them to 5.6k as that's the marshall value.

I really like the direct feel of the plate driven tone stack but i wonder if that's the effect I'm hearing vs the compressed feel of a cathode driven tone stack?
I'd love to hear from someone who has experiance with playing amps with plate driven tone stacks vs cathode driven stacks

[leftu2]

What's your goal?

To clean up the note definition and up the gain while keeping the note definition, no buzzy notes.
Again, this amp has a plate driven tones stack so the feel and response is much different than a cathode driven tone stack.
It has a very direct feel vs the compression of the cathode driven stack.
I'm hoping the issues isn't with the response of the plate driven tone stack as splawn uses it in his high gain amps and that issue doesn't exist.

[acheld]

I need to reduce some more blocking distortion.

Could you clarify this statement?   What symptoms are you hearing?

[leftu2]

I need to reduce some more blocking distortion.

Could you clarify this statement?   What symptoms are you hearing?

Low notes are buzzy even if the gain is from 9-12 oclock . Which isn't that much gain at all.  I believe that I have reduced enough bass in the preamp circuit with changing a few of the coupling caps, especially the first stage is now 0022uf. I don't think the low end is the issue. Again, I'm wondering if it's just the nature of a plate driven tone stack but then again splawn has gotten it right with the quick rod in the nitro, I have been looking at some of their schematics but of course being on a pcb board, I'm a bit limited at what I can change as I don't want to go crazy cutting traces and things of that nature to completely match the splawn layout.
Like I said in my original post, changing the high pass filter that was in between the gain control and the grid at v2b to a single 470k grid stopper definitely has helped the issue, I believe the input of that stage was being overdriven too hard causing the blocking distortion sound. It's definitely not normal distortion that I'm hearing.

Like I said in my original post, I want to know the best place to adjust the amp to get rid of blocking distortion, is it the grid stoppers or voltage dividers?  I'm assuming so at this point as I really don't want to change the cathode resistors and cathode bypass caps and plate load resistors as I like the harmonic content of the amp now, I'm ust trying to get more definition and get rid of the buzziness.

[stratomaster]

I don't think you're hearing blocking distortion, but rather the graininess of the cold clipper and other voicing choices you've made.

Try reverting the V2 cathode networks back to stock before the tonestack and add the grid cap to ground back in at the input stage.

You'll probably need R13 back as well, and V1B is biased cold which could be buzzy too.  Revert that to stock, or at least cut the resistor in half and leave the cap off to keep the reduced gain you were going for.

[shooter]

don't think you're hearing blocking distortion

scope it n know, listen'n just makes rabbit hole where no rabbits live

[leftu2]

don't think you're hearing blocking distortion

scope it n know, listen'n just makes rabbit hole where no rabbits live

...and that's why i ordered a scope. I'm tired of guessing.
Would you be so kind to tell me what to look for when scoping the amp to see if blocking distortion is occurring? I'm a  100% novice on how to use a scope. I did watch Jason's video from head first amplification where he kind of goes through a jtm 45 but he never says "x"  Is a bad sign etc.
I still would like to know where's the best place to adjust the preamp in the event of blocking distortion, nobody has answered that one yet 😉

[leftu2]

I don't think you're hearing blocking distortion, but rather the graininess of the cold clipper and other voicing choices you've made.

Try reverting the V2 cathode networks back to stock before the tonestack and add the grid cap to ground back in at the input stage.

You'll probably need R13 back as well, and V1B is biased cold which could be buzzy too.  Revert that to stock, or at least cut the resistor in half and leave the cap off to keep the reduced gain you were going for.

I was wondering if the 4.7k non bypassed resistor at v2b could be the cause as even in a slo 100, I believe all cathodes are 1.8k/ 1uf except the 39k cold clipper  if I remember correctly.
I didn't realize too many cold stages could cause a grainy distortion effect.
I thought the rule was a hot stage after a cold stage. That's why I swapped the 39k to v2a and put the v2a 1.5k/.68uf at v2b.

[shooter]

I don't have links anymore but believe the valve wizard covers it on his page, could be wrong though, I'm a farmer now 


this was from a build that didn't have enough Grid stop IIRC

[leftu2]

this was from a build that didn't have enough Grid stop IIRC

Thanks

[AlNewman]

It's interesting what you did.  What made you decide to mod it that way?  Did you replace 1 piece at a time, or did you do it all at once?
4 gain stages is really a greased pig. 

[leftu2]

It's interesting what you did.  What made you decide to mod it that way?  Did you replace 1 piece at a time, or did you do it all at once?
4 gain stages is really a greased pig.

I picked the amp up cheap and wanted to start messing around with it and see what I could get out of it as in stock configuration it was horrible, to me anyway. My goal was to get it a little more Marshall sounding but have a lot more gain than a jcm 800 with it having the 4th gain stage. I like the way the plate driven tone stack feels and wanted to start modeling this amp so that I could get more gain and more definition out of it while retaining the plate driven tone stack because I like the open and direct feel it has, that's what the goal of my mods are.

I did not do all the mods all at once. I started with coupling caps and trying to knock down some of the low end out of the amp to tighten it up, swap the big resistor for a choke, then I did the cathodyne pi to long tail pi conversion along with changing the negative feedback resistor, and coupling caps after the pi, those mods were done at the same time. Then i started tying to tame the highs, this amp is bright...lol. Then I started trying to smooth out the distortion which is where I'm at now.

[pdf64]

I need to reduce some more blocking distortion.

Could you clarify this statement?   What symptoms are you hearing?

Low notes are buzzy even if the gain is from 9-12 oclock .  ...

A sound clip might be really beneficial here.
Blocking distortion in this preamp seems really unlikely.
What folks refer to as blocking distortion is usually something else.

Blocking distortion at the output valves can usually only occur if the output valves themselves are being heavily overdriven.
So unless you're setting the master volume really high (?), it's really unlikely to be the issue you're having.