Cathode bias Plexi tuning

[SLW]

Hi everybody, The amp I have been working on lately is this cathode biased plexi with 6V6's.  I have been tinkering with the normal channel today.  The normal channel has too much bass response for my taste.  I have reduced the cathode bypass cap from 220uf all the way down to 1uf but there is still too much bass.  I could reduce the bypass cap further, I could lower the coupling cap for the stage.  I guess I could also lower the coupling caps in the PA.  What would you guys do?

[sluckey]

That preamp is the equivalent of a real 1987 Plexi with channels jumpered. It's also the same as the November amp. I personally would not change a single thing. If you want it brighter, turn up the volume on the bright channel. Or turn down the volume on the dark (normal) channel. You can get some really good sounds just by blending the bright and the dark channels.

But if you must, you can make it brighter by copying the bright channel. All the things you mentioned will brighten the sound of the normal channel.

[phsyconoodler]

Here's my 6v6 plexi build. It works exactly as it should.Blend the two channels with a jumper and you control the amount of treble with the volume controls.
  Mine has 320v on the plates,it's a Vox AC30 PT with overseas voltage capability.
It sounds incredibly good.

[tubenit]

Change  C8 from .02 to .03        You can use the Duncan Tone stack calculator to see what this does.

Change  C10 from .02 to .01

I think you will find this makes a significant difference.

with respect, Tubenit

[dnator]

I think your output tube couplers, C12 and C13 should be .02uF. That what they are in the 1987 and 1959 schematics I've seen.

http://www.el34world.com/charts/Schematics/files/marshall/jmp_lead_50w_1987.pdf

[HotBluePlates]

The normal channel has too much bass response for my taste.  I have reduced the cathode bypass cap from 220uf all the way down to 1uf ...

That preamp is the equivalent of a real 1987 Plexi with channels jumpered. ... I personally would not change a single thing. If you want it brighter, turn up the volume on the bright channel. Or turn down the volume on the dark (normal) channel. You can get some really good sounds just by blending the bright and the dark channels.

I agree completely with Sluckey.

I used to have a '73 50w Superbass, which despite the name was essentially the same as the Superlead. Anyway, I had the same gripe with that amp as you: Normal channel is mud, Brilliant channel is an icepick. But if you blend them, the sum sounds great.

So I jumpered channels, and fiddled the volumes of each channel to be my primary EQ. Worked great!

[tubenit]

Normal channel is mud, Brilliant channel is an icepick. But if you blend them, the sum sounds great

The normal channel 220uf cathode cap is already a 1uf now per schematic he posted.

With respect, Tubenit

[plexi50]

Change  C8 from .02 to .03        You can use the Duncan Tone stack calculator to see what this does.

Change  C10 from .02 to .01

I think you will find this makes a significant difference.

with respect, Tubenit

The .1 coupling caps should be .022. .1 allows way to much bass frequency to pass. .022 will tighten the bass up nicely

[SLW]

Thanks for the replies fellas. 

That preamp is the equivalent of a real 1987 Plexi with channels jumpered. It's also the same as the November amp. I personally would not change a single thing. If you want it brighter, turn up the volume on the bright channel. Or turn down the volume on the dark (normal) channel. You can get some really good sounds just by blending the bright and the dark channels.

That is how I intend to use it.  Right now if I use more than the smallest amount of the dark channel I have a huge amount of bass.  I did not want to change the preamp much.  I went from 220uf to 22uf and the bass did not decrease.  Then I went to 2.2uf without any decrease in bass.  Then I went to 1uf thinking that this should really change things but it did not.  That is when I stop and ask you guys since it seems that there is something else going on. 

I wanted to build a 6V6 cathode biased plexi.  I had not seen a schematic around for one so I came up with this one on my own.  I simply mated the plexi preamp to the PI from the TOS donor amp and changed the PA to match the values for a Deluxe.  I simply decided to start here and see where it took me.  It has been been fun so far.  Everything on the schematic is open to change. I am very willing to believe that the PI and/or the PA need to be adjusted to get the thing sounding best.  I did not even give any thought to the NFB and just used what was there.

Since a couple of you have suggested the 0.1 coupling caps should be changed to 0.022 values I will try that first.  If you have suggestions for the PI or NFB values I would like to hear them.

I am not the best with words so please forgive me if I have been confusing.  I value your guys experience and insight. 

[HotBluePlates]

...  I went from 220uf to 22uf and the bass did not decrease.  Then I went to 2.2uf without any decrease in bass.  Then I went to 1uf thinking that this should really change things but it did not.  That is when I stop and ask you guys since it seems that there is something else going on.  ...

No, that's pretty normal.

Even a 25uF cap is an effective bypass down to about 8Hz. So 220uF (or 330uF as seen in some versions) is ridiculously large, and likely intended to help kill hum due to heater-to-cathode leakage in the input tube.

Trimming down to 1uF with an 820Ω cathode resistor is -3dB down at ~195Hz. That sounds high, but low E is only about an octave lower, and -3dB is not a drastic reduction. It just sounds like "cleaner bass" instead of "less bass".

I can only tell you that a Strat in the Normal channel of my old Marshall was too muddy for me, and I liked thick-sounding guitars at that time. If I was playing a humbucker guitar, it probably would've only sounded right in the Brilliant channel.

Question: did you tailor your Brilliant channel to have a nice sound when used alone? What guitar/pickup are you using?

I ask because if either sounds "right" on its own, they probably won't be different-enough to be effective for blending to adjust the tonal balance. In other words, you almost need the bright to be too-bright and the dark to be too-dark if you want to have enough range to blend them to get a good resulting sound.

[phsyconoodler]

Hmm....the jumpered channels should do it.Recheck your work or make sure you don't have a bum patch cable or speaker issue.

[sluckey]

Here's a link to Mark Huss's plexi 6V6. It's fixed bias but other than that your amp is about the same. He uses some of the component values mentioned here. Should give you some ideas. Mark is a very reputable amp guy.

http://mhuss.com/Plexi6V6/index.html

Right now if I use more than the smallest amount of the dark channel I have a huge amount of bass.

My November amp's dark channel is way too dark for me too. But I like the sound of it being blended with the bright channel. You didn't use linear pots for the volume controls did you?

[tubenit]

I built a Texas Raptor that was essentially a cathode biased 6V6 Plexi with reverb and a paralleled V1.  

It was not too bassy at all. I thought it had a very balanced tone to it and it would cut thru a band mix very easily. The schematic and layout showing values are here:

http://www.el34world.com/Forum/index.php?topic=2442.0

With respect, Tubenit

[SLW]

Thanks again guys.

Queastion: did you tailor your Brilliant channel to have a nice sound when used alone? What guitar/pickup are you using?

Not really.  I have removed the treble bypass cap from the 470K mixing resistor on the bright channel.  That is the only change that I have made.  I am not sold on it.  I am testing it with all of my guitars, tele, strat, and humbuckers.  I play each for 30 min or so and make a decision what to do after that.

My November amp's dark channel is way too dark for me too. But I like the sound of it being blended with the bright channel. You didn't use linear pots for the volume controls did you?

The pots are log.

After looking over the schematics that you all linked to I did notice that the 0.022 coupling caps before the power tubes is common to all the versions so I am going to change it to those values.  I will report back

[plexi50]

Did i ever mention that i love plexies?

[phsyconoodler]

Me too.

Here's a gut shot of my 6V6 plexi. Cathode biased,AC30 PT,Allen OT. Choke......pure magic!

[SLW]

I did get to tinker with the amp yesterday.  I changed power tube coupling caps from 0.1 to 0.022uf.  That did correct the overwhelming bass.  It does make sense now that I study the circuit and bother to calculate some things.  That cap and the 220k resistor form a high pass filter.  The cutoff freq with the 0.1 cap is about 7.2 Hz.  With the 0.022 cap it is about 33 Hz.  That still seems pretty low for guitar use but I can hear a difference between the two.

Here's a gut shot of my 6V6 plexi. Cathode biased,AC30 PT,Allen OT. Choke......pure magic!

Will you share resistor values for the power stage?   I just used values from a deluxe.  I never could find a schematic of a cathode biased plexi to study.  I would like to see what you did.  How did you set up the NFB and the presence portions of the circuit?  I want to adjust those next.

[SLW]

Just a final follow-up to my cathode biased plexi project.  I wanted to share the final schematic.  It is nothing new or ground breaking but perhaps somebody will find it useful.  I will probably still tinker with the tone stack but I have not changed it in the last two weeks of playing it.  Thanks to all the members that gave their input.  It has been fun.