Master volume issues - Harsh

[Joe6v6]

So I built a Divided by 13 CJ11 using the attached schematic & layout. The amp works fine, noiseless & very stable. At lower volume with the master all the way up the clean sound is great, very bright , articulate & responds well to playing & tone controls. At lower or higher volume with the volume all the way up & using the master the overdriven / distorted sound is harsh, its as if its almost there but something is causing unpleasant notes & sizzle that shouldn't be there.


The power trans is 300-0-300 @ 120ma & a 5e3 classic tone output transformer.
Right now I have a 5y3 rectifier in that gives 296 on the plates & 278 on the screens, 16.7 across the cathode resistor.
With a GZ34  - I get 336 on the plates 319 screen & 19.3 across the cathode resistor. 
Is the bias with these numbers way high? with either rectifier tube the problem seems the same.


Looking for ideas to sort this out  .    .
Is the high power tube bias a suspect?


Add smoothing caps? / lower cathode caps? if so where to start?


What would be the result of removing the master volume? Is turning this master all the way up the same thing as eliminating it?  Initially I liked the idea of having a master vol that worked but as of now that hasent happened.


Any thoughts or suggestions are appreciated  .   .     Joe

[shooter]

Is the bias with these numbers way high?

336 - 19.3 = ~317Vdc
19.3/250 = ~ .08A
.08 * 317 = ~ 12.2w/tube
guessing modern 6V6?
you can hunt up the data sheet, it's finally a sweat free day with sunshine 

[Joe6v6]

it's finally a sweat free day with sunshine

Very few of those here in South Florida,   ,  So the bias seems reasonable, 

[jjasilli]

the overdriven / distorted sound is harsh. . .
One suspect is the cathodyne PI.  This has come up in some recent threads.  Try grid stoppers on the PI, per Merlin & Robinette.


Also: maybe a grid stopper on the tone recovery stage; larger ones on the power tubes.


Checkout Aiken: blocking distortion article.


Hope this helps.

[ac427v]

I built a version of the circuit and found the Lamar MV to be harsh also. I tried variations of the pre PI and did not like either. Then did the old fashioned post PI master with a dual 1 Meg pot and extra .2 coupling caps. Now my favorite amp. It allows smooth sweet jazzy distortion at very low volumes which works great with my mostly acoustic group. My final build has many other changes from the original but the MV had the greatest impact on the sound.

[shooter]

Very few of those here in South Florida

that's why I wait til Jan to migrate there

[Joe6v6]

I built a version of the circuit and found the Lamar MV to be harsh also. I tried variations of the pre PI and did not like either. Then did the old fashioned post PI master with a dual 1 Meg pot and extra .2 coupling caps. Now my favorite amp. It allows smooth sweet jazzy distortion at very low volumes which works great with my mostly acoustic group. My final build has many other changes from the original but the MV had the greatest impact on the sound.

Thanks for the info - So just to be clear , from what im seeing on the schematic you attached, the master stays in the same position & gets changed to 1meg, remove those 2m2 resistors & add .2 caps where this schematic shows the .1's. ?  Seems very similar to whats already there, maybe im missing something.   .     

[PRR]

There's one 47k in there. This with the 0.02u cap makes a strong bass-cut an octave up from the bottom of guitar. Depending on speaker and genre, this bass-shave *might* sound "harsh". Try changing 47k to 470k (200k-1Meg).

However I suspect this bass-cut is part of "the sound" of this legendary amplifier, so don't expect it to be "the same".

[ac427v]

You're right Joe6V6. Now that I look at the schematic with more care I can see that both master volumes are very similar. So maybe the sweetness is due to another factor.  I did keep the 47k resistor that PRR notes causes bass cut. Maybe the difference in sound is due to the third stage bias? I have a slightly larger resistor and no bypass cap there. Also, the amp runs at low voltage on all tube plates. I think high voltage plates sound harder and have more edge. How does that compare on your amp?

[SILVERGUN]

Have you done what was suggested in Reply#3?


The master volume itself would not be my main suspect.


I would not hesitate to add a grid stopper on the 2nd and 3rd stages.
Like 10K and 100K respectively.


Temporarily remove the cathode bypass cap from the 3rd stage and see if that helps.

[Joe6v6]

I would not hesitate to add a grid stopper on the 2nd and 3rd stages.Like 10K and 100K respectively.

I will have time in the next day or 2 to test out some of these suggestions. I see on AC's schematic a 22k on the 2nd stage & 15k on the 3rd while the schematic I used has none.

I have a slightly larger resistor and no bypass cap there. Also, the amp runs at low voltage on all tube plates.

Im seeing where you have a smaller 1k5 unbypassed as opposed to the 4k7/25uf on the schematic I used.  . . I measured most voltages but only wrote down the power section so thats something I will pay attention to and see where my voltages are compared to whats on your schematic.  Also I see the negative feedback on yours, seems easy enough to implement so as I go along I will try that also.
Its nice to know that your amp sounds so good & it is so close to what I built, gives me hope that I can get this one under control.    .   .    Joe

[Joe6v6]

So after taking the voltages in this amp I guess the first thing I want to do is try to bring my voltages in line with what AC's schematic shows. Im attaching the schematic I used and also AC's schematic with my voltages next to his in Red. All of AC's dropping resistors are different than mine but im not sure what that tremelo is doing to the voltages. So what are some ideas to get my voltages in line with AC's & then I will start in on bypass caps, grid stoppers ect.   .     Joe

[SILVERGUN]

If you're dead set on getting closer to his voltages then you can surely up the values of your dropping resistors. You will wind up with less overall gain, less headroom and your clean sound might not be as nice as it was.

I personally would simply unsolder that third stage bypass cap to see if you're just overloading the PI, just because it's a simple move and might get you a quick answer.

Your amp might actually wind up sounding better than his but you'll never know cause you're probably not neighbors.

[dude]

Then did the old fashioned post PI master with a dual 1 Meg pot and extra .2 coupling caps. Now my favorite amp.

.
This is the same PPIMV that Allen Amps uses, l have an Old Flame with this master vol, fantastic. I put this in my Plexi 6v6 but used .047 caps, didn’t like like it, Allen used (4) .1 caps, perhaps l should try .1 caps ?

[shooter]

my Plexi 6v6

not being a musician MV stuff is like Charlie Browns Techer for me 
that said, my Son who is musical took my plexi and in 20min with a few pedals was doing a great job with Jimi's Star Spangle banner at loud living room 

[jjasilli]

Then did the old fashioned post PI master with a dual 1 Meg pot and extra .2 coupling caps. Now my favorite amp.

.
This is the same PPIMV that Allen Amps uses, l have an Old Flame with this master vol, fantastic. I put this in my Plexi 6v6 but used .047 caps, didn’t like like it, Allen used (4) .1 caps, perhaps l should try .1 caps ?

Seems like apples & oranges.  Old Flame is 6L6 based on Super reverb.  6V6 Plexi is Marshal.  Different power tube drive voltages.  Different signal frequencies.  Suggestion: find & plagiarize MV known to work well ion a 6V6 Plexi.

[dude]

Suggestion: find & plagiarize MV known to work well ion a 6V6 Plexi.

Makes sense, answer: LaMar with the 220k's replaced with 250K dual gang pot, also for more flexibility possibly add a 500KA pre-PI MV, too. 

[tubeswell]

Then did the old fashioned post PI master with a dual 1 Meg pot and extra .2 coupling caps. Now my favorite amp.

.
This is the same PPIMV that Allen Amps uses, l have an Old Flame with this master vol, fantastic. I put this in my Plexi 6v6 but used .047 caps, didn’t like like it, Allen used (4) .1 caps, perhaps l should try .1 caps ?

Yes. The sum of capacitances in series is inversely proportional (so you want the 2nd cap to be about 5x larger than the 1st one to begin to get close to the original capacitance of the 1st one on its own.


.047uF + .047uF = .023uF


And .1uF + .1uF = .05uF


(Or better still .1uF + .68uF = .087uF)

[pullshocks]

Before you go changing the MV, try a 470K ohm grid stopper on the cathodyne.


I breadboarded the CeeJay11 from the same schematic.  After adding that grid stopper, I thought the amp sounded great at all volume settings.  I turn the master volume way down when cranking the volume control, and I did not notice any harshness.


I had about 330 volts on the plates of the 6v6s

[Joe6v6]

I personally would simply unsolder that third stage bypass cap to see if you're just overloading the PI, just because it's a simple move and might get you a quick answer.

This was definitely a move in the right direction but it still has an unpleasant hash to it.

try a 470K ohm grid stopper on the cathodyne.

Im going to try this next - several people have suggested grid stoppers 2nd & 3rd stage.  .   

[ac427v]

Have you also switched off the second stage bypass cap? I omitted that cap when I tweaked my amp. I have other amps that do that tone.I also reduced the plate voltage on the preamp tubes with larger power supply dropping resistors and the added supply load from the tremolo tube. This gives more of a 50s jazz sound to the distortion. The opposite of the Marshall edgy, percussive sound.
I experimented with negative feedback on the power amp but find that I never use that feature. It reduces distortion a bit at high volume but also cuts some of the high frequency and touch response of the amp. So it stays switched in the no feedback position.I enjoy sharing about my quest for "that sound" and reading about your experiences.