Looking to tighten amp up with filtering....

[leftu2]

good morning everyone....
I'm still a novice at amp building and literately read about this stuff everyday trying to understand how amps work, so go easy on me ....lol.

I built the design of Rob Robinette's Jcm 6V6 amp a few yrs ago that he has posted on his site.
I'm looking to tighten up the amp using higher value caps in the power section of the preamp and power amp.
The amp is built in a princeton chassis and uses a JJ 500V 40/20/20/20uf cap can in the pwr section and a
22uf 500V cap in the preamp.

I wanna make sure my thinking is right on this. As long as the replacement cap can and cap for the preamp meet the voltage requirements, the higher value of the uf of the new cap can should not have any ill effects. The only change will be the amp has a tighter feel? Is this correct?

I looked at a layout of ceriatone's lunchbox 2203 that uses 6V6s and he's using 47uf caps in the preamp and pwr amp sections.

I'm looking at this CE Manufacturing cap:
https://www.amplifiedparts.com/products/capacitor-ce-mfg-450v-40404040-f-electrolytic

In case anyone wants to look at these...

Rob's design:
https://robrobinette.com/images/Guitar/Marshall/RR2104_Master_Volume_Micro/RR2104_Master_Volume_6V6_Layout.png

Ceriatone's design(click the on the "layout" on the bottom of the page):
https://ceriatone.com/british-style-2202/

[acheld]

Since your amp is SS rectified, you can increase the capacitance in the B+1 node far beyond the 40uF (which is really a value based on a GZ34 rectifier).  How much difference that will make, I don't know.   

Increasing capacitance for remaining nodes (B+2-5) won't effect "tightness" much, IMO.

Excellent one page review:  http://www.valvewizard.co.uk/smoothing.html

[tubenit]

You can also increase bass by changing the post LTPI coupling caps from .022 to .033 or .047


With respect, Tubenit

[bmccowan]

Similar to Acheld's comment - no harm will be done, but I would not expect a big change. You have a choke in the mix?

[leftu2]

Similar to Acheld's comment - no harm will be done, but I would not expect a big change. You have a choke in the mix?

It does have a choke. It's 5H 120ma

[nandrewjackson]

What do you mean by "tighten up the amp"?


If you think it's got a woofy low end, try lowering the first two coupling caps' value.

[nandrewjackson]

If I built this and thought it needed tightened up, I'd do these 2 things.


Lower first 2 coupling caps to 0.003uF. Yes, it seems extreme on paper.


And , like tubenit suggested,  raise value of the coupling caps that come after Phase inverter.  0.4uF.

[leftu2]

Since your amp is SS rectified, you can increase the capacitance in the B+1 node far beyond the 40uF (which is really a value based on a GZ34 rectifier).  How much difference that will make, I don't know.   

Increasing capacitance for remaining nodes (B+2-5) won't effect "tightness" much, IMO.

Excellent one page review:  http://www.valvewizard.co.uk/smoothing.html

Thanks for the link...
If i"m correct, I can just wire in another capacitor in parallel with  the  B+1 terminal on my existing cap can to increase capacitance? Going from the ground terminal to the B+1 terminal

When you say "far beyond" that makes sense to me. I thought that if 40uf was the highest value on the cap can, that's much lower that other amps I've seen like 100uf and higher.
What would you recommend to start with as far as a total capacitance value for B+1?
I'll have to order some caps so I don't want to go overboard and waste $$ on too high uf values


What about the 22uf cap in the preamp ? Would increasing the uf help much?

[sluckey]

The Marshall JCM800-2203 used 100µF for first and second filter and 50µF for the last two filters. Going any bigger will not likely tighten up the power supply any more.

[leftu2]

What do you mean by "tighten up the amp"?


If you think it's got a woofy low end, try lowering the first two coupling caps' value.

When running the gain close to or wide open, even with a ts-9 , when chunking low pwr chords, there's too much hair/fizz/fuzz in the notes.
I do have a switchable .0022uf coupling cap on the V1b plate which helps. Maybe I'll add another .0022uf  for the 1st coupling cap as well and wire that to my existing switch for v1b.

I don't want to filter out so much bass with coupling caps that I destroy the low end level of the amp.

[leftu2]

The Marshall JCM800-2203 used 100µF for first and second filter and 50µF for the last two filters. Going any bigger will not likely tighten up the power supply any more.

Thanks

[leftu2]

If I built this and thought it needed tightened up, I'd do these 2 things.


Lower first 2 coupling caps to 0.003uF. Yes, it seems extreme on paper.


And , like tubenit suggested,  raise value of the coupling caps that come after Phase inverter.  0.4uF.

Thanks for the tip. So pull bass out in the preamp and add it back in the pwr section?
Am I understanding that right?

[acheld]

What would you recommend to start with as far as a total capacitance value for B+1?

100uF (as Sluckey has already stated).     Dumble inspired amps often use 110uF at B+1.   You can do this because you are using SS rectification.   Do NOT do this with a tube rectifier, which have specified limits on the size of the first filter cap. 

As to how best to add the additional capacitance, you can temporarily solder in components in parallel with your existing setup before you commit to major surgery.   

I think you'll have a more enjoyable experience by fiddling with the coupling caps first, one at a time.   Take your time with this and listen. 

And of course, you need to know what "tightening" up an amp means to you.  It could be another speaker would be better, different coupling cap values, a larger filter reservoir.   It's not so simple.   

[sluckey]

And , like tubenit suggested,  raise value of the coupling caps that come after Phase inverter.  0.4uF.

  Surely you meant to say .04µF rather than .4µF? I've never heard of using a huge .4 for those couplers.

[leftu2]

Thanks so much guys for all the help

[ac427v]

Too much "hair/fizz/fuzz" sounds like excessive high harmonic components of the signal aren't getting filtered out before the power amp stage. Usually turning down the presence control gives good results. Is yours working properly?

[leftu2]

Too much "hair/fizz/fuzz" sounds like excessive high harmonic components of the signal aren't getting filtered out before the power amp stage. Usually turning down the presence control gives good results. Is yours working properly?

The amp is working perfectly. Dead quiet at idle, every control works as it should.

I believe I'm running a 120pf cap for the treble-peaker cap. The stk value is 470pf, so I've knocked down the high end quite a bit.

I'm definitively gonna fool with coupling cap values as I've been suggest to do so...

I've done a little more research and I'm wondering about a few things that might also be causing my issue:

1. Pushing the 6V6s is possibly causing some of it as they are known to get a bit muddy quite a bit when pushed.
I'm running the allen output transformer which allows for El34s, 6L6s and 5881s. I might try some El34s I have and see if that helps.

2. I might fool with the grid stopper on V2a. It's at 470k right now, I might raise it a bit and cut some gain and see if that helps.


(I really need a scope so I can see what's going on....lol)

[acheld]

When running the gain close to or wide open, even with a ts-9 , when chunking low pwr chords, there's too much hair/fizz/fuzz in the notes.

Not sure what a ts-9 is, but . . .

With gain set very high, I would expect this amp to distort substantially -- I bet you're hearing square waves.   

Wouldn't this be designed in behavior?    A schematic would be helpful; I can't decipher "layouts" quickly.

[nandrewjackson]

Reply #11, yes


But, if it's a fizzy component to the sound that's bothering you,  I think different Speakers should be tried out.


Especially if you're running wide open. That's when the character of the speaker is most prevalent. 

[PRR]

When running the gain close to or wide open, even with a ts-9 , when chunking low pwr chords, there's too much hair/fizz/fuzz in the notes.

Not sure what a ts-9 is,

May be an Ibanez Tube Screamer pedal? https://www.google.com/search?q=ts-9

[ac427v]

Glad your presence control works. Did you use the 100k value for the feedback resistor? How does it sound with the control on "1"?
If circuit changes are needed, consider what your tweak of the treble peaking cap did. The stock 470pf value allows mid and high frequency signal to bypass the 470k resistor. A 120pf cap only allows the high frequency signal through. On your amp, the mids and bass frequencies are partially blocked by the 470k resistor and small value cap. If you use the stock 470pf value and turn down the Treble control and Presence control you may find your sweet spot.
If that doesn't help then I'll jump on the bandwagon for the overdriven square wave theory.

[leftu2]

Wouldn't this be designed in behavior?    A schematic would be helpful; I can't decipher "layouts" quickly.

Here's a link to the whole page on Rob's 6V6 JCM800 including his recommendations for tweaking the amp:
https://robrobinette.com/RR2104_Master_Volume_Micro.htm

Here's a link to the schematic only:
https://robrobinette.com/images/Guitar/Marshall/RR2104_Master_Volume_Micro/RR2104_Master_Volume_6V6_Schematic.png

[leftu2]

Glad your presence control works. Did you use the 100k value for the feedback resistor? How does it sound with the control on "1"?

Here's the deal on the negative feedback in this amp. I wanted adjustable negative feedback which is on a pot and a depth control as well. I took the amp to a friend who said he could do it. Maybe this is causing the issue? I don't know enough about amps yet to know if his design is causing my issue. I do know that when I turn up the negative feedback control, the amp gets more aggressive and the depth control adds low end when turning it up. Even though the negative feedback and depth control are working, is the design itself causing issues?
See the pic of the layout on what he did.

If circuit changes are needed, consider what your tweak of the treble peaking cap did. The stock 470pf value allows mid and high frequency signal to bypass the 470k resistor. A 120pf cap only allows the high frequency signal through. On your amp, the mids and bass frequencies are partially blocked by the 470k resistor and small value cap.

I was trying different caps trying to cut some of the shrill out of it (mids/high mids).

[leftu2]

Reply #11, yes


But, if it's a fizzy component to the sound that's bothering you,  I think different Speakers should be tried out.


Especially if you're running wide open. That's when the character of the speaker is most prevalent.

I know what you mean about speaker selection. If try chunking distorted pwr chords through my greenbacks, it's a muddy mess....lol
My main speaker combo (in a split 4x12) is a G12T-75 and G12-80. They work great with my other high gain amps like my framus dragon or home-brewed slo 100 (it has el34s)

[stratomaster]

Glad your presence control works. Did you use the 100k value for the feedback resistor? How does it sound with the control on "1"?

Here's the deal on the negative feedback in this amp. I wanted adjustable negative feedback which is on a pot and a depth control as well. I took the amp to a friend who said he could do it. Maybe this is causing the issue? I don't know enough about amps yet to know if his design is causing my issue. I do know that when I turn up the negative feedback control, the amp gets more aggressive and the depth control adds low end when turning it up. Even though the negative feedback and depth control are working, is the design itself causing issues?
See the pic of the layout on what he did.

From the looks of the layout your friend added you'd do better swapping the pots for resonance and feedback controls.  You can also play with the .0022uf cap to change which lows are being affected.

Filtering is the wrong place to look for addressing the problem you're describing.  I'd pull your 120pf treble peaker cap and add it in parallel to the PI snubber to see where that gets you. 

If you move the 1M pot to the resonance control and use the 100k pot to control your negative feedback your presence control will also be more effective at controlling the high frequency content.  You need some degree of appreciable negative feedback for these controls to do much.