Tweed mod for Vibro Champ

[mresistor]

Soliciting input and advice on a project. I have installed a 1M vol pot with a switch in my 65 VC. I have wired the switch so that in not pulled condition it is in the tweed mod mode , where the 15K resistor to ground on the tone stack is lifted. When volume pot is pulled out it reverts back to the original circuit.
In testing this, first with a Jensen C12Q and secondly with a 50w Alnico 12" speaker,  at around 7 on the vol knob the sound gets flabby bass response with undesirable distortion.
Another part of Gerald Webers mod is to replace the stock 250pf coupling cap in the tone stack with the 5F1 .02uf cap.
Will replacing this cap reduce the flabbiness? 

[mresistor]

Of course this is a little variation from GW's actual mod. But should be same effect. Tone stack completely bypassed with it's ground lifted.

[SILVERGUN]

Replace the .047(?) mid cap with a .002 for your Tweed mode (experiment with value .001-.005, to taste)
It allows you to retain some treble/bass control but pulls in plenty of mids / low mids

Check it out on the Tonestack Calculator, and you'll see the huge bump.
It's a great way to switch one wire, get a fuller sound, and not lift the whole tone stack so you can retain some control.

So when your switch is not pulled / activated, you go through the .002 cap (tweed mode)
By pulling the switch, you'll put that cap in parallel with the existing cap. (the low value won't make a huge difference when added to the stock value)

I would wire it so that you replace the stock value with the lowered value, and then have the stock value come right off the switch to ground.

[SILVERGUN]

For anyone who might be playing along at home and doesn't get the calc....
The only difference between the 'stock sweep' and the 'mod sweep' is the mid cap was changed
This also assumes that your fixed value mid resistor is a 15K pot

[jjasilli]

   maybe by this point as we're no longer specifically addressing the Weber Tweed Mod per se.  Another approach, which I used, is a RAW control.  Lifts the FMV mid-cut, which also results in less insertion loss in the tonestack.  I have not found it to sound flabby.  Works for me.

[mresistor]

Well  the champ doesn't have a "mid" control, only bass and treble. I could switch the .047 like you are suggesting but I will still have a tone stack bleeding part of the signla amplitude to ground, I want the boost in signal amplitude and the accompanying distortion that is present in the 5F1/5F2.   By lifting the tone stack ground I have increased the signal level to the vol pot and V1b. Webers mod was to make this a permanent change to "tweed" circuit, but I wan't to be able to switch it, to retain the AA764 sound. In webers mod he changes the small 250pf treble cap to the .02uf arrangement of the 5F1 coupling cap.  This should let more mid frequencies pass when in tweed mode (my non-switched, non-pulled state) I assume. So I will try it.
I'm wondering if the PS capacitance has something to do with the flabbiness. I have an extra filter node, using all 4 sections of the JJ 40-20x3 can. I need to make sure that I move the 40 to the preamp node.

[SILVERGUN]

Soliciting input and advice on a project.

Added GW sweep....I wouldn't use it just because he's more famous than me...
 
I also renamed mine the SG VC ST (super tweed) MOD, so now it's definitely better.
 

 
 

[mresistor]

SG  thanks   I'll have to look at that for a while.   


jj   also   thank you      I don't think I wan't to mount another pot as of yet. Certainly not of the front panel, but maybe in back.


Also  Weber talks about feedback. The tweed fb resistor is 22K and the blackface is 2.7K.  I am going to play around with that as well. I might up the fb res to 22K and see what effect it has on the tweed sound and the bf sound.

[SILVERGUN]

Well  the champ doesn't have a "mid" control, only bass and treble. I could switch the .047 like you are suggesting but I will still have a tone stack bleeding part of the signla amplitude to ground, I want the boost in signal amplitude and the accompanying distortion that is present in the 5F1/5F2.   By lifting the tone stack...

The result of the SG VC ST MOD IS a major boost in signal amplitude, due to the fact that a large range of the fundamental freq. of guitar are attenuated by the stock tone stack. Lifting the entire stack just takes away control, and lifting it with a variable resistance doesn't really address the mid dip.
 
I was ignoring the reference to the 5F1  because there is no tone stack, and we're just talking about the value of a single coupling cap.
 
You're welcome mr....I try to only suggest things that are relevant and that I have tried and had good experience with.
 

[mresistor]

SG - so just change the mid capacitance in the tone stack with eliminates the mid dip and because of the prescence of mids at the same amplitude as the lows and to some extent the highs, there is a volume boost.


I will try it.  I might just want it to sound like that permanently instead of switching it out.

[mresistor]

that's a .002uf cap..in your mod...   right?     


Also  what are all the (values) for all the green lines? And how does one know which line is what?

[SILVERGUN]

that's a .002uf cap..in your mod...   right?     


Also  what are all the (values) for all the green lines? And how does one know which line is what?

Yup, .002 (or .001 to .005 or higher to taste), .002 seems to fill it out nicely
 
The green lines just represent a display of how the entire frequency curve is effected by different pot positions throughout a bunch of different "sweep' combinations...I just used the sweep feature on the tone stack calc to generate those...it doesn't truly show all possible settings, but just gives you a visual reference
 
I think the ultimate control would be found with a combination of this mod and a raw control, as jj suggested

[mresistor]

Thanks SG  much appreciated.   I thought I had already downloaded that TS calc. but couldn't find it. So I did it again, and installed and will play around with it, a very useful tool.
How are you doing those sceen shots? 

[SILVERGUN]

How are you doing those sceen shots?

snipping tool....I think it came in Win Vista and 7

start/all programs/accessories/snipping tool

This thread made me go back and find my original post...I guess I didn't use the sweep button back then.
Nobody really wanted to talk about it way back then in 2015, but I thought it was important enough to create a new thread.

http://el34world.com/Forum/index.php?topic=18155.msg184663#msg184663

[jjasilli]

jj   also   thank you      I don't think I wan't to mount another pot as of yet. Certainly not of the front panel, but maybe in back.


Yes, I put my RAW pot on the rear panel.


Feedback:  Your large speaker cone (& maybe impedance mismatch?) may generate a lot of back EMF; so NFB, or more NFB, might help.  Note that the VibroChamp AB764 also has local NFB at the power tube -- cap, cathode to grid -- in addition to global NFB.

[mresistor]

JJ   I am using a 4/8 ohm OT  and have it set on 8 ohm out and 8K input.  I should have noted that earlier.   It's a classic tone  http://www.classictone.net/40-18030.html


wow  I just noticed that 330pf cap across the 6v6. pin 5 to pin 8 .  I don't have that cap on my amp .   ( I built this amp, on a turret board )

[mresistor]

JJ  I think it would be less Pout if I used the 5K primary.

[jjasilli]

Yes, the AA764 does not show that local (-)feedback cap.  NFB provides damping for the speaker.  For these purposes a guitar speaker is like a woofer.  Once set in motion at any frequency, it also tends to want to resonate -- pump back & forth -- at its own "personal" resonance frequency, which will be a bass frequency.  The speaker's driver assembly then acts like an electric generator, pumping electricity at that frequency back into the amp.  This is called back-EMF (electromotive force).  This back signal is positive feedback at that frequency, which comes back out of the amp as excess bass at that frequency.  This might contribute to the flabbiness you mention.   NFB is a negative signal to combat that positive back EMF signal. 


The long version:
The plate-OT Primary Impedance of 8K, is proper.  Downside: that hi impedance holds up the strength of the unwanted back EMF signal.  This is analogous to a large grid leak resistor maintaining the voltage of incoming signal.  A smaller shunt resistance would bleed more signal to ground.  As that resistance is lowered it becomes a heavier & heavier load on signal voltage.  SS amps tend to act as a short circuit to back EMF; and kill it, providing full damping to the speaker.  Triode power tubes have low plate impedance and so provide good speaker damping.  Small bottle pentodes have high plate impedance, which maintains back EMF voltage; so there's little speaker damping.  That's why more NFB might help.

[mresistor]

Thank you for the analogy JJ.  I think I will order some 330pf  500v caps if I cannot find one in my junk boxes. Then I can add to see what effect it has, hopefully positive.
My AA764 schematic shows that cap. 


http://el34world.com/charts/Schematics/files/fender/Fender_champ_vibro_aa764_schem.pdf

[PRR]

> I just noticed that 330pf cap across the 6v6. pin 5 to pin 8

That's super-sonic oscillation prevention.

The need for it will be very layout dependent.

Do we have hints of oscillations? If not, you don't need it.

[mresistor]

No osc. PRR    thanks 

[mresistor]

I've seen alot of these small caps on silverface amps that were wired like spaghetti ..poor lead dress, to prevent osciallation.    it was the neg feedback label that caused me to take pause.   

[mresistor]

I replaced the mid cap in the TS (tone stack) to 2n.   Also rewired the PS caps so that the 40uf is on the preamp. Tried it out. I left the switch on the vol pot installed.  The volume level in BF VC mode did get a significant volume boost with the change in mid capacitance and sounds pretty sweet. With the TS bypassed, oh wow. It is really loud. I like this set up. I haven't ever heard a 5watt champ this loud. And it sounds great.

[SILVERGUN]

 
...thanks for the update

[Willabe]

Also rewired the PS caps so that the 40uf is on the preamp.

What?

[mresistor]

I am using a CRC filter right after the rectifier. Using 20uf   250ohm  20uf  1Kohm  20uf  1Kohm  40uf  .  IAW an extra filter stage right off the rectifier out to quiet hum.  I moved the 40uf section to last, to the preamp node.

[Willabe]

I am using a CRC filter right after the rectifier. Using 20uf   250ohm  20uf  1Kohm  20uf  1Kohm  40uf  .  IAW an extra filter stage right off the rectifier out to quiet hum.

Yes, I get that.   

I moved the 40uf section to last, to the preamp node.

This is what I was wondering about. ^

That's not very common to do and you can do that, but the last B+ filter node is already the cleanest in the B+ string. So extra filtering is usually not needed there.

There are a number of guys here who feel that 20uF (or larger) in some/most amps at the very last B+ node will take away some of chime and picking touch sensitivity.

They like to go with only a 10uF or an 8uF in that position.   

[mresistor]

that is interesting info..   I have a JJ cap can and the stock capacitance for the preamp tube power node is 20uf, and it had plenty of sparkle and highs. But, I don't want too much capacitance on the 5Y3 at startup. So that is why I moved the 40uf.   it sounds pretty good   so    guess time will tell  how this works out.

[mresistor]

SilverGun  thanks a bunch for your help on this amp.  Much appreciated .