Tone stacks 5F6A vs Ab763

[Toxophilite]

I've been doing some reading and I'm curious as to the merits (or lack thereof) of the cathode follower tone stack of the bassman 5F6A
Is it functionally/tonally different than the Ab763 3 band tone stack?
And if so can anyone tell how? (other than it's placement in the circuit)


Thanks

[jjasilli]

Is this a trick question?     Comparing the venerable 5F6A to a Bandmaster could get one burned at the stake for heresy!


Anyway, in theory the output impedance of the plate driven tonestack vs. the cathode follower is rendered equal by the values of the slope resistors which set the input impedance of the tonestacks.  In practice, subjectivity and occult factors apply.  But as to this particular amp comparison, a host of other factors come into play, beyond the tonestacks.

[Willabe]

I've been doing some reading and I'm curious as to the merits (or lack thereof) of the cathode follower tone stack of the bassman 5F6A
Is it functionally/tonally different than the Ab763 3 band tone stack?
And if so can anyone tell how? (other than it's placement in the circuit)

Oh, now you did it!     

You just done opened up a BIG can of worms!     

Ok, I've never seen (or at least remember) what jjasilli answer is and I think it's very interesting.

Have to wait and see what the other guys have to say.


                     Brad   

[Toxophilite]

Gosh Sorry
I thought the Bassman 5F6A was obsolete
I made one into a set of shelves the other day
Of course I had to throw out all those old electronics first!

[Willabe]

Gosh Sorry
I thought the Bassman 5F6A was obsolete
I made one into a set of shelves the other day
Of course I had to throw out all those old electronics first!

         

[jjasilli]

Gosh Sorry
I thought the Bassman 5F6A was obsolete
I made one into a set of shelves the other day
Of course I had to throw out all those old electronics first!

When younger, I actually gave away a Silverface Bassman; but hell it was just SF.

[tommytornado]

This does a decent job of explaining it:  http://www.valvewizard.co.uk/accf.html  In my experience, in general..  CF tone stack amps are a little more touch sensitive and richer in harmonics (especially at lower volumes). 

[jjasilli]

and see this:  https://books.google.com/books?id=MZIYFyXUDhUC&pg=PA186&lpg=PA186&dq=tonestacks+cathode+plate&source=bl&ots=ZLLv8dkcWe&sig=jjDBD5ADfxEazscViPWOrX2Z1hQ&hl=en&sa=X&ei=i-oaVZ7VD8K4ggS3oILgDg&ved=0CCQQ6AEwAQ#v=onepage&q=tonestacks%20cathode%20plate&f=false

[Fresh_Start]

Merlin's essay is very helpful.

IIRC the cathode follower reduces the "insertion loss" from a TMB tone stack. I'm uncertain about whether or not switching to a 33k slope resistor from a 100k offsets this, but you can download Duncan's Tone Stack Calculator and find out.

http://www.duncanamps.com/tsc/index.html

You can also figure out what the slightly different Mid pot wiring does in terms of frequency response.

There also is a point where the cathode follower introduces some compression. R.G. Keen writes about this in his MOSFET Follies since it is the primary difference between a tube triode and a MOSFET as the cathode follower. This compression probably contributes to the touch response mentioned earlier.

Cheers,

Chip

[jjasilli]

I plotted the curves on Duncan's Tonestack calculator.  The top curve is the Bassman 5F6A; the bottom curve is the curve of the more standard Fender BF tonestack - controls at mid-position.  (Please ignore that the chart says Marshall).

There is a significant difference in insertion loss.   By way of illustration:  If this were power amp output (instead of voltage gain in the preamp), then ea loss of 3dB would require 2X the amp's power in Watts to make it up. The FMV Mid drop is about 12dB down from the Bassman.  That's 4X 3dB increments; so @ 50W, 800W would be needed to replace the insertion loss!!!


EDIT:  Note that swapping the Bassman 5F6A's 25K mid pot for 10K - all pots @ mid position -  results is a nearly identical curve, but with an additional 4dB mid-drop -- barely audible.  Also, changing the tone caps from 22nF to 47nf / 100nF also yields little change in the curve. This seems to indicate that the cathode driver circuit produces both a much smaller mid dip, and far less insertion loss overall in the tonestack. 


EDIT 2:  Speculation:  With additional notes, including harmonics riding on a single guitar note, the tube will see the notes superimposed on the signal curve and increasing its amplitude (swing across the loadline).  Given that the cathode driven tonestack already yields more mid signal voltage, then the extra swing provided by harmonics (or other fundamental notes in a chord, etc.) may be sufficient to drive a stage into saturation & compression. 

[Fresh_Start]

I plotted the curves on Duncan's Tonestack calculator.  The top curve is the Bassman 5F6A; the bottom curve is the curve of the more standard Fender BF tonestack - controls at mid-position.  (Please ignore that the chart says Marshall).

There is a significant difference in insertion loss.   By way of illustration:  If this were power amp output (instead of voltage gain in the preamp), then ea loss of 3dB would require 2X the amp's power in Watts to make it up. The FMV Mid drop is about 12dB down from the Bassman.  That's 4X 3dB increments; so @ 50W, 800W would be needed to replace the insertion loss!!!


EDIT:  Note that swapping the Bassman 5F6A's 25K mid pot for 10K - all pots @ mid position -  results is a nearly identical curve, but with an additional 4dB mid-drop -- barely audible.  Also, changing the tone caps from 22nF to 47nf / 100nF also yields little change in the curve. This seems to indicate that the cathode driver circuit produces both a much smaller mid dip, and far less insertion loss overall in the tonestack. 


EDIT 2:  Speculation:  With additional notes, including harmonics riding on a single guitar note, the tube will see the notes superimposed on the signal curve and increasing its amplitude (swing across the loadline).  Given that the cathode driven tonestack already yields more mid signal voltage, then the extra swing provided by harmonics (or other fundamental notes in a chord, etc.) may be sufficient to drive a stage into saturation & compression. 

Neat! 

Also note that while the Marshall/5F6A tone stack looks like it has a bigger loss of bass, most of that is below 20 hz.  IOW below the lowest frequency most of us can hear. Im just guessing, but cutting lower frequencies may have benefits in terms of unwanted distortion, etc. 

Cheers,

Chip

[jjasilli]

Yes, I agree.  Low E on guitar is about 80 Hz. Bass guitar goes down one octave to about 40Hz.  At that point the Bassman is about 3dB louder, a barely audible difference.  Then the steep bass roll-off will kill not only lower fundamental frequencies which may be lurking around as noise, but also their harmonics.  Those harmonics would muddy-up the desired frequency range.

[Fresh_Start]

JJ makes more good points.  Blocking distortion and harmonics from ugly low end noise are best avoided.

Before we start picking on the bandmaster, and its ab763 schematic, remember the bandmaster had a TB tonestack.  However if we pick on the twin, we find a tmb tonestack. 

I am just wondering it is easier to incorporate a vibrato circuit into the ab763 where the tone stacks are upstream of the vibrato. than a 5F6a tonestack? 

It is my understanding that the 5f6a tonestack can be designed and modified independently.  I am not so sure if the Ab763 tonestack is as independent.

It is also my understanding the Bassman (keyword bass where the low string has a fundamental of 30 or 40Hz,{I'm ignoring the heavy metal guys on purpose}), became a favorite of guitar players.  (unless you play piccolo Bass)       

If you mean tremolo, adding bias vary to the 5F6A circuit isn't any harder than adding it to the AB763 circuit and several of us have done that.  Works great actually.

Cheers,

Chip

[jjasilli]

Before we start picking on the bandmaster. . .

It is also my understanding the Bassman (keyword bass where the low string has a fundamental of 30 or 40Hz,{I'm ignoring the heavy metal guys on purpose}), became a favorite of guitar players.  (unless you play piccolo Bass)     

I'm not picking on the Bandmaster.  But the Bassman 5F6A occupies an especially exalted place in the history of electric guitar amplification, and for good reason. 


Heavy metal did not exist in those days.  But, e.g., Chicago Blues did.  And so did the introduction of the Strat, when before was the Tele alone (in the Fender line).  A significant new direction in the tone of electric guitar was born.   A point here is history, if not historicity: historical actuality, or striving to understand a person or thing, and its impact, within the context of the specific historic period in which it arose or existed. 

[Fresh_Start]

Hoffman's AB763 circuit with bias vary tremolo.  Based on Brown Vibroverb.

http://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf

There's an old thread in the archives about how to adjust it for faster, slower, different output tubes, etc IIRC. Someone else here developed a variation using a MOSFET buffer.

I only have a Visio file given to me by Sluckey (so you should be able to find it here), but there may be an SCH version in the schematics section. I know that there's a Princeton Reverb schematic in there, and that's a very similar tremolo circuit. Biggest difference is the voltage swing needed for 6V6s vs. bigger tubed.

JJ makes more good points.  Blocking distortion and harmonics from ugly low end noise are best avoided.

Before we start picking on the bandmaster, and its ab763 schematic, remember the bandmaster had a TB tonestack.  However if we pick on the twin, we find a tmb tonestack. 

I am just wondering it is easier to incorporate a vibrato circuit into the ab763 where the tone stacks are upstream of the vibrato. than a 5F6a tonestack? 

It is my understanding that the 5f6a tonestack can be designed and modified independently.  I am not so sure if the Ab763 tonestack is as independent.

It is also my understanding the Bassman (keyword bass where the low string has a fundamental of 30 or 40Hz,{I'm ignoring the heavy metal guys on purpose}), became a favorite of guitar players.  (unless you play piccolo Bass)       

If you mean tremolo, adding bias vary to the 5F6A circuit isn't any harder than adding it to the AB763 circuit and several of us have done that.  Works great actually.

Cheers,

Chip

I was using Fender's verbage. 

Do you have jschem of the add? 

[jjasilli]

Checkout Tubenit's thread with video:  http://el34world.com/Forum/index.php?topic=18369.msg187630#msg187630


The 5F6A "blooms".  It may not need reverb or tremolo.

[Toxophilite]

Cool stuff
Lot's of great information. This thread has gained a life of it's own!


I wonder if these properties show themselves when playing perfectly clean, not mostly clean, but perfectly clean, no audible distortion.
Like when you're playing below the amps rated wattage rather than pushing it into distortion

Just sos you knows, I wasn't comparing a 5F6A bassman with a AB763 bandmaster (I don't think I mentioned the bandmaster)


I was asking about the differences in the tone circuit placement, I thought it might be something to try


I don't know if I want compression as it seems to me be the reverse of touch sensitivity . Doesn't compression even out volume changes, raises lows, drops highs? That would seem to in actually make the instrument less sensitive IMHO.


For kicks the other day I tried playing my homebuilt AB763 deluxe at 10 with my band(I usually use pedals if I want distortion because I like my cleans to be REALLY CLEAN) When I hit full chords hard they didn't have as much power . it's a cool sound (EVIL!)but I just don't find it as versatile.Very fun though.

[jjasilli]

Just sos you knows, I wasn't comparing a 5F6A bassman with a AB763 bandmaster (I don't think I mentioned the bandmaster)


Your right!  Your originally said "3 band tonestack", and I read Bandmaster!!!     Not really.  The same info still applies


**********
Yes tonestack placement among gain stage s matters.  Checkout "Dave Funk's Tube Amp Workbook", Chapter 14, p. 100.  Gain stages may be pre- or post- EQ (tone controls).  Up to 5 stages pre- and 0-2 post-.  Generally cascading gain stages are used to clip the signal waveform.  Because of the loadline, etc., when overdriven the bottom of the waveform clips, the top tends to compress or saturate.  The next gain stage is revers phase, so now when the bottom clips, it was the unclipped top of the wave in the prior gain stage.  This results in symmetrical clipping of both the top and bottom of the waveform (or purposely honed asymmetrical clipping).