What's this tone circuit (single knob)??

[Geezer]

A friend just worked on one of these ( http://www.el34world.com/charts/Schematics/files/gretsch/gretsch6162.pdf ) & said the single knob tone circuit worked really well.

Anyone know what type this is? If not, I think I'll call it a "wagon wheel" tone control..... 

I think I'll try it in the TOS & see what it does....

TIA, G

[sluckey]

Pretty!

[phsyconoodler]

It's similar to the ones used with an EF86 or other pentode preamp.It would definitely have quite a range.Debateable whether you would need that drastic a tonal swing.

[jjasilli]

Looks to me as follows:  1. tone may first be darkened a bit by the rather large plate resistors serving the preamp tubes.  2. Then tone is affected by the filter string of series caps and shunt resistors following the tone driver stage.  3.  At the tonestack the 1st 500K pot is a variable slope resistor. 3.  The signal then gets divided into bass & hi's at the cut-off frequency set by the .0033 caps.  The relative strength of hi/lo signals is further fixed by the series and shunt resistors in the tonestack.  4.  The 2 signals are then re-combimed and mixed at the 2nd 500K pot.  So this is a rather unique 2 pot system. 

[echuta13]

I think I've heard it called a "tilt" control before.  Merlin mentions it in his book and I think it's similar to something Quad did in their HIFI stuff.  
I think there's some relation to the Big Muff Pi tone control as well.  I built one up, but haven't used it yet.  

http://roymal.tripod.com/ReverbTone.htm

http://www.muzique.com/lab/tone3.htm

http://gilmore2.chem.northwestern.edu/images/eq_tilt.gif

[Geezer]

So this is a rather unique 2 pot system.

Talked to my friend (email) & he said it IS actually 2x pots, but it is a 500k dual pot (single shaft to control both pots @ the same time).

I have room for 2x pots, so I may try separate pots, as more "interesting" tonal variations might be had, and if I want the original operation characteristics, I can just keep the controls @ ~~ the same/matching positions as I turn them.

G

[jjasilli]

Talked to my friend (email) & he said it IS actually 2x pots, but it is a 500k dual pot (single shaft to control both pots @ the same time).

A ganged pot makes sense.  I was thinking that separate pots would be interactive in counter-productive ways.  Turning up the 1st pot = increasing the value of the slope resistor > more hi's and gain (less insertion loss).  This makes sense as hi's have less power and could use more gain.  Turning up the 2nd pot also increases hi's (by dialing lo's out of the mix).  So it makes sense to turn the pots together.  Dialing the pots in opposite directions will probably only be duplicative of some other setting using ganged pots.

BTW:  I should state thatmy prior post is not definitive, but more of a general idea.  That 500pF cap on the hi side will further favor hi's over mids.  That may leave a set of mid frequencies holding steady (bandwidth filter), while hi's and lo's are adjusted around the mids. 

[sluckey]

Well, here's another take on that circuit.

I think that RC network off the plate of V4a is a hi pass filter that's intended to pass the guitar signal and reject the tremolo signal. The trem is injected at the cathode of V4a. I've seen other amps (Vox and Gibson) that use a similar filter to strip the LFO trem signal from the actual guitar signal.

The 500K pot just before the 'wheel' is a volume pot. Look closely. If you crank the wiper down to the bottom you kill the entire signal. Typical volume control style. Look at the second channel circuit without trem. I think (but without pics, can't verify) that the dual ganged pot your friend saw was the volume pots, one off the plate of V2b and the other just prior to the 'wheel'.

The 'wheel' which includes the second pot is the tone circuit.

[jjasilli]

I think that RC network off the plate of V4a is a hi pass filter that's intended to pass the guitar signal and reject the tremolo signal.
Yes caps in series with shunt resistors is a hi pass filter.  As to trem, why inject it somewhere only to block it?  I'm not getting the trem operation.  Is it induced into the shared cathodes of V2 & V4?

The 500K pot just before the 'wheel' is a volume pot. Look closely. If you crank the wiper down to the bottom you kill the entire signal.

Yes, but it's common for tone circuits to kill vol. You are correct that the 1st tone pot is a vol pot.  But IMHO that's not all it is.  It also sets the input impedance of the tonestack.  So I think it is also a variable slope resistor.

[sluckey]

As to trem, why inject it somewhere only to block it?  I'm not getting the trem operation.  Is it induced into the shared cathodes of V2 & V4?

You want the effect of the trem signal, not the trem signal. V1A is a low freq phase shift oscillator, typically a 3 to 8 Hz sine wave. That signal is applied to V2A, a cathode follower (just a buffer), via the footswitch jack (which is drawn incorrectly) and the Intensity pot. The signal is taken from V2A cathode and applied to the cathode of V4A signal gain stage. The slow AC trem signal adds to the V4A cathode bias voltage when it's positive and it subtracts from the V4A cathode bias voltage when it's negative. The gain of V4A is varied at the trem rate and causes the guitar signal at V4A plate to vary at the trem rate as well. That's the effect of the trem signal and that's what we want. BUT, the trem signal itself also apears at the plate of V4A and we don't want that. So, knock it down with that hi pass filter. At least that's the way I look at it.   

it's common for tone circuits to kill vol.

I don't think so. The only tone circuit I can think of that actually kills the signal to zero is the Fender TMB. And it only does so when all three knobs are set to zero. The TMBs have a resistor that's typically called a 'slope' resistor, but I'm not aware of a 'slope' resistor in other tone controls. That wheel sure dosen't make me think TMB. I still think it's a volume control.

[HotBluePlates]

I'm agreeing with sluckey on this. I also do not see anything in common with the linked tilt tone controls.

To me, this is an artifact of drawing. Let's talk in terms of sides of the wheels, but I'll describe thinkning about it differently. If I was a MS Paint wizard, I'd draw this up in a hurry.

Go around the "top" of the wheel. You have series 150k and 680k resistors, and a 0.0033uF cap to ground. Think of this as a "T" instead, with the series resistors forming the top of the T and the cap to ground forming the vertical stem of the T. Then you'll recognize this as a T-filter, which shunts highs to ground; it's the low-pass part of the tone circuit.

Go around the "bottom" of the wheel. You have series 0.0033uF and 500pF caps, and a 68k resistor to ground. Thinking as a T again, the caps form the top of the T, and the resistor to ground forms the stem. This T is flipped upside down for convenience of drawing. This T-filter is a high pass, and tends to shunt lows to ground.

An alternate way to view the arrangment is a pair of pi-filters, with 2 series elements, and 2 grounded elements (the unaccounted-for grounded elements are the ones right at the 500k pot).

Regardless, if you sweep the 500k pot to one side (bottom), then you get a trebly sound, and if you sweep to the other side (top) you get a bassy sound.

[Fresh_Start]

So we'll call this the "6T2" tone control - it's drawn as a hexagon, but functionally there are two "T" shaped pi filters  :drunken_smilie:

HBP - your explanation works for me.  And I agree that the pot feeding the hexagon is simply a volume control.

Cheers,

Chip

[jjasilli]

Looks like I'm out-voted!  BTW saw an old Gretch amp in fair conditon today @ the local Guitar Center.  2 channel, 2 speakers; power amp at the bottom of the cab; fair condition $600.00

[PRR]

Did anybody try a Search?

http://www.el34world.com/Forum/index.php?topic=9162.msg82604#msg82604

same as the BigMuff Pi, except that it uses 2nd order filters instead. (e.g, instead of RC it uses RCRC)

....awful "soft 2nd order". Still that gives quite sharp slopes

[Fresh_Start]

Did anybody try a Search?

I knew that hexagon looked familiar!

One heck of a mid-scoop.  Not tubenit's favorite I'll bet!

Cheers,

Chip

[Geezer]

Did anybody try a Search?

I knew that hexagon looked familiar!

One heck of a mid-scoop.  Not tubenit's favorite I'll bet!

Cheers,

Chip

Yes......but how about this?

If the 22k on the BigMuff (R2) is adjusted, then the "scoop" is greatly varied.

It seems that 10k to 35k is a "reasonable looking" response, so how about replacing R2 with a 25kL pot with a 10k resistor in series, then one would have a variable mid "scoop" controlwith a range of 10k up to 35k.
I also included a snapshot of a 50k pot @ R2 (10k to 60k adjustment range)

It would have to be tried to see how it actually responds & to determine if it would be useful.....

G

[Fresh_Start]

Geezer - not knocking your inventiveness at all!  However, the guy(s) at Muzique call that a "body control" if I'm understanding your post and this correctly:
http://www.muzique.com/lab/tone3.htm

How could we apply this notion - an adjustable mid scoop - to the 6T2 (hexagon) tone control?  It's too late here for me to figure anything out right now...

Cheers,

Chip

[Geezer]

Ha Ha!  

I had actually NOT seen that before, but that was the 1st place (the 22k R2) that looked promising to me for some tweaking, AND it proves Tubenit's theory that "pretty much everything has been done before".....

 

Geezer

[Willabe]

Hi All,

Just a thought what if you stand the ground end of the 1st pot on a resistor, say 100k to maybe 250k ? You would always have at least some signal now even with that pot turned all the way down. Might be helpful in some instances ? Or would it throw the balance (what ever that is) of this tone control off, leaving it useless ?

    Thanks,  Brad     

[sluckey]

Well, the amp would be pretty loud, even with the volume on zero.

[Willabe]

sluckey,

Im not understanding this. If no signal is sent to the input of this tone circuit from the preamp, why would the amp be pretty loud, even with the volume on zero ? Even if we stand the input tone pot on a 1M resistor, no signal in, no signal out. Wouldnt make any difference where the tone control is set.

  Thanks,    Brad          

[sluckey]

I'm not understanding this. If no signal is sent to the input of this tone circuit from the preamp, why would the amp be pretty loud, even with the volume on zero ? Even if we stand the input tone pot on a 1M resistor, no signal in, no signal out. Wouldnt make any difference where the tone control is set.

  Thanks,    Brad          

What you are calling an 'input tone pot' is a VOLUME pot. If you stand a 500K VOLUME pot on top of a 1M resistor (or any other sizable resistor), you can never turn the volume all the way down. That's all I was saying. Of course I assumed you would send a signal thru this tone circuit. That's what it's for, right?

[Willabe]

slucky,

Im sorry but, Im still not getting this. If the tone circuit uses a duel ganged 500K pot then whats it labled as? Im looking at the schme for the Gretsch G6162 and it looks like it has a vol. control in both channels early on just after the first preamp stage. So each channel has has its own vol. and tone controls ? Im not trying to argue, I honestly am not getting how a duel ganged pot is vol. and tone at the same time. How does / would that work ?  So as you turn this control up the vol. increases and you get more highs and as you turn it down the vol. goes down and you get more lows ? I must be misundering this somewhere . Sorry if Im being a pain, but this has got me confused, which is not hard to do.       

   Thanks,   Brad

[sluckey]

The tone circuit doesn't use a dual ganged pot. The volume pot is the dual ganged pots. (See pic below)

Here's a quote that google turned up from another forum from someone working on this amp...

"If you look at the upper channel on the G6162 drawing, you will see two 500k pots and then the last 500k for tone. The drawing does not depict that the 1st two 500k pots are actually one control, but they are in my amp. You are correct in that it looks like the reverb is an integral part of the CH2 chain. That's why the reverb being so weak is concerning, but from what you've said that might be common and not necessarily mean something else is malfunctioning. The dual ganged volume pot looked strange to me since there wasn't one on CH1."

[Willabe]

sluckey,

Ohh!   Thought I was losing my mind!

    Thanks,   Brad