Hi - Low Vol switch ?

[tubenit]

As I understand the PPMIV, it reduces the resistance to ground. I have a PPMIV on one amp & don't adjust it much. It's either all on or at around 1/2.

So could one use a mini-toggle DPDT and simply parallel the 220k resistors to ground?

With respect, Tubenit

[Geezer]

Yes, but you may need to try a few different values to see what gives you the volume level desired....IOW, 110k (1/2 resistance) may not be 1/2 volume, but may be too loud, or too soft.

G

[darryl]

As drawn, there is no voltage division, so no significant gain change - the switch is simply changing the grid resistances from 220K to 110K. If instead, there were resistors between the .022 coupling caps and the top of each 220K grid resistor, these extra resistors could be shorted by a DPST switch to change the gain. ( Sorry I'm not sufficiently familiar with ExpressSCH to whip up a quick drawing.   :sad7: )

[jjasilli]

The SW circuit is confusing me.  Let's say ea power tube has a 220K grid leak resistor and that's full signal.  Exchange ea 220K resistor with 2 series resistors which add in value to ~220K:  e.g.:  100K + 100K = 200K;  68K + 150K = 218K.  Then use a SW to short out (jumper) one resistor on ea leg.  That will be reduced power.

[Fresh_Start]

I drew up the electrical equivalent of having the PPIMV pot at 50%, partly to help me see the circuit better.  When the PPIMV is full on (500K), you essentially have 242K grid return resistors.  At 50%, that value drops to 163K.  You also have 39% of the signal going to the power tube grids (250K||470K) / [(250K||470K) + 250K].

There's also a change in the high pass filter formed by the .02 coupling caps and the resistor network.  At full volume, the .02 works with an effective resistance of 242K (500K||470K) but the effective resistance drops to 163K at 50%, raising the cutoff frequency (bad term, I know).  Does the amp get more trebly when the PPIMV is turned down?

This PPIMV is different from others I've seen because there isn't a second set of coupling caps isolating the volume pot from the 470K grid return resistors for the power tubes.  Alternative #1 shown below is a switched version of the PPIMV I'm more familiar with.  I also included a circuit snippet from Bruce Collins at Mission Amps which shows the alternative with dual ganged pots.  Note, however, that Bruce's drawing has equal value coupling caps which may have less than ideal results in terms of changing frequency response with changing PPIMV settings.  There's also a version of this which grounds the PPIMV volume pots on the bias supply instead of chassis ground.  I think that one's from Ken Fisher but am not sure.  Obviously doesn't apply to a cathode biased amp anyway.  

Speaking of cathode bias, is the second set of coupling caps left out of tubenit's version of the PPIMV because there isn't any DC (bias voltage) on the power tube grids?  I ask because I'm wondering if including the second, lower value set would improve the stability of the hi-cut filter here.

Hope this helps.

Chip

[tubesornothing]

PPIMV are audio taper pots, you'll need to adjust those resistor values, rather than 50% each side to something like 20% - 80%.  Run a little test on your current set up to make sure.

[jjasilli]

There's also a change in the high pass filter formed by the .02 coupling caps and the resistor network.  At full volume, the .02 works with an effective resistance of 242K (500K||470K) but the effective resistance drops to 163K at 50%, raising the cutoff frequency (bad term, I know).  Does the amp get more trebly when the PPIMV is turned down?

Yes.  I didn't go there, as this "issue" would exist with pot or SW, and no complaint was made in this regard.  For tone scaling:  double the size of the cap if you halve the value of the resistor.  But, the ciruit may be interactive elsewhere, complicating values; but at least it's a place to start.  This could be done with two caps in parallel (on ea leg from the PI) - one blocked by a large say 10M resistor which could be jumpered by a different pole of the same SW.

Speaking of cathode bias, is the second set of coupling caps left out of tubenit's version of the PPIMV because there isn't any DC (bias voltage) on the power tube grids?  Yes.  Though it seems to me that there is still DC running there, hence the term grid leak resistor -- for DC operation of the tube, electrons are leaking to ground, and presumbaly charging the MV pots.  (For AC -signal- operation the same resistor is called a load resistor!)  Anyway, if the pots aren't scratchy it doesn't seem to matter.

I ask because I'm wondering if including the second, lower value set would improve the stability of the hi-cut filter here. No.  The "second set" should be at least 10X the value of the first set to block fixed bias DC from the MV pots, but not alter tone.  TUT1, KOC.

EDIT:  I guess a second set of series caps could be jumpered as an alternative, but this allows DC onto the pots.

[Fresh_Start]

I ask because I'm wondering if including the second, lower value set would improve the stability of the hi-cut filter here. No.  The "second set" should be at least 10X the value of the first set to block fixed bias DC from the MV pots, but not alter tone.  TUT1, KOC.

Don't you wish that KOC provided an index or at least put things in logical places?  The PPIMV discussion is in the "Preamp Modifications" chapter (page 5-9)... ARGH! 

You are correct about which pair of caps KOC says should be 10x bigger; however, if you think of the MV pot and the caps preceding it as the "new" components, it makes more sense to me to keep the old coupling cap values immediately before the grid return resistors and have the caps before the MV create a hi-pass filter with a LOW cutoff frequency (e.g. 0.2uf with 500K here).  I drew Alternative #1 from memory and put the big/little caps in that order.  Does my reasoning make sense to anyone else?

Please note that I haven't published any books on guitar amplifiers recently... 

Cheers,

Chip

[tubenit]

Just another way of drawing the DPDT

Thanks for the help!  Tubenit

[jjasilli]

Just another way of drawing the DPDT
No, my suggestion deletes entirely the resistors where your strings of  220K's are and moved them to where your 470K's are.  In my view, there is only one string of resistors.  See attached schematic.

Nothing wrong with your second set of .068 caps.  They block DC from the power tube direction, and are in parallel with the .047 caps, for somewhere above .022 and below .047.

[sluckey]

As I understand the PPMIV, it reduces the resistance to ground.

Yes, but more importantly, it forms a variable voltage divider in the signal path, and that is why it works. Just like a standard volume pot.

Nothing wrong with your second set of .068 caps.  They block DC from the power tube direction

Oh, it's very wrong. The power tube grid must have a dc path back to the cathode for biasing to work. Ie, you must have a grid return resistor just like in any other grid circuit. And that grid better be at zero volts, otherwise, the grid is drawing current, which is a bad thing in a cathode biased tube stage.

[tubenit]

JJasilli,

Did you mean to draw the schematic like you did?  It doesn't make sense to me. I'm not saying it's incorrect, I am saying I don't understand it?

Respectfully, Tubenit

[Fresh_Start]

Nothing wrong with your second set of .068 caps.  They block DC from the power tube direction

Oh, it's very wrong. The power tube grid must have a dc path back to the cathode for biasing to work. Ie, you must have a grid return resistor just like in any other grid circuit. And that grid better be at zero volts, otherwise, the grid is drawing current, which is a bad thing in a cathode biased tube stage.

That's the only electrical difference between tubenit's last drawing and Alternative #1 above.  I put the second set of coupling caps between the switch and the grid return resistors.

Also, the coupling caps are in series, not parallel.  That's why I showed 0.2uf and 0.02uf.  Net capacitance is 0.018uf.  Using 0.68uf instead of 0.2uf works out closer to 0.02uf on a net basis.  

If you put .047 and .068 in series, the net capacitance is .028 - close to the original .02.  However, There are two high-pass filters in series - one with the first pair of coupling caps and the MV and the second with the .02 (or .068 in tubenit's latest drawing) and the grid return resistors.  I don't know how to do the math for that scenario, but my approach was to have the cutoff frequency so low on the first hi-pass filter that it wouldn't really matter and you would be left with the original .02/470K filter.

Is that reasoning solid despite my lack of formulas?

Chip

[Fresh_Start]

JJ - the PPIMV forms a voltage divider.  We need that to reduce the signal going to the power tube grids.

Here's a stripped down version:  Alternative #2.  It's not the same as the original PPIMV though in terms of how the hi-pass filter works though.  

Looking at the tubenit's original circuit, there's a .02/242K hi-pass filter when the PPIMV is on "10".  As you turn the PPIMV down though, the net resistance actually goes up.  At "5", the net resistance to ground the .02 cap "sees" is 250K + (250K||470K) = 413K.  That means the cutoff frequency goes down as you reduce the MV, contrary to what I typed earlier.

The first snippet I posted exactly mimics the current action of the PPIMV.  If you like how it works now in terms of frequency response when you turn it down, do it that way.  

Alternative #2 gets you where JJ was headed (I think), at least in terms of reduced parts count. However, in this case the cutoff frequency of the hi-pass filter will go up when you cut the volume with the switch (instead of going down).  Don't know whether that's good or bad in this amp.  

There's a downside to Alternative #2 - you're relying on the switch contacts to provide a grid return path for your power tubes.  Likely failure?  Probably not, but sluckey referred to it as a "bad thing" for the power tube grids to not have a return path.  My guess is that he meant "BAD!"  KOC wouldn't approve for sure   If it worries you, then Alternative #1  or the first snippet get you to almost the same place but with a higher parts count.

Cheers,

Chip

[sluckey]

Chip, your 'Alternative 1' drawing will work. Jeff's drawing in reply #8 will not work for the reason I stated. In fact, Jeff annotated that cap with "not needed?". I would say "not allowed."

I don't want to use the term 'cardinal rule', but this really is one...

YOU MUST HAVE A DC PATH BETWEEN GRID AND CATHODE. IOW, connect an ohmeter between grid and cathode and you must read some resistance. If not (maybe because of a cap in this example), you will have an undetermined bias condition and the tube will probably run wild. Look at all the working circuits out there. You will see a measurable resistance between grid and cathode. Most are straight forward. Fixed bias amps are tricker, but if you look closely, you'll see a DC resistance path thru the bias supply. A cap in series with a grid before any resistor will violate this rule.

[jjasilli]

Whoops, good points all.  Here's a new drawing.

[darryl]

Whoops, good points all.  Here's a new drawing.

This still won't work. There is no voltage divider. In the amended drawing shown below, the value of Rx determines the amount of attenuation when SW1 is open. With SW1 closed, the circuit is in its original configuration. When SW1 is open, the degree of attenuation is determined by the value of Rx ( a useful starting value would be 1M ). The perceived bass response should not change, because the extra resistance will shift the bass rolloff to a lower frequency - well below any audible musical information.

[sluckey]

This still won't work. There is no voltage divider.

It will work, although not as effective as a voltage divider. The mere fact that you are decreasing the grid resistor, which has more loading effect on the driving plate, will decrease the signal available to the next grid. I would prefer to use a voltage divider. A dual ganged pot is still the simplest solution.

[Fresh_Start]

darryl - I like that solution.  Low parts count and the grid leak/return resistors for the power tubes stay constant.  It would shift the hi-pass rolloff frequency lower, but that doesn't seem to be a problem for this amp anyway.

I've learned a bit more through this discussion, so I attached a PPIMV with dual ganged pots and two sets of coupling caps.  As JJ points out below, this is a cathode biased version of the PPIVM from The Ultimate Tone.  The only difference is the order of the "big" vs. "small" coupling caps.  I think that difference is significant in terms of keeping the frequency response more constant but am not certain of that conclusion.  This approach does keep the grid return resistors for the power tubes constant which may be a "good" thing or maybe not. 

The original PPIMV tubenit posted reduces the value of the grid return resistors which, in turn, reduces the gain of the power tubes.  That gain reduction works in conjunction with the voltage dividers created by the pots themselves. 

Cheers,

Chip

[jjasilli]

I attached a PPIMV with dual ganged pots and two sets of coupling caps.  It keeps the grid return resistors for the power tubes constant which may be a "good" thing or maybe not. This is the PPIMV circuit recommended by KOC in TUT1.  Conceptually it seems that the normal value coupling caps should follow the PI plates, and the larger caps should follow the pots.  But functionally either way is fine.  (Note that if you lift the junction of those 470K resistors from ground, then fixed bias voltage can be inserted there.  The pots are surrounded by caps so are protected from DC.) 

[jjasilli]

I like darryl's switchable circuit.  Low parts count like mine; but I like that it preserves the integrity of the 220K grid leak to ground connetion for DC operation.  The the signal voltage divider probably better presrves tonal integrity of the signal.  If hi's are lost due the lo-pass filtering of the series resistor, presumably they could be saved with a pF-range bypass cap across the series resistor.