POST PHASE INVERTER MASTER VOLUME SCHEMATIC LAYOUT FOR PLEXI

[zfiles1701]

Anyone have or know where to get a schem for a post phase inverter master volume schem for the 50 watt plexi? And a layout if possible?
Thanks

[TIMBO]

Hi Zfiles1701, Here's what i have on mine . I have not tried it yet as it will be part of a mod i plan to do.

[tubenit]

And did you check the ARCHIVES?

http://www.el34world.com/Forum/index.php?topic=3496.0

With respect, Tubenit

[sluckey]

Timbo, you gotta add the red caps to a fixed bias power amp to prevent changing the bias with the MV. Without the red caps, the EL34s will melt when the MV is turned down because the grid bias voltage will be zero.

[Fresh_Start]

Remember that the caps Sluckey inserted and the .02uf coupling caps already shown are in series.  That means putting in a second pair of .02 caps would result in an effective coupling cap of only .01.

Personally, I would put relatively BIG caps (like 0.68uf) in place of the 0.02uf caps that are there now and put 0.02uf caps where Sluckey put his red caps.  I believe that way minimizes the tonal combination of the master volume pots and the 0.68uf caps, but others including Kevin O'Connor show the smaller coupling caps "first" in the signal chain for this PPIMV.

Hope that helps,

Chip

[TIMBO]

Hi zfiles, I posted my Plexi with the PPIMV , hope it helps with your project. Timbo

[Fresh_Start]

Hi zfiles, I posted my Plexi with the PPIMV , hope it helps with your project. Timbo

That setup is fine for cathode bias, but look at sluckey's post above.

[zfiles1701]

Thanks for the all the info guys.  Plan on using this in combo with a VVR on a firends hoffman plexi when I see him in Sept.  Trying to order all the parts in the mean time.

[Twanger]

hi all,

The master volume that sluckey is showing seems to be the type 1 (as reference in the trainwreck pages)

I was always led to believe that type 2 was the best and it's what I always used, what are the difference/ advantage and disadvantage of both type ?

John

[sluckey]

The type 2 uses fewer components. It doesn't need the extra caps to prevent changing the bias voltage on a fixed bias amp. And the 'onboard' bias resistors are replaced by the pots. This is because the normally grounded side of the mv pots is actually connected to the bias supply. And that's fine, because the bias supply should be at signal ground due to the bias filter caps.

The only thing I don't like about the attached circuit is the fact that the shields are also connected to the bias. It's a personal thing. I don't want any voltage on any of my shielded cable shields. So, I would use the type 2 but connect the shields to chassis ground instead.

[Twanger]

sluckey,

thanks for the clarification,

what about tone , any difference ?

Thanks again

John

[sluckey]

I don't have any hands on experience with either of these MVs and my ears are more than slightly tone challenged. So, I can't really speak about subtle tone differences.

[Fresh_Start]

I hadn't connected the dots before, but Geezer & tubenit's Tweed Overdrive Special is using a cathode biased version of "Type 2".  The low parts count is nice.

Chip

[zfiles1701]

Remember that the caps Sluckey inserted and the .02uf coupling caps already shown are in series.  That means putting in a second pair of .02 caps would result in an effective coupling cap of only .01.

Personally, I would put relatively BIG caps (like 0.68uf) in place of the 0.02uf caps that are there now and put 0.02uf caps where Sluckey put his red caps.  I believe that way minimizes the tonal combination of the master volume pots and the 0.68uf caps, but others including Kevin O'Connor show the smaller coupling caps "first" in the signal chain for this PPIMV.

What will putting bigger caps do to the tone?  And, does anybody know where to get the dual ganged pots?  Looked at my suppliers and searched but could not find.  Even mouser showed no results.  besides the pots and the caps, I do not see anything else I will need.

[sluckey]

Mouser has plenty of those dual pots...

http://www.mouser.com/catalog/catalogUSD/642/687.pdf
http://www.mouser.com/Search/Refine.aspx?Keyword=313-2420F-250K

[Fresh_Start]

The idea behind using the bigger caps right before the MV pot(s) is to minimize the effects on tone.  The cutoff frequency is a function of capacitance & resistance:

1 / (2*pi*R*C) if I remember it right

The bigger "C" is, the lower the cutoff frequency.  Since "R" is changing (MV pot in parallel with 2.2meg resistor, I believe it's better to have that variable cutoff frequency so low it doesn't affect guitar frequencies.  Your "standard" 0.02uf coupling cap comes "after" the MV pots so the cutoff frequency created by them in tandem with the 220K grid return resistors stays constant.

Make any sense?

Chip

[plexi50]

I have never like the PPIM. To me it's nothing more than a volume control for the output tubes and unless i am deaf i hear nothing of any benifit. If i use a master it is the standard master that can produce some great distortion
As far as the PPIM goes i hear no distortion characteristics at all. Some one please tell me im deaf

[zfiles1701]

The cutoff frequency is a function of capacitance & resistance:

1 / (2*pi*R*C) if I remember it right

The bigger "C" is, the lower the cutoff frequency.  Since "R" is changing (MV pot in parallel with 2.2meg resistor, I believe it's better to have that variable cutoff frequency so low it doesn't affect guitar frequencies.  Your "standard" 0.02uf coupling cap comes "after" the MV pots so the cutoff frequency created by them in tandem with the 220K grid return resistors stays constant.

By that calc I found all the freqs to be below 10Hz no matter which combo of caps I will use.  If I put .047 for all four caps then I would get around .02 which is closest to the original circuit.  OK theory question:  The caps are in series when the MV pot is not dialed down.  In other words, when the dual gang pot is dialed to 0 resistance the caps are in series.  What about when you dial the MV pot to lets say half?  Arent' the caps seperated by the res and no longer in sereies?  Thanks for the info

[Geezer]

Arent' the caps seperated by the res and no longer in sereies?

Yes, that's the point of using a cap ~~ 10x the size of the "normal" value. I have used 0.02 & 0.2 in series with good results.
A .02uf & a .2uf in series is ~~.0181....close enuf to the desired .02uf

[phsyconoodler]

With the Lar Mar master the tone is fantastic at lower volumes because it loses none of the highs.The trick to retain the stock tone when the master is not used is to use the right value of caps.If they are too small,the amp will sound thinner.When the proper ones are used there is no change in the normal cranked tone.
  I use .1uf caps in my PPIMV's.
I absolutely love them in my 18 watt amps with EL84's.They are almost as good as variable voltage regulation.
 

[Tone Junkie]

Im confused phsyconoodler were are you using the .1 caps at in the lar/mar. Bill