Hoffman 6 v 6 Plexi W/ Lar Mar MV

[Big_Mike]

George M. uses a dual 500K log pot for his PPIMV, preferably a PEC dual ganged pot because they are tightly matched.  Alpha dual pots can have a wide variance, particularly at lower levels.

Also, one thing you can do about the lack of NFB at lower volume levels is use a pot for the NFB resistor value to allow more NFB back into the circuit at lower volume levels.

When I build my 6v6 Plexi, I am going to do just a PPIMV, but also have a pot for adjustable NFB.

[uki]

I have a Fender amp that came with 2 6AQ5.  It is/was a little bass amp.  Forget the name, but you have seen one. YOu can still get them fairly cheap.

Check out this file:

[jojokeo]

Only difference between the linear is it will get louder quicker then not so much response at the end towards full up. The audio will allow a better low to middle control adjustment is all and full open or closed will present the same load so both effectively the same there.

[dude]

Thanks, audio does work best, also I did try a 500K dual pot, range on the 1Meg was much better. I have another 6V6 PLexi Hammond conversion. I was thinking of using the below PPIMV, from an Allen Amp Old Flame, (Super Reverb copy). The Old Flame's Master really works great, I'd like to try it on another 6V6 Plexi conversion I have.


My question is: Do I use (four) .022 PI caps like the two in the Plexi or go with (four) .1 PI caps like the Old Flame.? The Old Flame is plenty bright and very close to a Fender Super Reverb tone, very nice and gradual Vol PPIMV cut, even though it uses four caps instead of two in the Rich II Mod.


*The more I look at the schematic the two .1 caps would be in series when the pot is wide open or at any position, wouldn't this half the value of the caps to .05? But the Super Reverb uses two .1 and no master. 


al

[sluckey]

I would use four .047µF caps.

[dude]

Finally figured it out, amp circuits are build around tubes. You can't make an amp that takes 6V6s, 6L6s and EL34s to sound killer with all three. IMO, you can get 6L6's and El34 to sound great but when you throw in 6V6's you lose something. Change the circuit a little and 6V6's sound great. You can't have your cake and eat it too. Been screwing with this 6V6 plexi for a while, when I get 6V6's to sound great  (circuit close to what's in the schematics here), you can't throw in 6L6s or EL34s and get killer tone from them. It's one or the other. They all will sound decent but not killer tone.


I learned the hard way, trail and error. Unless I'm ready to put several switches in, it ain't happening. Already did the switch thing. Anyway the Allen Old Flame PPIMV sounds great with 6L6s or EL34s but to get the 6V6s to shine you have to change the tail and the power tube grids among other changes. That's just my opinion, so just have two plexi's one with 6V6s and one with EL34's or 6L6s.


*Next day: I replaced the 6V6s as when I checked the bias today one was way low, bad 6V6. Replace with known matched set, 6V6s do sound fantastic as 6L6's and EL43s in his amp. Jumped the gun.   Also IMO, the PPIMV posted above is much better than the Rich II or LaMar MV. Much more of an even sweep, gradually lower the overall volume, not like the other two that the first quarter turn does it all. You will still lose the presence control more then half down but the bright caps on the PPIMV will help will help. I highly recommend trying this PPIMV, I did use .047uf caps in the PI as Sluckey suggested. Both pre MV and PPIMV blended will give cleans to grind, along with the two volumes this is the best amp I ever played and the most versatile. 


Thanks, al

[Big_Mike]

Finally figured it out, amp circuits are build around tubes. You can't make an amp that takes 6V6s, 6L6s and EL34s to sound killer with all three. IMO, you can get 6L6's and El34 to sound great but when you throw in 6V6's you lose something. Change the circuit a little and 6V6's sound great. You can't have your cake and eat it too. Been screwing with this 6V6 plexi for a while, when I get 6V6's to sound great  (circuit close to what's in the schematics here), you can't throw in 6L6s or EL34s and get killer tone from them. It's one or the other. They all will sound decent but not killer tone.


I learned the hard way, trail and error. Unless I'm ready to put several switches in, it ain't happening. Already did the switch thing. Anyway the Allen Old Flame PPIMV sounds great with 6L6s or EL34s but to get the 6V6s to shine you have to change the tail and the power tube grids among other changes. That's just my opinion, so just have two plexi's one with 6V6s and one with EL34's or 6L6s.


*Next day: I replaced the 6V6s as when I checked the bias today one was way low, bad 6V6. Replace with known matched set, 6V6s do sound fantastic as 6L6's and EL43s in his amp. Jumped the gun.   Also IMO, the PPIMV posted above is much better than the Rich II or LaMar MV. Much more of an even sweep, gradually lower the overall volume, not like the other two that the first quarter turn does it all. You will still lose the presence control more then half down but the bright caps on the PPIMV will help will help. I highly recommend trying this PPIMV, I did use .047uf caps in the PI as Sluckey suggested. Both pre MV and PPIMV blended will give cleans to grind, along with the two volumes this is the best amp I ever played and the most versatile. 


Thanks, al

Is there a way you could diagram the PPIMV master you installed on the Hoffman layout?  I am still s bit of a newbie at converting a schematic to actual board layout.  Also, did you use an Alpha dual gang 1 Meg pot?

[dude]

It's going to be difficult to show you a layout as the boards available for the 6V6 Plexi do not leave room for this MV, or for LaMar or Rich II. You have to improvise or have a bd made. It's very simple but takes some room on the bd so I recommend finding small .047uf caps and putting them on the pot and using shielded wire to the two plate R's and two grid bias R's. I used a 1MA alpha pot but luckley mine were closely matched. Just use Hoffman's bd and run shielded wires to the dual pot, make sure you follow the schematic or layout and connect the correct R's to the correct lugs.


When you do your build post a picture of the bd then I can help.


al

[Big_Mike]

Just wanted to add, I finished up a Hoffman 6v6 Plexi and am going to experiment with different PPIMV circuits.  I am going to try the Rich Mod II using a PEC 500K dual pot first and see if I like it.  If not, I will move over to a standard Lar/Mar using a 250K dual pot.  It will be interesting to see how they work.

[purpletele]

Just wanted to add, I finished up a Hoffman 6v6 Plexi and am going to experiment with different PPIMV circuits.  I am going to try the Rich Mod II using a PEC 500K dual pot first and see if I like it.  If not, I will move over to a standard Lar/Mar using a 250K dual pot.  It will be interesting to see how they work.

How does it sound?  I have been playing mine for the last couple of hours and it is quite gorgeous.

I pulled all of the MV circuitry and just went Commando on mine, I really like the pure cleans of that circuit.  It is meant to be a back up amp for live shows so it is perfect.

I also really like this amp with the Celestion Cream Alnico.

[Big_Mike]

The amp sounds really good...I was able to get everything running last night, and I am really impressed.  The amp is dead quiet at idle, and the cleans are gorgeous.

The only tinkering I am going to do is to the master volume. I am going to try both PPIMVs and pre-phase inverter masters.  If I stick with the pre phase master, I can always increase the gain in the reamp to compensate for the loss of gain from the phase inverter.

One thing I have is scratchiness with the presence pot.  Not sure if it is a bad pot or what.

[sluckey]

One thing I have is scratchiness with the presence pot.  Not sure if it is a bad pot or what.

That's normal. Nothing you can do about it unless you change to a different presence circuit.

[Big_Mike]

I am waiting on parts to try different master volume circuits, but in the interim I have been testing the amp.  Great sounding amp, I really like it.  Nails Plexi tones.  Just a great amp to play,, I am really glad I built it!

[Big_Mike]

I should receive a PEC dual 500K pot today, and plan on wiring up a Rich Mod II PPIMV.  I will post my results.

[Big_Mike]

So I have been playing around with the stock pre-phase inverter master volume on the 6V6 plexi.  I saw on another forum that one way to retain bass and minimize tone loss at low volume is to place a 220K resistor in series before lug 1 of the master volume, and place a small cap, around 5-10pf, between lugs 1 and 2.  I made that mod, and it certainly increased the bass coming through, but perhaps to much so.  With gain, the neck pickup on my Les Paul was very bass heavy, almost flubby.  Why does installing a resistor in series before the master volume pot increase bass?  I may have to tinker with other resistor values because it is almost too bass-heavy.

[purpletele]

So I have been playing around with the stock pre-phase inverter master volume on the 6V6 plexi.  I saw on another forum that one way to retain bass and minimize tone loss at low volume is to place a 220K resistor in series before lug 1 of the master volume, and place a small cap, around 5-10pf, between lugs 1 and 2.  I made that mod, and it certainly increased the bass coming through, but perhaps to much so.  With gain, the neck pickup on my Les Paul was very bass heavy, almost flubby.  Why does installing a resistor in series before the master volume pot increase bass?  I may have to tinker with other resistor values because it is almost too bass-heavy.

I'm actually playing mine right now and I have the bass set at 9:00, That is when I have a Humbucker engaged.  I move the bass up just a tad when I am Stratting around.


Isn't that what is expected from a Marshall Plexi type amp?

[Big_Mike]

Not sure whether the bass on a plexi is necessarily high such that it is necessary to turn down the bass knob on the tone stack.  The amp just seemed bass-heavy with the neck pickup on a Les Paul.  Not sure if higher or lower resistors increase the bass response, so I am tinkering a bit.

[Big_Mike]

Ok, so I installed a Rich Mod II PPIMV in the amp tonight with a PEC 500K dual gang pot.  Everything turned out well and it worked fine.  However, while you could get more gain with the PPIMV, I found that the standard pre-phase inverter master sounds better.  I think the real problem with PPIMV is the loss of negative feedback at low volumes which hurts tone.  The standard master to me just sounds Fuller and more natural. 

As far as gain, you can always goose the amp in other ways to get additional gain while keeping negative feedback intact with a standard pre-phase inverter master.

[purpletele]

Ok, so I installed a Rich Mod II PPIMV in the amp tonight with a PEC 500K dual gang pot.  Everything turned out well and it worked fine.  However, while you could get more gain with the PPIMV, I found that the standard pre-phase inverter master sounds better.  I think the real problem with PPIMV is the loss of negative feedback at low volumes which hurts tone.  The standard master to me just sounds Fuller and more natural. 

As far as gain, you can always goose the amp in other ways to get additional gain while keeping negative feedback intact with a standard pre-phase inverter master.

I like the unmolested tone of that amp, I had given up on the MV so far.  I happen to have a Fryette Power station that I use with that amp and that works well.  Once I start building again I might try the PPIMV again.

[dude]

Everyone's ears are different, tone is subjective, therefore one might say it's too bassy another, too bright. I agree with purpletele, after fooling with changes to the tone stack, adding PPIMV, you name it, I came to the conclusion that the unmolested JTM45 is best. From my experience most play their amps with the bass pot on a lower setting, especially with a Les Paul on the neck only.


The pre PI MV, wasn't on the original plexi but I do agree it adds to the amp, it's probably the only change from the original schematic I would keep. I've changed out components left and right on this amp only trying the amp the next day and putting it back to original. A little advice, IMO for what it's worth, would be to step away from the amp when you get the desire to change things, wait a day or two, you might find that you really don't need that change. I've been there, just use the magic of the amp, blend the vol's.


Bottom line "Shut up and play your guitar"  .

[Big_Mike]

Everyone's ears are different, tone is subjective, therefore one might say it's too bassy another, too bright. I agree with purpletele, after fooling with changes to the tone stack, adding PPIMV, you name it, I came to the conclusion that the unmolested JTM45 is best. From my experience most play their amps with the bass pot on a lower setting, especially with a Les Paul on the neck only.


The pre PI MV, wasn't on the original plexi but I do agree it adds to the amp, it's probably the only change from the original schematic I would keep. I've changed out components left and right on this amp only trying the amp the next day and putting it back to original. A little advice, IMO for what it's worth, would be to step away from the amp when you get the desire to change things, wait a day or two, you might find that you really don't need that change. I've been there, just use the magic of the amp, blend the vol's.


Bottom line "Shut up and play your guitar"  .

So much wisdom in this post.  I made a few tweaks to the amp only to go back to the stock values.  I am really happy with the stock amp, including the pre-phase master volume.

[purpletele]

Bottom line "Shut up and play your guitar"  .

So much wisdom in this post.  I made a few tweaks to the amp only to go back to the stock values.  I am really happy with the stock amp, including the pre-phase master volume.
[/quote]


For what it is worth I have been really into the performance of the Cream Alnico Speaker with Marshall type amps. 

[Ed_Chambley]

A Post PI master I find to really just cut the volume when implemented correctly.  Next time simply use 1 meg audio pots you have on hand and using 2, you can find your optimum sweep.  Even with 6V6Gt, I still use a 250K dual pot.  PEC is usually about $30 after shipping, which is sort of high.  If you use the 2 separate pot method and mark each 50k on the back of the pot, it is easy to find the best POST Pi Master.


Preamp Gain started with the pre master and truthfully, we were having much better results with Plexi Circuits.  2 rules of thought.  You can use a pre master (2204) or Post Master LARMAR (bias resisters will be fine).  Use either one, but I find when I use both it gets mushy.  Like D type amps the preamp distorts, but the output stays mainly clean.


Opposed to a Marshall 1987 Circuit or similar, these amps had 2 volumes (at least) and one side trimmed lows to get upper mids and a stronger 3rd harmonic for dynamics, the normal channel if you can get one really likes the white lettered RCA Backplate.  This tube had second order harmonics and is biased near center the 2nd harmonic is more than 2 times as large.  Tube plays a Role.  RCA is not the only one, Tung Sol had them in 1948 too, lots were made by them.  Better hurry, the audiofiles have found them and I am seeing $180 for nice ones.  Hell, I saw one with Zenith on it the other day.  Just look for shiny black plates.

I believe this to be the plexi sound simply because when pushed the output clips and compresses this signal with pleasant 2nd harmonics, but it is a rock amp so the third is what separates a Plexi from other amps which I hear as the MARSHALL tone.


The only amp I have built that distorts in preamp intentionally and power is Various Trainwrecks, but they do have a stout power supply in most cases.

[purpletele]

A Post PI master I find to really just cut the volume when implemented correctly.  Next time simply use 1 meg audio pots you have on hand and using 2, you can find your optimum sweep.  Even with 6V6Gt, I still use a 250K dual pot.  PEC is usually about $30 after shipping, which is sort of high.  If you use the 2 separate pot method and mark each 50k on the back of the pot, it is easy to find the best POST Pi Master.


Preamp Gain started with the pre master and truthfully, we were having much better results with Plexi Circuits.  2 rules of thought.  You can use a pre master (2204) or Post Master LARMAR (bias resisters will be fine).  Use either one, but I find when I use both it gets mushy.  Like D type amps the preamp distorts, but the output stays mainly clean.


Opposed to a Marshall 1987 Circuit or similar, these amps had 2 volumes (at least) and one side trimmed lows to get upper mids and a stronger 3rd harmonic for dynamics, the normal channel if you can get one really likes the white lettered RCA Backplate.  This tube had second order harmonics and is biased near center the 2nd harmonic is more than 2 times as large.  Tube plays a Role.  RCA is not the only one, Tung Sol had them in 1948 too, lots were made by them.  Better hurry, the audiofiles have found them and I am seeing $180 for nice ones.  Hell, I saw one with Zenith on it the other day.  Just look for shiny black plates.

I believe this to be the plexi sound simply because when pushed the output clips and compresses this signal with pleasant 2nd harmonics, but it is a rock amp so the third is what separates a Plexi from other amps which I hear as the MARSHALL tone.


The only amp I have built that distorts in preamp intentionally and power is Various Trainwrecks, but they do have a stout power supply in most cases.

Ed,

Interesting post, thank you.   

To be clear on the method of equalizing the circuit with a MV included you are starting with two 1 meg pots and mark a scale of 50K intervals on the back.  When the amp is operating at a performance level of tone and clarity then you determine the average setting on the two pots, which will most likely be a 250K?

I am going to post another Master Volume Question on the Dual 50 Thread that I started.

[Ed_Chambley]

A Post PI master I find to really just cut the volume when implemented correctly.  Next time simply use 1 meg audio pots you have on hand and using 2, you can find your optimum sweep.  Even with 6V6Gt, I still use a 250K dual pot.  PEC is usually about $30 after shipping, which is sort of high.  If you use the 2 separate pot method and mark each 50k on the back of the pot, it is easy to find the best POST Pi Master.


Preamp Gain started with the pre master and truthfully, we were having much better results with Plexi Circuits.  2 rules of thought.  You can use a pre master (2204) or Post Master LARMAR (bias resisters will be fine).  Use either one, but I find when I use both it gets mushy.  Like D type amps the preamp distorts, but the output stays mainly clean.


Opposed to a Marshall 1987 Circuit or similar, these amps had 2 volumes (at least) and one side trimmed lows to get upper mids and a stronger 3rd harmonic for dynamics, the normal channel if you can get one really likes the white lettered RCA Backplate.  This tube had second order harmonics and is biased near center the 2nd harmonic is more than 2 times as large.  Tube plays a Role.  RCA is not the only one, Tung Sol had them in 1948 too, lots were made by them.  Better hurry, the audiofiles have found them and I am seeing $180 for nice ones.  Hell, I saw one with Zenith on it the other day.  Just look for shiny black plates.

I believe this to be the plexi sound simply because when pushed the output clips and compresses this signal with pleasant 2nd harmonics, but it is a rock amp so the third is what separates a Plexi from other amps which I hear as the MARSHALL tone.


The only amp I have built that distorts in preamp intentionally and power is Various Trainwrecks, but they do have a stout power supply in most cases.

Ed,

Interesting post, thank you.   

To be clear on the method of equalizing the circuit with a MV included you are starting with two 1 meg pots and mark a scale of 50K intervals on the back.  When the amp is operating at a performance level of tone and clarity then you determine the average setting on the two pots, which will most likely be a 250K?

I am going to post another Master Volume Question on the Dual 50 Thread that I started.

Yes, I just use 2 separate pots if I want to check for what I prefer.  I use some older log taper 1 meg pots and simply test.  When I find what works, I use that.


Truth, I really do not like master volumes because I am not fond of 12A_7 clipping.  Said another way, I refer a Champ distorting a 6V6 to a Plexi distorting 12Ax7.