6 V 6 Plexi Master Volume Advice

[purpletele]

Greetings,

All of the amps that you all helped me build are working wonderfully and they are being played all of the time. 

I have two 6 v 6 Plexi's and they sound great, but really loud.  I would like to cut in a MV into one of the amps, so I have a few questions.

1. I am planning on removing the pair of 100K resistors that are fed from the bias supply and have the .022 caps on the other side.  I am assuming that I need to jump the posts from the bias supply to the .022 caps?

2. I am planning on using a 250K dual pot, is that too big of a pot?  Should I be looking for a 100K dual pot?




The soldering iron has been cold for well over a year.  The Trem-O-Nator was my last project and the other amps are working great. 

[sluckey]

I am assuming that I need to jump the posts from the bias supply to the .022 caps?

NO!

I am planning on using a 250K dual pot, is that too big of a pot?  Should I be looking for a 100K dual pot?

250K is fine.

[jordan86]

I think what you are referring to is the LarMar MV, which is much like (same as?) train wreck type 2. Here are some links.

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Lar-Mar_MV

Discussed and illustrated here at 11 min mark as well
https://m.youtube.com/watch?v=TSbYEM_R-eU

[purpletele]

I am assuming that I need to jump the posts from the bias supply to the .022 caps?

NO!

I am planning on using a 250K dual pot, is that too big of a pot?  Should I be looking for a 100K dual pot?

250K is fine.

Thank You !!

[purpletele]

I think what you are referring to is the LarMar MV, which is much like (same as?) train wreck type 2. Here are some links.

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Lar-Mar_MV

Discussed and illustrated here at 11 min mark as well
https://m.youtube.com/watch?v=TSbYEM_R-eU

Thank You

[PharmRock]

Since the grid resistors on the board are 100K, wouldn't a 100KL pot with the 2.2M "safety" resistors be the way to go?  It seems if a 250K pot with the 2.2M resistors are used, then the parallel resistance would be 225K.  Using a 100KL dual pot with 2.2M resistors would put the resistance at around 96K...close enough to 100K.
Or...use a 250KL pot with around a 180K parallel resistor to give a total resistance of 105K.
Maybe I'm understanding this wrong. 

[sluckey]

Since the grid resistors on the board are 100K, wouldn't a 100KL pot with the 2.2M "safety" resistors be the way to go?  It seems if a 250K pot with the 2.2M resistors are used, then the parallel resistance would be 225K.  Using a 100KL dual pot with 2.2M resistors would put the resistance at around 96K...close enough to 100K.
Or...use a 250KL pot with around a 180K parallel resistor to give a total resistance of 105K.
Maybe I'm understanding this wrong.

Ain't no big deal. There are far more PP 6V6 circuits using 220K grid resistors rather than 100K grid resistors.

[mresistor]

There's an 82K in the layout. Supposed to be 100K?

[PharmRock]

There's an 82K in the layout. Supposed to be 100K?

That's one of the plate resistors for the PI, next to the other 100K plate resistor. 

[mresistor]

Right, I'm just saying both PI plate resistors are shown as 100K on the schematic, but one is 82K in the layout. Also it appears the .022uf PI coupling caps are in the wrong place in the schematic.
Most likely someone else has already mentioned this and I'm just now seeing it.

[sluckey]

Right, I'm just saying both PI plate resistors are shown as 100K on the schematic, but one is 82K in the layout. Also it appears the .022uf PI coupling caps are in the wrong place in the schematic.
Most likely someone else has already mentioned this and I'm just now seeing it.

That's not what I see. The schematic shows a 82K on pin 1 and a 100K on pin 6. The layout shows exactly the same. The .022 caps are in the correct place on the schematic and the layout.

[mresistor]

You're right sorry   looking at the wrong resistors ..   

[dude]

Purpletele, 
I did Sluckey’s layout for a PPIMV, like his diagram shows. I used a dual 250ka pot, best results are pots matched within 10% of each other, closer the better. I ended up using a resistor to get both pots equal. It’s hard to get cheaper pots that have equal swings unless you spend $30 on a Ped pot.
I’m not sure which way l wired the the tubes grids and the point where the two 220ks go to the bd ( your case the two 100ks). At first the master didn’t have a good swing, didn’t cut volume till half way down, l reverse the tube grids and 220k connections and a slight turn of the master gave a much better cut in volume.
I can’t remember which way l made the connections, all l can say is if you change the tube grids and board 220k one way, you don’t need the 2.2m safety resistors, so that might be a clue of what l did, maybe Sluckey can explain if he knows what l’m trying to say. I left the 2.2m R’s in anyway.


But one way was a much better response using the PPIMV. The amp was a Plexi 6v6, same layout as Doug’s and Sluckey’s


What I did was put the pots sweep to the tubes and the pots opposite to ground ends to the bias pot. Doing it this way you don’t need the 2.2M R, cause if the pot fails not an issue, no problem leaving them on the pot either way. Just a better swing response with sweep lugs to the tubes, IMO.

[PRR]

In many forms of long-tail phase splitters, one of the plate resistors is smaller than the other. Lots of Fenders used 82k & 100k.

[PharmRock]

I've seen a few posts/videos where the shield of the two-conductor cable that runs from the caps to the pot actually carries the bias supply voltage as well.  Wonder if anyone here can comment on that.  The other strategy and I assume the more widely used way to do it is to have a dedicated wire for running the bias supply voltage to the dual gang pot, but if the shield is going to be grounded to the bias supply voltage anyways, then what would be the difference?
 
Curt Granger of Granger Amps here in Birmingham has a video on how he does it:
https://www.youtube.com/watch?v=Wb6L0nzkRfc
Go to about the 6 min mark where he discusses using the shield to carry the bias voltage.

What I did was put the pots sweep to the tubes and the pots opposite to ground ends to the bias pot. Doing it this way you don’t need the 2.2M R, cause if the pot fails not an issue, no problem leaving them on the pot either way. Just a better swing response with sweep lugs to the tubes, IMO.

Dude...when you say sweep are you referring to the wiper?

Purpletele...hope its ok to post these questions in your thread...This is about the last thing I need to wire up in my amp, so any info is helpful.
Thanks!

[sluckey]

I've seen a few posts/videos where the shield of the two-conductor cable that runs from the caps to the pot actually carries the bias supply voltage as well.  Wonder if anyone here can comment on that.

Good idea if done correctly. That's how I would do it. Granger's video shows exactly how to do it correctly and he clearly explains all the pitfalls to avoid.

[dude]

Dude...when you say sweep are you referring to the wiper?

Yes, me wrong should have said "wiper".  That's an awesome video on PPIMV, that's for posting.

[sluckey]

Hey purpletele. Here's the same modified Hoffman layout showing shielded two conductor cable for hookups.

[purpletele]

Hey purpletele. Here's the same modified Hoffman layout showing shielded two conductor cable for hookups.

Sweet!!  Thank You

I have some time to start today!  One sniff of solder and I'll probably start building a new amp.

[purpletele]

Ok, the installation went smooth, the results are as follows:

The MV had to be turned half way up to get any volume,

The tone was terribly tinny.  I poked at the solder joints with a stick and they didn't pop or cause change, so I am confident with the soldering. I have attached some photos.

I have a 100K Dual Pot if I need to try that.

[dude]

I just opened my 6v6 plexi, l screwed up in post above. It’s wired exactly as Sluckey’s and Grangers amps. I had the tubes grids to outside lugs and caps to wiper first time, worked but volume cut didn’t start it knob was halfway down, changed to the layouts here and sweep was much better, volume cut came with a slight turn rather than halfway.


I got the first way off a train wreck method that said you didn’t need safety resistors, use it and had the bad sweep.

[dude]

Purpletele, looks like you have the wiring wrong, look at Sluckey’s drawing. The wipers go to the tube grid resistors with the ground cut off an covered. The two .02 caps go to the outside lugs, the ground goes to the bias spot on the bd. safely covered in heat shrink so it touches nothing but bias. The two grounds at the pot should be safely covered connected to the ground side of the dual pot. Keep the wiring correct, remember the granger amps said you get a high pitched squeak if wiring is reversed. See at Sluckey’s drawing carefully. You don’t need a 100k pot, 250k is fine

[dude]

The more look at your pics, you might have it right but maybe need to switch the grids wires or cap wires, one or the other, remember the video, watch it again. It will work well.

[sluckey]

I don't know why it sounds so tinny. Does it sound tinny with the MV turned up to max? Sounds like you used a log taper pot. If you want the volume to begin increasing as soon as you turn the pot from zero to one, use a linear taper pot.

Cover up that exposed shield on both ends of the cable between the pot and board. Otherwise you risk shorting the bias to something, probably with undesirable results. See pic...

[purpletele]

I don't know why it sounds so tinny. Does it sound tinny with the MV turned up to max? yes

Sounds like you used a log taper pot. If you want the volume to begin increasing as soon as you turn the pot from zero to one, use a linear taper pot.

Cover up that exposed shield on both ends of the cable between the pot and board. Otherwise you risk shorting the bias to something, probably with undesirable results. See pic...  Thank you, I'll cover that exposed foil

Thanks, I'll start troubleshooting this afternoon.

[PharmRock]

it looks like the feed from the bias supply is going to the pot closest to the chassis.  did you put a jumper between the terminals so both pots receive the negative bias supply?  it's hard to tell from the pics. 
In Granger's video, he uses the leg of the rear most resistor to connect the terminals. 

[dude]

Also, you might want to use a solder sucker to clean up the eyelets around the area where your 100ks where, you could have a drip under the board touching the chassis..? Just clean up the area and re-solder. A lot of times excessive heat and too much solder can drop under the bd, slightly touching the chassis. Possible over heating might have damaged the .02 caps...? If wired correct, it should work. Use bigger heat shrink that will go over the cable just having the two leads sticking out where you’re not attaching the ground. Just like the video and Sluckey’s layout. Using a 250KA dual pot will give you the fastest decrease in vol from full on, seems that would be best as it would cut the volume with less CC rotation. Sometimes episodes like this can drive you crazy you just have recheck everything several times, but do clean up that area with a solder sucker and start over with pot lugs and eyelets as clean as possible, don’t overheat. Heat and pull the l component out first, then solder suck the area clean.
Is the he 250ka pot within 20% of each other, if way off from each other you can use a resistor to make them equal, Sluckey is better at explaining that and the math to get the right value...? I think they should’ve be like 5% of each other, very hard to buy one like that unless you spend $30 on a Ped pot.

[purpletele]

I wired up a 100K Dual pot with 2.2M resistors and that seem to do the trick.

I am hooking the amp up inside the house with a pedal board to see how it sounds cranked but MV turned down.

I don't know if it was the pot itself or the value, but I have something to work with now.

BV

[dude]

I wired up a 100K Dual pot with 2.2M resistors and that seem to do the trick.

It wasn't the pot value I guarantee that, most likely a bad solder joint or a wire was shorting out. But glad you got it up and running.

[purpletele]

I wired up a 100K Dual pot with 2.2M resistors and that seem to do the trick.

It wasn't the pot value I guarantee that, most likely a bad solder joint or a wire was shorting out. But glad you got it up and running.

I have a feeling the 250K might have been bad.
I have come to the conclusion that this particular amp doesn't need the MV for my use.  The amp sounds less lively with the MV and there is a bit of loss of highs.  I thought about trying a nice PEC Pot to see if that helps, but I think I may change it back to the stock configuration and be done with that experiment.

Thanks for the help!

[sluckey]

Before you give up on it try this...

Put a 100 to 150pf cap between the hot side of the pot and the wiper. Do this for both pots. This is the same as a bright cap found on many Fender amps. It will help preserve the highs when the MV is turned down.

[Big_Mike]

Purpletele -

Long time I have been away from the board, but I saw this thread and had to jump on.  I tried the PPIMV in the Hoffman Plexi 6v6, but decided that the standard Pre-Phase inverter master with some modifications sounded better.  Here are the mods:

1.  Connect a 220K resistor before the input wiper of the master volume pot.  This preserves bass frequencies and keeps a much fuller sound as you turn down the master.

2.  Connect a small cap between lugs 1 and 2 of the master volume pot.  5-10 p works great.  This will help preserve treble frequencies at lower volumes.

If you need more gain, just use a boost pedal.  The advantage to the Pre-Phase inverter master is that you keep the NFB circuit intact.

[purpletele]

Purpletele -

Long time I have been away from the board, but I saw this thread and had to jump on.  I tried the PPIMV in the Hoffman Plexi 6v6, but decided that the standard Pre-Phase inverter master with some modifications sounded better.  Here are the mods:

1.  Connect a 220K resistor before the input wiper of the master volume pot.  This preserves bass frequencies and keeps a much fuller sound as you turn down the master.

2.  Connect a small cap between lugs 1 and 2 of the master volume pot.  5-10 p works great.  This will help preserve treble frequencies at lower volumes.

If you need more gain, just use a boost pedal.  The advantage to the Pre-Phase inverter master is that you keep the NFB circuit intact.

I'll give it a try, thanks for the note.

[Bieworm]

IME master volumes post or pre PI take away all the juice you have built up in the preamp. OD  is gone...
I'd rather look into VVR  or a good attenuator

[purpletele]

IME master volumes post or pre PI take away all the juice you have built up in the preamp. OD  is gone...
I'd rather look into VVR  or a good attenuator

Thanks for the note.  This amp really doesn't need a MV the way I use it, but I like the idea that Bike Mike had, so I'll try that. 

[Bieworm]

If/when you're planning a next build you should check 18Watt.com and look up tremolo TMB reverb. It has a superb plexilike preamp OD.  I built 2 18W versions and 1 36W recently.. oh man!!!
Check this 36W clip:
https://www.dropbox.com/s/qg5bekvobi8tytr/Tremolo%20TMB%20reverb%2036W%20first%20take.m4a?dl=0

[purpletele]

Before you give up on it try this...

Put a 100 to 150pf cap between the hot side of the pot and the wiper. Do this for both pots. This is the same as a bright cap found on many Fender amps. It will help preserve the highs when the MV is turned down.

I have to rescind my statement that the MV is not needed on this amp.  I have been playing the amp with 100K dual pot MV quite a bit, then I disconnected the MV.  I can get some real nice bluesy tones with the Volumes up just past noon, which would previously blow me out. 

I am going to add back some small caps to the MV to see if I can get a little more high end.  In summary, the MV seems to be working as intended, and it seems to be useful.

I was thinking about trying some different tubes to see if I can effect a tonal change. Any recommendations for any special pre amp or power tubes that might give the amp a jolt.  I have Mesa 12AX7 -A pre-amp tubes and 6 V 6 GTW's.  My tube dealer had a connection to Mesa Engineering.


Thanks

BV

[dude]

Different 12ax7’s in 1st socket can brighten the amp, try what you have. As far as 6v6, again some can be brighter sounding to a lesser degree, IMO, try what you have. Try the bright cap Sluckey suggested on PPIMV.
Also keep in mind speakers have “a lot” to do with tone. Alnico’s are usually brighter.
I see no reason to take the PPIMV out, turning that to full on should be no difference than without. 
One other tidbit, change the .022 coupling cap to the bright channel to .0022, if it isn’t already.