Volume / Voltage Issues with Bassman 6g6B

[HommeMarrBuckley]

Hello Everyone,


It has been awhile but I am nearing the end (hopefully) of my scratch build Bassman 6g6B build.  This is my first build and it has been a learning experience.  I spent most of the summer building the cab and head which was a first in woodworking as well.  Anyway, I don't want to bore anyone and was hoping this wouldn't be a never ending debug due to my abilities.  But after a great deal of time, I was ready to fire up the amp through the cab to set the bias.  I ran into some issues.  Two shorts and a fuse blown later, here I am.  I have sound on both channels but it is nowhere the volume this amp should be producing. Second, the voltages on V IV seem to be off.


First here is a link to the schematic and layout.
http://www.gad.net/Blog/wp-content/uploads/2011/06/1963-Bassman-6G6B-Schematic.jpg
http://www.gad.net/Blog/wp-content/uploads/2011/06/1963-Bassman-6G6B-Layout.jpg


After completing the circuit and then the woodwork and a lot of time, I was a little impatient to try the amp out.  I went to test the voltages and set the bias which I have never done before.  I installed 1ohm resistors and hit the weeds.


Short 1 - On power tube V6, while testing voltage on pin 3 from OT, I didn't realize the lead was too long and close to touching pin 4.  Bright lights and smoke and I fried the top ring terminal of pin 3.
Short 2 - Voltage testing remix, voltage readings were going fine but when I went to test bias, I forgot to switch from mV to V on the Fluke.  Went to pin 3 of power tube V5 and fried top ring terminal 3 there.


So, I have two power tube sockets with both pin 3s top ring terminals fried.
After the second issue, the amp wouldn't kick over and realized the fuse was toast.  I replaced it and completed the voltage readings which are as follows. 
Parentheses are (Fenders) followed by mine.  Some I am not sure on how to designate and some I have missed on the board.


From Bias rectifier board (-54) -58.3 to point on board by (2) 220 k resistors (-54) 51.2
(layout) Red wire from bias to board (430) 470
yellow wire point to right of it on layout (428) 470
black choke / red OT on board (430) 471


V1
pin 1 (135) 142.1
pin 6 (230) 263.2


V2
1 (150) 171.8
6 (150) 177.6


V3
1 (230) 264.8
6(190) 220.6


V4 (here appear to be some issues) perhaps some components on the board
1 (300) 454
6 (280) 455
8 (30) 6.6


V5
3 (428) 469
6(428) 470
Bias 35.8


V6
3(428) 468
6(428) 469
Bias 36


Every component and part is brand new from the filter caps to the transformers to the jacks etc.


During all of this (shorts, fuse, readings) I was able to get sound.
The Bass channel is slightly louder than the normal channel.  Bass channel is dead quiet until about 3-4.  Sound comes in about middle-6 on normal channel.  Both channels maxed are nowhere near the window shattering volume it should be but they sound reminiscent of the tone of those channels if that makes sense.
I also built the speaker cable with this thing.  I wasn't sure to mention it or not but was wondering if it could be choking the signal.  I don't know.  It's pictured later and it was made from 2 strands of white 18 GA with 2 strands of black 18 GA and pancake plugs.


I am hoping with everything that I didn't damage something severely, especially the power tubes, I am using winged Cs and they're not in production anymore.
I know there is definitely something wrong around tube 4.  I just hope I am able to fix that along with the volume but if not I will probably have to venture to the tech which kinda defeats the purpose of scratch building an amp.


Im all ears and any input is appreciated.
happy holidays.
Matt

[sluckey]

Turn the amp off and measure resistance from V4 pin 3/8 to chassis. Should be about 12K. If not, check resistors in the highlighted path from V4 to presence pot (see pic). Should have a 820Ω, 6800Ω, and 4700Ω.

[Willabe]

I am hoping with everything that I didn't damage something severely, especially the power tubes, I am using winged Cs and they're not in production anymore.

I would use different power tubes, a cheap(er) or good used set, that way if you blow them up not as big a deal, until the amp was up and running with no problems.

The speaker cable made with the doubled up 18 gauge should be ok, but the single speaker wire (18 gauge?) connecting the 2x12's looks like it's a little thin (gauge) for the current going from the OT secondary to the speakers. (Not saying it's your volume problem, V4's voltages are off, do what Sluckey tells you.)

When posting the tube pin voltages you need to post the grid and cathodes (K) also, so pins 1-2-3 and 6-7-8
(for 12 _ _ 7, 1/6=plate, 2/7=grid, 3/8=K).

For the output tubes, grid 1 (input/control grid) and screen grid along with K and plate.

On 6L6's, 5=grid1, K=8, screen=4, plate=3.

And check/post your heater voltage, just to be sure it's wired correctly on a new build. Red meter lead to 1 heater pin and the black lead to the other heater pin, not to ground.

When testing voltages use the alligator clips on your meter leads so you don't short anything from pin to pin on a tube.


               Brad            

[Willabe]

On new builds or when rebuilding a power supply, changing a PT or firing up a none working amp or an old amp use a light bulb limiter. It can save the amp from going up in smoke if there's a short.   

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


                   Brad   

[HommeMarrBuckley]

Turn the amp off and measure resistance from V4 pin 3/8 to chassis. Should be about 12K.

Won't be able to do anything until Monday.  Just don't have the space here to work on it so keep it at my folks.  So, I will set the MM to ohms and measure that.  Can I measure resistors soldered to board?  Just set it to ohms and put probes on leads (even though they're soldered in)?

I would use different power tubes, a cheap(er) or good used set, that way if you blow them up not as big a deal, until the amp was up and running with no problems.

I was thinking the same thing. 

When posting the tube pin voltages you need to post the grid and cathodes (K) also, so pins 1-2-3 and 6-7-8
(for 12 _ _ 7, 1/6=plate, 2/7=grid, 3/8=K).

Set to volts, ground and pretty much get a reading on all pins besides heaters? ok.

And check/post your heater voltage, just to be sure it's wired correctly on a new build. Red meter lead to 1 heater pin and the black lead to the other heater pin, not to ground.
When testing voltages use the alligator clips on your meter leads so you don't short anything from pin to pin on a tube.

Pins 5 and 9 on preamp tubes and 2 and 7 on power.  Assume it doesnt matter which color lead to which pin.  Set to volts here as well, I guess.  I was using the probe for the readings (kept it still)  The clips were a little tricky and passed on them.  Not much space in and around there.  Saw benefit of adding bias probes to chassis, as well.


What's a good way to discharge voltage afterwards?  I set red lead to pin 1 of V1 and ground to chassis but the voltage slowly dropped off.  From like 17.87 to 17.86 to 17.85 and so on.


Here's a snippet of the cab joints.  Happy how that turned out without doing any woodworking before but will probably never buy lumber from big box store. Learned hard way with warped boards.
When all is right will post finished pics and sound.  Have some staining and then just a matter of screwing it together.  Besides this-working on plates.


Thanks

[HotBluePlates]

Turn the amp off and measure resistance from V4 pin 3/8 to chassis. Should be about 12K.

Can I measure resistors soldered to board?  Just set it to ohms and put probes on leads (even though they're soldered in)?

If you just put one meter lead on ground and the other on V4 pin 3/8 (as Sluckey said), it will at once verify the value of all those resistors, your soldering work, and the ground at the Presence pot. This is a shortcut to see if any further probing is needed.

[eleventeen]

Willabe makes a very good point when he says "When posting the tube pin voltages you need to post the grid and cathodes (K) also, so pins 1-2-3 and 6-7-8 (for 12 _ _ 7, 1/6=plate, 2/7=grid, 3/8=K). For the output tubes, grid 1 (input/control grid) and screen grid along with K and plate. On 6L6's, 5=grid1, K=8, screen=4, plate=3.


The voltages you find on the preamp tubes don't exist in a vacuum (no pun intended). You want, both as part of asking for the assistance and for your own understanding, to know that for all the 9 pin tubes, we have two triodes, with pin numbers 1-2-3 and 6-7-8 for plate-grid-cathode. With the exception of the reverb driver tube and the PI, we expect to see about 180 volts on the plates (but could be as low as 120-150 and as high as about 250-280----not in Fender amps, but in others) and a single-digit number of volts on the cathodes. We expect to see almost nothing on the grids.

When that condition exists, we can have good confidence that tube is "set up" in a way that if we apply an AC signal to the grid, there is reasonable chance that the tube will be able to amplify. We want to see the positive volts on the plate, we want to see the grid a bit negative relative to the cathode (and this is sometimes hard to see when you're learning) but with the cathode at 2-3 volts and the grid at zero volts....the grid is, actually, negative relative to the cathode.


But what is more important for your own understanding is the "grouping" of those tube pins. In other words, don't list your voltages 'plate V1A, plate V1B, plate V2A, plate V2B'. What V1 has on its plate has utterly nothing to do with what V2 has on its plate. What *is* important are the relative voltages on plate-grid-cathode of V1, plate-grid-cathode of V2, etc; etc.

The reverb driver is a special case because the tube gets very high volts (~~425-450) in a reverb amplifier; you don't have that in your amp. The PI is a little different because the cathode is elevated, 30 or so volts in your amp, almost 100 volts in an AB763. 

[HommeMarrBuckley]

V4 pin 3 .843 M resistance.
Wasn't sure to do something else with it powered off before checking voltages.

[Willabe]

V4 pin 3 .843 M resistance.

.843M = 843K and it should be around 12K, way off.

So now measure each of the 3 resistors Sluckey pointed out. You probably have a wrong value resistor in the string.


                   Brad     

[HommeMarrBuckley]

Sorry if this sounds stupid, but measure both leads of resistor or keep grounded and measure end point of each?

[Willabe]

Measure across the R, 1 meter lead to 1 R lead the other meter lead to the other R lead. Meter set for resistance.


                    Brad   

[HommeMarrBuckley]

From presence pot down. (Fender)
(4700) 4.37k
(6800) 6.83k
(820) .83 M 

[Willabe]

(820) .83 M

No, 0.830M = 830,000 ohms = 830K ohm's.

So does it read 820 ohms or 830K ohms?


                  Brad     

[HommeMarrBuckley]

This is what it shows

[Willabe]

What are the color bands on that R?

If it's an 820 ohm resistor they will be; gray/red/brown.


                   Brad     
           

[HommeMarrBuckley]

 

[Willabe]

I bet that R you put in colors are; gray/red/yellow.

My meter reads 0.820M with a 820K R and 0.820K with a 820 ohm R.


                 Brad   

[HommeMarrBuckley]

No, 0.830M = 830,000 ohms = 830K ohm's.

So does it read 820 ohms or 830K ohms.

I kept reading this and realized.
It looks looks to be gold yellow red and probably gray
which would be 820k
Probably ordered and placed wrong value.

[HommeMarrBuckley]

Bass pot bass channel
Looking elsewhere as well
Thank you

[Willabe]

It looks looks to be gold yellow red and probably gray
which would be 820k.

Your reading it backwards. Gray/Red/Yellow/Gold, gold is tolerance.

Probably ordered and placed wrong value.

Yes, wrong resistor value, it's an 820K (= 820,000) resistor.

Look at this chart on R color code posted below. It shows 5 band AND 4 band color code.


                Brad   

[PRR]

> gold yellow red and probably gray

We usually read the other way (gold last), but you are correct: 820K.

> This is what it shows

The meter shows 0.830Meg or 830K or 830,000 Ohms.

The schematic shows 820. Without a K or an M. It should be 820 Ohms.

I do not care if the first part is 82 or 83.

But by the number of zeros, you have a Thousand Times more ohms in there than Leo picked.

In this case, the PI tube is forced to run with a thousand times less current than Leo thought best. It's feeling awful weak.

If you do not have a 820 in the shack, 470 or 1K will work a LOT better than a resistor 1,000 times too big.

[sluckey]

This is what it shows

Did you replace the resistor yet? Sound OK now? Any more problems?

[HommeMarrBuckley]

Sorry for the delay but things are pretty good.
After removing the wrong resistors and installing the new ones, I was pretty impatient in testing out the amp so I plugged it in and it worked!!!
Sound and volume came through as I thought they should be.  I don't want to write too much about my reactions because it's off topic but I was impressed with the responsiveness of the amp.  Also, I read numerous times, the distaste for the bass channel but I didn't have that vibe.  I spent a great deal going through tones (I gave my strat 7 way mod some time ago). 


I went through voltage readings again but I was much more nervous this time.  I was just really afraid of after finally getting an end result that I might do something to damage it so I was much more skittish this time.


V1
1  143.6
2  0
3  1.48
6  267
7  143.6
8  146


V2
1  174.3
2  0
3  1.39
6  179.5
7  .003 (??)
8  1.3


V3
1  174.3
2  0
3  2.07
6  218
7  0
8  2


V4
1  331
2  22.3
3  34.5
6  312
7  23
8  34.5


V5
3  476
4  477
5  -50.9
6  478
Bias 36.7


V6
3  477
4  478
5  -50.9
6  478
Bias  36.5


In terms of heater voltages, (Im sorry Brad) here is where my nerves got the best of me.  On V6, I got 6.3 or 6.5, can't recall exactly because I remembered more the spark I got when I removed the clips.  I decided not to press on from there.  Im looking at better clips for the meter.  These have good insulation but are not very sturdy--meaning that they tend to sag and don't hold firm upright.  I still have to spray paint/create baffle and stain the wood and with winter in PA like it is, it might be some time before I am able to do those out in the garage.  So, I haven't quit on it because it's not getting screwed in the head cab any time soon so I might come back to it.  It helped me look at my layout and I see some things that I could have done better or correct.  I really should have splayed out the tube pins more.  I don't necessarily like my wiring around the bass input jacks in relation to the ground bus.


This leads to when I initially played through.  Upon flipping standby, there is hum.  Once I start and stop playing, it's not there.  Upon thinking about it, I haven't screwed the filter cap can on yet.  Does this play a role? Also, I read that it's good to put some foam insulation on the top of the can to help prevent rattling caps but I was concerned with fire hazard.   All in all, the hum made me look around grounding areas and tube sockets.  I checked the forum for Hoffman's ground diagram and all seem to be good in relation to it.  I just don't like the length of the ground wire from RG74 wire by bass jack and tube sockets seem to have much more going on in the center--inductance??  Reminds me of a flower bud--like mine are at nighttime when they should be spread out like daytime/noon.


Perhaps unrelated, but nearly every amp I have had I was able to pull in radio sound very very faintly when powered on.  This includes Boogie Tremoverb, SF Twin reverb, etc.--decent amps.  Now, the home is pretty high up on a hill but I noticed even this  happened when I was checking voltages (while reading I could barely hear radio).  Perhaps, it is hard to eliminate due to elevation.  Regardless, I don't have it when I power on so it really doesn't matter.


Anyway, Im posting too much here but just wanted to say that I am grateful for the help and everyone that chimed in to pinpoint a simple mistake.  I am really glad it wasn't something major.
This forum and you guys are great, Thanks again!!

[sluckey]

Voltages look good.

I haven't screwed the filter cap can on yet.  Does this play a role? Also, I read that it's good to put some foam insulation on the top of the can to help prevent rattling caps but I was concerned with fire hazard.

You're talking about the sardine can that we like to call the doghouse, right? That's not a factor with hum. The only reason those caps are covered is to prevent someone from getting a nasty shock. Foam insulation is good. There's no fire hazard if done properly. Just keep the foam off of the resistors. Caps should never be warm unless they are bad. I would use self-adhesive high density foam refrigeration/pipe tape just because I'm looking at a roll right now. My roll is 2" wide and 1/8th inch thick. Door and window insulation would work well too.

[Willabe]

Hey glad you got it working!   

Your not the 1st guy and wont be the last that had a wrong value R or 2 in a build, it happens to the best of them.   

In terms of heater voltages, (Im sorry Brad) here is where my nerves got the best of me.  On V6, I got 6.3 or 6.5, can't recall exactly because I remembered more the spark I got when I removed the clips. 

You don't have to apologize, we all get a little nervous after a spark.   

I brought up checking the heater voltages because the amp wasn't working, now that it's working and you verified ~6.3 at 1 power tube your good. You don't have to check them all.

Here's a few things for next time;

1. Yes get some better clips that you trust.
2. When using them put them on with the amp OFF then turn the amp on, take your reading, then turn it off again before you move the clips.
3. The power tubes have the highest voltages in the amp at their socket. So take your heater voltage reading at the 1st preamp tube, it has the lowest voltages in the amp and it's the last tube in the heater string so if it reads good then all the other tubes will normally be good also. (In a rare case the tube sockets heater pin could be broken and the tubes heater pin wont get the voltage, but this is extremely rare.)


                      Brad        

[Ed_Chambley]

 
Great news Matt.  Glad you got it going.  I feel a little responsible for you catching the Tube Amp Virus.  Now that you know how they work and are playing what others do you have in mind to build?

It is a great circuit, I wish I had occasion to play mine more.

[HommeMarrBuckley]

In evaluating hum issue, perhaps I will shoot a quick youtube vid of it so it's understood/heard better.

Great news Matt.  Glad you got it going.  I feel a little responsible for you catching the Tube Amp Virus.  Now that you know how they work and are playing what others do you have in mind to build?

Thanks Ed, I don't think it's the bug to create amps as much as people like you and the forum being supportive to those like myself that have no experience.  I value that.  I lurked and read on some other places in relation to this amp and didn't like the vibe I felt there.  If I would have saddled up there, I probably would have shut down a long time ago.  So cheers to this place, Im staying.


Next?  It will be a Rip Van Winkle Supro Thunderbolt.  RVW in that it will take me awhile but I saw another build that I would like to try to copy.  It will be a challenge with the point to point and looking hard at the schematic and layout on my own but I think it's within my grasp.  I pretty much have all supplies but it will wait until I finish this one.  I am sure I will have plenty of questions posted here.


I will post vid as soon as it's avail.  Hopefully next week.


Matt

[HommeMarrBuckley]

Link to vid


https://www.youtube.com/watch?v=KuZtGJC1bak


Not the greatest in vid/audio quality.  It is much louder in person.  Put phone cam in front of speaker so you can hear it without guitar plugged in.  Hum does not change when I turn volume knob.  Have no idea if it's 60 or 120.


Then plug guitar in and it's barely audible in person and in vid. 


Any suggestions?

[SILVERGUN]

Have no idea if it's 60 or 120.

Your Fluke 87 does. 
Take your meter leads and put em across the speaker leads.
Set the meter per the instructions below:

Here's the rest of the manual:
http://media.fluke.com/documents/80v_____umeng0200.pdf
Oops, different series...
Instructions are similar enough

[Ed_Chambley]

Looks like you may have either a ground loop or your input jacks are not closing.  Check the jacks first.

[HommeMarrBuckley]

Take your meter leads and put em across the speaker leads.

At work right now uggg, so this will have to wait until tomorrow.  Sorry SG, but not sure where I put the leads.  On amp speaker jack?  or 2 points on  cab jack?  Im sorry but you will almost have to tell me like you would a kid.

Looks like you may have either a ground loop or your input jacks are not closing.  Check the jacks first.

What do I need to look at/ look for in terms of them not closing?
I know in terms of the jacks especially the bass channel, that I was concerned of the distance of the bus bar to them.  Plus, using the rg wire there.  The ground wire to bus was somewhat long.  I think there were a few broken strands there too.  I can post a pic later.  Sorry for my ineptitude.
Thanks for the suggestions.
 

[SILVERGUN]

Take your meter leads and put em across the speaker leads.

At work right now uggg, so this will have to wait until tomorrow.  Sorry SG, but not sure where I put the leads.  On amp speaker jack?  or 2 points on  cab jack?  Im sorry but you will almost have to tell me like you would a kid.

If you have an open back cab and can see the speaker connection terminals, put one lead on each of those tabs.

If it's a closed back cab, you can just disconnect the speaker cable and put one lead on the tip and the other on the sleeve....or if the amp is still open you can read it right at the speaker out jack, one lead on the 'hot' and the other one on ground.

[DummyLoad]

it that chassis painted? if it is, you may not have a good ground connection on one of ground lugs or input jack(s).


--pete

[HommeMarrBuckley]

As far as I know, it wasn't painted.


Here's a pic of the bass channel input jacks.  I copied this on normal channel as well.
Did I do these right?
http://min.us/i/byJrZKKrBi9Mi


The RG74 is ground to the bus but then the white wire is coming off the jack to the bus too.  Should it be like that?


(before someone asks--I know it looks like it in the pic) the ground jack (white wire) is not connected to the middle clustering tabs.

[HommeMarrBuckley]

I have been reading and it might be looking up because it seems from what I have read that if you have 2 grounds at the same point (my jacks) you will create a loop.  I have to think this is what is happening from what I see and in the pic.
I hope this is it because I wasn't really looking forward to hunting this down somewhere else in the amp.


Do I just need to clip out that white wire?


off to bed
thanks

[shooter]

I haven't looked at your schematic, just lurking.  the easy way to tell "if you can clip the white, grounded wire", after you clip it, ohm it to ground, if it's " still grounded" then you just eliminated 1 ground loop.

[Willabe]

Read through this on grounding if you haven't already.

http://www.valvewizard.co.uk/Grounding.html


                  Brad   

[HommeMarrBuckley]

Read through this on grounding if you haven't already.

Brad,
I read over pages 4 (265) and 9 (270) with ground loops and ground bus.
Without much of EE background, I wish it would diagram coaxial with ground bus together.
However, I was wondering if my white wire ground off jack (in the pic) needs to connected together with the RG74 coax ground at same point.
I dunno.  It seems there is one connection (jack) with two different ground points and perhaps they need to have/share same ground point on the bus.

[sluckey]

That white wire is fine.

[mresistor]

It sounds like 120HZ hum to me. Same sound my champ was making.  And if that is the case a Pi filter is needed to filter it out.  An additional capacitor and resistor or choke (inductor) between the rectifier output and the first PS node.

http://www.valvewizard.co.uk/smoothing.html

Also the RG-174 shouldn't be grounded at the tube socket (the other end) only at the source.

[HommeMarrBuckley]

The other end at the tube socket is not grounded.

[mresistor]

Ok  I just looked at the schematic as I remembered the bassman has solid state rectification. How are the two 20uf capacitors wired up prior to the standby switch? Are the solder joints well done? Can you post up some good pics to show the detail of the PS filtering caps?  there shouldn't be that much hum in this amp at all. A pic of under the doghouse where the big caps reside   to show the wiring. 

[HommeMarrBuckley]

I plugged into all four jacks.  The "hi"s work on bass and normal channel and sound great, however, the "lo"s do not have any sound or volume at all.

[sluckey]

Jacks are not wired correctly. Wire your jacks exactly as shown in Hoffman's common hookups pdf...

http://el34world.com/Hoffman/files/CommonHookups.pdf

[Ed_Chambley]

You can still leave the 68K on the tube.  The diagram Sluckey pointed to shows 2 of the 68K resistore, but notice they connect together and them to the tube.  Just rework the jacks.

[HommeMarrBuckley]

I have the 68k grid stopper on the tube instead of using the two at the jack.


So, by that diagram on here for double inputs, I run the RG74 hot through Tip Lo jack ----- to Sleeve (Lo) and Tip (Hi) that are wired together.  I have that.


Ground of RG 74?? Just goes to ground of Hi Jack---and then to bus, correct?


And Ground Lo Jack connects to bus?  If so, I can keep mine as is--white wire in pic.


I understand 1 M resistor placement and mine is same.


So, from writing and reading this over and looking at my jack, it just looks liked I need to re-wire my RG-74 ground.  It should go to the Hi Jack Ground and on to the bus bar.

My RG74 ground is not that long--guessing just connect it to Hi Jack Ground and run piece of 20GA from there to bus, right?


I am not sure.  Was trying to look at similar double input jacks with coax.
This gain channel input of tweed deluxe is wired different.
http://robrobinette.com/images/Guitar/5E3P_Build/5e3p_Cascade_Coax_Layout.png

[Willabe]

If your using standard Switchcraft 1/4" jacks then the jack is automatically grounded to the chassis, make sure you use the lock star washer that came with it and tighten up the jack so it makes good contact with the chassis for a good ground.

So if you have a 4x input then you can run the pot ground buss to any of 1 the 4 jacks ground lug and it will be grounded.

If you use the plastic Cliff jacks like in Marshalls or isolating/insolating washers, then you have to wire up the ground lugs from each jack to each other.


                            Brad       

[HommeMarrBuckley]

Not trying to frustrate anyone and perhaps Im not seeing the wood for the trees but I assumed something is not correct right in my wiring because the Lo Jacks are not working.

[sluckey]

I have the 68k grid stopper on the tube instead of using the two at the jack.

I assumed something is not correct right in my wiring because the Lo Jacks are not working.

If you want it to work like the 6G6B hi/lo jacks you'll have to wire it IAW the schematic. You must use one 1MΩ and two 68KΩ. Doug's commonhookup pdf that I pointed to shows you how to do it in case the schematic is confusing to you.

Look at page 7 of this pdf if you want an explanation of how the hi/lo jacks work...

     http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[HommeMarrBuckley]

Alright, I think I get it.
So, if you look at my pic, I can cut the RG74 ground going to the bus bar and leave the white wire jack ground to bus

So if you have a 4x input then you can run the pot ground buss to any of 1 the 4 jacks ground lug and it will be grounded.

And with that, I can move over to the normal channel jacks and cut the ground lug pot to bus wire, as well, because I only need 1 of the four which I already have (bass channel white wire ground lug to bus)