New Fender Bassman 6G6-B Build Questions

[72Blazer]

This is a new Fender 6G6-B build in progress

1:  Should I/is it recommended to install 1.5KR grid stoppers on the power tubes like the AB165 and AA864? Benefits?

2.  Why does the original layout show pins 1 and 8 connected?

I am installing 1 Ohm resistors (pin 8 to ground) for Bias checking ease.

3.  If I install the grid stoppers and the 1 Ohm resistors (as described above), do pins 1 and 8 on the power tubes still need to be jumped together?

Hopefully my questions make sense and thanks in advance.

Layout Attached

Vr
J

[dwinstonwood]

2.  Why does the original layout show pins 1 and 8 connected?

Maybe back when this amp was made some people were still using the metal 6L6's? I don't really know.

[sluckey]

This is a new Fender 6G6-B build in progress

1:  Should I/is it recommended to install 1.5KR grid stoppers on the power tubes like the AB165 and AA864? Benefits?

2.  Why does the original layout show pins 1 and 8 connected?

I am installing 1 Ohm resistors (pin 8 to ground) for Bias checking ease.

3.  If I install the grid stoppers and the 1 Ohm resistors (as described above), do pins 1 and 8 on the power tubes still need to be jumped together?

1. I would. May prevent some hi-freq oscillations. I suggest installing the 1.5Ks directly on the socket, between pins 1 and 5 (just like the AB763 amps). Of course you cannot jumper pin 1 and pin 8.

2. Nothing special. All the 6G?? amps connected pins 1 and 8 together. This was a carryover from metal tube era. The metal part of the old 6L6s was connected to pin 1 and then to ground to act as a shield. I think if you look at the bottom of a glass 6L6 you'll see there ain't no pin 1.

3. Don't put a jumper. See #1 and #2.

[72Blazer]

Thanks Steve for the very helpful clarification here.  So for my own edification here anytime you use those metal tubes (6L6 or 6V6) in a fender AB763 circuit, you need to jumper pins 1 and 8?  I ask because I am using metal 6V6s in a FDR and I did not do that.  I did not notice any adverse affects either...

On another note, I think I figure out the connections for the normal channel treble pot.  The pot I purchased actually has 4 connection tabs-where as the layout (I attached earlier) has only three connection tabs-like all the other pots I am familiar with.  Once I finish wiring that pot up-I will post some pics so maybe it van be verified correct by you smarter guys.

Thanks again
Vr
J

[72Blazer]

Here is a pic of the treble pot.

After looking at the layout again, there is what looks like a 4th tab in the 7:00 position (looking at the layout pic I posted).

[sluckey]

Thanks Steve for the very helpful clarification here.  So for my own edification here anytime you use those metal tubes (6L6 or 6V6) in a fender AB763 circuit, you need to jumper pins 1 and 8?  I ask because I am using metal 6V6s in a FDR and I did not do that.  I did not notice any adverse affects either...

Simple solution. Don't put metal tubes in an AB763 amp.

On another note, I think I figure out the connections for the normal channel treble pot.  The pot I purchased actually has 4 connection tabs-where as the layout (I attached earlier) has only three connection tabs-like all the other pots I am familiar with.  Once I finish wiring that pot up-I will post some pics so maybe it van be verified correct by you smarter guys.

You better look closely at your layout. I see four tabs.

[72Blazer]

Ok, makes a little more sense now.  I found this pic online of another 6G6B build.  I'll try that.

[Dave]

Here is a small piece of advice that if you are unaware, will drive you crazy when the amp is finished. Look closely at the schematic and layout at the bass pot on the bass channel. You will see that there is a discrepancy between the two. Build it to the schematic and not the layout and you will be much happier.


Dave

[sluckey]

Look closely at the schematic and layout at the bass pot on the bass channel. You will see that there is a discrepancy between the two. Build it to the schematic and not the layout and you will be much happier.

What are you talking about? They look the same to me.

[Dave]

6G6 variants are known for being bass-weak in the bass channel and that channel is almost unusable.
Forget what I said about the schematic. I was talking without looking. The point is moving the wires.

If you move the two wires from the board each one lug to the right, the bass channel will wake right up.

Dave

[sluckey]

On the schematic, all three lugs are used. On the layout, only two. Every 6G6 variant that I have ever messed with are all built like what you see on the layout and the bass channel is almost unusable with no low end.

No they are not. There are only two wires connected to the pot on the schematic, just like on the layout. The schematic shows the pot being connected as a two terminal variable resistor. That's what the arrow drawn through the resistor indicates. There are more than one way to draw a variable resistor. Design engineer's choice. Here are a couple ways to draw that circuit. They are both the same.

[Dave]

Yes, I got it. I realized my mistake. I wasn't looking at the schematic when I spoke out of turn. The point is. Move the wires. Makes a huge difference.


Dave

[sluckey]

If you move the two wires from the board each one lug to the right, the bass channel will wake right up.

I don't get it. It's a linear pot. The only thing that changes is now you have max bass on 1 and minimum bass on 10.

[72Blazer]

Thought I'd post a pic of how I wired that pot

[Dave]

72Blazer, Looks like you have it wired as I was suggesting.


Sluckey, No. Wired as its shows on the layout/schematic, the bass pot does almost nothing and the channel is practically unusable unless you like nothing but treble. If you wire the pot as I am suggesting, and as 72Blazer has done, you get a usable pot with bass coming in throughout the sweep of the pot up to a point where there is quite a bit of bass. The issue is commonly known with the 6G6 amps and many people grumble about it all over the internet.

I guess it could be that what you are getting is actually lower midrange by doing it this way, but the bass channel just sounds bad otherwise. Besides, as a "mod" its about the easiest and most reversible mod that you could try.


Why would you think it would work in reverse?

Dave

[72Blazer]

Can't Tell if Tom Petty or Mike Campbell is playing the 6G6 Blonde Amp-but I like it.

[Dave]

Yeah, now I have been studying the schematic and it looks like to me that it is increasing the lower midrange. With the pot wires as Fender did it, you have 820r bleeding mids off to ground, and at max 25.82k between bass and ground. With the change, bass is at max all the time and midrange is now anywhere from 820r to 25.82k to ground. I'm guessing that the circuit is heavy on lower midrange frequencies and that the change rolls less of them off. So, as I am reading it, it looks like my way maxes out the bass and makes the lower midrange more adjustable.


Love to hear what anyone else has to say about that.


Dave

[72Blazer]

I could even be Stephen Stills at the 12:28 mark of this video

[72Blazer]

This is my first attempt at this circuit.  The layout had me confused so as I stated in repl,y#6, I wired it as I saw in another build.  I should have this ready for an initial check out soon.  I'll probably have issues to correct but I will report back my observations/impressions.  I appreciate the discussions though.

Vr
J

[PRR]

...Love to hear what anyone else has to say about that. ...

Love to see a drawing, even crayon. "Two turrets over" is not clear when I see dozens of turrets.

[Dave]

Talking about moving the two wires on the bass pot each one lug over. On the pot itself. Just wire from the wiper over to the right one. Wire to the 820 ohm moves over to the wiper.


Dave


https://el34world.com/Forum/index.php?action=dlattach;topic=30457.0;attach=107120;image

[PRR]

Like this?

[Dave]

Yes sir.

[72Blazer]

Negative, We are referring to the Normal channel here.  Specifically-the trebel pot wiring.

[Dave]

No... I've been talking about the bass channel the whole time. Bass channel Bass pot.


Dave

[72Blazer]

 
I'm trying to get the dang normal channel right and you injected the bass channel into the discussion.  Anyway..thanks Dave-I do appreciate the comments.  I will explore the Bass channel option once I get this thing operational.

Vr

J

[Dave]

Well, now I'm super curious about the wiring of the bass channel/bass pot. I know for a fact that moving those two wires makes a huge difference because I have done it on several. Now, though, I am curious if I was mistaking lower mid range content for bass content.


Still hoping others chime in.



Dave

[sluckey]

Yes sir.

Well, now it's not wired like the schematic or the layout!

[Dave]

Yes Sluckey, everything I said about the schematic and the layout was a mistake. I moved the conversation, when I discovered that fact, to the simple moving of the two wires in order to move past my mistake about the schematic. The picture PRR posted is a good representation of what I was meaning to say all along. Sorry for the rolling wave of confusion.


Dave

[72Blazer]

As a direct result of all this conversation and in double checking my work-I caught some mistakes I made on the main board.  The mistakes were in the Bass channel circuit wiring.  This is the correction and It can be be compared to the chassis pic in reply#13.

[sluckey]

Yes Sluckey, everything I said about the schematic and the layout was a mistake. I moved the conversation, when I discovered that fact, to the simple moving of the two wires in order to move past my mistake about the schematic. The picture PRR posted is a good representation of what I was meaning to say all along. Sorry for the rolling wave of confusion.

I never doubted that you had made a change that improved the sound. My only point was the schematic and layout were not different. I had to look closely because I've seen several cases where the original Fender schematics did not match the layouts.

[72Blazer]

New Question:

For the power to this Bias Board:  Should I use the Red Pair 720v OR the Red/White 660v Stripe Pair?

Also I think this is related.  On the Primary of the PT, there is an option for 120v (Brown) OR 125v (Blue)

I plan on using 6L6GC Tubes

[72Blazer]

My plan is use Brown and white for 120v and then use the red wires to power the bias board but I welcome comments here.

In fact, I think I'll go ahead and do that and measure voltages throughout.

[Dave]

Red and Yellow goes to ground. That's the center tap for the high voltage winding which is the red wires. Blue/Red is the lead that feeds the bias all by itself (only need one). I don't see red/white wires on the diagram. It looks like to me, according to the drawing, the final high voltage would be determined by whether you use the "120" or "125" volt primary. As far as that goes, whichever one gets you closest to 428 volts when its all over. In other words, build it and check the voltages after you get it up and running then choose which primary gets you closest to target. It won't hurt anything if you choose the wrong one to start with.

By the way, the layout has the color code for the wires written right on it and it is the same as the transformer in your drawing. except for the choice of primary winding.

Dave

[72Blazer]

Dave,

Thanks. 

I posted the wrong dang transformer I am using for this build.  Here is the correct PT.  Now my question in reply #31 should make sense now.

Vr
J

[sluckey]

I would use the red/white wires. You can always change later if you feel the B+ voltage is not high enough.

[72Blazer]

Sluckey, I am also using the 120v tap vs the 125v tap.  Thoughts?

Vr
J

[sluckey]

Switch to the 125V tap if your filament voltage is too high.

[72Blazer]

I had an issue on inital power up while using the LBL and the power switch on and in standby I was able to mesure all the heaters.  I then toggled the stqandby switch and heard a pop and the LBL lit really bright.  The fuse didn't blow but there seems to be an issue.  I could not locate anything buned nor did I ever smell anything. 

I decided to try and test the OT and PT before proceeding any further. 

I de-soldered all OT and PT wires for this testing.  The transformers are still bolted on the chassis.  OT tests out fine but I have a question on testing the PT. 

Should I have continuity between the green filiment wires?  I am seeing/hearing continuity when I probe both green wires with my volt meter.  I decided to test the yellow wires (which are not used for this amp/no rectifier tube) and they have continuity too.  Since I have never had to test a PT before is what I am seeing normal?  Thanks!

[sluckey]

Should I have continuity between the green filiment wires?

yes

I decided to test the yellow wires and they have continuity too.

yes

Since I have never had to test a PT before is what I am seeing normal?

yes

[72Blazer]

Thanks Sluckey.  Now to find my errors...

[sluckey]

Pull all the tubes. Measure negative voltage on pin 5 of the output tube sockets. Measure the B+ on every filter cap.

[72Blazer]

Will do.  Side note:  I had never even installed the tubes when I did a first time power up.  I am in the process now of re-hooking up the B+ / filter cap wires to the board etc.  I wanted to test all the filter caps since there was a pop-but they all checked out good.  Using this wiring diagram:

[72Blazer]

Well I powered up the amp with no tubes.

I do have -50v on pin 5 of the PTs.  But I do the B plus voltages look ok?  I measured the voltages where the wires connect to the board and on pin 6 of v1

[sluckey]

B+ looks good.

[72Blazer]

Thanks.  Gonna put some tubes in and cross my fingers.  I'll let y'all know how it goes.  Hopefully there will be no early 4th of July fireworks...
 
Vr
J

[sluckey]

I suggest connecting the screen node cap negative lead to the power ground rather than the preamp ground.

[72Blazer]

Will do.  The amp appears to be good to go.  All knobs function and it's really quiet. No hums, no motor boating, and no electrical storms.  Still need to Bias this thing better.  Only got 16mA on pin 8s.  Plate current is 385v.

Sounds like a Bassman with a presence knob.  I'll have to do a lot more playing on this but so far I really can't tell the difference in tone yet between this and an AA864...None the less, I am amazed I got this baby working relatively smoothly/not many errors.  As always-thanks for those who took the time to comment/advise. 

Vr
J

[72Blazer]

Perhaps I should use the pair of Red PT wires vs the red/white stripe pair wires?

[sluckey]

You are biased at 6W! Way too cold. There's a 27K across the bias cap. Change that to 15K and check bias again. If too hot, try 18K or 22K.

If you would like to have an adjustable bias replace that 27K with a 10K in series with a 10K pot.