Blonde 6G6/6G6-A Bassman filter cap board

[Slider]

I'm working on a Blonde Bassman 6G6/6G6-A with a tube rectifier. I have built a few amps, but the filter cap board on this one is presenting a challenge. I have read threads of other Bassman builds, but they mostly seem to be 6G6-B versions. My basic question is this: Is the filter capacitor board for the 6G6-A, or 6G6, the same as for the 6G6-B? If that is the case I'm good to go. If not, can someone please describe the difference and perhaps include a sketch. I will be eternally grateful for help on this.
I have looked at the schematics for all three versions of the Blonde Bassman and they look the same to me, but I am not great with schematics and there is, of course, no layout of the filter cap board readily available.
I would just go ahead and build the 6G6-B and put in the tube rectifier, but I already purchased the 6G6 eyelet board from Doug and have it pretty much populated.
I see that the 6G6 and the 6G6-A are really similar with the obvious difference of the tube rectifier and some component values.
One more question: On the 6G6 pin 8 of V1 connects to a .0001 cap and then a 56K resistor. On the 6G6-A the 56K is replaced with two 27K resistors. Does the way they are connected make a difference?
This is my first post here, so I apologize for assuming these schematics/layouts are readily available to everyone so I haven't included any of those. I read some earlier threads where some of you helped novices like myself with 6G6-B builds so I know you're familiar with the amp.
Thanks in advance for your help with this. I'm looking forward to getting this solved.

[PRR]

Welcome.

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_6g6a.pdf
https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_6g6b.pdf

It is re-drawn, but the differences are little or none. Same filter cap line-up, even same color doghouse wires. If you are trying to fake an antique to fool a buyer, you want to study genuine examples; maybe Leo changed eyelet spacing from 0.4" to 0.375" multiples. For ordinary fixing or cloning I'd say the cap-stuff is same either way.

The tone-stack is related to the classic TMB except fixed M resistor and oddly a fixed T network instead of a pot. In "A" the network gives 50% treble. In "B" the network gives 18% treble. Maybe "A" was found to be too-bright, maybe "B" got a brighter speaker and had to be toned-down. This would certainly be a tweak in final tone adjustment.

Oh, and a bigger T cap. So the T boost starts lower frequency but does not rise as much. Again, this depends a lot on the *speaker* (and room and player and historical genre).

[sluckey]

My basic question is this: Is the filter capacitor board for the 6G6-A, or 6G6, the same as for the 6G6-B?

Since the power supply filter cap schematic is the same for all 3 versions, it's logical that the cap boards would all be identical too.

I already purchased the 6G6 eyelet board from Doug and have it pretty much populated...

One more question: On the 6G6 pin 8 of V1 connects to a .0001 cap and then a 56K resistor. On the 6G6-A the 56K is replaced with two 27K resistors. Does the way they are connected make a difference?

You have a 6G6 board so stick with the 6G6 layout. It's different from the 6G6-A layout

[Slider]

Thanks for your help. I will stick with the 6G6. I'm just building a clone to use in my studio. I think only one more question. I thought I was good to go until I noticed that one of the yellow wires coming off the last filter cap, shown next to the preamp ground from the filter cap board on the 6G6-A and -B, is not shown on the 6G6 layout. Is it connected under the board to the two 100K and 1M resistors connected to V2 pins 1 and 6?

[sluckey]

You have a 6G6 board so stick with the 6G6 layout. It's different from the 6G6-A layout.

QUIT LOOKING AT THE 6G6A AND 6G6B LAYOUTS!!!

The 6G6 layout and schematic are slightly different. Just enough to keep you confused and wondering. I've attached the 6G6 board. The yellow highlight shows how V1 gets B+ from the cap board. The blue highlight shows how V2 and V3 get B+ from the cap board. Notice the two points labled "A" on the board. Those two points are jumpered together on TOP of the board but you can put that jumper on the bottom if you prefer.

FWIW, I think the 6G6 amp made a bad mistake by taking the poorly filtered B+ for V1 from the screen grid node. The 6G6a and 6G6B versions corrected this. Nevertheless, I recommend you have a 6G6 board so stick with the 6G6 layout. Once you have a working 6G6 you can begin "improving" it if you feel it's necessary.

[Slider]

Thanks for your help sluckey. Mystery finally solved. And I have the honor of receiving a message in all caps with 3 exclamation signs. I get it. I was confused by the one extra yellow wire on the filter cap board; it just left me stumped and stalled the project so I started comparing layouts and schematics. Honestly I should have done all the comparisons BEFORE I decided to build the 6G6. Old prejudices and preconceptions got in the way yet again; a tube rectifier must be better/cooler than a solid state. Right? Looking back at it my mistake led me to learn a lot more than I would have otherwise. Once again, the help is greatly appreciated.

[Dave]

I agree with Sluckey in that you should stick with what you have until you get it up and running. That is to avoid confusion. But, after you get it up and running, in my most humble opinion, you should change that fixed treble arrangement PRR was talking about. The 6G6 version doesn't work very well. The 6G6-A and 6G6-B versions use fixed resistors to simulate a pot in a fixed position and both work well. 6G6 doesn't simulate a pot very well because the single resistor only has two ends.


The reason, to the best of my understanding, that they did it like this is because they were trying to reduce interaction between the tone controls.


Again, once you have it up and running, there are a couple of things I would suggest. 6G6-A being my all time favorite amp, I have some thoughts.


The bass channel, in spite of popular opinion, is a monster for guitar but not as-is. But one little mod will take care of that. Replace the lead that goes from the faux treble network to the volume pot with a small capacitor, I use .0033, this will serve as a high pass filter and squeeze off the super low frequency content that is fine for bass guitar, but makes guitar sounds flubby and nasty. There are a lot of things people do to the bass channel on these amps to make them more guitar friendly, but most people change the whole tone section. This capacitor, for me, is a one component fix and makes the bass channel sing like a demon bird.


On the schematic, you will see either a 47pf or a 100pf cap across the plates of the phase invertor. If you are using this amp for guitar, I would forget to include it. It is a snubber cap and will eat up the nice sparkly high frequency content.


The junction of the two 470k channel separation resistors is where the PI connects to the preamp on 6G6 and 6G6-a. On the 6G6-b there is a cap there. All (that I have ever heard of) the 6G6 and 6G6-a amps had oscillation problems that would cause the amp to flake out at high volume. The 6G6-B version added the capacitor to fix the problem. It works. Use it.


Dave

[sluckey]

And I have the honor of receiving a message in all caps with 3 exclamation signs. I get it.

No prizes. You're not the first. Figured I was too subtle first time I suggested this, since you came back with more mysteries about the 6G6A/B. So I pumped it up enough for you to see it. No harm intended, just wanted you to get my message. 

[Slider]

Thanks again - for all the input. Dave, I’m not seeing the lead that goes from the treble pot to volume on the bass channel. I do see one on the normal channel. What am I missing?

[Dave]

There's no pot. The resistor network that is substituting a treble pot. 56k on the 6G6.  27k and 27k on the 6g6-A. So, from the tone network to the volume pot, there is a wire. That wire becomes a capacitor.


Dave

[PRR]

> 6G6 doesn't simulate a pot very well because the single resistor only has two ends.

They are all equivalent to a 56k (54k, 57k) pot turned to 100%, 50%, or 17%.

[Dave]

Oh yeah, they are, huh? Hadn't looked at it like that before.


Dave

[Slider]

After the 6G6 is built and working, then considering mods, I have questions:

From Dave:
“you should change that fixed treble arrangement PRR was talking about.”

How would one do that since there is not another eyelet on the 6G6 board?

From sluckey:
“ FWIW, I think the 6G6 amp made a bad mistake by taking the poorly filtered B+ for V1 from the screen grid node.”

Would it not be feasible to change the connection of the last filter cap? Eliminate the jumper between A - A on the 6G6 and connect to the same point on the two 100K resistors to pins 1 and 6 on V2 like the 6G6A? What am I missing there?

[Dave]

How would one do that since there is not another eyelet on the 6G6 board?

One way I have done it is to epoxy a 50k pot to the chassis as close as you can get it to that spot. Then take the wires off the resistors and put them on the pot. Then you can adjust to taste and just leave the pot there. Its not hurting anything, and will let you get it right where you want it.


Don't forget to try the cap from there to the volume pot. Makes it a whole new amp. I make mine switchable in case I actually want to play bass through it.



Make sure you get rid of that snubber cap. Its a tone killer.



Not sure what Sluckey is getting at, but I'm sure he will clarify. A lot of people would increase the screen filter to 40uF. Maybe that's what he is talking about. 20uF of filtering and too much going on for that filter node.

Dave

[sluckey]

From Dave:
“you should change that fixed treble arrangement PRR was talking about.”

How would one do that since there is not another eyelet on the 6G6 board?

I would solder one end of each resistor into an eyelet but leave the two resistors standing up. Now twist the two free ends together and solder the wire from the VOL pot to the twisted ends.

From sluckey:
“ FWIW, I think the 6G6 amp made a bad mistake by taking the poorly filtered B+ for V1 from the screen grid node.”

Disregard what I said about the filter caps. Looking at the schematics more closely I see that the only difference is the way V2 gets it's B+. V1 gets it's B+ from the same point in all three schematics. Sorry for that confusion.

[Dave]

They are all equivalent to a 56k (54k, 57k) pot turned to 100%, 50%, or 17%.

This got me to thinking and since I had my main amp out tinkering with it anyway. I decided to have a look and see where I had mine set at. About 12% is the answer. I have mine attached to a pot inside the chassis (set it and forget it style) and I measured it today to see where it is because I adjusted mine for ?best? sound when I built it. about 12% is where I like it.


How did I get it there. I used a 50k that actually measured about 47k pot and put 4.7k resistors on either end to make approximately the right size pot. Mine was adjusted so that there was 6.2k on the side that goes to the Bass pot. About 12%.


Across the range of the pot, the sound changes dramatically, so I think playing around with it and adjusting to taste is worth while.



Dave

[Slider]

I finally finished this 6G6 Bassman build. This was a challenge for me. I stuck to the Fender 6G6 layout except for adding adjustable bias and increasing resistors to 1 watt in most places and higher wattages in a couple spots.
I used a Mercury Magnetics 8ohm transformer. Plugged into an 8ohm Jensen speaker it sounds great. The Normal channel sounds especially good.
There is one modification I would like to make. The Bass channel has a lot of treble. If I turn the treble knob all the way down it is about right, but I would like more control. Can someone please point me toward a mod to reduce the amount of treble at the Bass channel treble pot?