Understanding Revision Differences - Bassman 6g6

[Yeatzee]

Hello! I'm working on a '61 6G6 right now, and as I tear apart and redo the poor work done to the amp previously I'm realizing that at one point someone was trying to convert it to 6G6a specs. I pulled up the two layouts and I see the differences range from minor to what I'd assume to be quite large, based on the value changes, but I don't know enough to full comprehend what's happening and why fender made those changes. Here are the two schematics:

6G6: https://el34world.com/charts/Schematics/files/fender/Fender_bassman_6g6_schem.pdf
6G6a: https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_6g6a.pdf

I've attached a quick drawing of what I notice to be different on the layout (ignoring the tube rectified vs SS rectified difference). Green = same, yellow/red = different. I would love if someone can explain what each component and wiring change is doing.

The biggest obvious change to me is the bypass cap move on V3. It goes from a 0.1uF in the 6g6 to a 25uF in the 6g6a, while all else in that area appears to be equal. If I'm not mistaken, the frequencies they boost will be very different with the 0.1uF letting through a lot less lows/low mids... but does that actually translate directly to the tone coming out of the amp in a big way? Was that done because it's a "bassman" so they didn't want to cut (not boost) those low frequencies? So would 6g6 be better for guitar?

Other changes that confuse me include the 1meg spanning between the plate resistors of V2 on 6g6, but it's removed on the 6g6a revision. And then on the 6g6 a wire connects to the eyelet where the two plate resistors meet and goes over to where the 220k and 100k plate resistors of V3 meet. On 6g6a that wire connection is not made, but instead a wire is run from the two 100k plates of V2 direct to the doghouse caps. Why? What does this change do? Is there a strong reason to do one way over the other? Presumably Leo wouldn't do a revision without making an improvement, just trying to understand.

Help as always greatly appreciated, thank you for your patience as I try to learn these things :)

[sluckey]

Fender was changing over to using all 25µF bypass caps about that time. The 25µF will boost lower frequencies by bypassing the lower frequencies around the cathode resistor, thus reducing the natural degenerative feedback for the lower frequencies. Notice this was done to the Normal channel. Many people prefer smaller cathode bypass caps for guitar.

The 1meg spanning between the plate resistors of V2 on 6g6 is actually a local negative feedback loop from the plate of the second triode back to the grid of the second triode. This LNFB reduces the gain of the second triode and improves the signal. I guess Leo decided it wasn't really necessary and dropped it from later versions. This shaved a few pennies of the production cost as well.

As for the yellow wire to the doghouse in the 6G6A, it's easier to see what changed if you look at the schematic (not the layout). In the 6G6A both tubes in the Bass preamp get their B+ from the same filter cap and the tube on the Normal preamp gets its B+ from a different filter cap. See attached.

[dwinstonwood]

In the 6G6A both tubes in the Bass preamp get their B+ from the same filter cap and the tube on the Normal preamp gets its B+ from a different filter cap.

I'm currently finishing up schematic and layout drawings for the 6G6-B.

Here in my schematic you can see what sluckey is talking about. The Bass preamp is fed from node "D" while the Normal preamp is fed from node "B" which goes through an additional 56K dropping resistor connected to another 20uF cap.

The big change I've made is to have one input with a channel switch (stolen from sluckey's Dual Lite).

[Dave]

The first two versions of these amps have a problem that pops up frequently. Turn the volume up pretty loud and hit a chord with a lot of bass in it. Say an E chord or something like that and the amp craps out and then slowly fades back in.


If you look at the schematic for the B version, there is a .0005 cap at the input of the PI. It is not present in the previous two versions.


It needs to be added if the amp exhibits this behavior. Personally, I have never seen one that didn't, but I'm trying to be open-minded.


Dave

[sluckey]

If you look at the schematic for the B version, there is a .0005 cap at the input of the PI. It is not present in the previous two versions.

That PI input cap was not needed in the previous versions because each channel had a plate cap prior to the 470K mixing resistors. Therefore there was no B+ plate voltage that could get to the PI grid. However, the 6G6B version removed the plate coupling caps for each channel and mixed the two channels with B+ plate voltage on the 470K mixing resistors. That PI input cap became very important to keep previous stages plate voltage away from the PI grid.

[Dave]

That PI input cap was not needed in the previous versions because each channel had a plate cap prior to the 470K mixing resistors. Therefore there was no B+ plate voltage that could get to the PI grid. However, the 6G6B version removed the plate coupling caps for each channel and mixed the two channels with B+ plate voltage on the 470K mixing resistors. That PI input cap became very important to keep previous stages plate voltage away from the PI grid.

Well, that very well be true, but... I haven't messed with a single one of those amps that didn't have the problem I described and that wasn't fixed with the addition of the cap. Sorry if the science doesn't add up on paper, but in practice, and as far as I'm concerned, that is the only solution I have found.


Dave


Edit: To be more clear. I have worked on many. Early on, I thought that leaky coupling caps (plate caps) must be to blame. Many times, I replaced them only to find that the problem persisted. The only solution I have ever found that works is the addition of the small cap before the PI.

[sluckey]

I didn't dispute what you said. I simply explained why that input cap to the PI is absolutely necessary.

[Dave]

I wasn't offended. I just want to make sure people know that there is a solution. Because the amps, as awesome as they are, can be frustrating.


Dave

[Dave]

On another note, there are 3 "Official" versions of those amps, but results can vary from one to the next. I feel like there were at least 5 or 6 revisions along the way, but only 3 were made official. I, personally, have an "A" version with a tube rectifier and I can't find any evidence of modification.


Dave

[Esquirefreak]

There was also the C revision. It had the bass pot on the BASS channel wired differently.

/Max

[Dave]

There was also the C revision. It had the bass pot on the BASS channel wired differently.

/Max

Interesting note.


I have always thought that the Bass control on the Bass channel was odd on the other three versions, and frankly, it makes the bass channel sound weird. There is a simple one-wire mod that some people use to loosen things up, but I didn't know that Fender addressed the issue on a later model.


Pretty cool.


Dave

[PRR]

....Turn the volume up pretty loud and hit a chord with a lot of bass in it. ....the amp craps out and then slowly fades back in. .... If you look at the schematic for the B version, there is a .0005 cap at the input of the PI.

I think you play harder than Leo (or his testers) did.

Yes, the LTP input can be overdriven if you try. Because its high (higher than you might think) input impedance, 0.1 or 0.05uFd coupling (even on the far side of mix resistors) will grid-block for a long time. Maybe a second? Which would be a distinct hole in the music. 0.005uFd un-blocks quicker, maybe 50mS. That should be audible, but my analysis is over-simple.

[Yeatzee]

On another note, there are 3 "Official" versions of those amps, but results can vary from one to the next. I feel like there were at least 5 or 6 revisions along the way, but only 3 were made official. I, personally, have an "A" version with a tube rectifier and I can't find any evidence of modification.


Dave

Thanks everyone for chiming in! Dave mine appears to be very similar, a mixture of 6g6 and 6g6A. I did a video working on the amp and talk about it a bit.

The bass channel input section has the extra eyelet for the two 27k's like the 6g6a, but otherwise appears to be 6g6. There's been some tomfoolery with the amp so it's not 100% original allowing me to say for certain. I decided to revert it to 6g6 specs while obviously leaving the bass channel input 6g6a since it has the hardware required to do that. First fire up, and it's got issues.

Bass channel bass pot seems to do basically nothing. Going to try and dive back in to it tonight, but would love to hear if anyone has any ideas or insights! Normal channel has a 500k volume pot replacement I didn't catch with no tooth washer and stripped threads so it's a mess making testing that channel dubious until I can replace the pot.

[Yeatzee]

Fender was changing over to using all 25µF bypass caps about that time. The 25µF will boost lower frequencies by bypassing the lower frequencies around the cathode resistor, thus reducing the natural degenerative feedback for the lower frequencies. Notice this was done to the Normal channel. Many people prefer smaller cathode bypass caps for guitar.

The 1meg spanning between the plate resistors of V2 on 6g6 is actually a local negative feedback loop from the plate of the second triode back to the grid of the second triode. This LNFB reduces the gain of the second triode and improves the signal. I guess Leo decided it wasn't really necessary and dropped it from later versions. This shaved a few pennies of the production cost as well.

As for the yellow wire to the doghouse in the 6G6A, it's easier to see what changed if you look at the schematic (not the layout). In the 6G6A both tubes in the Bass preamp get their B+ from the same filter cap and the tube on the Normal preamp gets its B+ from a different filter cap. See attached.

As always, appreciate you insight! By the way my phone got a notification saying you commented on YT, but I can't see/find the comment anywhere. Not on the notification feed and not on the video itself. Just an FYI, if you didn't delete it YT is still messing with your stuff for some reason!

[sluckey]

I had commented about the "mystery" eyelet on the righthand side of the board, telling you to look at the 6G6-A layout to solve the mystery. As I continued watching the video I saw that you did just that so I removed the comment which was no longer useful.

[Dave]

Phase inverter grid "2" has a wire going to 1m. From there in continues under the board to the junction of two 470K. Replacing that under-the-board lead with a .0005 fixes the phase inverter issue I talked about earlier on.


The .0001 across the 100k and 82k of the PI is a snubber. You may find that the amp sounds better for guitar with it removed.


There are two identical caps in the tone circuit of the bass channel for the "mid" cap and the "bass" cap. One goes to the wiper of the bass pot and the other goes to the lug that is grounded through an 820r. This is a little weird considering that the two tone caps are the same value. Instead of dumping mid frequencies to ground, it seems to dump mids and lows both to ground and leaves the bass control not doing much. A lot of people think that the bass channel is too thin for guitar (ironically).


You could change the value of the mid cap, but I like to leave my amp as original as possible. Moving the grounded (820r) lead to the wiper with the other one together works real well to open that channel up and make the bass pot work a lot better.


Dave

[Yeatzee]

Phase inverter grid "2" has a wire going to 1m. From there in continues under the board to the junction of two 470K. Replacing that under-the-board lead with a .0005 fixes the phase inverter issue I talked about earlier on.


The .0001 across the 100k and 82k of the PI is a snubber. You may find that the amp sounds better for guitar with it removed.


There are two identical caps in the tone circuit of the bass channel for the "mid" cap and the "bass" cap. One goes to the wiper of the bass pot and the other goes to the lug that is grounded through an 820r. This is a little weird considering that the two tone caps are the same value. Instead of dumping mid frequencies to ground, it seems to dump mids and lows both to ground and leaves the bass control not doing much. A lot of people think that the bass channel is too thin for guitar (ironically).


You could change the value of the mid cap, but I like to leave my amp as original as possible. Moving the grounded (820r) lead to the wiper with the other one together works real well to open that channel up and make the bass pot work a lot better.


Dave

Wow, thank you Dave! Great things for me to try there. So perhaps the bass channel is working as (originally) intended, definitely thin and the bass knob does basically nothing. Seems easy enough to test moving the 820r resistor to the wiper!

[Dave]

Don't move the resistor. Move the wire. There will be two wires on the wiper. and no wires on the other lugs.


The resistor stays where it is.


Dave

[Yeatzee]

Don't move the resistor. Move the wire. There will be two wires on the wiper. and no wires on the other lugs.


The resistor stays where it is.


Dave

Got it, sorry misunderstood!

[Dave]

If you try any of those adjustments, please report back with your observations.


Dave

[Yeatzee]

If you try any of those adjustments, please report back with your observations.


Dave

Amp turns out to have an issue with the OT so things are on hold until the owner decides what to do. Was putting out about 3w, but voltages all read fine at first glance. Ended up tacking in another OT and bam came to life. 42watts iirc, different world.

[Dave]

Sorry to hear that. I would cry if I lost any of the transformers in mine.


Dave

[Yeatzee]

Sorry to hear that. I would cry if I lost any of the transformers in mine.


Dave

The PT is already a MM replacement too. Poor guy got hosed by the shop.

[Dave]

Yikes. It'll still make a fine amp when you get it going, but won't have much value.


Dave