Using Ground switch on SF Bassman 50 for NFB switch

[Locrian99]

Hello,

First time poster here.   And still getting my feet wet when it comes to a lot of this stuff..

Recently I acquired a Fender Bassman 50 head (72 silverface).   I've gone through and recapped it, replaced a few components that had drifted far out of spec.   Replaced power cord etc.   Working great, now I'd like to do a little tweaking to it.   Most of which I feel pretty comfortable with but I had a questions regarding using th existing ground switch which isnt currently doing anything as a switch for removing the NFB, having the 47k stock resistor value, and putting an 820R in as is seen in the BF circuits etc.

The ground switch however has the fuse, standby switch and power switches between it and the speaker outputs.   I have an open eyelet on the board in a perfect location for a second resistor and wire from the switch to go and meet up with the 47k.  Anyways my concern is that I'm just going to be asking for noise running a wire from the output over all that power and then also the proximity to the power tubes.    Would rg174 be the solution for this?   where would be the best place to attach the shield to ground (or does it really matter)?   I'd have to do a run of one wire from the speaker output jacks to the switch,   and then two wires from the switch back to the middleish of the eyelet board.   With the coax cable do I need to worry about routing?   


Thank you for your time   

[sluckey]

The Bassman 50 NFB circuit is quite different from an AB763 (blackface) circuit, so don't switch a 820Ω resistor with a 47K resistor. I suggest switching a 100K instead. Shielded cable is not necessary.

Of course, you can always convert that power amp circuit to an AB763 circuit, but it's more involved and may not be worth the effort.

[Locrian99]

Oh wow you are very right that is quite a bit different now that I look at it again.   

My thought was I wanted more clean head room, and increasing the resistor would get me there?   With the difference do I need to be concerned with having the loop completely listed in the "off" position of the switch?   

This is the schematic of my amp. 

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman50.pdf

[sluckey]

I suspect that amp already has all the clean headroom you can squeeze out of it.

[PRR]

.

[Locrian99]

I suspect that amp already has all the clean headroom you can squeeze out of it.

It actually starts breaking up around 5 on the normal channel and most of what I have read states that is normal. 

Edit:  Guess I'll just install the switch and experiment a bit with values.   Now that I know I don't need to be worried about shielding between the Jack switch and eyelet. 

Thank you both :)

[Locrian99]

So I kind of held off on doing any mods  yet.   But I really would like to find an option to have a bit more headroom with this amp but keep the normal channel as is.   

Obviously I could “blackface” the power amp section.    I’m somewhat curious if in the bass channel I bypassed the gain stage between the tone stack and the power amp.   I’m assuming I’d lose a little volume but would also get a bit cleaner. 

[TitaniumValhalla]

At 5 on the volume knob with a Bassman your eardrums should be approaching blowout. My Pro Reverb is literally shaking stuff off my walls at 5. Are you using an attenuator? What kind of speakers are you working with? There should not be any shortage of clean headroom if all is working as designed on a 40-50 watt Silverface.

[Locrian99]

This is where I’m a bit confused.   I’ve heard that.   And then I questioned if there was an issue.   

Digging around I think this particular model the bassman 50 is a bit different than just about all others.


 
https://el34world.com/charts/Schematics/files/Fender/Fender_bassman50.pdf

Vs

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_100.pdf

Or the

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_10.pdf

This one is the closest to it, but still is a bit different with the negative feedback loop.   I don't know enough to know what difference this would make. 

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_ac568.pdf


This video makes me think it’s working as intended.

i=bCXMYB9-QhG1muof

[Locrian99]

There is an extra gain stage in the preamp using half of a 7025 and the negative feedback loop is very different …

Not to be argumentative, I'm still learning here, just been very frustrating trying to figure this out a bit.  As I've gotten mixed info on what the sound of this amp should be.   But after seeing that this is the only model bassman I have seen with the negative feedback loop as it is.  The 10/70 etc. Don't have that extra gain stage between the tone stack and the PI.  Unfortunately I haven't played a bassman 50 before and I don't know of anyone around me with one to compare it to.   So I have to kind of go off videos and what people say it should sound like.    After listening to the video above, I am starting to think this particular model is a bit different than the rest of them, and it is functioning correct.   I could be wrong.   At 5 it is louder than I would typically be practicing with for sure, but it is far from ear bleeding loud. 

I am using at the moment an 8 ohm 1x12 cab.   It has an alnico eminence speaker one of those fender special designed ones that came out of a handwired fender twin, I believe its a 70 watt speaker.   I have a 71 bandmaster 2x12 cab but the speakers in it are the original oxfords and I think someone tried to recone them or something they sound awful.   Will be replacing them in the near future.

Little background on the amp itself I picked it up off facebook marketplace broken.   Person I bought it from said he was playing it one day heard a loud pop, a god awful smell, and he put it in a closet 25 years ago and hadn't touched it since.   Fuse was completely roached.   Had a blown filter cap.   Bulging, read 0 capacitance  etc.    Replaced all filtercaps and resistors in the doghouse.   Few other resistors that were way out of spec I replaced.   Everything else measured well.   Tubes it has all the original rca pre amp tubes (well at least I believe them to be original), and an RCA power tube, as well as a triad power tube for the other.   

I like the sound of it as it sits, but was hoping to get a more traditional fender clean out of it as well.   Which I think I'm starting to realize is not going to be an easy task to get both worlds here. 

[TitaniumValhalla]

It appears the Bassman 50 expects a 4ohm load. If you're running it into an 8ohm speaker load, the mismatch is going to affect output power as well as the sound of the amp. I would start by fixing that, as well as maybe check the values of internal components for any other potential mods and put it back to stock to see how it behaves before you mod.

[Locrian99]

Fair enough on the speaker.   Just haven’t decided which speaker I am going to out into the cab yet.   Didn’t think the difference would be all that drastic but worth checking first.   

As far as internal go I have measured every component in it.   Anything way out of spec I have replaced.    There’s a few resistors pushing the envelope of the 10% tolerance but everything is within tolerance that is still in it.   I’ve replaced a few that were out of spec.   

[pdf64]

… this is the only model bassman I have seen with the negative feedback loop as it is.  The 10/70 etc… 

What’s the 10/70?

more clean head room

Guitarists use the term headroom to cover both an amps gain and / or its clean power output.

So what do you want it to do differently, without using the ‘headroom’ term?

If the amp’s only putting out eg 25W, reducing / removing negative feedback will just result in the 25W overdrive point being reached on 3 rather than 5. ie it changes the gain (and to some extent tone / response), not the power output.

[AlNewman]

Kinda looks like an ab763 minus the reverb.
So, normally the 2nd half of the 7025 would be a recovery/mixing stage, in this case it looks to my simple brain like it's mostly gain.  They changed up the NFB and dumped it directly into the plates...
You could try just lifting the nfb and see what happens.  Crank it, try all the settings.  I bet they put it there for a reason.

I would imagine there's a way to do a fairly simple change up to the 3rd gain stage and the NFB to make a workable amp more in line with what you're after. 

[Locrian99]

Thx.   I am going to follow the previous advice and try running through 4 ohm cabinet.   Especially since this was in my plans anyways.    I’ve got a couple speakers in the way and I’ll get them in the. A master cab and I guess resist this if I still want a change after.   

I am somewhat wondering if changed the bass channel tone stack up to something closer to the normal and just bypassed that gain stage.   But then I think I need the nfb to come in like it does in the ac568.    Anyways I’ll
Get there if/when I need to cross that bridge.   First just going to try running it through the appropriate speaker impedance that makes the most sense before making any changes.   

Thx

[mresistor]

Curious  is there an AX7 or an AT7 in the PI slot?   Also curious as to why CBS engineers would move the feedback injection point on the Bassman 50 to the input side of the PI coupling cap while the AC568 injection point is on the output side of the PI coupling cap?

[Locrian99]

Curious  is there an AX7 or an AT7 in the PI slot?   

It has an AT7 there.   It actually still has what I think are all of the original pre amp tubes...

Looking at the 568 schematic I could easily move the feeback loop so it comes in at the same place which is something I may do just to see if it makes any difference.    What I wasn't sure if I do decide to go that route (which won't happen until I get the new speakers and see what my thoughts are then.)  The local feedback  of that 1/2 a 7025 and the grid leak resistor on the 568 are each 470k ,where as they are 220k here. .    Not sure what change that would make as well.   

[Locrian99]

… this is the only model bassman I have seen with the negative feedback loop as it is.  The 10/70 etc… 

What’s the 10/70?

more clean head room

Guitarists use the term headroom to cover both an amps gain and / or its clean power output.

So what do you want it to do differently, without using the ‘headroom’ term?

If the amp’s only putting out eg 25W, reducing / removing negative feedback will just result in the 25W overdrive point being reached on 3 rather than 5. ie it changes the gain (and to some extent tone / response), not the power output.

The 10 and 70 are other silverface models of the bassman.   

I probably am throwing the term headroom around perhaps incorrectly.    What I am after am having a bit more volume before pushing the amp into "break up".   

At this point I am curious what difference I will see in that regards running it into the 2x12 cab at 4 ohms vs. the 1x12 at 8 ohms.   So that may actually take care of that. 

The other part of the feedback switch I was thinking would do at least how I understood it is the feedback loop helps keep it from going into to break up, so I also thought it might be nice for when I wanted more of the "break up" to lift the feedback loop.   As I said I'm pretty green here just trying to figure out how these all work.   Read through several of the articles on rob robinettes page to try and familiarize myself with it.    I can read a schematic, follow it, verify stuff is following it, but don't always understand exactly what the parts are making happen. 

[Locrian99]

At 5 on the volume knob with a Bassman your eardrums should be approaching blowout. My Pro Reverb is literally shaking stuff off my walls at 5. Are you using an attenuator? What kind of speakers are you working with? There should not be any shortage of clean headroom if all is working as designed on a 40-50 watt Silverface.

Well I take it back.   

I think there is a bad power tube.   Removing the triad and leaving the rca in amp works exactly as it has.   Removing the rca and leaving the triad in.   No sound. 

[WimWalther]

I think there is a bad power tube.   Removing the triad and leaving the rca in amp works exactly as it has.   Removing the rca and leaving the triad in.   No sound.

Haven't followed the thread - but can I assume you've tried both RCA & Triad in alternate sockets, and the issue follows the Triad tube and not a socket?

In that case, yes, it's safe to proceed as if you have a bad power tube.

[Locrian99]

Yes in either position the rca works, in either the triad does not.   

I guess I should’ve looked at the easier options first for getting more the sound I was expecting.   

So does this appear to be red plating? 

Hard to tell through the getter for my untrained eye from the images I tried to compare it to online.  Since I was getting sound and it was lighting up I guess naively thought that was good.

Sounds like if I do want to have lower volume break up it’s ok to just run it off one power tube as well for playing around the house ?

[pdf64]

It looks to be ‘red heatering’ to me?

[mresistor]

That is not red plating. The plates will glow orangish red.   

[Locrian99]

Ok thank you all for answering my complete noob questions.   I wondered be side it def is glowing brighter than the other power tube (different brand mind you) and not working apparently.   Didn’t think so from the online pics I saw but me being wrong on this wouldn’t be new.  :)

[Locrian99]

Just a minor update.   

Some yea my problems weren’t to be fixed by mods it was I just needed to get the amp correct.   

The one power tube was bad.   Lots of very loud clean head room especially now that it is running at 4 ohms through a 2x12.   No reason I could see myself turning it up past 3 at this point.   Thanks all.