Fender 5F6A bassman

[Grees01]

Hello everyone. New to the forum. Introduced myself in the into area. Would like to build a hand wired amp. Been reading up on Rob Robinettes website and following Uncle Dougs videos. Considering what amp to build. I'm looking for lovely 3d clean and a little grit when I dig in with the pick. Some harmonic overtone depending on my playing dynamics. I've been considering the bassman circuit with a few mods that Rob lays out. I need a multi output OT so I can use it with my existing cabinet. I want adjustable bias so I can run different power tubes. The modification to the power tube sockets so I can run el34 power tubes if desired. An adjustable feedback loop circuit. Backup diodes on the rectifier tube, and possibly diode rectification thats swichable. Pentode / triode switching on the power tubes for a 1/2 power mode. Possibly voicing a lead channel. I would like some feedback from you guys on what you think about this choice for a guitar amp. Suggestions and recommendations welcome.
G. Rees

[mresistor]

I built a 5F6A using the hoffman board that I also built myself and I have always run EL34s in it and its a great amp. Mine is a little different in that it has a very large choke input which I think gives it a little character, and smoothness. I haven't really turned it up to as loud as it can go but on the attenuator it does growl a bit. I think it's perfect for pedals.

[PharmRock]

Hello Grees,
Welcome to the forum!  I'm sure you will find (like I did) that the members and moderators of this forum will give you great advice on building and troubleshooting an amp.  You've come to the right place.

I am literally in the middle of a JTM45 build, which is the UK sibling of the Fender Bassman.  While opinions vary, either one of these amps is considered by many the Holy Grail of classic guitar tone. 

Prior to starting my JTM45 build, I was thinking of incorporating many of the same things you listed....switchable rectifiers, switching for different NFB resistors, switchable 6V6/KT66, and also things like switchable cathode resistors on V1, different "eras" for tone stacks, etc.  I settled on just building pretty much a bone stock JTM45.  Why?...two reasons...1) the amp is classic for a reason.  Why implement a build with all of these bells and whistles I might not even want or like, and 2) adding these other features exponentially complicates things, especially for a novice builder trying to get through the troubleshooting phase.

Once you build the amp and get it up and running, you can start implementing these other features.  If you are creating your own turret board (like I did for my JTM build) you can think ahead and have spaces on your board layout that will be ready to go when you get to the point of trying them out.  For example, I have a spot on my board for a split cathode on V1, but will start off with the shared cathode. 

I think your best bet for easily incorporated mods would be the switchable NFB, possible switch for shared/split V1 cathode, and maybe a switchable bypass cap on V2a ("lead voicing" maybe?).  You could install temporary switches on the side of the chassis (vs front or rear) to avoid any permanent changes to the panels, and once you decide on the ones you like, you can incorporate those on the front or real panel.

Hope this helps, and best of luck with the process!  Once you get started on this, it becomes an addiction.

[PharmRock]

I should add that a non-stock feature you mention that would be a "must do" addition would be a bias trim pot, as you pointed out for handling various tube types.

You can find Doug's board with this incorporated here: https://el34world.com/schematics.htm#Hoffman_5F6A_Bassman_

[uki]

Welcome to the forum!

Here a very handy link:  ALL-TIME great information

[Grees01]

I built a 5F6A using the hoffman board that I also built myself and I have always run EL34s in it and its a great amp. Mine is a little different in that it has a very large choke input which I think gives it a little character, and smoothness. I haven't really turned it up to as loud as it can go but on the attenuator it does growl a bit. I think it's perfect for pedals.

Thank you for the feedback. Did you build this as a head unit? Or did you build it as a combo?. Any changes to the original circuit? Other than the different choke? Can you post a sound clip? Or video of you playing thru it? How is the clean sound? vs the driven sound? Does it sound good at lower volume levels, or do you have to crank it pretty loud for it to sound good. Overall are you happy with it? In love with it? Or just okay with it?
Glen

[Grees01]

I am literally in the middle of a JTM45 build, which is the UK sibling of the Fender Bassman.  While opinions vary, either one of these amps is considered by many the Holy Grail of classic guitar tone. 

Prior to starting my JTM45 build, I was thinking of incorporating many of the same things you listed....switchable rectifiers, switching for different NFB resistors, switchable 6V6/KT66, and also things like switchable cathode resistors on V1, different "eras" for tone stacks, etc.  I settled on just building pretty much a bone stock JTM45.  Why?...two reasons...1) the amp is classic for a reason.  Why implement a build with all of these bells and whistles I might not even want or like, and 2) adding these other features exponentially complicates things, especially for a novice builder trying to get through the troubleshooting phase.

Once you build the amp and get it up and running, you can start implementing these other features.  If you are creating your own turret board (like I did for my JTM build) you can think ahead and have spaces on your board layout that will be ready to go when you get to the point of trying them out.  For example, I have a spot on my board for a split cathode on V1, but will start off with the shared cathode. 

I think your best bet for easily incorporated mods would be the switchable NFB, possible switch for shared/split V1 cathode, and maybe a switchable bypass cap on V2a ("lead voicing" maybe?).  You could install temporary switches on the side of the chassis (vs front or rear) to avoid any permanent changes to the panels, and once you decide on the ones you like, you can incorporate those on the front or real panel.

Hope this helps, and best of luck with the process!  Once you get started on this, it becomes an addiction.

Hello PharmRock. Thanks for the feedback. Very good helpful information. I am considering the JTM45 as well, for a couple of reasons, 1) would be that the chassis design is for a head, which is what I want. 2) The JTM45 was designed as a "guitar amplifier" even though the circuits are very, very similar. Your suggestion to build it very close to stock makes good sense, but if I buy a JTM45 chassis then all the holes are already drilled and if I want to add something I have to work with what space remains. I was considering buying a blank chassis and then drilling and cutting as needed to fit the things that I want in there. But probably its better to go with a chassis that is already made for the job. Seems to plenty of space on the rear to add a few things in the future. I think one of the biggest things that's holding me back at this point is the idea of spending the money to buy the parts to build one, (I don't care about my own labor time) getting finished and for what ever reason just not liking the sound or feel of the end product. I guess knowing that these two amps are considered by many to be the holy grail of guitar amps should provide some solace to me, but there are so many choices in components ie: which brand caps, which type of resistors, what brand transformers, ect ect, that all interact with each other and hopefully come together to produce sound that you love. Thanks again for the feedback. Good information. 

[Grees01]

Welcome to the forum!

Here a very handy link:  ALL-TIME great information

Thanks uki I will check it out!

[PharmRock]

I think one of the biggest things that's holding me back at this point is the idea of spending the money to buy the parts to build one, (I don't care about my own labor time) getting finished and for what ever reason just not liking the sound or feel of the end product.

Reminds me of something I read by Doug here on his site:
"First of all let me start by saying, it is usually more expensive to build an amp from scratch than to just go buy an amplifier. Imagine buying all the parts for an automobile one by one and then putting it all together to make a car. Do you know how to do that? If you don't know how to assemble an automobile from scratch it would be a good idea to attend an automobile mechanics class and start your learning process."

https://el34world.com/projects/scratchbuild.htm

[mresistor]

Thank you for the feedback. Did you build this as a head unit? Or did you build it as a combo?. Any changes to the original circuit? Other than the different choke? Can you post a sound clip? Or video of you playing thru it? How is the clean sound? vs the driven sound? Does it sound good at lower volume levels, or do you have to crank it pretty loud for it to sound good. Overall are you happy with it? In love with it? Or just okay with it?
Glen

It's Hoffmans 5F6A on a turret board, and it's a head so it was a little fun to make as it's inverted. I converted a Baldwin organ power supply and thats why it has the big iron.  Hoffmans board only has two input jacks - normal and bright.  Since I play mostly clean I tend to like the bright channel as the normal channel is somewhat dark.  I don't know the style of music you play but this bassman sounds good plugged straight in at low volumes which is normal for playing at the casa. I haven't played it loud much at all.  I play it through 1x12 or 2x12 and the speakers are all ferrite   Celestion 75  Weber 12F150 and an Emminence Tejas Heat.  I don't have any vids sorry. Hoffmans board has a bias adjust trim pot already on it.  I like the amp but honestly I play my Super Reverb head much more often and it's a Hoffman AB763. I haven't played a JTM45 to be able to compare it to the 5F6A. I am quite happy with the bassman..  The normal chan has sort of a tubular tint to it, maybe that's the EL34 sound and I should probably try some 5881s in it.  The bassman and probably the JTM45 are not as clean as the Super thats for sure. You will prolly get better advice on the 5F6A from a perfoming musician than from me. 



The reason the turret board in my bassman is inverted is because I built it to be a head in the Marshall style with the tubes standing up no hanging down like a Fender.  You can build a Fender circuit and install it in a head cab with no speakers Fender style which is probably what you were thinking of. I don't suggest your first amp build be like mine.

[mresistor]

I installed a triode pentode switch in my Super and I will probably never use it.  The difference is not that great and the triode mode sounds a little thin comparably. and not toneful to my ear.  I seriously doubt if would be much different in the bassman.  Something more useful would be a LarMar PPIMV and/or some switching from fixed bias to cathode bias. Also a switchable cathode circuit on the first amplification stage. The safety diodes can't hurt. Hoffmans board also has the 1 ohm bias sense resistors which are very handy.

[PharmRock]

Something more useful would be a LarMar PPIMV

+1 on the LarMar PPIMV.  Still new at this, but I've installed this on 2 amps so far and very pleased.  Inexpensive, easy, and reversible.  If done correctly it introduces no noise/hum. Use one of the speaker jack holes in the back panel to mount the pot.

[valvetones]

MResister—- You did Choke input filter?
Tell me more about this please.
I did it once , the amp goes thud when you turn it on and off.
Did you find a work around for this?
I’ve read a lot about inrush current and choke input filter, but never really found enough info to deal with it.
What’s the secret, can you tell me or point me in a good direction?
Thanks
VT

[valvetones]

GReed01-
I’ve built the 5F6A three times now, and I find it to not really be my flavor.
In my opinion, it has way too much low end. I think it get fame and acclaim because it was the basis for so many great amps that came after it. I get that you wanna do some things to make it different, so did I, three times.
I personally would suggest going a different route. Build plexi or JCM800 if you wanna rock. Build a super or a vibroverb if you want awesome cleans.
Maybe the best place to start is, why do you want to build a bassman? Does it really suite your style of playing, or are you doing it because it’s talked about alot?

[Grees01]

Reminds me of something I read by Doug here on his site:
"First of all let me start by saying, it is usually more expensive to build an amp from scratch than to just go buy an amplifier. Imagine buying all the parts for an automobile one by one and then putting it all together to make a car. Do you know how to do that? If you don't know how to assemble an automobile from scratch it would be a good idea to attend an automobile mechanics class and start your learning process."

Hmmm. Well. I'd really have to compare the cost's to believe that. But that's not the reason why I want to build a tube amp. I'm a builder by nature. I've been building things most of my life. Computers, RC aircraft, Helicopters, guitars, tooling ect. I've been DIY for a lot of things in my life. I want to be able to put together a hand wired tube amp and have it sound amazing! And I'm determined to do so. I'm just trying to be careful and not foolish and just jump on the first idea that crosses my mind. As far as I know there aren't many amp manufacturers around that still hand wire amps. Fender does a few, Marshall also does a few, Soldano at least did when Mike was running the business, I'm not sure now a days. Dave Friedman does. A BE 50 deluxe will set you back 3599.00 plus tax. Do you think it would cost 3600.00 to build it? Possibly, but I doubt it. He has to make a profit, and he has to pay someone to assemble it. Dr Z amps which I know nothing about are supposed to be hand wired. But for the most part, the industry has been relegated to printed circuit boards. My MB Triple Crown is on a printed circuit board. But it seems like we all want vintage stuff out of the 50-60's to some degree. Maybe with a modern twist. I have a set of old RCA 6L6GC power tubes short bottle left from some of my dad's old radio stuff still. I don't know how good they are as I don't have a tube tester, and they stopped having them at the local pharmacy long ago. But they are currently in my Hot Rod Deville biased at 70 mv. They are still working, and I'm not willing to part with them. Mmmmm. I'm rambling on..... I digress.

[Grees01]

I installed a triode pentode switch in my Super and I will probably never use it.  The difference is not that great and the triode mode sounds a little thin comparably. and not toneful to my ear.  I seriously doubt if would be much different in the bassman.  Something more useful would be a LarMar PPIMV and/or some switching from fixed bias to cathode bias. Also a switchable cathode circuit on the first amplification stage. The safety diodes can't hurt. Hoffmans board also has the 1 ohm bias sense resistors which are very handy.

Hmmm. Thank you for sharing that information. I will have to search for that (LarMar PPIMV) as I have no clue what that is or how it works.

[Grees01]

GReed01-
I’ve built the 5F6A three times now, and I find it to not really be my flavor.
In my opinion, it has way too much low end. I think it get fame and acclaim because it was the basis for so many great amps that came after it. I get that you wanna do some things to make it different, so did I, three times.
I personally would suggest going a different route. Build plexi or JCM800 if you wanna rock. Build a super or a vibroverb if you want awesome cleans.
Maybe the best place to start is, why do you want to build a bassman? Does it really suite your style of playing, or are you doing it because it’s talked about alot?

That's the thing, without having ever played thru one I don't know for sure. It certainly has its stature. Marshall copied it for there JTM 45. If I could have it anyway I wanted I would like a 3 channel amp that does amazing clean, great crunch, and smoking high gain. That's why I bought a triple crown, but of the TC 3 channels really I only love the clean channel with the drive set to about 1:00pm and the master at approx 10:00pm. Channel 2 I just cant get a crunch tone that I'm happy with either with my custom shop 59 reissue Les Paul or with my Strat, nor a high gain sound out of channel 3 that I love. There not horrible, but I'm not in love with both those channels. More often that not I play the clean channel which has just a little bit of drive and nice touch sensitivity on the guitar and with varying my playing style I can get lush cleans with creamy upper register notes and leads, and good touch harmonics.

[sluckey]

You may want to listen to some Dumble amp clips. Plenty of info and resources at The Amp Garage. Very knowledgeable and friendly people. Good musicians too.

[TitaniumValhalla]

I’m not sure what kind of spaces you’re planning to play this amp in, but I built a 5F6A about a year ago and it was way too loud to use in my home. Bear in mind a cranked 15w amp can deafen everyone in your house easily, and a 40w amp like a 5F6A will get the police called on you by your neighbors. I couldn’t play mine above about 30% on the dial. I added a master volume and a lead channel but just ended up realizing it was the wrong amp for my needs and I got rid of it. I love the sound of a cranked 5F6A but if I had to do it again I would look at some of the “micro” Bassman designs out there instead if you are after that sound in particular, or one of the many great 15-20w designs that are very popular to build.

Just something to think about - if you don’t already have any amps in this power range, swing by a guitar shop and play one for a few minutes to get any idea of what you’re in for.

[mresistor]

MResister—- You did Choke input filter?
Tell me more about this please.
I did it once , the amp goes thud when you turn it on and off.
Did you find a work around for this?
I’ve read a lot about inrush current and choke input filter, but never really found enough info to deal with it.
What’s the secret, can you tell me or point me in a good direction?
Thanks
VT

The Baldwin organ had this great big choke in the power supply and I used it as the primary input filter at the suggestion of Sluckey who helped me with that build. The PT outputs near 700v and the big choke drops it down into the 400s. I don't know the exact plate V as I haven't been in the amp in years.  This amp does not exhibit that symptom, no clunk when turning it on or off. Can't help ya with that.

[Grees01]

You may want to listen to some Dumble amp clips. Plenty of info and resources at The Amp Garage. Very knowledgeable and friendly people.
Good musicians too.

Thank so much sluckey. I looked quickly at the forum for some of the videos and sound clips but haven't found much yet that helps. A quick YouTube video search however found this video which was excellent and I would recommend watching.

[Grees01]

Boy that Dumble sounds sweeeeet 👌 100 watt head played at a level that matches the level of the dialog. Unless some editing was done. Just because you have some power available doesn't mean you have to max out the volume and gain controls to get great tone. IMHO.
G. Rees

[valvetones]

Perhaps a 6F6GT power section for the bassman is the ticket!
What would that be, 14 watts or so maybe.
Hmmmm🤔
So many options.
I too am in the process of rebuilding my 5F6A bassman. I had it set up pretty dang good but as others have said , it’s beastly loud. To loud to use at home unless your nearest neighbor is 10 miles away.
Any how. Listen, when you build it,
 1. use 1 Meg LINEAR taper pot not audio taper for the volume.
2. Use a 1 watt 6800 ohm tail resistor instead of 10K
3. And use 100k slope resistor instead of 56k.
Connected to one side of that slope resistor, you’ll see two .02 caps, one to the mid and the other to the bass control. Consider changing the bass cap to .1uf.

Another thing to consider is your speaker loud and it’s relation to the negative feed back loop resistor.
The schematic shows 27k, but that is for a 2 ohm speaker load.
If your going to run 4 or 8 ohm speaker loads, you may want to change that value, make it higher.
I’m no authority on what values would be appropriate for a 4, 8, or 16 ohm speaker load but perhaps someone else can chime in.

As far as the three deviations from the schematic listed above, these are well documented as what was actually done on the 5F6A from the factory.
Fender never updated the layout or schematic to reflect these deviations, but that’s what they did.

Now,,, I’m curious,, what transformers will you be using for this build?

[Grees01]

I too am in the process of rebuilding my 5F6A bassman. I had it set up pretty dang good but as others have said , it’s beastly loud. To loud to use at home unless your nearest neighbor is 10 miles away.
Any how. Listen, when you build it,
 1. use 1 Meg LINEAR taper pot not audio taper for the volume.
2. Use a 1 watt 6800 ohm tail resistor instead of 10K
3. And use 100k slope resistor instead of 56k.
Connected to one side of that slope resistor, you’ll see two .02 caps, one to the mid and the other to the bass control. Consider changing the bass cap to .1uf.

Another thing to consider is your speaker loud and it’s relation to the negative feed back loop resistor.
The schematic shows 27k, but that is for a 2 ohm speaker load.
If your going to run 4 or 8 ohm speaker loads, you may want to change that value, make it higher.
I’m no authority on what values would be appropriate for a 4, 8, or 16 ohm speaker load but perhaps someone else can chime in.

As far as the three deviations from the schematic listed above, these are well documented as what was actually done on the 5F6A from the factory.
Fender never updated the layout or schematic to reflect these deviations, but that’s what they did.

Now,,, I’m curious,, what transformers will you be using for this build?

Hey Valvetones. The modification that you just outlined are exactly what Rob Robinette lists and shows in his schematic. Take a look here
https://robrobinette.com/5F6A_Modifications.htm Seeing how Fender designed this amp for intended use with the bass guitar, it make sense to me that some changes would be required to make it work out well for guitar.
With regards to transformer I've only looked at 2 thus far, as far as the power amp is concerned. One was a Mercury Magnetics which I can't seem to find the right one at this moment or maybe I made a mistake with regards to the high voltage output number. The other one that seems to fit the bill is a Hammond 290dx https://www.hammfg.com/electronics/transformers/classic/290 I can't find a schematic for this at the moment either, but the specs are listed. Heater current available is only 4 amps though. Which according to Rob is just enough to be able to run a pair of EL34's if desired.  Thanks for the feedback, information, and dialogue.
G. Rees

[valvetones]

Good luck on your build G’Rees.
Keep us posted on the outcome and your thoughts.
I’ll check out the link you sent.

[pdf64]

… To loud to use at home unless your nearest neighbor is 10 miles away.
Any how. Listen, when you build it,
 1. use 1 Meg LINEAR taper pot not audio taper for the volume.
…

Why linear, wouldn’t a decent smooth audio taper pot will facilitate lower volume use?

[66Strat]

I don't know where you are located, but; here in the USA finding transformers and other components at present time is near impossible. You may be ahead of the game if you buy a used Bassman Reissue. I believe that most, if not all, have Schumacher transformers. The later LTD series have finger jointed pine cabinets with reissue Jensen alnico speakers. The earlier reissues have plywood cabinets (Baltic Birch?) and blue frame Eminence Legend alnico speakers. Either would be a good platform to mod or rebuild using a new circuit board. Buy one and try it out to see if you like the amp without sinking a lot of time and money into a potential white elephant. If you do not like the amp, you can sell it and recoup your money without incurring much, if any, of a loss.

[Grees01]

Good luck on your build G’Rees.
Keep us posted on the outcome and your thoughts.
I’ll check out the link you sent.

Hey there Valvetones. Hope posting the link to the information on Robs site and the confirmation that what you said and he says both agree wasn't offensive to you or anyone else in any way. I'm not 100% committed to building a 5F6A circuit quite yet as I stated in my original post. Still considering and thinking. I wrote an email to Mercury Magnetics last night with regards to the PT. I'll post once they reply. Please share any feedback on your own bassman build. What you like, what you don't, any other changes that you made other than the ones that just mentioned. And if you have time to look at the Hammond PT let me know what you think. Thanks again.
G.Rees

[Grees01]

I don't know where you are located, but; here in the USA finding transformers and other components at present time is near impossible. You may be ahead of the game if you buy a used Bassman Reissue. I believe that most, if not all, have Schumacher transformers. The later LTD series have finger jointed pine cabinets with reissue Jensen alnico speakers. The earlier reissues have plywood cabinets (Baltic Birch?) and blue frame Eminence Legend alnico speakers. Either would be a good platform to mod or rebuild using a new circuit board. Buy one and try it out to see if you like the amp without sinking a lot of time and money into a potential white elephant. If you do not like the amp, you can sell it and recoup your money without incurring much, if any, of a loss.

Hey there 66 strat! Thanks for the feedback. Interesting idea. Certainly worth considering. Could certainly lower your financial liability if it turned out that you just really didn't like the amp. But then again if the circuit ends up needing some mods to make it work for you, you'd have to be pretty careful with your work to be able to return it to stock for resale. Why do you think getting components here in the USA is next to impossible? 
G.Rees

[66Strat]

Hey there 66 strat! Thanks for the feedback. Interesting idea. Certainly worth considering. Could certainly lower your financial liability if it turned out that you just really didn't like the amp. But then again if the circuit ends up needing some mods to make it work for you, you'd have to be pretty careful with your work to be able to return it to stock for resale. Why do you think getting components here in the USA is next to impossible? 
G.Rees

If you don't like the basic stock amp, why would you go any further? The deeper that you dive into mods, the further under water you will be regarding your investment in the project. Have you ever played a 5F6-A Bassman? They are relatively clean sounding amps until you get up towards the higher end of the volume control. At that point, they are LOUD. They are great sounding amps, but if you are wanting overdrive be prepared for the volume or get an OD pedal. If you are in the US, find one at a Guitar Center near you and play through it.

As to transformers, the supply chain backlog has left vendor inventories depleted. Mojotone does not have any 5F6A or JTM 45 output transformers in stock.
https://www.mojotone.com/amp-shop/electronics/transformers/output-transformers?page=1

Same goes for power transformers.https://www.mojotone.com/amp-shop/electronics/transformers/power-transformers?page=1

[Grees01]

Have you ever played a 5F6-A Bassman? They are relatively clean sounding amps until you get up towards the higher end of the volume control. At that point, they are LOUD. They are great sounding amps, but if you are wanting overdrive be prepared for the volume or get an OD pedal. If you are in the US, find one at a Guitar Center near you and play through it.

I have not played thru a 5F6-A Bassman. I didn't see one at my local Guitar Center the last time that I was there. And the last time I looked online at Fender's website currently they produce 3 versions of the Bassman. 2 heads, One that is all tube at 300 watts, One that is hybrid tube and solid state at 800 watts, and a copy of the 59 combo at 45 watts, but it's not hand wired, and I'm not sure at this point what changes have been made to the design. According to Fender's nomenclature a 5F6-A is a 1956 Bassman, which was hand wired at that time. (correct me if I'm mistaken) The combo that they produce would be closest to the 56 design. And I'm due for a trip to GC anyways, been wanting to stop by for that last week or so. So I will go today and take a look, I'll bring my Strat and if they have one I'll plug in and see (which is a great suggestion and I was planning to do that already) who knows maybe I'll get lucky and they do have one on the floor.
With regards to transformers. Hammond makes one that appears to me to fit the bill, and they are available, see prior post. As I've said a few times now I'm not dead set on a 5F6-A amp, but considering one. Also waiting on a return email from Mercury Magnetics with regards to transformers- which I will post the reply when it arrives. Thank you for the feedback, appreciate it.
G. Rees

[Willabe]

1. use 1 Meg LINEAR taper pot not audio taper for the volume.
2. Use a 1 watt 6800 ohm tail resistor instead of 10K
3. And use 100k slope resistor instead of 56k.
Connected to one side of that slope resistor, you’ll see two .02 caps, one to the mid and the other to the bass control. Consider changing the bass cap to .1uf.

As far as the three deviations from the schematic listed above, these are well documented as what was actually done on the 5F6A from the factory.
Fender never updated the layout or schematic to reflect these deviations, but that’s what they did.

That's news to me.

Since the early 90's, I've never read in any book on tube amps (name it, I probably have it) or any tube amp forum about those 'well documented' changes.

There is talk of tweed 5F6a Bassmans actually having the latter .1/.047 caps and not the 2 x .022 caps shown for the TS in the Fender schematics.

A tweed Bassman has no TS tail R, it uses a 25K mid pot.

You can change the slope R from 56K to 100K, but if you do, now your into Brown face 6G series and Black face AB763 series Fender amps. (Not talking about the volume pot taper.)

The 56K TS slope R and 25K mid pot in the TS are a large part of why guys love the old tweed amps. Way less mid scoop than if using the latter 100K and 6K8 R's TS values.

You want mid scoop, AB763 circuits.   

[Willabe]

Seeing how Fender designed this amp for intended use with the bass guitar, it make sense to me that some changes would be required to make it work out well for guitar.

No, no, no, it works great with guitar as is!

That's why guitar players were, and still are, playing through the tweed Bassmans back when they came out. And that's why Marshall copied it.

Look at the 5F8a tweed Twin, all circuit values are the same. What Fender did for the 5F6a Bassman was give it a larger PT and OT. That helped keep the power supply a little stiffer, to handle the low bottom end. So it had a stronger/tighter bottom end and stayed a little cleaner/clearer, a little longer across the spectrum. Guys liked the clearer low end.

 https://el34world.com/charts/Schematics/files/Fender/Fender_twin_5f8a_schem.pdf

The only thing you might want to change and that goes for any Fender, and many other amps is the preamp K bypass cap values. If your going to be cranking the amp, lower the 25uF to 10uf or 4.7uF. It takes the low end mud out.

[66Strat]

Seeing how Fender designed this amp for intended use with the bass guitar, it make sense to me that some changes would be required to make it work out well for guitar.

No, no, no, it works great with guitar as is!

That's why guitar players were, and still are, playing through the tweed Bassmans back when they came out. And that's why Marshall copied it.

Look at the 5F8a tweed Twin, all circuit values are the same. What Fender did for the 5F6a Bassman was give it a larger PT and OT. That helped keep the power supply a little stiffer, to handle the low bottom end. So it had a stronger/tighter bottom end and stayed a little cleaner/clearer, a little longer across the spectrum. Guys liked the clearer low end.

 https://el34world.com/charts/Schematics/files/Fender/Fender_twin_5f8a_schem.pdf

The only thing you might want to change and that goes for any Fender, and many other amps is the preamp K bypass cap values. If your going to be cranking the amp, lower the 25uF to 10uf or 4.7uF. It takes the low end mud out.

The first JTM45 was pretty much a dead nuts copy of the 5F6-A circuit. Key differences were 12AY7 tube in the V1 slot for the Bassman vs 12AX7 for the JTM45 and four 8 ohm Jensen 10 inch alnico speakers wired parallel for the Bassman vs two 16 ohm alnico Celestion Blues wired parallel for the JTM45.

While you're at Guitar Center, see if they have a JTM45 to play through. The amp won't have Celestion Blues but will likely be connected to a cabinet loaded with greenback Celestions. Speakers make a huge difference with this circuit IMO.

[valvetones]

Willabe

If by TS you mean tone stack, you are confused. TS don’t have tail resistors, but Phase Inverters do!
And yes it is well documented, despite the lack of that knowledge in your library.
Here’s some food for thought regarding what you may see in a real 5F6A

https://m.youtube.com/watch?v=tU7wXFFZ08w

Notice that 6.8 k where the schematic shows 10k?
See how the 1960 has linear pots?

There are other images and videos you can check out that will have these same changes.
And as the man said, Rob Robinette has said the same thing.

Now to the builder— if you decide to build a bassman I can almost 100% guarantee that once you have it working, within a week , you will be considering the very simple mods that made the JTM45, The Plexi, and the JCM800.

And even if you like one of those, you will still wish you built a Quieter amp.
I suggest scaling down the power section and using 6v6 or 6F6

As for the transformers, I have enough to build 30 amps.
What do you need, and do you want merit, Stancor, thordarson , or Triad?
 

[valvetones]

Personally I think the 5F6A has way to much low end. It gets kinda farty distortion when you finally get it to break up.
I’ve gone through and played with the Duncan TS calc and ripped up and rebuild several times now.
This particular time, I’m going to change the cap that feeds the Phase inverter from .02 to something in the PF range, somewhere between .0047 and 250PF.  This one cap is the bottleneck for the whole preamp, and looks like a good spot to cut some low end.
What gave me this idea? Last summer I build a 6G16 vibroverb, and it’s glorious, I’m way happier with that than I ever was with the bassman builds, and it feeds the Phase Inverter with a .0005 mfd cap.
50 pico farads yes?
So if that small of a bottle neck works well for this, and many others, perhaps it will suit me better this time around, and is worth experimenting with values to come up with the right stuff, which is subjective of course.
Also, to my ears, cutting the 250mfd cathode cap down to between 2md and 25mfd may be another spot to shave some Low end.
Now I will say the stock circuit has some great cleans, and if that’s what you like, make it so.
I’ll be using 6F6G tubes and will report back.
Do tell us what you decide on and if you plan on changing things after the initial build.

[Grees01]

Hey guys. Understanding Fender's circuit nomenclature is a bit of pain in the $^^%#. I was doing some reading and came upon this on Fender's website which I found helpful : https://www.fender.com/articles/gear/going-low-the-history-of-the-bassman. Pretty good information, hopefully correct.

[Grees01]

Willabe

If by TS you mean tone stack, you are confused. TS don’t have tail resistors, but Phase Inverters do!
And yes it is well documented, despite the lack of that knowledge in your library.
Here’s some food for thought regarding what you may see in a real 5F6A

https://m.youtube.com/watch?v=tU7wXFFZ08w
Notice that 6.8 k where the schematic shows 10k?
See how the 1960 has linear pots?

There are other images and videos you can check out that will have these same changes.
And as the man said, Rob Robinette has said the same thing.

Now to the builder— if you decide to build a bassman I can almost 100% guarantee that once you have it working, within a week , you will be considering the very simple mods that made the JTM45, The Plexi, and the JCM800.

And even if you like one of those, you will still wish you built a Quieter amp.
I suggest scaling down the power section and using 6v6 or 6F6

As for the transformers, I have enough to build 30 amps.
What do you need, and do you want merit, Stancor, thordarson , or Triad?

Wow!!! How super cool is that? I had just finished watching part one of the same video just now!!! Hey Valvetones, that's super nice of you to offer that help on the transformers. You know quite a few people now have mentioned on this topic, the concern with how LOUD!!!! this amp would be (Rated at approx 40 watts) right? My Deville 4 x 10 is rated at 60 watts if I remember correctly. My MB Triple crown is rated at 50 watts. I play both of them in my little bedroom studio> Do they get loud? Yea they do if you crank the volume knob up a little ways. But I don't play either one of them on 1-2. Bedroom level on the Fender clean channel usually 3, up to 4 and that's loud enough for sure unless for some reason I just want to rattle the windows or piss off my neighbors who are all actually pretty cool and know that I play. MB clean channel is about 1:00pm on the gain, 12:00pm on the channel volume, and 11:00am on the master volume. Sounds great and doesn't hurt your ears. Also the MB isn't as searing on the high notes as the fender FWIW. An Lar-mar Master volume might be a good idea, I need to read up more to understand how that works, and how it affects the tone. And finally the offer to help with me with some transformer needs, is really super kind of you. I will definitely keep that in mind, although brand to brand I have no clue what the better or best choice might be>
G.Rees

[Grees01]

While you're at Guitar Center, see if they have a JTM45 to play through. The amp won't have Celestion Blues but will likely be connected to a cabinet loaded with greenback Celestions. Speakers make a huge difference with this circuit IMO.
[/quote]

Well I had planned to get over to Guitar Center this afternoon, but it just didn't happen. I had a few other things that I needed to take care of. I will make it over there tomorrow!!!!

[shooter]

Speakers make a huge difference with this circuit IMO.

Speakers make a huge difference in most builds.  Once an amp is up and bug-free, start with speakers, lots of speakers, it's easy, it's productive, you won't burn holes in your polyester pants since you don't need to solder.
Once you find "the speaker", now you're ready to tweak Tone.

[TitaniumValhalla]

Willabe

If by TS you mean tone stack, you are confused. TS don’t have tail resistors, but Phase Inverters do!
And yes it is well documented, despite the lack of that knowledge in your library.
Here’s some food for thought regarding what you may see in a real 5F6A

https://m.youtube.com/watch?v=tU7wXFFZ08w
Notice that 6.8 k where the schematic shows 10k?
See how the 1960 has linear pots?

There are other images and videos you can check out that will have these same changes.
And as the man said, Rob Robinette has said the same thing.

Now to the builder— if you decide to build a bassman I can almost 100% guarantee that once you have it working, within a week , you will be considering the very simple mods that made the JTM45, The Plexi, and the JCM800.

And even if you like one of those, you will still wish you built a Quieter amp.
I suggest scaling down the power section and using 6v6 or 6F6

As for the transformers, I have enough to build 30 amps.
What do you need, and do you want merit, Stancor, thordarson , or Triad?

Wow!!! How super cool is that? I had just finished watching part one of the same video just now!!! Hey Valvetones, that's super nice of you to offer that help on the transformers. You know quite a few people now have mentioned on this topic, the concern with how LOUD!!!! this amp would be (Rated at approx 40 watts) right? My Deville 4 x 10 is rated at 60 watts if I remember correctly. My MB Triple crown is rated at 50 watts. I play both of them in my little bedroom studio> Do they get loud? Yea they do if you crank the volume knob up a little ways. But I don't play either one of them on 1-2. Bedroom level on the Fender clean channel usually 3, up to 4 and that's loud enough for sure unless for some reason I just want to rattle the windows or piss off my neighbors who are all actually pretty cool and know that I play. MB clean channel is about 1:00pm on the gain, 12:00pm on the channel volume, and 11:00am on the master volume. Sounds great and doesn't hurt your ears. Also the MB isn't as searing on the high notes as the fender FWIW. An Lar-mar Master volume might be a good idea, I need to read up more to understand how that works, and how it affects the tone. And finally the offer to help with me with some transformer needs, is really super kind of you. I will definitely keep that in mind, although brand to brand I have no clue what the better or best choice might be>
G.Rees

If you have a Deville, turn the master volume all the way up, the drive down, and then use the channel volume to set your overall volume. That would be an approximation of how the Bassman is going to behave. It will be loud and clean.

[HotBluePlates]

...
1. use 1 Meg LINEAR taper pot not audio taper for the volume.
2. Use a 1 watt 6800 ohm tail resistor instead of 10K
3. And use 100k slope resistor instead of 56k.
Connected to one side of that slope resistor, you’ll see two .02 caps, one to the mid and the other to the bass control. Consider changing the bass cap to .1uf.
...
As far as the three deviations from the schematic listed above, these are well documented as what was actually done on the 5F6A from the factory. ...

That's news to me.

Since the early 90's, I've never read in any book on tube amps (name it, I probably have it) or any tube amp forum about those 'well documented' changes.

One of Gerald Weber's books mentions the different capacitor & slope resistor values in the tone stack.  It says there were 3 different ways 5F6-A Bassman tone stacks were configured, even though only one is shown on the schematic.

Else, you have to go talking to folks who own the original tweed amps & have seen A LOT of them.  Some Fender models had notable deviations fro the schematic, others didn't.

[pdf64]

Given Fender's high model turnover in the 50s and 60s, that Bassman model was in production for a long time, nearly 3 years. Leo is reported to have liked to tinker and tweak things. I get the impression that the Sept 57 5F6-A documentation, whilst applying to earlier batches, may not be 100% accurate for the entirely of the production cycle. Hence the modern RI 59 Bassman and its various differences (eg tone stack, presence / LTP) to the Sept 57 circuit. ie it was a reissue of the '59 Bassman design, rather than the '57.
Perhaps in an ideal world, the documentation would have kept up with the changes, eg 5F6-B, 5F6-C, perhaps it did but never made its way into the public domain, but most probably the changes, if documented at all, were in the form of hand annotations to the factory floor copy.

[Willabe]

If by TS you mean tone stack, you are confused. TS don’t have tail resistors, but Phase Inverters do!

Coming on kind of strong for a new member, don't you think so?   

You could have wrote that a little gentler and a just touch more respectful in response.   

Yes I meant tone stack and some TS's do have tail R's.

Fender brown/black face amps that do not have a mid range pot have a 6K8 tail R that the TS stands on. It's the mid range set R. That R is the tail of the TS. Look at Vox top boost and latter Gibson GA77 amps (Gibson copied the Vox top boost TS). The mid range set R is in || with the bass pot, the TS does not stand on the mid set R, so it is not a tail R.

Amps that have a TS with a mid pot don't have the tail R, but you could, and guys do, put a 6K8 R under a 25K or 50K mid pot so turning the mid pot to zero, you have the standard Fender mid value. Turn up that mid pot and you start removing the mid range scoop.         

For clarity, it would have been helpful if you would have said/wrote LTPI in your post. I saw 6K8, 10K, common Fender TS tail R and mid pot values and thought TS. 

[valvetones]

 Willabe,
Perhaps, abit strong, I guess I read your post as coming on abit strong and just wanted to point out that these changes are known.
Certainly not looking to hurt any feelings. Just an amp thread. And only trying to be helpful.
Thank you for the additional detail about the (other ) tail resistor.

Cheers!

[Willabe]

And yes it is well documented, despite the lack of that knowledge in your library.

https://m.youtube.com/watch?v=tU7wXFFZ08w

No, turns out I have it in Webers 1st book, just forgot. 

I must have dismissed it at the time of reading it years ago when he said; Two .02s for the bass cap and middle cap with a 56K ohm slope resistor are the values generally regarded as the best sounding because the bass is clearer."

Personally I think the 5F6A has way to much low end. It gets kinda farty distortion when you finally get it to break up.

3. And use 100k slope resistor instead of 56k.
Connected to one side of that slope resistor, you’ll see two .02 caps, one to the mid and the other to the bass control. Consider changing the bass cap to .1uf.

Considering Webers comment of which TS values give clearer bass in the 5F6/5f6A Bassmans, maybe you should reconsider going back to the 2x .02/56K slope R values?    And maybe lower the 1st preamp tube K bypass cap from 250uF down to 10uF of 4.7uF?

Weber probably had his hands in dozens of original 5F6/5F6A Bassman already by the time he wrote that 1st book. He would have known by then.

See below;

From Webers 1st book; A Desktop Reference of Hip Vintage Guitar Amps, 1994, The Definitive 4x10 Bassman, page 52;

"You never know what you will see in a 5F6 or 5F6A for tone caps. The treble cap was always a silver mica 250pF ; however, I know of three different sets of vales used for the middle and bass caps and the slope resistor. Fender's schematic shows two caps (.02mfd) and a 56K slope resistor in the schematic; however, a .1 mfd bass cap and the .02 middle cap with a 100K slope resistor were very common. Though I have never seen one, there are a few 5F6 series Bassmans that used a .1 mfd bass cap and a .047 middle cap with a 100K ohm slope resistor for the tone circuit. Two .02s for the bass cap and middle cap with a 56K ohm slope resistor are the values generally regarded as the best sounding because the bass is clearer."

Never disputed that there are 5F6/5F6A Bassman with different TS values out there, I said there was talk of that. I know HBP has brought it up before.

What I am disputing is that it's well documented. Now that HBP said it's in a Weber book, I remember it, sort of. (I read that in the mid 90's, that's a long time ago. ) So I got the book out and read it just now.

I wouldn't call a couple of paragraphs in 1 tube amp book "well documented." Documented yes, well documented no. And that link you posted, yes he has a couple of real tweed Bassmans, but he's talking about talking with a "Texas amp builder" in his comments, so he's clearly drawing from Weber.

There are other images and videos you can check out that will have these same changes.
And as the man said, Rob Robinette has said the same thing.

For all we know, Robinette got it from that same book as many others could have too.

[Willabe]

Here’s some food for thought regarding what you may see in a real 5F6A.

So the 5F6/5F6A Bassmans with the Fender schematic values in the TS aren't real tweed Bassmans?

Note that Weber writes the ".1 mfd bass cap and the .02 middle cap with a 100K slope resistor were very common." But doesn't write their just as common/just as many or there's more of those than the 2x .02/56K TS. And the 3rd TS value version, he writes there are a few but only the 5F6, not the 5F6A.

So it seems of the 3 TS value versions, the 2x .02/56K R were most common.

I think it's more like HBP wrote;

Else, you have to go talking to folks who own the original tweed amps & have seen A LOT of them. 
Some Fender models had notable deviations fro the schematic, others didn't.

It's known by guys who had them in their hands and opened them up. Not because it's been documented in tube amp books.

If the other TS values were the 'secret sauce' of a 5F6/5F6A Bassman we'd be talking about it a LOT here as a TS option and it would be in most tube amp books and in tube amp companies clones of tweed bassmans.

Marshall copied Fenders 2x .02/56K TS values, and stayed with those values. They even lowered the slope R to 33K on some amps, opposite of going up to 100K slope R. Then many copied Marshall. Luck of the draw that Marshall had a 2x .02/56K to copy?     

And the reissue Bassmans are bright and hash sounding. I would not be copying those amps.    Some gut them and put in a eyelet/turret board and new PT to get the dcv's down, then they like/love them.

At that time, tweed amp series, Leo was still in the pretty early stages of evolving his amps. We know he did switch to the TS values found in a few of the 5F6 Bassmans, .1 mfd bass cap and a .047 middle cap with a 100K slope R, with the brown/blackface amps. But we also know that besides experimenting he would use different values at times that he had in stock just to get the orders out the door. Could have been a little of both?   

[Willabe]

Willabe,
Perhaps, abit strong, I guess I read your post as coming on abit strong and just wanted to point out that these changes are known.
Certainly not looking to hurt any feelings. Just an amp thread. And only trying to be helpful.
Thank you for the additional detail about the (other ) tail resistor.

Welcome to the forum. 

Very happy to see you so excited about tube amps, just try not to nip at us old dogs?   

[pdf64]

…
And as the man said, Rob Robinette has said the same thing.
…

Rob has kindly provided a lot of well drawn layouts, compiled ideas for mods, builds etc, and hosted The Trainwreck Pages, on his site.
He’s also written a lot of text, lots of explanations for how stuff works.
A lot of people seem to have found it helpful, and he’s got a lot of people enthused to make a start at mods and builds, but I find too many mistakes, errors / misapprehensions and opinion / speculation stated as fact, for me to suggest it as a reference source. In the way that the Aiken or valve wizard sites can be.

[valvetones]

Glad to share the passion for great audio with you all.

Thank you.