De Tweeded My 5F6A Bassman

[Casey4s]

I got snowed in Dec. 24, and have only been able to get out 6 times until this week. So I got nuts looking for something to do so I tore into the 5F6A Tweed Bassman clone I scratch built in 1998/99. When I got finished it turned into a killer sounding combo amp but all the "Tweed" is gone. I am not quite sure what to call this amp now so I just refer to it as my "DeTweed" combo amp.

 

I started at the first stage and gave myself 3 different switchable voices (load lines) to choose from.

 

Then I added the Weber Active Tone Stack in place of the usual drain to ground tone circuits from Fender, Vox, Marshall etc. Wow what an impact it has on my over all tone. The midrange is the most "dynamic" of the three controls.

 

I changed the Presence circuit from the Tweed style to the 6G6 style and that made some improvement.

 

I left the PI alone, but I added a Post PI Master Volume control to the power amp making the original volume control the master GAIN control. This improvement alone is wwell worth the few dollars in parts and little time it took to do this mod.

 

Then I went back and orphaned V1-B to use later for an up comming modification.

 

I had to stiffen up the power filtering a bit, and I am waiting on parts for the final version, it's a bit "stiff" right now.

 

I added a Hum Balance pot which eliminated the little bit of heater noise I did have. Another good choice in my case.

 

As long as I was playing around in the power supply I decided to add a Solid State - Tube rectifier switch just for shits and giggles. A simple no brainer install and was dirt cheap to do.

 

While I was doing all this over a several week period I changed all of the signal and coupling caps. In the Active Tone stack I used the AES foil in oil MOD capacitors. The .1 coupling caps are paper in oil (PIO) and the .022 caps are those Russian PIO caps that are flooding the market.

 

I forgot to mention in the beginning of this post that when I originally built my 5F6A I built it with a 4x6V6 power amp instead of 2x5881/6L6. I also added 1.5K swamper resistors and 1 ohm bias test resistors to the power amp.

 

 

I know every one knows what a Tweed bassman schematic looks like,

 

http://www.el34world.com/charts/Schematics/files/fender/bassman_5f6a_schem.pdf

 

So I 'll just post my modified schematics.

 

 

 

I am hoping to get someone to help me shoot a demo video/sound clip in the next week or so and I 'll post it when I get it done.

[SHinybulldozer]

That looks awesome! Love the voicing switch idea too. Ive got a couple questions though. Thats a super clean schematic are you using something like Visio? And why did you choice 6v6s instead of the 6l6?

[Casey4s]

 

That looks awesome! Love the voicing switch idea too. Ive got a couple questions though. Thats a super clean schematic are you using something like Visio? And why did you choice 6v6s instead of the 6l6?

 

 

I just started using Express SCH few weeks ago, before that I used a pencil and paper. It's not really full featured like some programs but it's free and so easy to use there is almost no learning curve.

 

I like the tone of 6V6 type tubes better than 6L6 types. So when I was doing the research for the bassman parts I discovered that I can run 4x6V6 on the same OT as a pair of 6L6/5881 type tubes with no real impedance mismatch. I also had about 60 NOS USA and European 6V6GT's at that time so It was a good move that way too.

 

With just a twist of the bias adjust I can go from 4x6V6 (14 W) at 39 Watt Diss (all at 70%)

 

pull those and install a pair of REAL 5881's (23 W) for 32 Watt Diss

 

or a pair of 6L6GC's for about 42 Watt Diss.

 

That is of course a reference and does not translate directly to RMS watts out.

 

 

The whole idea of the re-design is to get more flexability from this amp. It was a One trick Pony before.

[Casey4s]

Here's the express link. It has free CAD and PCB design sofware.

 

http://www.expresspcb.com/expresspcbhtm/Free_schematic_software.htm

[AMZ-FX]

Thanks for sharing your mods. Looks like a nice project!

 

You should put V1B to use as another gain stage (selectable). Or you could wire it as an oscillator and use it to drive the bias inputs on the 6V6 power amp for some tremolo action.

 

regards, Jack

[WRGKMC]

[cx04332]

Looks interesting. Thanks for sharing your schematics.

[Casey4s]

I enjoy sharing this stuff with the DIY forum especially my schematics which I have load of fun drawing.

 

I hope someone else can use any of this information to mod or design an amp.

 

I am at the public now because my DSL modem crashed Monday. So I might not post much in the next few days.

 

I do have a freshened up schematic to post. I forgot the standby switch and I did some better labeling on the drawings.

[Casey4s]

I finally got back on line a few hours ago after 8 days.

 

I re-drew the schematics as revisin I to reflect a few new changes.

 

I forgot to draw the standby and had a few things drawn incorrectly.

 

I removed the Hum Balance pot because it was causing my PT to run hot for some reason.

 

I also redid my filtering again.

 

I changed the two plate caps (coupling caps) from .1 to .022. With the .1's it seemed "dark" sounding by comparrison.

 

I added two .022 Coupling caps to the control grids to prevent Bis voltage bleed through.

 

The third drawing is my intended Boost stage by paralleling V1 A&B together on a switch.

 

 

 

 

[Casey4s]

Here's revision III of the power amp.

 

[Slaymoar]

Hey, just got your PM, are you having any issues?

[Zozobra]

Looks pretty good man. Looking forward to hearing it.

I was wondering where you had been! You still working on your winter build too?

[Casey4s]

 

Hey, just got your PM, are you having any issues?

 

 

 

Mostly that I value your opinion.

 

I did have run away bias problems, LOL...if you look at power amp I and power amp III you'll see the frigging problem.

[Casey4s]

Looks pretty good man. Looking forward to hearing it. I was wondering where you had been! You still working on your winter build too?

 

What actually happened is that I pretty much converted the 5F6A into a version of that amp. I wanted to "beta-test" a few things like the Active tone control and MV, and the next thing I knew I had a new amp.

 

I have the amp all pulled apart at the moment. I figured I might as well re-do sme of the things on the cabinet I have always wanted to change. I made a new cane grille that is flush in front instead of recessed like the the old one. I mounted the new grill on a seperate grill board instead of directly onto the speaker baffle like before. I am redesigning the rear panels too because one got water damage. I also made a new speaker baffle to replace the ugly old one, and painted the front black this time.

 

OLD Grill

 

 

 

NEW Grill

 

 

[Slaymoar]

Mostly that I value your opinion. I did have run away bias problems, LOL...if you look at power amp I and power amp III you'll see the frigging problem.

 

Thanks!

 

Ok, most everything looks good, a few quick things:

 

- The 470 grid resistors (power), the outer tubes and the inner tubes are going through the inner grid resistors. This doubles the load on them. This is not a problem, but it also creates a bit of an non-balance where the outer tubes have a greater voltage drop at the grids. Again, wont make much of a difference, just wondering if you noticed.

 

- Why the 47p cap at the P/I?

 

Likes:

 

- Active tone circuit, nice! I bet you get killer EQ range with that!

- Voicing options are great, I would make it: 0.47, 1u (or 2u2), then 22u. Try to use poly for the 0.47 and 1u. (you wouldn't notice a difference between 20u and 30u, they are going below 30Hz anyways)

- PPIMV -> Not my thing at all, but as long as you keep your voicing on the low bass side, it should sound great.

 

Other:

 

- I'm not sure how your proposed boost circuit works, but I have a few ideas which might give you some great results. The only issue what that is the PPIMV (loose/muddy distortion).

- I'd keep the filter caps at 20u or above. As for the bias, you can increase those to above 40u.

- 4007 (1000V peak right?), you only need 1 per side, but 2 for good measure.

- Maybe make the 100p cap at V1a switchable if pedals dont work well..

 

 

Nice work! Will be watching for pics

 

-D

[Casey4s]

Thanks! Ok, most everything looks good, a few quick things: - The 470 grid resistors (power), the outer tubes and the inner tubes are going through the inner grid resistors. This doubles the load on them. This is not a problem, but it also creates a bit of an non-balance where the outer tubes have a greater voltage drop at the grids. Again, wont make much of a difference, just wondering if you noticed.

 

Actually I need to redraw that, it is actually wired correctly. (good eye)

 

 

- Why the 47p cap at the P/I?

 

It was on the original 5F6A to supress oscillations ( I think), so I just never removed it.

Likes: - Active tone circuit, nice! I bet you get killer EQ range with that! - Voicing options are great, I would make it: 0.47, 1u (or 2u2), then 22u. Try to use poly for the 0.47 and 1u. (you wouldn't notice a difference between 20u and 30u, they are going below 30Hz anyways) - PPIMV -> Not my thing at all, but as long as you keep your voicing on the low bass side, it should sound great.

 

The active tone circuit is very cool but a bit "qwerky". I had to tweak it a bit to get it to work from the original Weber drawing and still nedds to be looked at again after I get some other things done. The most dynamic control is the midrange but the added gain from the tone circuit really gives this amp some useable gain and more overdrive than I'll ever need.

Other: - I'm not sure how your proposed boost circuit works, but I have a few ideas which might give you some great results. The only issue what that is the PPIMV (loose/muddy distortion). - I'd keep the filter caps at 20u or above. As for the bias, you can increase those to above 40u. - 4007 (1000V peak right?), you only need 1 per side, but 2 for good measure. - Maybe make the 100p cap at V1a switchable if pedals dont work well..

 

 

I just always use 3 diodes per side simply because Leo F. did it that way.

 

Nice work! Will be watching for pics -D

 

Thanks Mr Diaz.

[Slaymoar]

Any pics of the amp itself?

[Casey4s]

 

Any pics of the amp itself?

 

 

Probably later today. Every time I take a few pictures, I change something else and I need to re shoot ...LOL..

[Casey4s]

Here's a couple, it got pretty sloppy with all the changes but it's quiet.

 

 

 

 

 

 

The tone stack

 

 

[Slaymoar]

Still looks pretty clean to me, its going to remain quiet as long as you keep a proper ground topology and you keep the gain in check - then the infamous heater wires haha... do you have any shots showing your heater wiring? Its my next step on my build - not exactly sure how to go about it. I've twisted all the way from the TXer to the first tube socket, then from there its untwisted to the next power tube, after that I'm not sure if I should either twist or keep a linear type.

[Casey4s]

Still looks pretty clean to me, its going to remain quiet as long as you keep a proper ground topology and you keep the gain in check - then the infamous heater wires haha... do you have any shots showing your heater wiring? Its my next step on my build - not exactly sure how to go about it. I've twisted all the way from the TXer to the first tube socket, then from there its untwisted to the next power tube, after that I'm not sure if I should either twist or keep a linear type.

 

I twist all my heater wires, I just always have. If you leave loops in the wire at the sockets it can generate humm.

 

I used a "perimeter" grounding system on this one back in '98 and it's really quiet. It is a contiuous loop of tinned 18GA wire on six absorbers, and runs all the way around the chassis perimeter. It's quiet, but a pain in the ass sometimes to solder to because there is so much mass to dissipate the irons heat.

 

The other local bus and single absorber method I showed you a few months ago is better but it needs to be adusted per application which is simple enough t do.