Bassman 70 project

[jjackson7]

Allright I'm gonna get started this week end on my bassman project. Look at this mess!


In hindsight I probably didn't do myself much good with those heaters  . I was pretty new to all this back then.

My plan is to gut everything but the transformers and the filter caps (which I know to be good) and rebuild. Here's the thing I would like to build something that people on this forum are comfortable with and could help out with. I am looking at the 5F6A or the Plexi 50. I would actually like a combo of the two but I think I want to start with the 5F6A and tweak from there.

So problem #1. The controls and tube layout don't match. I thought about using the master volume on this guy as a reverb control and using the spare 12ax7 for a reverb driver. Also just gonna blank the second set of inputs. I'll probably just make a new faceplate. The controls would then be In, In, Vol, Vol, Pre, Bass, Mid, Tre, Rev.

Oh I'm gonna use a solid state rec too.

I also am a little apprehensive just gutting and trashing everything. Does it seem wise to save these components if they check out? I don't want to be the guy throwing away money cuz he doesn't know any better.

Thanks for the input,

Jordan.

[TIMBO]

Hi jjackson7,I think doing Doug's PLEXI with HOT SW. would be perfect if you were to gut it and start again. I'm not sure how close the bassman board and the plexi are in the layout so i don't know how tech you are if another board is needed. My idea for a build would be(using the existing holes) :-
IN/hi, IN/lo, Vol, HOT SW., REVERB DWELL, REVERB LEVEL, FX SEND, TREBLE, MID, BASS, PRESENCE.
                                                                             FX RETURN                            ( PPIMV )

I'm not sure the distance between REV DWELL and REV LEVEL would be the same (first hole of second set inputs ) and thinking you could have FX SEND/RETURN stacked on top of each other (using second hole second set inputs).
Presence could be added or maybe a PPIMV which is always useful. Hope this helps. 

[jjackson7]

like this


I don't really have use for an fx loop so i'm gonna skip that.

I'm running into a couple real estate problems. I don't have room for a choke or reverb xformer as it stands. Can I use existing filter caps only and a ss reverb send/return circuit? Can I just drop B+ to feed the op-amp supplies.

[Willabe]

I'm running into a couple real estate problems. I don't have room for a choke or reverb xformer as it stands.

You do if you get rid of the filter cap dog house on the outside of the chassis. Just put the filter caps inside the chassis.


                    Brad       

[jjackson7]

Okay I thought about that but wasn't sure I could fit everything in the chassis. I just took some measurements and it looks like my chassis is comparable to the 5F6A chassis sold here. I'll get it worked out and draw it up this weekend. Nevermind about SS reverb I hated that while I was typing it and it doesn't solve my extra 12ax7 purpose. If anyone can point me to pics of their finished builds of either the 5F6A or the Plexi 50 I would appreciate some inspiration. I'll look around on my own too. Thanks.

[jjackson7]

Allright I think I've decided on Tubenit's Bassman w/Reverb (http://www.el34world.com/Forum/index.php?topic=571.0). I want to go ahead and mark the transformer leads and start gutting this weekend. Before I do though I need a little help understanding the power transformer in the bassman 70. (http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/fender_bassman70.pdf). I understand the grn and grn/yel are heater wiring, the reds are ac to the rectifier. I am unsure how to treat the red/yel and browns though. I can't really find any info but the schem which sadly isn't enough for me. There are some major changes to the power supply circuit here from the 5F6A. This has got me wondering if these transformers are going to work for this circuit.

[Willabe]

The red/yellow is the center tap for the high voltage winding, reds. It goes to ground.

The browns are for the -bias. Instead of having a single -bias tap wire off of the high voltage winding, they gave this PT a seperate bias winding.


                    Brad     

[jjackson7]

Many thanks. This is making more sense by the minute. Can I implement tubenit's design by grounding both the 10uf caps and 47k res as well as the other brown wire that is going to ground now?

[jjackson7]

Okay stripped the chassis


and then took a wire brush and drill to it



I think I've decided to use the Plexi50 build because I can use the combo 50uf caps where the cap can was and save space inside the chassis. I should then still have room for a choke and reverb transformer next to the filter caps. When I was measuring I couldn't find space in the chassis for 5 20uf caps for the 56FA build.

I'm still trying to figure out the bias winding on the bassman 70 power tranny and how to add reverb to the plexi50. I got my crayons out and here is what I came up with. I also added a NFB switch.



I basically stole the reverb circuit from Tubenits 56FAreverb and tried to insert it in the plexi layout. I have no idea if this will work. And again sorry for the childish drawing. If it seems like I'm on the right track I will get a real board layout done.

Thanks,
Jordan.

note - missing paralled 10uf cap and 1.5k res on pin 3 of reverb tube and NFB switch should be in parallel with the NFB res.

[Willabe]

Many thanks. This is making more sense by the minute. Can I implement tubenit's design by grounding both the 10uf caps and 47k res as well as the other brown wire that is going to ground now?

Yes, bring them all together as a ground star point. When you have a tap from the high voltage wind, it is referenced to ground by the CT. With a separate winding you ground the 2'nd leg/wire as the ground reference.

Nice job on cleaning up the chassis.   


                        Brad     

[tubenit]

Here is what I probably would have as a first try.  I indicated in red where I would experiment with values to tweak to taste.

And I'd probably add an additional filter cap.  I think one of the smaller 20uf filter caps should work OK?

CHECK for errors!

With respect, Tubenit

[sluckey]

The red/yellow is the center tap for the high voltage winding, reds. It goes to ground.

Better take a closer look at this PT!

This PT is connected to a FWB rectifier with a 520VDC output. The bridge is connected to two series 220µF caps and the PT center tap is connected to the junction of the two caps to balance the voltage across each cap. Center tap is NOT connected to ground. So, what does all this mean? 

Doing some backwards math... 520VDC from a SS bridge rectifier means the AC input to the bridge must be 520\1.414=368ACV. But that's 368VAC across the entire HT winding. Since this winding has a center tap, the voltage could also be stated as 184-0-184. IOW, you will have to use a FWB to get enough B+ to run the Plexi or 5F6A and that will put your B+ right back at 520VDC. That will work but may be higher than you are expecting.

[tubenit]

Here is an idea of how you could keep the Hoffman style Turret board and incorporate the reverb circuit.  Look at the Hoffman Plexi 50 layout board that Doug has and then wedge this into that design. 

Check for errors!    With respect, Tubenit

[jjackson7]

Here is an idea of how you could keep the Hoffman style Turret board and incorporate the reverb circuit.  Look at the Hoffman Plexi 50 layout board that Doug has and then wedge this into that design. 

Check for errors!    With respect, Tubenit

Man thanks a million Tubenit!!! That's exactly what I was thinking about doing.

The red/yellow is the center tap for the high voltage winding, reds. It goes to ground.

Better take a closer look at this PT!

Okay had to look up some transformer info in my Electronic Troubleshooting book to understand your post, but I get it - Vdc = 1.41*Vrms. I don't know how 520B+ vs 480B+ is going to affect the amp. What is the usual way of tailoring B+? Is it as simple a adding a voltage divider before the filter caps? If I'm using the FWB what do I do with the center tap?

Thanks again everyone for the input! I can tell I'm going to learn a lot with this build.

[jjackson7]

Did a quick search and found this https://taweber.powweb.com/store/vdump.htm. Could this be the answer?

[sluckey]

What is the usual way of tailoring B+? Is it as simple a adding a voltage divider before the filter caps?

The usual and best way of tailoring B+ is to use a PT designed to work with the circuit. If I were doing this project and had to use that Bassman PT,  I'd probably go for EL34s, although 6L6s worked in the original with 520VDC. I would not try to drop the B+ in a 50 watt amp.

If I'm using the FWB what do I do with the center tap?

That PT is designed to work with a FWB. Your best bet is to build the power supply exactly like the original.

[Willabe]

Thanks for catching my mistake sluckey. Sorry about that jjackson7.


                Brad

[jjackson7]

The usual and best way of tailoring B+ is to use a PT designed to work with the circuit.

That PT is designed to work with a FWB. Your best bet is to build the power supply exactly like the original.

Yeah I'm just trying to save a hundred bucks by using this PT. When you say build the power supply exactly like the original are you talking about this only


Can I then use the bias supply as in Tubenit's design?

I would not try to drop the B+ in a 50 watt amp.

Why not? Just curious.

[Willabe]

You have to include the 2 100K R's, there the voltage sharing R's for/across the 2 filter caps.


                    Brad     

[sluckey]

When you say build the power supply exactly like the original are you talking about this only

Yes. Basically FWB (I'd probably drop the caps on the diodes) feeding two series caps for the first filter and HT center tap connected to the junction of the caps to force equal voltage across each cap. At that point, I'd tie into the stock B+ rail of whichever amp you chose to build. I'd also add the two 100K bleeder resistors across the first two filter caps although they're not needed to equalize the voltage since the HT center tap does that.

Can I then use the bias supply as in Tubenit's design?

Sure. But why not use the dedicated bias winding since you have it? Hoffman's Plexi 50 and Tubenits schematic use the HT winding as the AC source to power the bias circuit. That big voltage requires a big 220K resistor to knock the voltage down to bias levels. It's perfectly OK to feed the bias circuit from the HT winding. Many amps do so. But since it's available, it just makes more sense to me to use that dedicated low voltage bias winding to feed the bias circuit. The only change needed would be to reduce that 220K resistor to about 1K. That's how I would do it, but the net results are the same. If you don't use the bias winding just tape off the two brown leads.

I would not try to drop the B+ in a 50 watt amp.

Why not? Just curious.

Just seems wasteful to me.
 

[jjackson7]

Yeah what I meant was use the bias supply on the Bassman PT and the bias circuit that Tubenit drew. Thanks for explaining that. I think I have enough information to start drawing the schematic and layout but I'm about to go to class so it may be later in the week before I get these things accomplished. Man I really appreciate everyone's help so far!!! I don't know anyone in my area that I could consult with and would be lost without this site/forum.