Modding my 66 bassman

[corduroyew]

Now what I want is even more gain and more compression from the bass side of the amp. I've seen people say that a great way to do this is to add the unused tube into the circuit just after the tone stack. I was wondering if anybody could give me any information on how to do that?

I have also seen people suggest a tube rectifier and switch so that I can go back and fourth between the tube rectifier and the solid state rectifier. What I need to know is what tube would I want to use for this? Anybody have any tips on how to do it? Will wiring it in be pretty straight forward or is there something I need to look out for?

When I got the amp it wasn’t working properly and I’ve already had to replace a lot of parts so I’m not interested in “preserving a blackface amp.” I’ve had to do so much to it just to make it sound good that it isn’t really all that collectable, and the AB165 was never *that* collectable to begin with. That said, I still want everything I do to be reversible so I don’t want to drill any new holes in the chassis or anything. I plan on running the wires out the hole that was drilled for the ground switch, and then I’ll put all the stuff (like the extra tube and the switches I might need) in a box that will be mounted to the top of the amp. This way everything will be secure and I won’t have to do anything that isn’t reversible.

So what do you guys think? I’m still pretty new at the amp modding thing and I’m just starting to learn the jargon so please be gentle If you think these mods are horrible ideas then please let me know why. If you think they might work then please give me an idea about how to do it.

[MWJB]

You don't want hi voltage wires running to and from a rectifier outside the chassis, if they become chaffed & exposed you could kill someone. Fitting a rectifier tube will require either:

1) an additional transformer for the 5V 3A winding. You may be pushed to find room for it in the chassis?

2) A new PT with a 5V 3A rectifier winding.

3) If there is no blanked off hole for the rectifier in the chassis as it is, you will have to cut a new one (assuming that there is room), which you apparently don't want to do.

An easier solution and a reasonable project for a novice would be to fit a cathode bias switch (could go in the old "ground switch" hole. See the londonpower website, sift through Kevin's FAQ's and there is a schematic for exactly this.

Cathode bias will give you more compression, even plenty with the stock SS rectifier & PT.

Use a 20W resistor (maybe 470 to 600ohms-ish if B+ is 470v or more? Start high & reduce in value if current allows) and a 100v cathode bypass cap, keep it away from the resistor as the resistor gets very hot and can cook the cap.

[Satamax]

Corduroyew, i've done the add a tube rectifier. And it's nice, thought requires punching a hole and drilling another one for the 5V transformer.



Well it's quite simple, and i've used a dpdt in the grounding switch spot to go back and forth to the tube or solid state rectifiers. I added a variable NFB, and i also had a switch between regular AA864 and AA763 for the bass chanel. The amp was quite sweet, and i've done another mod on my bandmaster wich would be nice to try on teh bassman, and it's soooooooo simple to do

http://satamax.free.fr/bandmastermod.jpg

By pluging a patch cord between input two of the modded chanel and plugging this into the normal chanel, you have two gain stages added Nasty is the word

HTH.

Max.

[corduroyew]

That gain mod looks interesting. Is this where I would need to put it on a bassman?

[Satamax]

Yep, you would feed the bass chanel into the normal chanel. That would be nice.

[corduroyew]

Cool. I'll have to try that.

What tube did you use for the rectifier on your modded amp? Any idea where I might be able to find a schematic or layout for an added rectifier tube? I just don't want to do anything stupid and kill myself, or even worse, hurt my amp.

[Satamax]

Cool. I'll have to try that.

What tube did you use for the rectifier on your modded amp? Any idea where I might be able to find a schematic or layout for an added rectifier tube? I just don't want to do anything stupid and kill myself, or even worse, hurt my amp.

Try to use this one

http://www.ampwares.com/ffg/schem/bassman_6g6_schem.gif

[corduroyew]

Thanks. Now, if I do this (I'm still don't understand the mod enough to know for sure that I will or won't) then am I correct in assuming that I will need to build a bias supply that looks like the one in the 6G6 and have it switch after the bias supply?

Also, could somebody point me in a good direction to get the extra transformer I would need?

Another question. I was thinking about putting a presence pot on this amp. Could I add it in the same place as it is on the 6G6 or should I abandon that idea all together?

[Satamax]

Well, nope you can't put teh presence where it is You would have to do the mod from AB165 to teh mix of AA864/AB165.

Here you go for the GZ34 mod. Thought, forget the presence, it's badly thought.

[corduroyew]

I think I should be albe to follow that. Now I just need to figure out where I can buy the 5v transformer.

I tried puting the guitar side and the bass side of my amp in series the way you did with your bandmaster and it didn't work very well. It let me use the guitar volume control like a master volume which was cool, for some reason it sounded out of phase. It also did strange things so the EQ like if I turned the treble up past 5 the volume would shut off and the bass knob acted more like the tone knob on a strat than the bass knob on an amp. I'm guessing there is something realy simple I could be doing to fix all that but untill I know what that is, I've got it back the way it was before I started.

[Satamax]

Well, i don't know what you've done, but it's weird that it doesn't work. You're sure the wire which returns to the mixing resistor was disconected? How did you plug? Guitar into bass input NR1, then patch cord between inputs two? As for teh transformers you must be able to find them everywhere, if you do'nt know how to choose a transformer for proper amp rating go with angela, or hammond or someone who would have a 5V "filament transformer"

[corduroyew]

I'm sure that I did it exactly like it is shown in the bassman schematic that I posted. I even took it apart and redid it and it still didn't work. I was plugging the guitar into Bass input 1 and using a jump from bass 2 to guitar 1.


Will that hammond 266M5 work for my filament transformer?

[Satamax]

Yep trany is what you are looking for! Funny how your mod wasn't working? Well, we're too far apart for me to show you. Well, sorry mate, my mistake, you need to add a .1µ on the line going to bass jack #2 tip. Because there's no insulation from DC otherwise and you had the full 260vdc on the grid of normal chanel V1, which must have messed with the bias and other things for sure!

[corduroyew]

Cool. I'll try it with the cap tommorrow. It's getting too late tonight. Thanks for the help, and I'll let you know how it goes.

[corduroyew]

I've done some more modding. Here is a layout of what I've done so far. It's got the AB165 to AA 864 mod and it's got the "how to make your bass chanel sound like the guitar chanel with an extra gain stage" mod that I found on the old forum.



I had changed the balance pot into a bias pot but then found out that the plate current between my tubes was 10ma different! So they were not a very good match and it meant that (for now anyway) a balance pot is the best way for me to go. Because I have to cut into the chassis to add the rectifier tube and transformer, I think I might add another bias pot so that there will be one for each output tube.


I like it the way it is now, but I still want to figure out how to add the unused tube into the mix. I've kinda got an idea about how I might do it but I don't really know. So if somebody could give me some pointers or get me going in the right direction it would be greatly apprecitated. I'd also like to make it switchable so that I don't HAVE to have the extra gain stage all the time but if it's too dangerouse or just a stupid idea then I won't do it.

I've also converted the layout onto the schematic that Satamax gave me with the rectifier tube on it. So this is what (I think) the schematic should look like once I’m done adding the rectifier tube.