Help sorting out whats going on with heavily modded bassman

[Lynxtrap]

It's a bit late to chime in now, but the best thing to do with bias balance in SF Fenders is to add voltage adjustment. Bias balance is not a bad thing, so it's great to have both.

[john l]

Having a 100% matched tubes isnt really that important to me right now just having a regular adustable bias is fine but thanks. Now Im pretty much trying to correct any obvious mistakes to so I have a nice stable platform to work from. If anyone wants to bring anything to my attention Id really appreciate it. Thanks very much.

[Lynxtrap]

I wouldn't worry too much about lead dress unless there's a problem with it.
The only unusual in there is the extra board, as far as I can see.
You haven't mentioned the filter caps underneath the chassi? I would check those while at it.

Otherwise, if it sounds good, it is good

[john l]

Im waiting for new filter caps to come in the mail. Its been recapped with the same orange spragues that were on the board so I know its been recapped at least once, most of the values have been upped considerably from stock but I actually like a lot of filtering on the mains and screen filters so Im not really in hurry to change much just replace whats there with newbies I guess.

I started messing around with the bass side today by pot mounting its tone stack like I did the normal channels and changing the values around a little to be close to stock (.022s-100k slope-250pf trebble cap) One thing Ive like about almost every bassman Ive tried is that the cleans arent brite and chimey like most fenders, they have this dark and chewy thing going on that my ears dont get so tired of so I wanted the brite switch to have a more full frequencey boost, tried out a bunch of different cap values accross there and ended up going with a 1ooopf. Finally I tried a .68uf bypass cap on the first gain stage and over all Im liking what Ive got going on now very much.


As it stands I guess Ive taken the same route most modded bassmans take with a channel dedicated OD and and stockish clean side that I can A/B . Now if I could just make it as pretty as what most of you guys build Id be stoked lol.

[john l]

So quick question about the skinny purple wire the bundles the filter cap wires on the pot side of the board. I've noticed that its grounded on both ends. Why is this? Isn't it only suposed to be grounded on one end if its for shielding?
I also noticed the bypass caps that are under the bonus board don't go to the ground by the bass channels bass pot directly in front of them. I have two black wires "Y" from where the bypass caps apex, one goes to the left lug on the normal channels bass pot and one goes to a ground by the input jacks.

[Firestorm]

So quick question about the skinny purple wire the bundles the filter cap wires on the pot side of the board. I've noticed that its grounded on both ends. Why is this? Isn't it only suposed to be grounded on one end if its for shielding?
I also noticed the bypass caps that are under the bonus board don't go to the ground by the bass channels bass pot directly in front of them. I have two black wires "Y" from where the bypass caps apex, one goes to the left lug on the normal channels bass pot and one goes to a ground by the input jacks.

It probably doesn't matter, Fender grounds are almost random to begin with. They either wanted to connect two ground points together, or (more likely) needed two places to solder the ends of the wire so it didn't unravel. It is a shield, intended to keep post-rectifier buzz on the main filter run from getting into the jacks and tone controls. As to the "Y" ground: hard to say if it's factory or done at the time of the mods. If you have unwanted AC potential between two ground points, the lazy man's fix is to connect them together.

[john l]

So please bare with me, as I told you guys that I have plenty of holes in my game here. Id like to rewire the PI section of my amp to the typical BF (AA864)circuit but obviously theres some circuit differences along with some under the board wires that are completely throwing me from doing this.

Anyone with some serious patience for idiocy or amazing drawing kung fu want to chime in lol.

Heres the layout of my amp
http://www.prowessamplifiers.com/schema ... layout.pdf

And here is where I want to go
http://www.prowessamplifiers.com/schema ... layout.pdf

[john l]

I have no problem changing the values but as far as the wiring goes...
Initially I want to say that I should ditch the .1 that apexes with the .01 comming off pin 2 of the PI and replace it with a jumper. Then I want to come of pin 7 and break joint where the 22k and .1 meet, run both of those leads to the right side of the NFB reisistor and ground it with a 100ohm resistor at the bypass caps.

How far off am I here and what the hell is that long under the board wire going to the 220k doing?

[Firestorm]

In both models, the eyelets for the PI are in almost exactly the same places, so you can simply remove everything and rebuild that section according to the AA864 layout. But, as you've noticed, there are significant differences before the PI as well. In the 864, the Normal Channel has two gain stages feeding the PI and the Bass Channel has three; in the 371 (and in the other Silverfaces going back to the AB165), both channels share that third stage and mix there. The long wire going to the 220K doesn't only go to 220K. It first goes to the plate of the third stage, so it carries the mixed channel signal from the third stage to the PI input. The wire between the third stage plate and the 220K is a feedback loop around the third stage.

So you have to decide if you want to keep the third stage channel mixer from the AB165, AC568, AA270 and AA371; or revert fully to the AA864 circuit. How you wire the PI depends on that decision.

[john l]

Thanks firestorm. Since Im dealing with a modded circuit I have no idea how that will effect what I have going on now. What are the sonic trade offs supposed to be by doing the 864 PI conversion on silver faces? Where would that wire and resistor want to go if I went through with the conversion? It seems since all the mods are in the preamp that everything should still work if I go this route.
Thanks very much man.
EDIT
Hey also since your here. There is a .01 across V1s plate resistor, a 500pf across V3s plate and a 100pf across the NFB resistor. Are these supression caps for ocsillations? If so are they typically removed when blackfacing?

[Firestorm]

Oh yes, that amp ... no clue what it will sound like with the blackface PI, but here's how to do it:

Like I said, the eyelets are in the same places, so just remove the PI components and rebuild it like the AA864 EXCEPT for these differences:

1) on the 864, there are two 220K resistors forming a "V" and marked "x" and "y." Those will not be there. The long wire you mentioned will still go to the point marked "y," followed by your coupling cap.

2) the 864 coupling cap is 500pF; the 371 cap is .01. You may want something between the two. You can experiment with this.

3) feedback on the 371 comes into the inverting side of the PI; on the 864 it goes to the non-inverting side so one is 180 degrees out of phase with the other. When you convert to the 864, you will have to switch the green and black wires going to the speaker jacks to keep the amp from squealing. Alternatively, you can switch the blue and brown wires going to Pin 3 of the output tubes. Either way, not both.

[Firestorm]

EDIT
Hey also since your here. There is a .01 across V1s plate resistor, a 500pf across V3s plate and a 100pf across the NFB resistor. Are these supression caps for ocsillations? If so are they typically removed when blackfacing?

The .01 and the .0005 cut high frequency gain on those stages. The .01 is on the Bass Channel, so its cut is more pronounced. These probably are there to cure parasitic oscillations. If they're still there, you should probably leave them. The .0001 across the feedback resistor lets high frequencies feedback unattenuated and is also probably an oscillation fix. Leave this one out for now. There are other cures if necessary.

[john l]

Oh yes, that amp ...

LOL Yeah "that" one . Okay here comes the idiocy I warned any who were nice enough to help me with this about.

[IMG:799:449]http://img.photobucket.com/albums/v299/ ... 39_657.jpg[/img]

So your saying that I should leave the wire under the board where its at and proceed bulding the PI. I ask because "y" is an empty eyelet on my board and "X" is the right side of my NFB resistor.... I think. Was the way I initially mentioned wiring it correct?

[Firestorm]

Sorry, the Fender layout shows the "y" eyelet used for this rather than the "x." But it doesn't matter. Use whichever eyelet has the long input wire going to it. In adding the AA864, PI to this amp, one of the two will wind up empty.

It really is easier to unsolder all the PI components and then look at the empty eyelets and see how they correspond to the 864 layout.

[john l]

Sorry, the Fender layout shows the "y" eyelet used for this rather than the "x." But it doesn't matter. Use whichever eyelet has the long input wire going to it. In adding the AA864, PI to this amp, one of the two will wind up empty.

It really is easier to unsolder all the PI components and then look at the empty eyelets and see how they correspond to the 864 layout.

Im with you and Ill deffinately strip that part of the board to rebuild it but Im just making sure my ducks are in a row ya know.

If I removed the .1uf cap comming off pin 2 and carried the .01uf caps lead over to the right side of the NFB resistor and then moved pin 7s .1uf cap and 22k resistor's apex to the right side of the NFB Resistor as well that would be correct right? All that would be left to do is to remove the yellow wire that went from the .1uf/22k apex to ground and add 100ohm resistor from the bypass cap ground to the right side of the NFB resistor. Im sure I totally butchard this question but Ill throw it at you any lol.