Help sorting out whats going on with heavily modded bassman

[Firestorm]

Also should I clip that jumper on the output jack, I seem to have continuety on every terminal on it???

The jumper grounds the hot terminal when nothing is plugged in. Some protection against running the amp into an open load if you forget to connect your speaker. Leave it alone.

[john l]

Still can't see where the feedback wire is connected. Didn't know about the resonance pot. You should pull that out now since it's connected to the output and probably screwing things up.

done uploading pics. What about that jumper on the jack connecting the center and hot lugs?
[IMG:799:449]http://img.photobucket.com/albums/v299/ ... 20_593.jpg[/img]

[Firestorm]

Leave the jumper alone (see above). How does the amp sound now? If you have high frequency issues and don't want to rewire everything, you'll use some "crutches."

1) Put a 47pF to 100pF cap across the 82K and 100K resistors on the PI. This will phase cancel high frequencies.

2) Shorten the grid wires feeding the output tubes as much as you can.

3) To make them really short, move the grid stopper resistor on the output tubes off Pin 1 and connect the resistor and the lead feeding it right to Pin 5 (heat shrink it for insulation and stability). You can also turn the output side of the PI "upside down" so the 82K/100K Rs are at the top and the 220Ks are at the bottom. You will need longer plate wires. (Sometimes Silverface to Blackface conversions benefit from changing the 220K "bias feed" resistors to 150K, but this is more interesting on 6V6-based amps than on 6L6 ones.)

4) If you still have problems, put the 2000pF grid to ground resistors back on the output tubes the way CBS had them originally. Gerald Weber says this kills tone, but I know how loud Gerald plays and I guarantee you his ears cannot hear it. Mostly "dog ear" frequencies are affected.

[john l]

Thanks Ill start in after I handle a few things around here and report back. Should i do all this at once or once at a time to keep from doing going over board with supression?

[Firestorm]

Always one thing at a time. Make sure what you're hearing is a problem rather than a "feature" designed in from previous mods.

[john l]

heh?
[IMG:799:449]http://img.photobucket.com/albums/v299/ ... 52_262.jpg[/img]

So I moved the 220ks to the bottom of board, put the 82k on the top right and the 100k on the top left then swapped the brown and green wires that met them at their apexes. I didnt want to have the grids too tight but I did manage to chop at least 2"s off both the wires while at the same time lengthend the plate wires. Ive also read webers books and this is supposed to add inductance which is good for killing parasistics.

Its weird whatevers going on, the amp now seems to stablize after being played for a little. When I first flip the stand by to play I can make the amp totally wig out by playing with that pot for the extra 12ax7 as well as playing with the trebble pot but after a few minutes it simply stops.

[john l]

Well I give up, the amp apears to be completey stable at all but the highest settings now with none of the initial power up isses I mentioned. Typically I dont sit in the problems fixing themselves camp but all Ive done is shorten some additonal grids in the preamp (very slightly) and chopstick around to see if I could find a bad joint anywhere so I doubt its that.

The only thing Im still not a 100% on is whether or not I should have a 220k in one of the empty eyelets like it shows in the layout. I see theres a 220k the run hits on the other side of the board, is that why wouldnt need to add it at the PI side?

Over all I think I like this tweek very much as the feel of the amp at overdrive is unexpectedly nice. There seems to be a little less low end now or it could be more highs present?....dunno. Maybe this is due to the absence of the .1 and changing the coupling cap from .oo1 to 500pf.

[Firestorm]

The channels are mixed earlier so there's no need for the 220K over on that side. Go ahead and experiment with different coupling cap values if you want to add some bottom: .001, .002 and so on. Gerald Weber used to suggest .01 and even .02 which makes the bass overwhelming. I wouldn't go that high.

[john l]

The channels are mixed earlier so there's no need for the 220K over on that side. Go ahead and experiment with different coupling cap values if you want to add some bottom: .001, .002 and so on. Gerald Weber used to suggest .01 and even .02 which makes the bass overwhelming. I wouldn't go that high.

Thanks Ill give that a shot and see if I can steer it a little more to my liking. Also what does that 22k hitting the NFB resistor do and what effect would changing its value have?

You really have been extreamly kind helping me as much as you have Firestorm Im sure its frustrating trying explain the basics to someone with little working knowledge.

[john l]

Yep thats deffinately where the low end went thanks for that. I have a .002 there now and like it alot better. I wish my decade box went down to these lower values then I could cruise through it faster

[Firestorm]

Also what does that 22k hitting the NFB resistor do and what effect would changing its value have?

The 22K is the tail resistor for the PI; the feedback resistor connection there is mostly coincidental (most of the feedback signal enters at the grid, but a little signal comes in that way, too). Changing the tail resistor affects the balance of the PI, which in turn affects harmonic distortion. You could put a 15K there, even a 10K (or 27K for that matter), but the results will be very subtle. If it ain't broke ...

[john l]

I was just checking out the 6g6 layout and noticed it had 6.8k there and was wondering why is all.

[Firestorm]

I was just checking out the 6g6 layout and noticed it had 6.8k there and was wondering why is all.

Really 11K5 since it sits on top of a 4K7 NFB shunt resistor.