Tweed Disease

[J Rindt]

I am having trouble with a Bassman head I rebuilt for a friend. It exhibits a problem, on both channels, that sounds like "blocking distortion". I may have installed a wrong part, or made some other simple mistake, but I have been over this amp a few times and have not been able to find a mistake like that.
I have played with the power tube grid resistors and and the PI input and coupling cap values. (more on the resistors, less on the caps) It made no change at all. It is a distortion that seems to come on as the note peeks and stays as the note decays.
Is there a test to tell if those black fiber boards (is it vulcanized rubber.?) are bad.?
Thanks

[mac0611]

the only alternative would involve checking the board for continuity between the eyelets. If the board is in good shape and dry it should be ok.  I would start by checking all of your solder joints and connections. Especially in the signl path.  It sounds like a loose connection somewhere or a bad solder joint.

[J Rindt]

Hey mac0611 -
Your right, it does sound like a bad connection of some sort. And I have re-flowed, re-soldered, re-connected, just about everything on the board twice. Tweed Disease was not my first thought. It is kind of my last hope.
However, something I did in this amp that i have never done in a Fender before, was to install a ground buss. Hard to believe that is the problem. But maybe a connection somewhere along the buss.......? Though, I have soldered all those connections a few times as well.
Thank You

[PRR]

Supersonic oscillation. Move the OT primary leads. Move the tone-stack leads. If it gets worse, move the other way.

[J Rindt]

Supersonic oscillation. Move the OT primary leads. Move the tone-stack leads. If it gets worse, move the other way.

I have played with lead dress quite a bit, but I will take another shot at this. I have nothing to loose.
Thank You

[phsyconoodler]

What model bassman is it? I have had exactly the same symptoms on bassmans that have none,or too small grid stopper resistors to pin 5 of the power tubes.
  Usually Fender put 1.5k right on the tube sockets from pin 1 to pin 5 and the grid wires hooked to pin 1.
I take those off and use at least a 2k,1/2 watt resistor directly on the tube socket,pin 5,and put it up in the air like an antenna.Hook the driveline grid wires to the top of the resistor.
  I've had to go all the way up to 100k on some amps to cure the oscillation that gives the distortion artifact on note decay.

[jojokeo]

  I've had to go all the way up to 100k on some amps to cure the oscillation that gives the distortion artifact on note decay.

Didn't that affect the tone and high end response?

[phsyconoodler]

Up to 100k it doesn't seem to make much difference.Haven't needed over 100k so I can't comment beyond that.

[J Rindt]

I mostly used the AB165 schem with the AA864 NFB. I have 4.7k on each of the power tube grids. I do not have the resistor up in the air like you do though. I ran the grid wires as a twisted pair, then connected the 4.7k right to Pin 5. I thought it might be worse to have the grid stoppers up in the air "antenna style", but it does seem to be a common way of doing it these days.

What model bassman is it? I have had exactly the same symptoms on bassmans that have none,or too small grid stopper resistors to pin 5 of the power tubes.
  Usually Fender put 1.5k right on the tube sockets from pin 1 to pin 5 and the grid wires hooked to pin 1.
I take those off and use at least a 2k,1/2 watt resistor directly on the tube socket,pin 5,and put it up in the air like an antenna.Hook the driveline grid wires to the top of the resistor.
  I've had to go all the way up to 100k on some amps to cure the oscillation that gives the distortion artifact on note decay.

[phsyconoodler]

I never twist the grid wires.That may be an issue...or not.
try upping the grid stopper resistors to 100k and see if it makes a difference.If yes,then you know you are on the right track.If not then you have a lead dress or cold solder issue.

[J Rindt]

I never twist the grid wires.That may be an issue...or not.
try upping the grid stopper resistors to 100k and see if it makes a difference.If yes,then you know you are on the right track.If not then you have a lead dress or cold solder issue.

10-4
I will try this as well.
Thank You