Blackface Fender Twin rattle

[WholeToneMusic]

I have been de- modding a Blackface Twin Reverb That had some mods done to it back in the late 80's when people were getting there Fenders "BOOGIED". An extra gain stage was added and the vibrato circuit was gutted, along with a presence control addition and a Master Volume. I re-assembled the vibrato circuit which is working fine now. I have been careful to make proper lead dress on the layout. Also i put in shielded coax from input to grid tying the shielded ends to the plate of the same tube. All other coax implemented is to ground to help minimize hum etc.

The problem I am having is that on some of the low notes i am getting an annoying speaker rattle. Sounds like the speaker is blown, Primarily the A string F# note and the E string G notes. When i turn the volume and bass over 3 it starts in on both channels. It almost sounds like a built in snare drum. I have changed out speaker cabinets, cables, guitars,Power tubes and preamp tubes etc. with no luck. That rules out the above. I noticed that on a Super Reverb of the same era that when you max out the volume and bring up the bass it still gets flubby but not as bad as this. Does anyone have any advice that could help remedy this low frequency oscillation? Any help would be much appreciated.

Best Regards,
Brian

[Chuck H]

Tracing the problem would be alot easier with a scope. Since you've tried it through another cab there is almost certainly something happening in the circuit to create that rattle "effect". It's called a parasitic oscillation. A scope would help to pinpoint the problem. But it can still be another part of the circuit "talking" to that area of the circuit. Parasitics can be tough to find sometimes.

You mentioned that your layout is "blackface", but no blackface amp ever left the factory with the input lead shielded at the "plate of the same tube". FWIW, at best this is like putting a small capacitor between the plate and grid. It will dampen high end and dull the classic "Twin Reverb" sound. At worst you could have the shield linked to the other triodes plate (two triodes in "that" tube). That would create a positive feedback that will almost surely cause an oscillation. You should remove the shield from the plate and test. If you like you could ground that shield instead.

So...Since that particular shield trick is uncommon to a blackface amp, I have to wonder what else may actually be different. Be double sure on your lead dress and grounding scheme. If you can"t fix the problem in this way, and you don't have a scope, You may want to take it to a tech that does have a scope. It's a great amp, and you've taken it this far from where it was. But it might be time to let a pro with more experience put the polish on it.

Chuck

[WholeToneMusic]

Follow up

I wanted to do a follow up on the 65' Twin Reverb restore. I re-did some of the lead dress and then I ran a signal generator into the amp. I wanted to target the lower frequencies so i swept through the troublesome notes that was causing the rattle. I took an oven mit and firmly grasped each tube when I could here the rattle. I found that V2 preamp tube was causing the oscillation so I swapped it with another tube and all is well. Here is a pic of he before and after of the amp. Thanks for the input Chuck, Much appreciated! Also I took out the coax wire ( even though I think it is lowers hum in the amp.) Your right it didn't come from the factory like that, Probably because they didn't know that it would reduce hum at the time. I have seen some Fender's that Paul Rivera did when he was at the Fender factory that had shielded inputs.

Best Regards,
Brian