Bassman 135 Noise in first Channel.

[DerEber]

Hey Friends,

I need some new inspiration with my current repair.
The first Channel is making noise.

First here is the schematic:

I'll try to state how the problem occures to me and what I already did.

First of all, when I turn the volume of the bass and the normal channel down and only the master up the thing is dead quiet!
So quiet that it realy impresses me.

Now when I dial in the volume of the first (Bass) channel. I get some sort of chrackling noise.
I'll try to record that later.
Sometimes it is fine for a short time right after turning the amp on and then it starts.

The noise is present very quietly in the Normal channel also, but goes away if I pull the Bass-channel Tube.
So it looks to me like the secound channel is affected only due to grounding.

What is realy strange is that the noise nearly disappears if I turn the volume of the Channel all the way up to 10.
Then it is still dependent on the Trebble Pot. But not dependent on the mid and bass pot or the switch.

I might have suspected the pot's, but they go so smooth and nearly no crackling while turning.

I did replace the V1 and V2 catode cap. Changed all tubes. Tapped all resistors and even sprayed coolant spray on the pot's and resistors. No change.
It also seems that it's not the first input resistors, cause I can turn the Mid and Bass down without any change.
I cleaned the sockets. But the unit looks reasonably clean.

PS caps are still original. I want to leave them. Also the last filter-cap is feeding Bass AND Normal channel. So any noise should be the same or simmilar in both channels. I meassured directly on all the PS-caps with the scope and the noise is not meassurable there.

Plus I did some other things I allready forgot  ;D

Any new input for me is welcome!
I realy do not feel like ordering a all Pots and resistors, changing everything and in the end it was something else.

best greetings from Germany!!!
Stephan

 

[mjrippe]

Try replacing the plate resistors (100k 1W) with new metal film types.

 

[DerEber]

Ah... cause it's the most stressed resistor there, right?
I'll try.

 

[DerEber]

Nope.... new Plate Resistor didn't do it. Too bad.

But I had another idea. I grounded the upper side of the pot. (schematic wise)
Now I have noise as soon as I turn the pot away from either end.

So that should be the pot, but wait...now I desoldered the Tap from the pot and the noise is still there.
Now there is only the wire left to blame  ;D

Lets see....

 

[DerEber]

Ok....
without the wire attached to the tube socet (V1B Grind) its quiet.
Now I have noise on the other channel.
Reattached the wire and now noise on both cannels.

So either all Pots gone mad or now it's finaly time to change the PS-Filter Caps....
Isn't it?

 

[DerEber]

Thats how it does.
https://hearthis.at/dereber-ot/135-0011s12/R14/

 

[radardoug]

Is this a Fender unit with the wiring on their custom turret board? Has the unit not been run for a time? Those boards have a problem with moisture retention, try leaving it on for a couple of days and see if that fixes the problem.

 

[PRR]

Now I have noise as soon as I turn the pot away from either end.
Now there is only the wire left to blame  ;D

Right. When noise is worst mid-turn, odds are it is the wire picking up crap. Move it. Shield it. Move any crap-radiating wires in the area. Normally that is only heater wiring. Power light/switch wiring should not be near good audio. B+ filters should be very low-crap; if they are crappy, fix the caps.

A combination of crap in the stage before the pot, and crap in the wire off the pot, can give the odd case of a crap-null at a very narrow point on the turn. Stages generally invert so you can get out-of-phase crap and a null when pot loss equals stage gain.

 

[DerEber]

Hey thanks for your help!

Yes, it is a Fender unit with costum turrent board.

I cleaned the board and shielded this one wire with was also very touch sensitive (tap tap) and left it open over night.

Now the noise is gone!  8)
Hurray!

Without your input I might have started to randomly replace parts. And that would still now have solved the issue.
So thanks again!

Case closed  ;D

 

[andyfromdenver]

Oh looks like you solved it.  I have this amp and have done way above and beyond work on it.  There is potential for crud to be under the board, in the pots, tube sockets.  I shielded signal wires and routed things like the Brownface era.
I should send a pic someday cause I did the top tolex wrap in baby blue and moved the handle to over the power transformers so it carries more balanced.  Also, if you take the grill cloth off the front, it should have a super sweet appearance with holes for ventilation.  Mine looks super tricked out.  I'm personally a fan of keeping 1W carbon comps on the plates.
If yours still has the heater balance pot, I would lose that pot and just put two 100ohms to ground for an artificial center tap on the heater taps.  Those pots burn out.

 

[andyfromdenver]

found a pic from the tdpri days

 

[DerEber]

Wow so cool!!  8)

What do you mean by "Brownface area"?

Greetings!

 

[andyfromdenver]

Wow so cool!!  8)

What do you mean by "Brownface area"?

Greetings!

Thank you!
I mean the lead dress from the late 50s to mid 60s.  The wiring of this era of amps is peak quality for Fender.  Careful attention to wire routing to minimize/ prevent oscillations, noise, etc. 

I also forgot to add the amp actually has a 4ohm and an 8ohm tap on the output transformer.  I did away with the silly switching system and made dedicated single 4ohm and 8ohm outputs.  Good versatility for cabs.