Twin Reverb BF 1991 PRE186 with Bad Static Noise

[BeeMan1024]

Hi Hi ...

Got a 65 Twin Reverb (1991) PR186 with bad static crackling noise ..

So to help everyone shorten their reading time here is what I have already did :

1. Crackling sound off standby switch without any plug in. Volume knob does not affect crackling
2. Clean all 10 control POTs and 4 input sockets
3. Replace R33 with dry soldered joint
4. Reflow solder on all tube sockets
5. Retension and cleaned all 10 sockets, no crackling when rattling tubes .. Good.
6. Replace all screen resistor with 1W R1.5k on V7 to V10
7. Replace all Plate R470 ohm with 1W on V7 to V10
8. Remove Output power tubes V8 and V9 to half the output.
9. Remove V1 ... just so to test CH2 only.
10. Change all 6x Preamp and 4x Power Tubes and re-bias to 65%. Tested and no microphonic fault.
11. Further chopstick poking with the amp ON ... did not find any loose components or loose ribbon cables.

Tested the Amp and sounded really sweet late into the evening ... happy and slept well. 🙂

Played again in the morning .... the damned static crackling noise just came back ... pops occur at random and not regular.

I am pulling my hair out because I could not find where the fault is .... 😕

Do I need to go into the doghouse to look at the caps ... or change out the plate resistors pre-amp tubes ?

Any help will be greatly appreciated ...

Thanks in advance ...

 

[Enzo]

Instead of throwing parts at it, isolate the problem.

Do you have a scope?

You pulled V1 with no help, so pull V2 as well, still noise? Turn reverb to zero, still noise? Pull V4, still noise? Pull V6, still noise?

personally I'd be suspicious of R42, the 82k plate resistor of the phase inverter. But it could be many things. You need to isolate it to one stage.

 

[29285]

Instead of throwing parts at it, isolate the problem.

Do you have a scope?

You pulled V1 with no help, so pull V2 as well, still noise? Turn reverb to zero, still noise? Pull V4, still noise? Pull V6, still noise?

personally I'd be suspicious of R42, the 82k plate resistor of the phase inverter. But it could be many things. You need to isolate it to one stage.

Cheap carbon composite anode resistors are a candidate.

 

[Enzo]

As far as I know all the resistors in the amp are film type other than power resistors.

 

[BeeMan1024]

Hi ...

Greatly appreciate all the help here ...

Well ... yes .. its both the anode resistors that are causing the crackling and intermittent operation ...

Change out R4 , R11 , R15 and R22 for V1 and V2 (12AX7) with 1W version.

Finger crossed ... Power On and ta-da no more crackling ...

When I was at it ... change out R33 too for V4B.

Apologises ... I do not own a scope anymore ... work bench accident ... broke the screen.

Hopefully, this help the rest who is troubleshooting their Twin Reverb for crackling noise issues.

Thanks again for the swift reply ...