BFDR with crackle in the Vibrato channel driving me nuts

[bnwitt]

I think this latest blackfaced Deluxe Reverb AB763 is going to get my goose.  I have been chasing a crackle at high output in the vibrato channel for hours now.  The normal channel is fine.  Nice and clean with just a little tube distortion when cranked.  The vibrato channel has a crackle with way more distortion and slightly lower output when cranked.  I've disconnected the reverb and vibrato circuits, I've pulled V3 and V5.  I've changed ever single component in the normal channel circuit including cathode resistors and capacitors on V2A and V2B.  I've changed the volume, treble and bass pots on the channel.  I've changed every coupling and tone cap.  From the grid of V2A to the 3.3meg/10pf RC at the grid of V4B all components are replaced.  All solder joints have been checked.  I've checked for wire wiskers everwhere. 

All voltages are in range except on the V5 Vibrato oscillator tube which should be 270vdc and 345vdc on pins 1 and 6 respectively according to the schematic.  I'm getting 340 and 427 with the vibrato off but I don't think that is an issue.  I can't get it to do it with my signal generator so, I can't trace the origin.  It only does it with a guitar.  I'm sure that is due the combination of string frequencies from the guitar pushing the preamp as opposed to a single frequency sign wave from the generator which can't drive the preamp as much.

The amp is quiet on idle, the reverb works fine and so does the tremolo (vibrato).  Just this crackle at high input on the normal channel

[phsyconoodler]

I just finished a 100 watt amp that had the same problem.I agonized over it and tugged on everything looking for a loose component.I finally decided to fire up my 100 watt Weller pistol iron and touch up all the solder joints.Well it worked.Crackle gone.
  I bet it was a flux bubble on a joint in the power section.With the extra heat I found it and cured it.
I know it's very hot,but I didn't have to stay on any joint for long.

[dwp]

i have a 67 DR, intermittent crackle, only at high volume. First time I located the problem and solved it, was loose connection/bad contacts at the RCA jacks. I would squeeze the tabs on the RCA plug, twist, de ox, fixed. second time the exact same crackle, loose preamp tubes. re-tensioned, problem solved.....not for long. I'm now thinking I need to replace the tube sockets. Or....neither item above is the problem.
you try twisting input tubes? shields on? shields off?

hope you prevail......

[tubeswell]

Hi Barry

Have you tried re-tensioning the socket pins for that channel?

[bnwitt]

Thanks guys.  Man I've resoldered every dad gum solder joint in this amp at least 3 times each.  I've jiggled every tube and wire, I've done more chopsticking then a Beijing restaurant all you can eat buffet crowd.  I've definitely narrowed it down to the vibrato channel.  It doesn't do it on the normal channel so I'm assuming it's not in that channel, the PI or the power section.  I've had the chassis out of the cabinet and plugged into a separate cab and it still does it so it's not mechanical.  I am at my wits end, no pun intended.

[bnwitt]

Tubeswell, I'll try that and see if it helps.

[bnwitt]

Hello everyone.  Been out with some health issues for the last couple of weeks and am glad to be back.  Here is where I am at on the BFDR crackle noise.

The amp's normal channel sounds fine always.  The amps vibrato channel, however, sounds fine unless I plug in the reverb pan's "In" cord.  Then I get a crackling noise in the output signal when driving the amp hard.  This amp is an exact replica of an AB763 Fender layout BFDR except for one thing; and that is I am using the original reissue style 1/4" TRS jack and foot pedal for switching the effects instead of the two RCA jacks and plugs like the original BFDR.

I have replaced the reverb trannie, swaped tubes, rerouted wires put two original style RCA jacks on the back panel like the original BFDR in leau of the PCB jack assembly that came with the reissue, all to no avail.  If I plug in the "Out" reverb pan cord by itself, everything is still fine.  It is only when I plug in the "In" cord that I get the noise and it is there even if the "Out" cord is unplugged.  I am now thinking about replacing the 1/4" TRS jack with two RCA jacks like the original to see if that helps.

Has anyone ever experienced this before?

Barry

[tubeswell]

Hello everyone.  Been out with some health issues for the last couple of weeks and am glad to be back.  Here is where I am at on the BFDR crackle noise.

The amp's normal channel sounds fine always.  The amps vibrato channel, however, sounds fine unless I plug in the reverb pan's "In" cord.  Then I get a crackling noise in the output signal when driving the amp hard.  This amp is an exact replica of an AB763 Fender layout BFDR except for one thing; and that is I am using the original reissue style 1/4" TRS jack and foot pedal for switching the effects instead of the two RCA jacks and plugs like the original BFDR.

I have replaced the reverb trannie, swaped tubes, rerouted wires put two original style RCA jacks on the back panel like the original BFDR in leau of the PCB jack assembly that came with the reissue, all to no avail.  If I plug in the "Out" reverb pan cord by itself, everything is still fine.  It is only when I plug in the "In" cord that I get the noise and it is there even if the "Out" cord is unplugged.  I am now thinking about replacing the 1/4" TRS jack with two RCA jacks like the original to see if that helps.

Hi Barry

Glad to hear you are feeling better. Since it only happens when you make a circuit with the secondary of the RT, I wonder whether its the RT itself that is shorted/shorting, or something to do with the grid leak resistor for the reverb driver tube?

[bnwitt]

TW, I'm on my second RT and the problem continues.  I haven't checked the grid leak resistor on the Driver tube but I'll do that.  Thanks for the feedback.

[jojokeo]

Hi Barry, glad you're feeling better also.
I'm curious if you have another reverb tank to plug into? I'm sure you switched or flip-flopped chords right? I'm not clear if you get it when you just plug the chord in w/out the reverb tank and it's okay. Then you plug the tank's input in and then you get it. It may be something to do w/ the reverb tank and it's drain or draw? They have those little matching transformers on them too. Something could be going on side the tank maybe?

[bnwitt]

Jojokeo,
Thanks for the kind words and the help.

I have tried a brand new pan as well as the pan that came in the amp.  Different cords too.  It doesn't happen if the "In" cord is not plugged into the pan but it is plugged into the amp.  So, the only way to make it do this is with a cord plugged into the amp's "In" jack and also plugged into a pan's "In" jack.  It doesn't matter if the Reverb control is on zero or any other setting either.  It only does this when the amp is at high volume along with the guitar.  It doesn't do this on the normal channel at all.  I'm a little perplexed as to how this could happen when only the reverb send connection is made between the amp and the pan without the return signal being connected from the pan to the amp.  The only thing I can figure is something is happening to V3 when the input cord and pan are plugged in and the crackling is making its way from V3 back through the 500pf capacitor and the 10pf/3.3m capacitor/resistor assembly into V4B,

[bnwitt]

This is from Enzo on the Music electronics forum:

"Sounds like your hard driven signal is clipping the drive stage, and that might be coupling through the B+ or ground return or even radiating."

I'm using a Hoffman back of pot grounding system with the 4 board grounds going to it, so I guess I might have to move those 4 board grounds to individulal chassis spots and see if that eliminates the problem.

[bnwitt]

Well I separated all of the board grounds, soldered ring terminals on the wires and them screwed them individually to the chassis right near each board connection.  I still have the same problem.

I'm racking my brain with this.  It just doesn't make sense that if I unplug the "In" cord the noise goes away and with it connected it's there.  What does having the "In"cord and pan input hanging on the reverb trannie jack which is connected to the B+ and driver tube (without a return signal from the pan) change in the circuit to turn the noise on and off?  If the Reverb driver tube is clipping and being picked up through a shared ground or B+ why doesn't it keep doing so without the "In" cord and pan "In" connected to the jack?

[tubeswell]

 I still have the same problem...  What does having the "In"cord and pan input hanging on the reverb trannie jack which is connected to the B+ and driver tube (without a return signal from the pan) change in the circuit to turn the noise on and off? 

This is what got me thinking about the interaction of the Reverb driver stage with the vibrato channel. When you plug the RT cable in, you're completing a circuit (not that one should run any type of output tranny without a load). And I thought "What part of the RF driver circuit could cause interference with the vibrato channel?" and that's when I started thinking about the load on the TS recovery stage of the vibrato channel. Hence I thought of the 12AT7 grid leak resistor. Anyway - did you check that out?

[plexi50]

Have you checked all the preamp wires to be sure there is not one of them broke inside the insulation and making an intermitant arching connection ? This drives me nuts especially when its all new parts including wire. Could there be a manufacturer error in the wire?

[stingray_65]

Trouble shooting is not my best skill, in amps at least.

This may sound way out, BUT was the original reverb tray original? and did you replace it with the same model as what was in there in the first place? so maybe you replaced the problem with the same problem?

I understand you're an experienced amp tech and likely replaced the reverb pan with what should be in there regardless of what was in there.

I'd be curious too, what if you soldered an 8ohm resistor to  an RCA plug and plugged that in to simulate a reverb pan, like a dummy load.

It wouldn't be reactive like the reverb pan changing impedance with frequency.

Good luck with you're quest.

I'll be happy when you have you're AH HA! moment and share the key to this mystery.

I'm also happy to hear you're doing well, Don't let the stress of this crackle get to you too much, the answer to this problem will be simple and straight forward, It always is.

Ray

[bnwitt]

Plexi, I'd have to rule that out sense everything is fine unless I load the reverb driver tube with the pan.

Stingray, I've put in a new 4AB3C1B pan from my stock which I believe is the correct pan for the BFDR.  I may try the dummy load on the pan "In" jack.

I always seem to have issues with the Fender AB763 amps.  I never have issues with the Hoffman AB763 amps.  Maybe my aha moment will be to change over.

[Fresh_Start]

I always seem to have issues with the Fender AB763 amps.  I never have issues with the Hoffman AB763 amps.  Maybe my aha moment will be to change over.

The 2 big differences between the two are the tremolo and the grounding scheme.  Please don't be offended, but I'm not sure the ring terminal approach described would solve a grounding issue.  

I'm racking my brain with this.  It just doesn't make sense that if I unplug the "In" cord the noise goes away and with it connected it's there.  What does having the "In"cord and pan input hanging on the reverb trannie jack which is connected to the B+ and driver tube (without a return signal from the pan) change in the circuit to turn the noise on and off?

I didn't understand the second set comments until I re-read them.  

Reverb pan 4AB3C1B has an insulated input.  http://www.accutronicsreverb.com/

One thought is that somehow connecting the pan to the reverb transformer is injecting noise into the power supply.

Something that bothered me in both AB763 circuits was having the reverb transformer supplied with B+ from the poweramp but grounded almost at the other end of the chassis.  Can you experiment with a separate dropping resistor and filter cap for the reverb trannie, grounding both the filter cap and reverb transformer with the rest of the reverb send circuit?  IOW make that node on the power rail part of the preamp instead of part of the poweramp.  That's one of the things I did on my 3rd Blackface reverb build and it was the only one which didn't have squealing reverb problems.

Also, isolating the reverb power supply node from the screen node also isolates it from the tremolo.

I've never experienced a bad power supply filter cap, but could the screen node cap be part of the problem?

Don't know if this will help at all.  Just grasping at straws frankly... your comment about not having trouble with Hoffman style AB763 builds got me thinking about the differences.

Chip