Hum in a GA-19RVT.......

[Jack_Hester]

I picked up a Gibson Falcon, cheap, that had been some cannibalized.  All the tubes were gone, along with knobs, back panel, and original power cord.  The grounded cord on it had the ground lead clipped on the chassis end.  It was not a good cord, so I replaced it.  I installed all the tubes and powered it up on my lamp limiter.  I've been working from a schematic the same as this one:

http://www.el34world.com/charts/Schematics/files/gibson/ga19rvt.pdf

It became obvious that it needed a cap job.  I replaced all electrolytics in the power supply, and on the cathodes that had them, with the exception of the one on pin 9 of V2.  Just overlooked it, but will get back to it.  Fired it up again on the lamp limiter, and as I progressed up in bulb wattage, a very noticeable hum became evident.  The amp worked and sounded great, when I got it up to full current.  The amp had no footswitch, but Reverb worked.  Just weak.  The tank appeared to be the newest thing about the amp, as it had no oxidation on it, like the rest of the components.  I swapped it out with one that I had from a Hammond organ, and it came alive.  Sounds really good.  

Next, I installed a footswitch that was a cheap offshore model.  This is when I noticed that the Tremolo was not wired like the schematic.  I corrected that and fired it up again.  It now has a really nice sounding Tremolo.  Turns on and off as it should, along with the Reverb.  That's when the worst of the hum disappeared, when I turned the Reverb off (grounding C23).  The same hum is in the background, but very subtle.  

Thinking that it may be coming from the Reverb, I disconnected the output of the tank.  No change.  I replaced C23, just for grins.  No change.  Lifted pin 6 of V2.  No change.  I did notice that the schematic called for the CT of the 6.3v winding to be connected to the 6V6 cathodes.  It was tied to ground.  I lifted it and connected to the cathodes.  No change in hum.  Let me clarify.  I'm leaving the Reverb on, as to get the max hum while troubleshooting.  So, I lifted the CT, taped it back, and created an artificial ground, tying a couple of 220 ohm resistors from ground to each 6.3v lead.  No change in hum, but now I have a very noticeable hiss.  

I pulled V1, to eliminate those signal sources.  Same hum.  I know I pulled V2, but can't remember any change.  I've done too many things and need to step back.  

That's where I stopped today.  Will go back and remove the artificial ground and tie the CT to the 6V6 cathodes, as drawn.  I'm going to work on this until the problem is solved.  

I don't have any plans for this amp other than I know a young Pastor that has a Church about 20 miles from here who is in need of an amp.  My wife and I attended a Relay for Life meal and song service at his Church, and he and his wife are fantastic musicians.  He had an acoustic bass and an acoustic guitar, both with pickups.  But, no amp and you could not hear his playing very well.  Though well enough to know that he has talent.  His wife plays the violin.  So, I'm thinking this will make a good addition to their instruments.  

Anyway, if anyone has suggestion as to what to look for,  I would greatly appreciate it.  It's been raining each day, after work, so I can't mow.  Makes for some good shop time.  If I chance onto the problem, I'll post it in this thread.  Thanks for any and all help.

Jack

[Series_of_Tubes]

Check the PSU grounding.  I''ve seen multiple Gibson amps from this era where the filter cap negative leads follow a thin brown (or white) wire all the way back to the ground on the input jack.  If that wire is still there, cut it and ground the filters to a power transformer bolt.

[tubenit]

Unless I missed something, it appears that that there are only 3 caps on the B+ rail.  I would consider adding a 4th one for the reverb only in an effort to reduce hum. 

I'd use Hoffman's grounding scheme with a buss wire across the back of the pots. 

I'd check to see if there is a leaky cap prior to a pot that may be causing hum.

With respect, Tubenit

[Jack_Hester]

Check the PSU grounding.  I''ve seen multiple Gibson amps from this era where the filter cap negative leads follow a thin brown (or white) wire all the way back to the ground on the input jack.  If that wire is still there, cut it and ground the filters to a power transformer bolt.

This amp had two multi-caps.  One had two 20uf and the other had two 10uf. 

The two 20's were grounded to an empty terminal on one of the 6V6's, along with several other wires.  The PT is grounded to one of the transformer studs, but no other wires attached there.  My plans are to run a jumper from there to the 6V6 terminal.

The two 10's were grounded to the input jacks, only.  I removed this when I removed the multi-cap. 

I used a canned cap that had two 20's and two 10's in it.  I will get a multi-cap with leads, to replace this, when the rest of the bugs are ironed out.  I ran a ground jumper between the two input jacks to the chassis, as this is where the signal ground is attached.  The single common in the canned cap took care of the PS filter common. 

Unless I missed something, it appears that that there are only 3 caps on the B+ rail.  I would consider adding a 4th one for the reverb only in an effort to reduce hum. 

I'd use Hoffman's grounding scheme with a buss wire across the back of the pots. 

I'd check to see if there is a leaky cap prior to a pot that may be causing hum.

There's a 10 up at V4, and a 10 up at V1, on the schematic.  Good advice on the pot buss wire.  That will happen the next time I have it open (this afternoon, I hope). 

My next troubleshooting move is to lift the signal wire from the pre-amp, but leave the signal wire from the Reverb attached.  That will isolate the two and hopefully prove the hum origin.

Just got a call, as I'm at work.  I'm off the clock, but that still doesn't stop the calls.  More later. 

Thanks for the input, and will be back with an update this evening.  Have a good one.

Jack

[Jack_Hester]

Here's what I accomplished this afternoon:

http://www.el34world.com/charts/Schematics/files/gibson/ga19rvt.pdf

1 - Connected a jumper from the PT post where the CT is connected, to pin 1 of V6, where a number of signal grounds are connected.

2 - Connected a jumper from this same post to the screw that I'm using as a star ground, on the other end of the chassis.  This is where the input jacks and the monitor jack are grounded.  

3 - Made a buss wire connection across all the pots, except the Tremolo Frequency pot, as it has no ground connections anyway.  Just the wiper and the #1 leg are connected.  #3 leg is open ended.  Connected this buss wire to the star ground named above.  

4 - Removed the virtual ground resistors and connected the 6.3v CT to pin 8 of V6.  

Gave everything a once over and fired it up.  With the Reverb grounded, the amp is now super quiet.  Only with the main volume at full do I hear a subtle hum.  Approaching 3/4 volume, the hum is gone.  So, the previous suggestions made a world of difference.  Thanks guys.

Now, with Reverb ungrounded, the hum is loud as ever.  So, I lifted the signal wire from the pre-amp (it was the easiest to do, first).  Hum as loud as ever (Reverb ungrounded).  Ground Reverb and amp is quiet.  I reconnect this wire and lift the signal wire from the Reverb.  Amp is super quiet with or without Tremolo.  Tremolo still sounds really good.  

I reconnected the Reverb signal wire and lifted C23.  Hum loud as ever.  Looks like it's narrowed down to the footswitch wire, unless someone can see something that I'm overlooking.  Again, because I've looked at too much this afternoon, I'll have to go back and revisit the footswitch connections.  I'm beginning to wonder if part or all of the footswitch cable needs to be shielded.  Is there something like a co-ax 3/C cable that has individually shielded leads?  I don't think I've ever looked for such.  

I'll take a quick look at it, again, in the morning before leaving for work, and jot some notes as to footswitch connections.  Short term memory is sorely lacking, so I'll make a quick post as to how they are connected.  Mull this over, please, and make suggestions.  Thanks, muchly, for the help.

Jack

[spacelabstudio]

I'm interested to know if you resolve this.  I never really did get hum to an acceptable level on my Scout with the reverb.  (It's dead quiet with the reverb off.)  I'm not convinced I can do it without rebuilding the darn thing so it's not a rat's nest.  I did move some grounds around and got it quieter, but never quiet enough.  I don't use reverb much anyway, so I just pulled the reverb tube and forgot about it.

[Willabe]

Hi Jack,

I would say yes, the reverb fsw needs to be shielded. It's going to/coming from the grid of 1/2 of v2. So when it leaves the shielding of the chassis it acts like an antenna, plus it's long. Fenders are the same and need shielded wire for the fsw because their grounding the grid to kill the verb.  

Man you really grabed the bull by the horns on this amp. Gonna by a great sounding little amp.  

    
                Brad      

[sluckey]

I'm beginning to wonder if part or all of the footswitch cable needs to be shielded.  Is there something like a co-ax 3/C cable that has individually shielded leads?

BINGO! Doug has what you need. That reverb circuit needs the shield. Trem circuit doesn't. You could also just get a standard Fender two button FS. Doug has that too.

[Jack_Hester]

I would say yes, the reverb fsw needs to be shielded. It's going to/coming from the grid of 1/2 of v2. So when it leaves the shielding of the chassis it acts like an antenna, plus it's long. Fenders are the same and need shielded wire for the fsw because their grounding the grid to kill the verb.

Man you really grabbed the bull by the horns on this amp. Gonna be a great sounding little amp.

BINGO! Doug has what you need. That reverb circuit needs the shield. Trem circuit doesn't. You could also just get a standard Fender two button FS. Doug has that too.

So, I can use the cable shield as my ground  for each?  I only ever used a cable shield as just a shield.  And, the shielded conductors as the signal wires.  I don't think that I've ever tied a shield to something on both ends.  This will be a new experience for me.  I just placed an order for the Fender 2-button footswitch. 

If it's raining tomorrow, I'll lift the Reverb lead in the chassis that is coming from the footswitch, and observe the difference.  Otherwise, when Doug's footswitch arrives, I'll swap it out. 

I'm interested to know if you resolve this.  I never really did get hum to an acceptable level on my Scout with the reverb.  (It's dead quiet with the reverb off.)  I'm not convinced I can do it without rebuilding the darn thing so it's not a rat's nest.  I did move some grounds around and got it quieter, but never quiet enough.  I don't use reverb much anyway, so I just pulled the reverb tube and forgot about it.

I believe these guys have answered my problem and yours.  Do you have the shielded footswitch wire?
 

Brad -

My wife says that I'm relentless, when I focus on a problem.  To a pain.  She also says that I'm like all boys (of all ages).  I have to conquer something, then throw it in the pile of toys, and look for the next thing to conquer. 

Steve -

Thanks for putting me on the cable and footswitch.  I like the looks of the Fender, over the cheap offshore one that I have.  My friend Don is making some replica wooden Gibson single-switch footswitches for me.  I'll get him to make me a double, and swap it out when I get it. 

Thanks again, for all the input.  I'll post the next results. 

Jack

[HotBluePlates]

So, I can use the cable shield as my ground  for each? ...  I don't think that I've ever tied a shield to something on both ends. ...

Yes. It's connected to ground inside the amp, and acts as the ground point for both trem and reverb footswitches inside the footswitch box.

If this doesn't do the trick, look for where the 100k grid reference (R19?) for the V2 reverb return triode is grounded. In some Fender amps, the equivalent resistor is grounded at the reverb return jack, across the chassis from where the rest of the reverb return triode grounding is done. Often, moving this resistor's ground to the rest of its associated triode's ground cures reverb hum. If needed, this could be done with a wire running from the resistor to the new ground point.

[Jack_Hester]

If this doesn't do the trick, look for where the 100k grid reference (R19?) for the V2 reverb return triode is grounded. In some Fender amps, the equivalent resistor is grounded at the reverb return jack, across the chassis from where the rest of the reverb return triode grounding is done. Often, moving this resistor's ground to the rest of its associated triode's ground cures reverb hum. If needed, this could be done with a wire running from the resistor to the new ground point.

I'll trace this one next, while I'm waiting for the footswitch from Doug.  

In my original post, I mentioned oxidation.  I wouldn't say that it is really bad.  But, a little bit can be a real pain, when the whole signal ground path is dependant on a good metal to metal connection.  So far, the hard-wired signal grounds that I've added have cleaned things right up.  I won't be surprised if R19 is not grounded along with the other components that it should be.  

I went out to the shop at 04:15 hrs., just to take a quick look.  Found my soldering iron still hot.  Imagine that stroke of good fortune.  Lifted the Reverb footswitch lead in the chassis.  Fired the amp up and the big hum is still there.  Of course, the Reverb is always on with this grounding wire lifted.  Touched it back and closed the switch.  Big hum gone.  I'll land it back this evening, just to have it as a reminder of where the next one will go.  Been a fun job for a tired old head.  I like troubleshooting.  Just not enough time in the day to do it all, until it's finished.  Now, I'm wondering if I unplugged the soldering iron.

I'm afraid that I will be forced to mow, when I get home.  Calling for clear skies, today.  I guess I need a break.  More to come.  Have a good one.

Jack

[Willabe]

My wife says that I'm relentless, when I focus on a problem.  To a pain.  She also says that I'm like all boys (of all ages).  I have to conquer something, then throw it in the pile of toys, and look for the next thing to conquer. 

Next.   


             Brad       

[Jack_Hester]

look for where the 100k grid reference (R19?) for the V2 reverb return triode is grounded. In some Fender amps, the equivalent resistor is grounded at the reverb return jack, across the chassis from where the rest of the reverb return triode grounding is done. Often, moving this resistor's ground to the rest of its associated triode's ground cures reverb hum. If needed, this could be done with a wire running from the resistor to the new ground point.

Just checked this junction, awhile ago.  R19 is grounded along with the other components shown in the schematic.  Thanks for giving me the heads-up on that to look for.  A ground lead was tied from this junction to the case of the Reverb pot, which I now have grounded with a buss wire (Thanks again, Jeff) to my signal ground screw.  When I get back out there, I'll do the same check for the other 'like' junctions. 

Jack

[spacelabstudio]

It may be that I haven't understood about the footswitch assembly, but in my Scout I converted the footswitch from being hardwired to using a jack and plug, so I can unplug the footswitch.  With the footswitch unplugged the reverb is on and I get the hum.  The hum goes away when I plug in the footswitch and turn off the reverb.  So my guess is it's something else on my Scout.  It's a rat's nest of wiring with no apparent consideration for grounds.

[Jack_Hester]

With the footswitch unplugged the reverb is on and I get the hum.  The hum goes away when I plug in the footswitch and turn off the reverb.  So my guess is it's something else on my Scout.  It's a rat's nest of wiring with no apparent consideration for grounds.

This is the only issue left with this Falcon.  I found all other components wired to their respective common ground tie points.  Each was tied to the nearest pot case.  So, the buss wire carried them all to my star ground.  Again, this is now a super quiet amp, with only the Reverb hum, when the signal is ungrounded (footswitch open).  That hum is not as loud, but still much more than I want.  When I get the Fender style footswitch (with a shielded cable) installed, I'll find out if that cures my issue. 

Jack

[Willabe]

One more thing. The verb tanks output end should be furthest away from any/all ac, ie, power cord and PT.


                  Brad     
 

[Jack_Hester]

One more thing. The verb tanks output end should be furthest away from any/all ac, ie, power cord and PT.

Good to know, for future reference.  The Falcon has the tank input/output cables on the opposite end of the chassis, from the PT and power cord.  Cables are routed down along the bottom of the cab and up to the the chassis.

Jack

[Jack_Hester]

Ok.  Took the time to install the footswitch (thanks, Doug).  I unsoldered the plugs from the ends of the wires, and soldered them to the appropriate points.  Warmed the amp up, and tested the switches.  Hum is still there, though I think it is a bit lower.  Still disappears when the Reverb is grounded by the switch.  Tremolo is still very hum free.  Still sounding good.  Reverb still sounds very good, when on.  Amp sounds really nice.  So, back to the schematic for more troubleshooting.  I'll walk away from this one, until next week.  Need a little thinking time.  The footswitch is a keeper.  It goes very well with the amp.  I'll report back on my next de-hum attempt. 

Please comment, if anyone sees anything else that stands out.  Thanks for all the help.  I'll post any and all results.  Have a good one.

Jack

[Jack_Hester]

The Falcon is back on the bench, after a weekend away from it.  I'll do some more troubleshooting this evening and report back with my findings.  I did some troubleshooting on and Ampeg this weekend.  Another thread started on that.

Jack

[Jack_Hester]

Recap of what I've done so far (and hope that I have it right):

http://www.el34world.com/charts/Schematics/files/gibson/ga19rvt.pdf

1 – Replaced all tubes with known good ones.

2 – Replaced power cord with ground tied to same transformer mounting bolt as the CT for the PT secondaries.

3 – All power supply coupling capacitors replaced (two 20uf and two 10uf), the common being tied to pin 1 of V6.

4 – Replaced Reverb tank.  Nothing to do with the hum.  Previous one was weak.

5 – Rewired Tremolo to match schematic.  Tremolo works really well.

6 – Hum goes away when Reverb signal is grounded at C23.

7 – Disconnect Reverb tank output.  No change in hum.

8 – Replaced C23, just for grins.  No change.  Still loud hum when Reverb ungrounded.

9 – CT of 6.3vAC filament supply tied to ground.  Lifted and tied to cathodes of V5 and V6.  No change.

10 – Lifted wire from pin 6 of V2.  No change in hum (Reverb ungrounded for all these tests).

11 – Lifted CT on 6.3vAC and taped back.  Created virtual ground with a 220 ohm on each lead of the 6.3v and the other end to the chassis.  No change
in hum, but picked up a hiss.  Removed the virtual ground resistors and connected the 6.3v CT to pin 8 of V6.  

12 – Pulled V1 (power off before pulling).  No change in hum.

13 – Connected a jumper from the PT post where the CT is connected, to pin 1 of V6, where a number of signal grounds are connected.  

14 – Connected a jumper from this same post to the screw that I'm using as a star ground, on the other end of the chassis.  This is where the input jacks and the monitor jack are grounded.  

15 – Made a buss wire connection across all the pots, except the Tremolo Frequency pot, as it has no ground connections anyway.  Just the wiper and the #1 leg are connected.  #3 leg is open ended.  Connected this buss wire to the star ground named above.  

16 – With the Reverb grounded, the amp is now super quiet.  Only with the main volume at full do I hear a subtle hum.  Approaching 3/4 volume, the hum is gone.

17 – I lifted the Reverb signal wire from pin 8 of V2.  Amp is super quiet with or without Tremolo.  

18 – Reconnected the Reverb signal wire and lifted C23.  Hum is gone (I originally said that hum was loud as ever. Not so).

19 – Verified that all components at each cathode were grounded together where their respective grounds should be.

20 – New footswitch installed with shield tied to the star ground.  Reverb ungrounded, hum still there.

This is where I’ll start my troubleshooting, this evening.  I need to go back and revisit step 18, to see if something is touching R41.  More to come.

Jack

[Jack_Hester]

Revisited step 18, and revised.  Hum goes away when lifting C23.  

21 - Nothing was touching R41, other than one end is connected to C23 and the other is connected to the junction of the 100K resistor (R20?) and the shielded signal wire going to pin 8 of V2.  I lifted R41 from this junction and amp goes quiet.  Reconnected.

So, source of hum is before C23.  

22 - Lifted R19 and C6, leaving the signal wire, from the RETURN of the Reverb tank, attached to pin 5 of V2.  Still hummed.  Landed them back.  

23 - Lifted the signal wire, from the RETURN of the Reverb tank, going to pin 5 of V2.  Reverb ungrounded, and hum is still there.  Landed it back.  

24 - Swapped tubes V2 and V4.  Same hum.  Put them back.


I'm done for today.  Need to ponder it more.  Hum is originating from 1/2 of V2.  

If anyone can see something, please post.  I'll be back on it tomorrow afternoon.  Have a good one.

Jack

[Jack_Hester]

Did a little thinking this morning, over my first cup of coffee (which usually starts around 03:30 hrs).  I lifted C6 and R19, together, leaving the Reverb signal wire attached to pin 5 of V2.  Then, I landed them back and lifted the signal wire.  But, I never lifted all of them at the same time.  I don't believe that it makes a difference though, but I'm open for comments/suggestions, if someone thinks that it would. 

C14 (pin 4 of V2) is new.  Replaced it when I replaced the other electrolytics.  R28 is the only component on this half of V2, that I haven't touched.  I can lift C14 easy enough, as I installed it for easy removal.  Nothing else about this build was put together that way. 

I was thinking of putting a jumper across them both, as another test.  Not sure of how it will affect the tube.  I've put together a couple low voltage projects that had the cathodes tied straight to ground.  But, I've not tried it with a higher voltage configuration. 

Anyway, if I have opportunity this afternoon, I'll tackle these two components, and report the results.  Have a good one.

Jack

[tubenit]

I would be tempted to replace everything connected directly to the V2 tube on all the pins including the heater wiring.  Not sure what else to tell you to try?


With respect, Tubenit

[Jack_Hester]

I would be tempted to replace everything connected directly to the V2 tube on all the pins including the heater wiring.  Not sure what else to tell you to try?

Jeff -

Two things that I plan to do first:

 - I will jumper from pin 4 of V2 (cathode) to the same signal ground as all the rest of the components related to that half of V2. 

 - If it doesn't go quiet, I plan to replace R28 with something close in value, if I don't have a 2.2K.  I've got a 1.5K. 

If no success, I will work my way around to each component, as you suggest.  I'll inspect the heater wiring.  I think that I already have, but having looked at so much, I need to revisit it.  I'm even pondering a RETURN cable replacement, though it looks good, and made no effect when lifted.

This is a real stumper.  But, something simple will rear it's ugly head, eventually.  And, it will be a really nice sounding/functioning amp.  Even if it still looks cannibalized.  More to come.

Jack

[tubenit]

The 6EU7 is a nice tube. I presume you know you can rewire that for a 12AX7 and then have the option of 12A_7 variations? 

I rewired a Gibson Scout with some 12AX7's. It is also an example of a one tube reverb in a production amp. That reverb works quite well, IMO.

With respect, Tubenit

[Jack_Hester]

The 6EU7 is a nice tube. I presume you know you can rewire that for a 12AX7 and then have the option of 12A_7 variations? 

If it were a keeper, I would consider this route.  I've actually spent more time on it than I expected to.  When I bought it, I only wanted the plastic insert of the handle, for the missing one on my GA-78.  I still took that part.  Everything else that I have done is because I opened it up and the chassis components were in place and very intact.  So, in went the missing tubes.  Then, the coupling caps.  Then, the rest of the things that I have done, so far.  I just couldn't stand to leave it 'unfixed'.  When this last bug is ironed out, I plan to give it away, missing the back panel, the incorrect knobs, and the plastic insert that I robbed.  The tolex looks nice, now that it's cleaned. 

Your suggestion is actually the best route, for availability of the 12A_7 tubes.  And, I still haven't approached the young Pastor about taking it, until it is trouble free.  I can't say how much he would use it, but I'm thinking that it may get a good workout.  I don't think that he even has an amp.  He surely needs one.  If he and his wife do use it on a fairly regular basis, I'll replace all the missing components over time, until it is in a 'non-cannibalized' condition.  I've got quite a few 6EU7's on hand, so I can support him for a few more years. 

Again, thanks for the suggestions and help.  It has been a puzzle, but not a worry.  Have a good one.

Jack

[spacelabstudio]

I haven't done an A/B comparison in the same circuit with 6EU7 vs 12AX7, but the 6EU7 preamp in my Gibson scout sounds quite nice. 

[Jack_Hester]

Looked over my previous notes and began the next steps: 

I post the drawing link from time to time, so that it will be ready for viewing - http://www.el34world.com/charts/Schematics/files/gibson/ga19rvt.pdf

25 - Lifted everything from pin 5 of V2. Had not done that previously.  C6 and R19 are in parallel, and connect directly to pin 5.  The Reverb RETURN was connected directly to pin 5.  I had lifted one or the other, but not both.  It does make a big difference, with everything lifted.  Not a good sound at full volume.  Started out as a mild oscillation and went into a wild screaming one.  Landed those back, as the hum stayed the whole time.  Why would it go into said oscillation, with no input to this triode?

26 - Jumpered R28 and C14.  Hum still there.

27 - Grounded pin 5.  Sounds like I may have two hums, the louder of the two remaining, with the lesser going away.  Remember that when C23 is grounded, amp is quiet.

The only thing I haven't done now is add more filter before the plate resistor, R29.  It has a 10uf downstream, in the schematic, labeled as C10b.

28 - Just for grins, I checked the DC voltage at pin 6.  175vDC, just like the schematic states. 

29 - Noticed that pin 3 on V2 was being used as a junction point for grounds.  Floated those away from the pin.  No change.  Will land them back later.

I'm beginning to wonder if the tube socket is defective.  Tired mind trying reason this out.

Had to stop here.  Time to go to the house.  I'll give it a fresh ponder in the morning, early.  A cup of French Roast, from fresh ground beans, will improve the thinking considerably.  Have a good one. 

Jack

[Jack_Hester]

Well, after the fresh coffee beans cleared the cobwebs, I'm tending now towards a tube socket inspection/change.  The little mounting bushings on this one are just about falling off, so I'm going to have to find some for replacements.  This socket has the tube shield base as part of the socket.  I have a regular socket, and a shield base that mounts separate.  This one should be easy to remove, as it has hollow rivets holding it.  A self-tapping screw in one side to hold it, and an oversized drill on the other, to shave the lip off.  I'll use small screws with nuts, and blue Loctite to hold them.  Just got to find new bushings.  Shouldn't be too hard.  Hope that I see something obvious and fixable with the socket.  I like reusing original.  Anyway, I'm heading in that direction.  Please comment or advise, if I'm overlooking something. 

I apologize for the lengthy thread.  I don't do component level troubleshooting or repair, at work, like I use to.  We are card changers and calibrate with hand-held instruments or laptops.  I've only in recent few years begun this level of repairs, just for pleasure.  It's been 20 years or so before that, since I've 'fixed' something like this.  In those days and before, we numbered each step of troubleshooting on a pad, and if you walked away it for any length of time, you left the pad on your bench for others to follow, if need be.  That's kinda what I've done here.  Leaving the 'pad' behind, until I get back to it.

I'm not as neat or thorough as I once was.  And now, it serves as my short term memory.  Typically, I wouldn't fill up a post with such.  But, this one was better 'said out loud', hoping something would jump out.  Anyway, more to come.  Have a good one. 

Jack

[tubenit]

Jack,

I admire your tenacity fixing this thing.  I've reread this thread numerous times, but it's lengthy and I admit I may have missed something.

Have you tried removing the shorting switch from being after that .01 plate coupling cap and instead install it prior to the V2-a grid like on Fender amps and others?   Not sure why that would help, but obviously something odd is going on so it wouldn't hurt to try it.

OR you could jumper bypass the shorting switch and then jumper to ground the V2-5.

Are we positive that the problem is V2-a (reverb triode return)?  Can we isolate that?


In an effort to isolate exactly where the problem is, I'd do the following: 

I'd take an insulated alligator jacked wire and ground  V2-5 directly to ground.  Does hum go away?

Then ground V3-6 to ground.  Does hum go away?

Does pulling V3 make the hum go away?

Then ground V1-8 to ground.  Does hum go away?

Different reverb tanks are grounded different ways.  Look at the Accutronics website for different examples.  Are you positive that yours is grounded correctly?

Something to note, You could convert this reverb to a Fender type without a problem.  You could tie the 6EU7 triode to the 6C4 like a paralleled 12AT7.  Convert the current (dwell type) Gibson reverb pot to a Fender style reverb pot.  You can do a one tube reverb like the Dumblish style or the Gibson Scout type (similar to Falcon).

The one thing I would not do is mentally get locked in that I have to keep it as original as possible and not change the topology or component parts. 

I am rooting for you and I'm confident that you will get this resolved.

With respect, Tubenit

[Jack_Hester]

Jeff -

I'll answer the first part of your post, as I only have a minute before going into a meeting.

Step 27 above - I grounded V2 pin 5.  The louder of the two hums is what I'm trying to get rid of.  And, it is still there.  The lesser hum may be the subtle hum that I hear, when the Reverb is turned off.  I may resolve both, when I fix the loud one.

For now, I will troubleshoot it as if it has to stay as designed.  Just to resolve it, for future reference.  Got to run.  I'll read over the rest, later today.  Thanks for the input.  Have a good one.

Jack

[Jack_Hester]

I haven't done a very good job of listing everything that I've done.  Mostly everything around the Reverb side of V2. 

I've grounded the V3 pin 6 (I believe that's the grid number), and I've grounded V1 pin 8 (Reverb side of that triode). 

I've pulled V3, and still loud hum.  Let me clarify the hum again.  Somewhere along the way, after doing the capacitor replacements (decoupling and cathode, and one coupling), and after making the various new ground connections (star ground and buss ground), the amp went from working with a hum that got really loud with the Reverb switch open, to an amp that is ever so quiet with switch closed, and loud (but not as before grounds) with switch open.  That's the hum I'm chassing. 

I'll go back and revisit grounding the grids on each, and report back on that. 

Along the lines of keeping it original, I'm not really locked in to that.  Actually, I had no plans to fix it.  Just take the handle insert, and put it aside for parts.  I made the fatal error of wondering why someone would rob parts from it.  What got it to where it was when I did my part to rob another piece.  I figure that if I can resolve this issue, as a stock amp, I'll have notes (I promise to clean them up) for future reference on other Gibson models that have a similar design to this one. 

I have a '63 Epiphone EA-28RVT Pathfinder that, with the exception of one cigarette burn on the top front, is as new as the day it was made, appearance and (I hope) sound.  I believe that it may be identical in build, to this one.  I don't have it on hand, as it is loaned out to a a studio in Durham.  Even if it were on hand, I would hesitate to open it up.  Too untouched.  Sometimes, I'm like that. 

Now, my buddy Don (over in Durham, too) has what I think is a '65 model, a little later than this one, I think.  Different schematic from this one.  They did basically the same amp scheme, but configured the tube usage different, and the 6C4 is gone.  His has a 7199 in it, both triodes used in the Reverb. 

So, originality is not a goal, for now.  Just making it work, as is, then go from there.  Thank you for your support, and please keep the suggestions coming.  Even if I've covered some of the ground, it doesn't bother me one bit to recheck.  Have a good one.

Jack

[Jack_Hester]

Jeff -

Took only a short trip out to the shop, after getting home.  Here's what I checked:

30 - Grounded V1 pin 8, V2 pin 5, and V3 pin 6.  All at the same time, back to the star ground.  Same loud hum, goes away completely with Reverb off. 

31 - Put a jumper from the Reverb tank to the same star ground.  Still hums.  But!  A higher pitched, very low hum disappeared.  I forgot to turn the Reverb off.  That may be the background lesser hum, when the amp is quiet.  You're a genius, Bud!  Eliminated another one.  The loud one is a dull, low pitched hum that I'm thinking is 60 Hz.  This lesser one may be a 120 Hz, but I haven't listened to a known 120 Hz, to know for sure.  I closed up and came to the house.  Very tiring day.  I'll step out there in the morning (I usually do) and listen with the Reverb off.  I can lift the tank ground, to be sure. 

This amp did not have a grounded tank, nor have any of the several 60's Gibsons that I've been inside.  Including the GA-78.  But, that's another whole HUM story.  Not a very good design, from the beginning, for hum-proofing.  The 50's Gibsons, that I've had my hands in, did not have Reverb.  The Epiphone Pathfinder did not have a grounded tank.  It will in the future.

I will say, this tank is laying out on the bench, and the chassis is in a cradle, with another speaker temporaried to it.  So, everything is an antenna.  I turn off the lights and use only an incandescent lamp when de-humming. 

I'm glad that I'm not charging myself for this job.  I don't believe that I would want to pay the labor bill.  I'm only working for me, anyway.  I've had to do similar on bikes, in my younger days.  That's why I stopped service work, and moved to restorations, only.  The customers just don't understand why you did what you had to do. 

Anyway, we've got a Unit coming down at the plant, for an outage.  And, the next two weekends are shot, along with next week.  I do plan to borrow a high voltage megger.  I will lift each wire from the Reverb half of V2.  Then, I'll megger each of those pins to ground, using the 500v scale.  Then maybe, the 1000v scale.  That will prove for sure if I have a pin that has tracked to ground, or if there is something that I can't see with a magnifier. 

Don't know when I'll get to the next test, but I'll post my findings.  Thanks again, for the help.  And, thanks to everybody who reads my very wordy thread, and posts.  Have a good one.

Jack

[Willabe]

No, thank you Jack.

Very interesting read.


                  Brad     

[Jack_Hester]

Just got a moment to report what I found this morning (early). 

Turned the amp on, grounded the Reverb tank to the chassis, and turned the Reverb off.  Quiet amp, with hum that can be heard with my ear to the speaker. Ungrounded the tank.  Could hear no difference in hum. 

Grounded the tank and turned the Reverb on.  Same loud hum.  Ungrounded the tank and a higher pitched hum occured, along with the lower pitched hum that was already there.  The higher pitched hum quite possibly can not be heard, without having my ear to the speaker.  Just that quiet, too.  But, the grounded tank eliminates it completely.  I'm thinking that it was picking up the overhead fluorescent lights. 

Either way, I will ground the tank when the chassis goes back in the cab for the last time. 

Jack

[Jack_Hester]

We got our generating unit back online, so I'll be back on the Falcon towards the end of the week.  I plan to label and lift everything off of the V2 tube socket, and remove the mounting rivets.  Both shock mount bushings are not in the best of shape.  I've search a few catalogs, and find nothing listed for these.  I'm beginning to think that these are nothing more than small o-rings.  I would like to think that I can find some a bit tougher and longer lived, than rubber o-rings. 

Also, where would I find hollow rivets like these, and the proper tool(s) to install them.  I could swage them with a punch, but I'd rather not risk bending the chassis. 

I ordered a few sockets, that have the shield base mounted to the socket.  I still plan to megger the original, and see if it is something that can be cleaned, if in fact that is the source of my Reverb hum.  But, I'll have a new one on hand, if it is defective, and cannot be repaired.

Thanks for any help and info. 

Jack

[Jack_Hester]

Took a little time to get back out in the shop Saturday (03Nov).  Still a bit burnt out from the outage, but figured this would be something to clear the cobwebs.

Removed the V2 tube socket and replaced it with a new one.  Used the old shock mounts and a couple screws (until I can find proper replacements).  Fired it up and the hum is still there.  So, I'm thinking my next step is to see if the B+ is the source.  Also, I need to revisit all my notes and maybe organize them a bit.  This is a real puzzle, but it's just a matter of elimination.  Still a really good sounding amp, with the Reverb signal grounded.  

I cleared another spot on the bench to put the Ampeg on.  This will give me opportunity to have something else to work on, while I'm thinking on the Falcon.  More to come.

Jack