Gibson Thor Bass amp........Bad Hum.......

[Jack_Hester]

Not trying to work on any major projects, as there are plenty 'Holiday' projects to keep me otherwise occupied. 

However, I have what was a very trashed Gibson Thor, that I'm trying to clean up the amp.  The cabinet was destroyed beyound salvage.  So, that's long gone.  I did save two working 8 ohm speakers, though one has a tear in it.  I believe that I can patch that with no issues.  The front panel (attached to the cab and not the chassis) had a broken fuse holder and Power (polarity) switch.  I have a fused (3A) box that I'm using to power the chassis.  So, I'm using a jumper to bypass those and using the power switch on the box.  This is my lamp limiter box with a circuit breaker installed in the lamp socket. 

Here's the schematic from Doug's library, that I'm using:

http://el34world.com/charts/Schematics/files/gibson/Gibson_THOR_BASS-AMP.pdf

If you will look at the 2nd half of V1, labeled as 'Voltage Amp', I replaced the .047uf coupling cap, as the original had a broken lead.  Went ahead and replaced the 500pf cap, just downstream of it. 

Replaced all the caps in the power supply.  The amp originally had two 3x20uf canned caps.  I replaced one of those with a 4x20uf that I had on hand.  And, the 40uf with a single.  Replaced the resistors in the PS, as well.  So, it's all new. 

Connected the front panel cable that contained Vol, Bass, Treble controls, leaving the power cable disconnected.  Installed my power jumper and brought it up on the limiter.  25W, 40W, 60W, and 100W bulbs.  Then, put the CB in place of the bulbs and up on full line voltage/current. 

One of the mods that I made was to make the Bias adjustable, so with the plate voltage stable at 475vDC, I set the JJ 6CA7 tubes for 36ma.  The two 6EU7's are old stock.  The amp works really great, and the Tone controls have a good range. 

The problem is a moderately loud hum, that is there regardless of volume level.  I start pulling tubes, beginning with both 6EU7's.  Hum went away.  Reinstalled the PI.  Big hum is back. 

At the downstream end of the .047uf coupling cap (leaving the V1-pin 6), I have installed an FX loop.   The Send being connected to the .047uf, and the Return to the junction of the 500pf/470K, by means of another .047uf coupling cap.  Shielded signal cables in between. 

With no tube in the V1 socket, and using a jumper with one end connected to ground, and the other with a .1uf cap for a test probe, I began shorting out the signal.  I started at the junction of the 500pf/470K components, just downstream of the .047uf coupling cap from the Return.  Hum gone.  Connected to V1-pin 6, and hum went away.  Just out of curiousity, I connected to the top side of the 100K plate resistor.  Made no difference.  Hum still there. 

Any suggestions as to what may be going on here?  I can put the Pre-amp tube back in and hear no change in the hum level.  Again, the amp sounds really good, and would make a nice project for a head cabinet.  Thanks for looking.  Have a good one.

Jack

[shooter]

The last one of those I looked at needed the mouse nest removed, then the spiders!  Did you bring it up & test before you added the adjustable bias?  The one I looked at had some pretty crumbly shielded wires, is it an inductive sorta hum, like when you get your fingers "close" it'll change?  That's about all I got for ideas.

[Jack_Hester]

The last one of those I looked at needed the mouse nest removed, then the spiders!  Did you bring it up & test before you added the adjustable bias?  The one I looked at had some pretty crumbly shielded wires, is it an inductive sorta hum, like when you get your fingers "close" it'll change?  That's about all I got for ideas.

This one had a fine maze of spider webs.  Those are gone. 

One of the canned caps had its bottom blown out, literally.  I do a visual inspection before putting on the limiter.  The PS was replaced before any initial tests. 

And no, to your question on the adjustable bias.  It was a simple mod that I've made on quite a few old fixed bias amps that I plan to keep.  I made that right after replacing the PS.

The original shielded wires appear to be in good shape.  My issues come after the 'Voltage Amp'.  So, the hum of which you speak is not the source.  That would be in the pre-amp, before the voltage amp.  The pre-amp/voltage amp tube is pulled, and the hum remains. 

But, thanks for looking.  Please continue to do so.

Jack

[shooter]

I've followed your work, I am definitely the student here!  Am interested in your signal tracing, are you jumping out the pins of v1(G/P)? then shorting out along the signal path?  Only other thing that stands out is the NFB line, after that it looks like all the usual suspects have alibis'

[Jack_Hester]

Am interested in your signal tracing, are you jumping out the pins of v1(G/P)? then shorting out along the signal path?  Only other thing that stands out is the NFB line, after that it looks like all the usual suspects have alibis'

I'm using a capacitor as my signal grounding probe, attached to one end of my jumper, the other end to signal ground.  In this case, a chassis ground. 

I eliminated my FX loop, for now.  One less path to troubleshoot.  I temporary'ed in a .047uf coupling cap, back in the place where the original was connected.  V1 tube is removed, as it makes no difference in the hum.

Power on and hum is present.  Once again, I touch the my signal grounding probe to pin 6 and amp is dead silent.  Touch it to the B+ side of the same plate resistor, and no change in hum.

I had one other thought.  When I first started my repairs, I considered replacing the socket for V1.  Just to install a shielded one.  Once I saw the small rats nest of components (point-to-point build), I decided not to.  But now, I'm back to that thought again.  The reason being that both V1 and V2 sockets are brown fiberboard, in construction.  And, I'm now wondering if the V1 socket is conducting, and I'm getting filament AC induced hum.  Those leads are on pins 1 and 2 of the socket, and easy to get to.  I'm thinking that my next move is to get those out of the way and give it a try.  I'll be back.

Jack

[Jack_Hester]

I had one other thought.  When I first started my repairs, I considered replacing the socket for V1.  Just to install a shielded one.  Once I saw the small rats nest of components (point-to-point build), I decided not to.  But now, I'm back to that thought again.  The reason being that both V1 and V2 sockets are brown fiberboard, in construction.  And, I'm now wondering if the V1 socket is conducting, and I'm getting filament AC induced hum.  Those leads are on pins 1 and 2 of the socket, and easy to get to.  I'm thinking that my next move is to get those out of the way and give it a try.

Well, that won't it.  Same hum without the filament leads connected.  Soldered them back. 

More head scratching.

Jack

[Jack_Hester]

Ok.  Stared at the schematic a bit more. 

Lifted the plate resistor from V1-pin 6, leaving the coupling cap connected.  Fired it up, and once the power tubes began conducting, big hum came back.  Connected the grounding cap to the tube-side of the plate resistor, hum disappeared.  Connected to the B+ side, no change in hum. 

Back to more staring and thinking. 

Jack

[shooter]

liked the conductive thought, staying with that, what about gator clipping or tacking in a couple 100ohms on the filaments n confirm or eliminate the filaments?

my brain is flu infected, I think they gave me the wrong rev flu shot, this rev flu doesn't seem to be bothered!

good luck!

[jjasilli]

A picture is worth a thousand words.  I suggest posting a schematic of the amp as rebuilt.

[Jack_Hester]

liked the conductive thought, staying with that, what about gator clipping or tacking in a couple 100ohms on the filaments n confirm or eliminate the filaments?

The amp originally had one filament lead connected to chassis/signal ground.  I clipped that and installed a 100 ohm wire-wound Hum balance pot.  No change from it, but I plan to leave it in. 

A picture is worth a thousand words.  I suggest posting a schematic of the amp as rebuilt.

That will be my chore for today (in between all the other Christmas Eve chores that materialize).  Really not much change in the main circuit, as I've disconnected the FX loop.  I had intended to make a new drawing, when all repairs and mods were complete.  I'll go ahead and make one like I hope to leave it. 

One thing that I hope to do today is to remove the .047uf cap (that I have temporary'ed in) and attach my listener to the V1-pin 6.  And, see if the hum remains.  I'll do that before I complete the drawing. 

Back soon. 

Jack

[Jack_Hester]

A picture is worth a thousand words.  I suggest posting a schematic of the amp as rebuilt.

Here's what I have drawn so far.  Everything up to the junction at the 500pf (C7 on my drawing).  However, I have disconnected the FX loop and the additional .047uf coupling cap (C6).  So, there is no connection between C5 and C7, for now.  The hum is present at C5 and it's connection to V1-pin 6.  I'm taking a break, but will connect my listener to pin 6 and the top side of the plate resistor, when I start again.  Thanks for looking. 

Jack

Note: See Reply #14 for the complete drawing, voltage chart, and original drawing.

[Jack_Hester]

Connected my listener to a small tube amp.  I removed the (.047uf coupling capacitor) to eliminate the amplified hum.  Refering to the original drawing:

http://el34world.com/charts/Schematics/files/gibson/Gibson_THOR_BASS-AMP.pdf

it is the one connected to V1-pin 6 on one end, and the junction of 500pf/470K.  My drawing shows it as C5.

With the listener grounded to the Thor chassis, I touched the probe to pin 6.  Got the hum.  Touched it to the B+ side.  Major hum.  That's when I decided to walk my way through all the taps on the power supply.  Expecting for it to get louder as I worked my way back. 

First, I connected to the junction of the 40uf/220K, which goes to the Pre-amp plate resistor.  Very quiet.  Had to crank the volume up on the external amp to hear any hum. 

Of course, the next one is the noisy junction with a lead going to the plate resistor of the Voltage Amp.

The next junction goes to each plate resistor of the Phase Inverter.  Not as much hum as that at the junction for the Voltage Amp, but still there.

The next junction goes to the common power tube screen resistor, as well as the CT of the OT.  Major hum, but I expected that. 

The next two are the ones before and after the choke.  Super loud hum, but that too was expected. 

So oddly enough, the PI junction before Voltage Amp junction is not as noisy.   


I'm now wondering if I have a defective canned capacitor.  As another test, I'm thinking that I will lift the wire leaving the 330K/220K junction.  As I have a spare lug on a solder terminal strip close by, I can add another small resistor (in value) from this junction to the terminal strip.  Put a 20uf (if I have one) from this new point to chassis ground.  And, connect my wire back to this new junction.  Thoughts, please.

I'm going to start digging to see if I have a cap.  I'll be back.   

Jack

[Jack_Hester]

I had a 22uf-450v cap, and decided to use a 1K-3W resistor.  And, the 3W resistor leads were long enough to reach from the 330K/220K junction to the solder terminal that the plate resistor is connected to.  I removed the wire and landed the resistor in place.  Then, soldered the cap (+) to the plate resistor terminal and the other end (-) to the canned cap ground lug. 

Fired it up, and I now have one seriously quiet amp.  No need for a listener now.  Gave it a quick test with the guitar, and shut it back down.

Now, I'll reconnect my FX loop and hope that all stays just as quiet.  I'll be back when that's done.

I'll update my drawing with all mods to the amp and PS.  Have a good one. 

Jack

[Jack_Hester]

I added my FX loop back to the amp, which includes an additional coupling capacitor, labeled C6 on my drawing.  Must have done something right with this, too.  The amp is still really quiet.  And now that I can hear it clearly, I'm very pleased with the clarity and the great tone. 

I'll have all mods included on the completed drawing.  I've got to replace the Polarity/Power switch.  Gibson used a switch with a splined shaft, and my replacement is a smooth one.  It was missing two knobs anyway, so I'll replace them all with something alike.  And save the two originals.  When that's done, I'll give the FX loop a try.  Sure wish that I was a woodworker.  No cab for this one.

I've done enough for today.  Time for a nap.  Have a good one. 

Jack

Note: See Reply #14 for the complete drawing, voltage chart, and original drawing.

[Jack_Hester]

Ok.  I couldn't leave it.  Here is the amp, as-built.  Other than my mods, one difference in the power supply (from the original) is that the OT center tap is fed from before the choke, and not after.  Not my mod, but original to the amp. 

Also, on the original drawing, one of the input jacks is an open circuit jack.  They are both closed circuit, as I have them drawn. 

I added a thermistor to the primary side of the PT, and a 100R wire-wound Hum Balance pot to the Filament supply. 

Anyway, if I try anything else on it, I'll update the drawing and post it on this thread.  Have a good one. 

Jack

[Jack_Hester]

Made a change to the drawing, to rotate the original schematic.  Now, all three are have the same orientation. 

Otherwise, no change.

Jack