Hoffman AB763 Twin Reverb HUM when intensity is turned all the way down

[proaudioguy]

I have noticeable 60/120 cycle hum when the Intensity control is at 1.  10 has no hum.  I did not record that but I did record the other noises.


I’m chasing my tail here guys.

&feature=youtu.be


Let me know what you think please.

[Latole]

To check amp noise, never plug guitar. Test amp alone.

Tubes amp; alway begin by trying know good tubes.

Reverb ; Clean RCA jacks amp back plate and at reverb tank. At cable end too.
If you don't know how, we can explain.

You can try new cable. Standard stereo cable work fine.
Try reverb tank outside the cab.

Did filter caps are originals ? It is a must to replace them. Don't forget bias circuit filter cap.
Old filter caps can cause strange problems that one would not guess. I saw it.

Good luck .

[proaudioguy]

This is a brand new amp.  All the connectors and cables are all new.  Its a hoffman AB763.  The only thing retained was the original transformers, except the reverb transformer, and the chassis.  The original cabinet was re-covered.  I did a bunch of the work in 2004, then the rest 8-10 years ago and did things off and on to it until maybe 2013.  It was done and was good but the tremolo was weak so I was still dicking around with it trying different things for a while.  There are ghosts in this thing.  Circuit looks great.  I’ll take a gut shot and post it up.  Maybe someone will see something.

[proaudioguy]

Most of that noise is there whether there is a guitar plugged in or not as I demonstrated.  There is the additional ground related RIZZ noise when the guitar is plugged in and not being touched.  Touching the chassis makes that go away too.  Doesn’t seem right to me.  I would like to know hot to address that. The other issue is that massive HISHHH.   The only noise I want to hear is the hum and buzz my guitar pickups put through.  I have several high gain fender amps and on the clean channels, they are clean as can be.  No pickup noise at all.  On the high gain, the noise is about the same when I am on this amp, on the non-humbucking position.  With the humbucking position on the other amps on the clean channel you wouldn’t know they were on unless I really cranked them.

[proaudioguy]

Video

[SoundCity85]

Do you also measure plate voltage with your fingers? I got nervous watching this video

[Latole]

This is a brand new amp.  All the connectors and cables are all new.  Its a hoffman AB763. 

I read only AB763 and to me it is a vintage, Sorry

[Latole]

Don't work with your finger, lethal voltage there

I do a better wire routing.
How to explain ?
All yellow wires must run close to the chassis, never side by side.
If wire must cross another wire it mut be in 90 degrees.

_________________________

"There is the additional ground related RIZZ noise when the guitar is plugged in and not being touched.  Touching the chassis makes that go away too."
-Proaudioguy

Look a bad ground somewhere. What about the ground wire of the power cord ?

[Latole]

I don't like wires routing at the tubes's sockets.
They are not close to the chassis, to say one thing, they may act like antenna for noise.

[proaudioguy]

I don't like wires routing at the tubes's sockets.
They are not close to the chassis, to say one thing, they may act like antenna for noise.

I am open to suggestions.  I have moved them all around.  This is an ongong process.  When I built it they were all against the steel.  There is no way to make them not run in the same direction so I have them separated.  If you have photographic examples of how they should be run that will reduce the hiss, cool.

Do you also measure plate voltage with your fingers? I got nervous watching this video

As for putting my fingers in there, I routinely tie in 400A 3 phase for work.  Sometimes its hot (not under load).  I am very well aware of what is there and where my hand is at all times.  Nothing is willy nilly even if it looks that way.  Like watching roofers or tree service guys.  They are like monkeys.  I haven’t had a shock since I was 18 working for a plumber, doing what I was told, and before that I was a small child, almost 3.  I learned real fast.  I took apart TV sets as a kid and never got shocked.  I know what not to touch.  Even so, your answer provided no information about my issue with that cap collecting buzz like a buzz antenna.  I would prefer it was isolated by a shield.  I didn’t think that type of cap was at all directional but I’m open to suggestions.


Here is me at work.  Hope we can go back to work soon.

[proaudioguy]

This is a brand new amp.  All the connectors and cables are all new.  Its a hoffman AB763. 

I read only AB763 and to me it is a vintage, Sorry

My bad!  I was on the old forum.  I’ve been on this one since 2012.  I just assume the hand full of people that usually answer me, know the project.  That’s on me.  I think I have 3 posts up on this page.  I am sorting one issue at a time.  The story is this.  I bought the amp in the mid 1990s.  It was a basket case AA270.  The tolex was torn up, the amp had lots of problems.  It smelled amazing.  Still had great tone even with lots of nasty noise.  I wanted it for a project and I wasnted channel switching.  I bought one of those slimey Torres Super Texan kits, thinking I was going to be “rebuilding the amp into a 2 channel blue monster” or something like that.  What I got were piss poor instructions and hardly any components.  If I saw the guy today, I’d punch him in the face.  I called them and told them it was as if a child wrote the instructions and they didn’t like that.  It was years before I realized the guy doesn’t speak english natively and his grammer is HORRIBLE, but he’s a marketing genius.  I installed the kit and it sucked plus it didn’t solve the problems I had.  I got side tracked but I was reading a lot of tube amp books and such as I could afford to buy them.  Then in 2004 I found this or another website and joined and asked some questions and a nice guy contacted me directly and offered to build me boards and I traded him some brand new Kendrick Brown frame speakers I had bought for the amp.  He maybe got a way better deal than me but he hand wrote me explicit instructions which walked me through the entire process.  I had bought a black face reissue panel in the mid 1990s from my fender dealer cause the old one was beat.  I stripped the cabinet and used wood filler as needed and I had bought the tolex and grill cloth in the 90s and never used it.  I finally used the tolex and had lost the grill clothe so I had to buy that again.  I gutted the chassis totally, then we had 4 hurricanes in 6 weeks and our lives were upended and actually we have never gotten back to normal since then.  So finally several years later, I lost track I ordered the under $100 worth of parts from Mr Hoffman to build the turret boards.  I lived with it for a while, still had a few issues but was mostly good.  I bought some nice RCA 6L6GCs and biased them and there was no temolo.  THe tubes also sounded a bit dark for my tastes.  I made a bunch of posts then.  I ended up pulling the bias vary and installing the roach circuit.  This was a bad idea.  I hated it.  The tremolo worked but it killed the tone.  The tremolo didn’t have that deep cycling and the 50k pot to ground really sucked down the tone.  I then sold those tubes and ripped that circuit out and had the intention of putting the bias vary right back but life side tracked me again.  I lost my music room due to mice taking up residents in there.  Once I had the whole room empty and cleaned up, my wife moved in and I never got it back.      Then my dog died and I literally quit playing guitar for 6 years.  I really only noodle anyway but I totally quit.  It was super depressing every time I picked up the guitar.  My pup was my blues buddy and would sing along.  So now I am just working on finishing projects and trying to get things done.Knowing what I know now, I would have replaced all the old Electrolytics and left the old beat up tolex, etc and played the old amp with its funny smells.  It had character.  During the rebuild I replaced all the tube sockets, the boards with turret boards, all the pots, all the knobs, all the jacks, all the switches.  THe chassis and transformers were original.  Back about 8-10 years ago I replaced the reverb transformer.  I had some bad microphonics and that solved it.  That’s why I say its way easier to start with all new parts for a new build than to try and make old parts into a new amp.  I’m chasing gremlins throughout this build.Knowing what I know now, I would have heat shrink around the ends of every wire to keep them from fraying.

[Latole]

I don't like wires routing at the tubes's sockets.
They are not close to the chassis, to say one thing, they may act like antenna for noise.

I am open to suggestions. 
 

Answer #7

[Latole]

Easier to read for me like that : 


My bad!  I was on the old forum.  I’ve been on this one since 2012.

 I just assume the hand full of people that usually answer me, know the project.  That’s on me.  I think I have 3 posts up on this page.
  I am sorting one issue at a time.

 The story is this.
 I bought the amp in the mid 1990s.  It was a basket case AA270.  The tolex was torn up, the amp had lots of problems.  It smelled amazing.  Still had great tone even with lots of nasty noise.  I wanted it for a project and I wasnted channel switching.  I bought one of those slimey Torres Super Texan kits, thinking I was going to be “rebuilding the amp into a 2 channel blue monster” or something like that.
 What I got were piss poor instructions and hardly any components.  If I saw the guy today, I’d punch him in the face.  I called them and told them it was as if a child wrote the instructions and they didn’t like that.  It was years before I realized the guy doesn’t speak english natively and his grammer is HORRIBLE, but he’s a marketing genius.  I installed the kit and it sucked plus it didn’t solve the problems I had.  I got side tracked but I was reading a lot of tube amp books and such as I could afford to buy them.

 Then in 2004 I found this or another website and joined and asked some questions and a nice guy contacted me directly and offered to build me boards and I traded him some brand new Kendrick Brown frame speakers I had bought for the amp.  He maybe got a way better deal than me but he hand wrote me explicit instructions which walked me through the entire process.

 I had bought a black face reissue panel in the mid 1990s from my fender dealer cause the old one was beat.  I stripped the cabinet and used wood filler as needed and I had bought the tolex and grill cloth in the 90s and never used it.  I finally used the tolex and had lost the grill clothe so I had to buy that again.  I gutted the chassis totally, then we had 4 hurricanes in 6 weeks and our lives were upended and actually we have never gotten back to normal since then.

 So finally several years later, I lost track I ordered the under $100 worth of parts from Mr Hoffman to build the turret boards.  I lived with it for a while, still had a few issues but was mostly good.  I bought some nice RCA 6L6GCs and biased them and there was no temolo.  THe tubes also sounded a bit dark for my tastes.  I made a bunch of posts then.  I ended up pulling the bias vary and installing the roach circuit.  This was a bad idea.  I hated it.  The tremolo worked but it killed the tone.  The tremolo didn’t have that deep cycling and the 50k pot to ground really sucked down the tone.  I then sold those tubes and ripped that circuit out and had the intention of putting the bias vary right back but life side tracked me again.

 I lost my music room due to mice taking up residents in there.  Once I had the whole room empty and cleaned up, my wife moved in and I never got it back. Then my dog died and I literally quit playing guitar for 6 years.  I really only noodle anyway but I totally quit.  It was super depressing every time I picked up the guitar.  My pup was my blues buddy and would sing along. 

So now I am just working on finishing projects and trying to get things done.Knowing what I know now, I would have replaced all the old Electrolytics and left the old beat up tolex, etc and played the old amp with its funny smells.  It had character.  During the rebuild I replaced all the tube sockets, the boards with turret boards, all the pots, all the knobs, all the jacks, all the switches.  THe chassis and transformers were original.  Back about 8-10 years ago I replaced the reverb transformer.  I had some bad microphonics and that solved it.
 That’s why I say its way easier to start with all new parts for a new build than to try and make old parts into a new amp.
 I’m chasing gremlins throughout this build.Knowing what I know now, I would have heat shrink around the ends of every wire to keep them from fraying.

[acheld]

Hey there.  Your project will take some time to fix, BUT DO NOT give up.  That circuit is lovely and as you have noted it can be dead quiet when everything is copacetic.

I'm not one of the stars on this platform, but at least some of your hissing noise may be from carbon comp resistors in the first stage of both the Normal and Reverb circuits.   My own preference is to use metal film resistors in that position.  This may or may not work for you, but it would be a relatively simple start to working on your amp.  These are R3,4,5,6 on the Hoffman AB763 schematic.

This is a nice write-up regarding grid stoppers (which I had a tough time understanding at first):

https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

It is worth viewing sluckey's build photos to see how seriously he takes the role of these resistors.

I do have one comment about your video:   I totally trust your judgment about what is safe for you -- that's your beeswax, not mine.  Just remember that there are a lot of folks who travel this forum who have NO high voltage experience whatsoever, and that's why you will get some pushback.

[proaudioguy]

That makes perfect sense.


I will order metal film for all the signal path resisters.

[glass54]

Hi Proaudioguy,
....and don't forget Metal Oxides resistors for Plate loads and Output tube screens. More robust at higher volts and potentially better noise performance than Carbon Comp. 
(by the way, I've had great success with Hoffman's AB763 boards  )
Kind regards
Mirek

[proaudioguy]

Talking about lead dress.....


I saw THIS on a Mojo blog post.  I intentionally did NOT route my stuff like this because there is so much parrallel routing here that would cause magnetic coupling noise.  Have I been misunderstanding this issue all along?  My routing was intentionally more random to avoid having any wire “close” to any other, as well as only crossing at right angles to avoid coupling.  This routing on Mojotone’s amp seems counter intuitive to me.  I love the way it looks though.  This is how we do our racks but every cable is balanced and shielded and running typically at line levels.

[tubeswell]

Does the hum vary with various combinations of intensity pot rotation with rotation of other pots?

[proaudioguy]

Does the hum vary with various combinations of intensity pot rotation with rotation of other pots?

No.  Just comes UP when I turn the intensity down, regardless of other controls.  In fact I have another noise that is driving me nuts.  Sounds like a storm.  Real quiet but there.  I have V4 pulled out right now and the HUM is still there when intensity is turned down and the storm is still there.  If I pull out V5 there is no change on the storm.  Didn’t check the hum with V5 out.  If I pull V6, the amp is dead quiet.  That would be the LTP PI.


In regards to the static storm noise....
V1 makes no difference.
V2 makes no difference.
V3 makes no difference.
V4 makes no difference.
V5 makes no difference.
V6 makes the entire amp dead quiet when removed.


There is something before V6 and after V4B that is getting through the phase invertor and making a racket.  Its very low but I’m listening in a quiet room.  Its not “ok”.


Additionally, my CH 2 (vib) Volume pot has a bit of a crackle (all tubes in) when turned and as I work it back and forth it is getting stiffer.  All other pots are super smooth.  These pots are brand new, albeit I purchased them like 10 years ago.  They have little to no use on them.  As much of a hassle as it is, I think I will replace that POT.  When I go into the amp again, I would like to tackle whatever is making this noise.  I’m not sure I can record it with my phone but its very audible to my ear.

[proaudioguy]

I should note, the storm noise appears after the amp is hot.  When its first turned on, that is not there.  Again, if I pull V6, it goes away.  I thought about the resisters on the power tubes getting hot but I would think they would make noise even if V6 was out.  Also I don’t have them across the tube socket like Fender does.  Mine are off to one side so they won’t get that direct heat.  Also they are metal oxide and pretty big.  Also, they aren’t that used.  Everything other than the transformers in this amp are new (purchased around 8-10 years ago).  The amp has only a few hours of play on it and most of that is between making tweaks.

[proaudioguy]

Here is a video of the noise.  You will need to wear headphones and crank it up to hear it.  I used the phone and it doesn’t record that great.
One thing the mic picks up that I cannot hear over the refrigerator in the next room is that 120Hz hum that is going along with the noise.  Later in the video I turn down the intensity pot and you can hear the 60cycle hum.

&feature=youtu.be

[proaudioguy]

Just occurred to me that V1B is still in the circuit even if the volume is turned all the way down.  That tube has been swapped before but I’m going to pull it out and see if that makes a difference for the bacon frying noise.  It won’t affect the HUM though.

[proaudioguy]

Progress.  Pulling V1 makes the bacon frying go away.  I put a different tube in V1 and the bacon frying comes back.  Doesn’t appear to me to be a bad tube.  What do you all think?  Volume pot is down so has to be between the volume pot and the 220K mix resister.  I could hear the 120Hz hum when I got close after pulling out V1.  The intensity 60Hz hum is still there.  This is why I posted that question about DC heaters.  I would like the amp to have nothing but resister SSSS when the gain is wide open.


So, any ideas?

[kkillebrew]

Hi there, sorry you are having amp problems.  Regarding the crackle, since it goes away when you remove/change V1 then check or replace the plate resistor(s) for that tube.  They should be 2W metal film 100K or RND65 at the least.  Even if they are correct in type I would still look there first since the problem appears after warm-up. If it's not that look for a leaky coupling cap that is connected to either stage of the tube.  It could also be a cold solder joint.
.
Any kind of crackle you get from a pot that is known good and clean is likely to be a DC leak from somewhere, most likely a coupling cap, and this can hold true with a leaky cap for the entire circuit - not just when you turn a knob. 

A quick test for the reverb hum would be to lift the ground leg of the reverb pot and attach it to another ground point, ideally to the cathode of the the recovery tube (V5?).  I have had this occur in lots of amps I have built...  The Hoffman layout shows a ground buss that appears to connect to the chassis and if yours is that way AND you have connections to ground via the chassis at other places, like the input jacks, you are inviting ground loop problems - I know fender did it, but you have replaced their original design, and as a general rule you should have only one chassis ground connection, and that should be at the input jack. But that's another topic.

Lastly, if you get a reduction in noise when you touch the amp chassis with no guitar plugged in, then check continuity between the chassis and the AC ground - or round bottom connection of the AC plug.  The AC cord should have the green (ground) wire connected directly to the chassis somewhere near the power transformer, but definitely not to the the amp ground buss.

Good luck!

[proaudioguy]

GREAT ideas.  I am pretty sure those plate resisters are not likely 2W.  They are carbon comp and they aren’t huge.  I have an order in for 1/2W 100K Metal Film to replace them but if they should be 2W then I’ll need to source them.  Mojotone doesn’t appear to sell them so I won’t be able to get them on that order.


The hum is when I turn down the intensity on the tremolo.  The reverb is pretty good.  V4 is the recovery tube for the reverb.  V5 is the oscillator for the trem.  One thing I noticed on the hoffman layout is there is a cap on the bias pot.  I have a cap on the bias board.  Mine is a larger value I think.  I assumed they serve the same purpose and omitted the one on the bias pot.  Mine is in a slightly different location in the circuit though.  It appears to me it is doing the same job.


Your point about the grounds is well taken and has always been a concern.  That being said this IS a real 1969 Twin reverb chassis and transformer set and cabinet and I am extremely hesitant to drill the pot holes out to lift the ground.  Same for the jacks.  If I could get jack isolators that fit the holes and allowed the jacks to tighten up, I would be willing to remove the back shell grounding I have across all the pots.  This amp also has the bress plate.  You could consider this to be 90% original other than the turret boards and components, but I did replace all the tube sockets cause the old ones had too many issues and I replaced all the pots cause its just a lot easier than rebuilding the old ones.  I have the original boards.  They are all warped.  The amp is WAY WAY cleaner than it was with the old stuff on it and that was in the 1990s.  Some would say I am nit picking.


Another question.  If I have it biased too low and it sounds totally fine up to about 7 or 8 (and real loud) and then falls apart when the volume is dimed, is that what happens with crossover distortion?  For some reason I thought it happened at low volume.  I have to bias it low to get the tremolo to work.  After all the “noise” is gone I’ll figure out what I want to do about the tremolo circuit.  I would love to have it working on another tube instead of on the bias of the power tubes.  I am considering making a 15” baffle and trying out my EV 15L instead of the 2 12Ls and taking out 2 of the tubes.  I assume (perhaps incorectly) that doin this will affect the bias setting and the tremolo as well so I figured I’ll wait and see.  Just trying to tackle each item one at a time.  This is by far the best sounding amp I have, and I have several.  I still want a Deluxe Reverb though.

Hi there, sorry you are having amp problems.  Regarding the crackle, since it goes away when you remove/change V1 then check or replace the plate resistor(s) for that tube.  They should be 2W metal film 100K or RND65 at the least.  Even if they are correct in type I would still look there first since the problem appears after warm-up. If it's not that look for a leaky coupling cap that is connected to either stage of the tube.  It could also be a cold solder joint.
.
Any kind of crackle you get from a pot that is known good and clean is likely to be a DC leak from somewhere, most likely a coupling cap, and this can hold true with a leaky cap for the entire circuit - not just when you turn a knob. 

A quick test for the reverb hum would be to lift the ground leg of the reverb pot and attach it to another ground point, ideally to the cathode of the the recovery tube (V5?).  I have had this occur in lots of amps I have built...  The Hoffman layout shows a ground buss that appears to connect to the chassis and if yours is that way AND you have connections to ground via the chassis at other places, like the input jacks, you are inviting ground loop problems - I know fender did it, but you have replaced their original design, and as a general rule you should have only one chassis ground connection, and that should be at the input jack. But that's another topic.

Lastly, if you get a reduction in noise when you touch the amp chassis with no guitar plugged in, then check continuity between the chassis and the AC ground - or round bottom connection of the AC plug.  The AC cord should have the green (ground) wire connected directly to the chassis somewhere near the power transformer, but definitely not to the the amp ground buss.

Good luck!

[proaudioguy]

Hello Friends!


I have an update.


I removed the carbon comp 68k resisters from all 4 inputs and replaced them with metal film at the tube pins. I had to run an individual shielded cable from each jack to the resisters then tied them together at the grid V1A and V2A.


I replaced the carbon comp 100K plate resisters on V1 and V2.  I will probably do V4 as well but I figured that was enough to check for noise cause I could just turn off the reverb.


I removed the back of POT ground bus and all the solder on the pots and made a floating ground bus.


The result, the storm noise is still there.  The hum/ buzz appears to be increased slightly.  No change to the hum that comes up when I turn the tremolo intensity all the way down.


I pulled each tube starting with V1 and the noise remained until I pulled V6.  Previously I was sure the bulk of the noise was coming from V1 and V2.


I’m open to suggestions!!!


Questions about the ground bus.


Some drawings show different ways of doing this.  This is how it IS though.
The negative from the 2 PSU caps in the can grounds to the ground bus behind the pots.  Hoffman shows all the caps on the same ground all the way to the main grounding lug.  The original Twin reverb has this grounded at the brass ground bus near input of channel 2, then the wire continues under the board presumably back to the main ground.  I have a grounding bus on the turret board.  Channel 1 input ground is the start of the floating bus and also is the attachment point to the ground bus on the board at the other side of the cathod resister cap pair for V1 just like the hoffman layout.  This is how it was before as well except I had the bus on the backs of the pots, and I figured that was causing possible additional paths to ground through the chassis.  The backs of the pots are not grounded to the bus anymore.  On the hoffman layout at the far right of the ground bus behind the pots, there is a ground symbol and it says ground bus.  Should that also be attached to the main ground?  That would create a loop would it not?  I believe my ground from the bus to the cathode/ resister pair, is grounded to the main lug with an under board wire, but its been so long, I guess I should double check that.  Looks like the current layout does NOT have that wire.  Maybe that is part of my problem?  So it is possible the path to ground is the wrong way but this was how I was told to do it at the time.


Any ideas about the storm?  What about the hum when the intensity is turned down all the way?

[proaudioguy]

Of course the chassis is in contact with the input jack so there are 3 paths to ground there.  The input jacks for Normal chan through the chassis and through the bus wire to the board and to the input jacks of channel 2 which also make contact with the chassis.  There is no way around this unless I drill the chassis jack holes out for isolators but its a 68 or 69 chassis and I’ve managed not to make any new holes.  I’ve even got the turret board mounted to stand offs using the original screw holes that screws the eyelet board down.


I forgot to mention, I sprayed Deoxit Fader lube F5 into all the pots and they don’t appear to be making any noise now at least not in regards to dust.

[sluckey]

https://el34world.com/charts/grounds.htm

[HotBluePlates]

The hum is when I turn down the intensity on the tremolo. 

No-hum with Intensity full-up, Hum with Intensity full-down.  So look at what is at either end of the Intensity pot:  a bias-supply (hummy side), and a cap-to-ground (no-hum side; tremolo circuit).

The above suggests, "the hum might be from the bias-supply, where there should only be d.c. volts."  So set your meter for Volts AC, and measure the output of the bias supply.  If you have significant a.c. voltage there, you need to fix your bias supply which is not filtering properly.

It's easier to see what I'm talking about if you look at the schematic (on Page 7) of the Hoffman AB763 rather than just gazing at a layout.  I know Doug originally never supplied schematics (because "it's same-as the AB763"), except the connection from Trem Intensity to the bias-supply is not-same-as-AB763.

[proaudioguy]

https://el34world.com/charts/grounds.htm

OK so I have 2 channels of input jacks.  Should I break the ground between them?


Also channel 1 is connected to the preamp ground and then the preamp ground has an under board wire back to the chassis ground point.  I should eliminate that?


The other confusing thing is you should 3 caps grounded by the preamp and 2 grounded on the chassis ground, but on the Twin reverb, its the other way around.  I even have gut shots of an original unmolested TR and it is that way as well.  On the twin reverb there are 2 large and 1 smaller that ground to the chassis ground and 2 smaller that ground near the preamps.  I’ll look at it again in the next couple days.  Now its off to see the grands.


Do ya’ll have any idea where the “storm” might be coming from given that its there even with all but the PI tube pulled out?  Its got a JAN Phillips 12at7 and I’ve had several different ones in there.  All NOS.

[proaudioguy]

The hum is when I turn down the intensity on the tremolo. 

No-hum with Intensity full-up, Hum with Intensity full-down.  So look at what is at either end of the Intensity pot:  a bias-supply (hummy side), and a cap-to-ground (no-hum side; tremolo circuit).

The above suggests, "the hum might be from the bias-supply, where there should only be d.c. volts."  So set your meter for Volts AC, and measure the output of the bias supply.  If you have significant a.c. voltage there, you need to fix your bias supply which is not filtering properly.

It's easier to see what I'm talking about if you look at the schematic (on Page 7) of the Hoffman AB763 rather than just gazing at a layout.  I know Doug originally never supplied schematics (because "it's same-as the AB763"), except the connection from Trem Intensity to the bias-supply is not-same-as-AB763.

Yes perfect logic.  Problem is my bias supply doesn’t match the hoffman and I can’t find anything with all the detail in one place.  My supply is for a twin reverb.  The filter cap is not attached in the same location as the hoffman schematic, but otherwise is the same.  I’ll see about drawing it and uploading a pic of how its attached.

[sluckey]

Just put a 47µF 100V cap between the bias pot wiper and ground.

[proaudioguy]

Just put a 47µF 100V cap between the bias pot wiper and ground.

Right so that is an ADDITIONAL Cap in the bias supply then.  The one that is there is like this with the bias connected to the bias pot this way but of course after the bias pot the intensity gets involved which is not the case on the stock twin reverb.


So, do I MOVE the cap in teh bias supply or ADD another cap?

[HotBluePlates]

...  The one that is there is like this with the bias connected to the bias pot this way ...

I've encountered the same hum you have one time in a vintage Fender amp.  It was a Princeton Reverb, and the hum was present even if I pulled the phase inverter tube from the socket.

The cause was a failed capacitor in the bias supply (the only cap there, same as the one you circled).  Capacitor replaced, hum gone.

The Fender bias supply is a half-wave rectifier: the output is 60Hz, same as line frequency.

     -  The bias supply is the only place besides the heaters (and power cord/PT wiring) where there is strong 60Hz.
     -  Hum injected from output tube heaters to the audio should be small relative to audio signal.
     -  Yank the phase inverter, that leaves only under-filtered bias supply to inject 60Hz right to the output tube grids.

[sluckey]

Temporarily add another cap. Connect as I said above. Observe polarity! If no joy, remove it.

[proaudioguy]

...  The one that is there is like this with the bias connected to the bias pot this way ...

I've encountered the same hum you have one time in a vintage Fender amp.  It was a Princeton Reverb, and the hum was present even if I pulled the phase inverter tube from the socket.

The cause was a failed capacitor in the bias supply (the only cap there, same as the one you circled).  Capacitor replaced, hum gone.

The Fender bias supply is a half-wave rectifier: the output is 60Hz, same as line frequency.

     -  The bias supply is the only place besides the heaters (and power cord/PT wiring) where there is strong 60Hz.
     -  Hum injected from output tube heaters to the audio should be small relative to audio signal.
     -  Yank the phase inverter, that leaves only under-filtered bias supply to inject 60Hz right to the output tube grids.

Did not occur to me that a brand new bias cap was bad but it makes perfect sense!  I’ll dig back in hopefully before end of Monday.  Need to see if I can find another electrolytic.

[HotBluePlates]

Did not occur to me that a brand new bias cap was bad but it makes perfect sense!  I’ll dig back in hopefully before end of Monday.  Need to see if I can find another electrolytic.

That's why I said first to measure with a meter, and find out for-sure if the 60Hz is coming from the bias supply.

I supplied a theory, but every theory should be tested.  That way you don't blindly replace parts hoping to stumble on a solution.


(This is a huge problem on every forum I've visited; people try "recommended fixes" without actually doing the diagnostic steps to figure out what is causing the problem.  Somehow they are surprised when they try the fix, but the problem remains.)

[proaudioguy]

I do not have a reliable meter but I will attempt.  I’m trying to find a Fluke 87V.  I’m done with cheap crap, but I
‘m not going to pay $400+ for one.  The pawn shops say “people must be working” cause no one is selling their gear.  Wish I was one of them!


I wanted to clarify my bias setup as it is and just get a final instruction before I tear into it.  See attached.  Simply moving the wire from the outside lug to the middle lug with the other wire, would do the trick?  I have been wondering about this for a while but it seemed like it would be 6 of 1 half dozen of another.  OR....add additional cap to center lug (missing part?)?  Or disconnect cap from diode and leave diode on the outside lug and add a wire to put the cap on the center lug?

[sluckey]

 I know what your bias circuit looks like. Easy to see from the pic you posted. I can't state it any simpler and clearer than I already did. Just do it!

[proaudioguy]

I moved the cap to the center lug and left the rest of the bias supply on the outside lug.  As far as I can tell without the amp in the cabinet, the hum is gone.  Can’t be 100% sure but it appears that way.  If I’m wrong, I’ll post back.  Here is a video.  I wonder if this is NORMAL or not.  Its been like this since day one with this circuit.  First time I’ve had it plugged into a raw driver so it can be easily seen.  There is nothing plugged into the input here.  More after the video....

&feature=youtu.be




So I plugged the PI back in and the quiet storm is of course back.  That is the last demon to chase out I think.  I don’t know where to start. I could swap the 100K plate resister for a metal film but I do not have a metal film 82K.  I don’t have metal film 1Meg Grid stoppers either.  Do I need to get this stuff?  I mean these amps from the factory don’t have metal film but they don’t have this static storm.  I would think the resisters would just cause straight HISS.  I pulled the oscillator tube back out so you could hear the storm.  Not sure if its loud enough over the cars going down the road but trust me, it is audible in real life.


I let it warm up with just the PI and the power tubes in and flipped it off standby.

&feature=youtu.be

[sluckey]

That sound could definitely be caused by noisy CC resistors. The 100K and 82K are the most likely suspects, but it could also be the two 1Ms. And maybe even the 470Ω and 22K. They're so cheap that I would change all six resistors. Hoffman sell them. Just follow the resistor link below this message.

The node C filter cap is also a suspect if the resistors don't fix it.

[proaudioguy]

OK thanks, I’ll order all of them.  I just find ist crazy that all these resisters are bad.  I’m sure it will be a few weeks before I can get back to that due to the ordering and shipping aspect.

[shooter]

It's probably one or two, the solder iron is hot, you're already there..........

[proaudioguy]

I just replaced the 100K on the PI Plate and the crackle appears to be GONE!!!  The 82K is still carbon comp.
Now I can just hear the RIZZ, now this is with ONLY the PI installed, everything else out.


The rizz (in my line of work rizz is harmoncs of hum, often appears when motors or fluorescent lights (K-mart levels) are on circuit) is VERY low but audible, but only if I stick my head in the speaker.   


I would like to get rid of it but not sure what else to do except maybe get better matched tubes?  Upping the capacitance on the A supply node won’t help, right?

[proaudioguy]

As I add more tubes, the rizz gets louder.  Quite audible now.  I have V3, V4, V6 in and all the controls all the way down and the riz is quite noticeable.  Never really heard it this loud before but that “storm” and the hiss was possibly masking it.  I still feel like I’ve made progress today




Edit, I think the RIZZ is my soldering iron!  I’ll be back in a few and edit again.


Here is a pic before I put it back in the cabinet.  If you all have any advice as to any wire routing, I’m open to that. as well.  The distortions on the photo are because its a panorama.

[proaudioguy]

So the HUM is still there when the intensity is rolled all the way down, but there is also a tick from the speakers moving when at full intensity AND the tremolo effect is MUCH weaker than before.  I already had the bias really low prior to this to get it strong.  I would not think moving the cap would affect the strength of the bias.   


I guess I just couldn’t hear the hum in that 10” raw driver.  It may well be reduced.  The hum is there with or without the oscillator tube installed.


Should I increase the capacitance?  I have a 100uF 100V sitting here I could use.


The tick is literally the more responsive of my speakers (JBL I just put in with the EV) and I don’t think I have actually played the tremolo since I put that speaker in there cause that was just last week.  The EVs are a lot stiffer.  They were also moving (and the one in there still is) but not making the noise.  They have a much stiffer spider and a heavier cone.


So is that movement normal or not?

[HotBluePlates]

I do not have a reliable meter ...

...  I just find ist crazy that all these resisters are bad. ...

Maybe they're bad, maybe they're not.

There's a way to test resistors for noise in resistors (bottom), but it means a need for more test equipment (and the test exposes you to high voltage, so it's not safe).  You also have to remove & test each resistor, then maybe re-install them if they're good.  So that's a bit of time.

If your test winds up finding noisy resistors, you'll have to replace them.  So you're going to need more resistors on hand anyway.

Where Sluckey is going is resistors are cheaper than test gear.  They may be "cheaper than your hourly rate" that would be spent removing, testing, reinstalling even if you had the equipment to do the test.

     So it's your call how to proceed:
          - "Economize" by saving the cost of more resistors, by buying more test gear and spending more time.
          - "Economize" by saving the cost of gear & time, by simply replacing the suspect resistors & see if the noise goes away.

The latter is contrary to my argument earlier that one should use test equipment to find the actual problem.  But it still probably better than spending $100+ finding the item to do the test and spending another~$100 of your time, when the resistors could be replaced in 5-6 minutes (if you had resistors on-hand).

(Lately, I've bought some test gear others assumed I always had.  I want to spend the time because right now because it suits my interest.  Some other time, I may just want to get-it-done.)

[HotBluePlates]

... Now I can just hear the RIZZ, now this is with ONLY the PI installed, everything else out.

The rizz (in my line of work rizz is harmoncs of hum, often appears when motors or fluorescent lights (K-mart levels) are on circuit) is VERY low but audible, but only if I stick my head in the speaker.  ...

You've got an open chassis, and hummy, buzzy stuff is in the air all the time.  See the video and how much crap the guy picks up with the probes for his signal tracer, unless he shields the probe with his hand, or supplies extra capacitance-to-ground by holding the grounded side of the probe.

So what happens if you put a conductive metal plate over the open side of that chassis?  You know, to shield the open side?  Which is normally in contact with the metal screen stapled to the inside of the cabinet?

[proaudioguy]

I swapped out the 50/50 for a 100/100 in the bias supply.  I still have it going to the center lug of the bias pot and the voltage going to the outer lug and the other outer lug going to the bias resister to ground. Hum is the same.


Still not sure if I should have 1 cap in the normal supply tied to the diode and going to the outer lug and a second cap on the center lug, or if the idea is just to have the one single cap and just have it moved to a different spot.


Now that I have the 100uF, on the center, I could add another 100uF to the diode where the original bias supply was.  That would increase the capacitance very high though right?  There would be some resistance between them when the 10KL bias pot was not all the way to one side.




And yes, I tested with the chassis back in the cabinet with the metal shield in place on top.


Edit, clipped 50uf cap on middle lug, move new 100uF cap back to stock locaton, hum gone.


So it seems you need both caps.

[proaudioguy]

Adding the additional cap to the center post, otherwise leaving the bias stock, solved the hum that comes up when the intensity goes down.  This should be added to the PDF when using the BIAS board.  One or the other did not work.


Also, putting the bias back to stock (albeit with a 100UF cap)  got rid of the tick from the speaker and brought the intensity back UP.


The speakers still MOVE and I still want to know if this is NORMAL or some other issue I have in this amp.  I have no point of reference other than all you guys that have built these.


I will order metal film resisters for all the grids, the 82K on the PI and the 22K and the 470 Ohm.  Not of this will get rid of the RIZZ though. I could totally live with the steady hiss I have if it wasn’t rizzing.  I’ll also order 2 50uF 100V caps for the bias unless you all think the 100/100 on the main supply is OK and the 50/50 on the center lug is also OK.  When I bought these parts, I ordered a KIT then substituted a bunch of the parts here and there but the BIAS was as it came in the kit.  It was way easier for me to order the parts kid and not use all the parts than to try and sort what resisters needed to be 1W, what needed to be 1/2W, and all the voltage ratings of various caps.  This was my first amp.  I ordered all the parts back in 2008 or so IIRC.  So to be fair the bias cap is 12 years old, but thats brand new for the most part cause its barely been played and I’m sure they haven’t dried out in my home.


I’ll shoot another video from the front of the amp so demonstrate.  The amp sounds gorgeous btw.  Getting rid of that STORM and the HUM is huge.  I could live with the hum keeping the intensity at full but that storm was random, not any sort of pattern.  I’ll let the amp sit on for a couple hours and see if anything comes back.


You guys are awesome!  Thanks for being patient with me.  Interestingly, the Princeton Reverb I built in 2014 had only 1 issue upon first power up which was a reverb send wire was on the wrong eyelet or disconnected and that amp has been perfect since I got it together. This Twin was the one I made all my first timer mistakes on.  I cleaned up a lot of solder the last few weeks.  Some of the turrets just had TOO MUCH so I sucked it off and reapplied just a tiny bit.  Has some PVC that was melted right at the solder connection, stripped those back and cleaned them up.  Little things here and there, none of which improved or disturbed the sound, but it looked awful.