How to filter diode buzz from amp?

[proaudioguy]

First off, does the fact that the amp doesn’t have a CT on the PT dictate that a Twin style full wave rectifier cannot be used and a bridge rectifier then has to be used?


This Hoffman Pro Jr just buzzes like mad.  The V1B grid wire is microphonic. It is shielded and the ground is soldered at the buss wire behind the pots.  The tube is not microphonic, only the grid wire.  It buzzes substantially and when I move the wire around the buzz changes.  Even though its shielded, its picking it up and that is with the volume all the way down.


I read the switching of the rectifier causes the buzzing.  The hum is actually very low since I change the 47µF to a 100µF, but the buzz (harmonics of the 60Hz) is unbearably loud, unless you dime the amp volume, then the buzz disappears and the SHH sound takes over.  This is all with the input grid grounded.  BTW, if I ground the V1B grid, the noise is WORSE.  That doesn’t make any sense to me!  How can I clean up this amp?


https://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf

[Latole]

Both Twin and Hoffman power supplys look same or I don't unserstand .

I read that it's a home-made amp.
Most problems in this type of assembly come from assembly errors. I've read it dozens of times in forums, and even if the author claims to have checked everything several times.


65 Twin Reerb Reisue
You link to Hoffman

[pdf64]

First off, does the fact that the amp doesn’t have a CT on the PT dictate that a Twin style full wave rectifier cannot be used and a bridge rectifier then has to be used?

Correct, a 2 phase rectifier won’t provide full wave rectification when supplied only with a single phase. Or indeed any DC output at all if connected across the hot inputs of a 2 phase rectifier.

This Hoffman Pro Jr just buzzes like mad.  The V1B grid wire is microphonic. It is shielded and the ground is soldered at the buss wire behind the pots.  The tube is not microphonic, only the grid wire.  It buzzes substantially and when I move the wire around the buzz changes.  Even though its shielded, its picking it up and that is with the volume all the way down.


I read the switching of the rectifier causes the buzzing.  The hum is actually very low since I change the 47µF to a 100µF, but the buzz (harmonics of the 60Hz) is unbearably loud, unless you dime the amp volume, then the buzz disappears and the SHH sound takes over.  This is all with the input grid grounded.  BTW, if I ground the V1B grid, the noise is WORSE.  That doesn’t make any sense to me!  How can I clean up this amp?


https://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf

Its unbypassed cathodes may the V1 stages susceptible to picking up heater buzz.
Have you tried some different valves in V1?
What is the resistance of either heater wire to the chassis?

You’re almost certainly chasing a red herring in thinking rectifier buzz could cause the magnitude of contamination you’re describing.

[WimWalther]

As a general rule, 60hz hum is produced by heater-cathode leakage or noise picked up from internal AC wiring. It can also result from failing (or undersize) filter caps, faulty grounding schemes, ground loops and so on.

120hz buzz is almost always diode noise from a fullwave rectifier. It can be produced by the amp itself, or by other electronics being powered from the AC lines. And the source may not be in the same room, or even within your own home, for that matter.

[acheld]

I agree with the above comments.

One thing to do is to be absolutely sure that your hum is 60 Hz, 120Hz, or something else.   Distinguishing 60 from 120, although it sounds stupidly simple, is not always so obvious.  I've used phone based spectrum analyzers at times to be absolutely sure.

As Wim states, the approach to 120 vs 60 Hz hum is completely different.

All that being said, diodes can induce noise in your rectifier.   I don't hear this often, but it does happen.  You can quiet that effect down by using UF4007 diodes protected by high voltage ceramic caps. 

There is no disadvantage to using UF diodes in guitar amps -- their Vf is higher than the 1N4XXX series, but that does not matter to us.  They cost about the same as a quality 1N4 diode. 

[proaudioguy]

As a general rule, 60hz hum is produced by heater-cathode leakage or noise picked up from internal AC wiring. It can also result from failing (or undersize) filter caps, faulty grounding schemes, ground loops and so on.

120hz buzz is almost always diode noise from a fullwave rectifier. It can be produced by the amp itself, or by other electronics being powered from the AC lines. And the source may not be in the same room, or even within your own home, for that matter.

I have 5 other amps in this room and they are dead quiet and 3 of them have bridge rectifiers, 1 full wave (the twin) and 1 has a 5U4.  The only noise on those amps is the usual hiss when diming it, and if a guitar is plugged in, the pickups.  The buzz is 120, 180 and even higher.  The 60 cycle is just about inaudible.  All the parts are brand new ordered from Hoffman.  Values are all measured as they were placed and I’ve checked all the part relative to the layout a few times.

[proaudioguy]

I agree with the above comments.

One thing to do is to be absolutely sure that your hum is 60 Hz, 120Hz, or something else.   Distinguishing 60 from 120, although it sounds stupidly simple, is not always so obvious.  I've used phone based spectrum analyzers at times to be absolutely sure.

As Wim states, the approach to 120 vs 60 Hz hum is completely different.

All that being said, diodes can induce noise in your rectifier.   I don't hear this often, but it does happen.  You can quiet that effect down by using UF4007 diodes protected by high voltage ceramic caps. 

There is no disadvantage to using UF diodes in guitar amps -- their Vf is higher than the 1N4XXX series, but that does not matter to us.  They cost about the same as a quality 1N4 diode.

I am well aware of what all of these frequencies sound like, but I agree entirely with your point.  I’m a Smaart instructor,  My measurement rig consists of 5 Eathrowks mics, 1 M30 and 4 M23 (about $500 each).  I also have 4 of them on wireless Lectrosonics system ($12,000 dealer) I train audio engineers how to tune PAs in rooms.  Smaart is the leading sound system optimization software for the last 25 years. 99% concert you will see anywhere more than about 1500 cap will have used Smaart to dial in the PA and often the FOH engineer and system engineer will have it running during the show.  My primary gig is FOH engineer.  I have no other job other than teaching other system engineers.  I have made my living this way for 25 years and I’ve been teaching for 16 of those.  Prior to that 25 years ago, I had other jobs simultaniously.  I’ve been twisting knobs for almost 43 years.


In any case, it was humming and buzzing (we call it RIZZ at work,...ZZZZZZ), but the hum is pretty much inaudible changing from the 47µF to the 100µF in the first stage of smoothing.  It is getting in before the PI.  Grounding the PI Grids eliminates it.  I really need a scope to see if this is on the heaters, I don’t have one.  Doug said the noise was bad on his build with the bridge where it was so he moved it way off the board.  For this kit he put it back on the board but its a lot farther from the tone pot.  Its really no where near anything except the first stage capacitor.  The Rizz/ Buzz has a lot more high frequency content from the V1 Grid.  Pull V1 and its just normal 120/180Hz, still loud.  Moving the V2B grid wire around changes the sound of it.  The harmonic levels change.


The blog I read said the diode switching puts all this noise back into the secondary and it couples with the heater windings.  Does that sound right?


I think I’ll order the snubbers fender added on the later models across the diodes (which apparently still buzzes) and see if that helps.


https://jpfamps.com/fender-pro-junior-hum-investigation/


I’ll try the UF diodes and the snubbers.  EDIT:Ordered 10 UF4007 for around $350 and 4 10nF disc caps for about $10.  Shipping...another $20 into the amp.  I really hope this makes the buzz go away.

[Latole]

I’ll try the UF diodes and the snubbers.  EDIT:Ordered 10 UF4007 for around $350 and 4 10nF disc caps for about $10.  Shipping...another $20 into the amp.  I really hope this makes the buzz go away.

Are you kidding ? IMO you make some mistake

[proaudioguy]

I’ll try the UF diodes and the snubbers.  EDIT:Ordered 10 UF4007 for around $350 and 4 10nF disc caps for about $10.  Shipping...another $20 into the amp.  I really hope this makes the buzz go away.

Are you kidding ? IMO you make some mistake

You think? 

  Here you can hear the nasty buzzing when the volume is anywhere below full on.  You can also hear the amp is full functional.  Are you sure its not a poor design?  Are you sure I made a mistake?  Feel free to let me know my error so I can fix it.  I included plenty of photos.  Admittedly my phone mic is covered by my leg as I was trying to hold it between my ankles so it wouldn’t fall over.


Here is a longer video.  Found another problem. Never heard that before.  I just changed the speaker for this one that sounds better.  The other one had way too much mids.

This one may not be uploaded yet.

[Latole]

I read you order 10 UHF diode for around $350

Digikey sell them $0.64 !

Or you forget a dot after the 3 ?

[proaudioguy]

I read you order 10 UHF diode for around $350

Digikey sell them $0.64 !

Or you forget a dot after the 3 ?

You are giviing me crap over a typo?  Thanks bro.  I said the total with shipping was about $20.

[Latole]

Standard diodes are quiet, no need UHF diodes.

You issues are not from the diodes;
Show the wiring
It is a hand made amp riight ?

[acheld]

I am well aware of what all of these frequencies sound like

Yes, I know that from your comments in other threads. I hope to learn from you. 

My point wasn't directed at you per se, but at all the readers who come by later.   Many of whom have little or no electronics background or amp troubleshooting experience.   

That's really what this forum is about, IMO. 

Anyway, figuring out rizzzz/hum/noise can be a challenge for any of us. 


   

[Esquirefreak]

I think it sounds like one or several bad grounds. The high freq hiss/rizz might be inaudible oscillation or cross talk. Just thinking out loud.

/Max

[proaudioguy]

I think it sounds like one or several bad grounds. The high freq hiss/rizz might be inaudible oscillation or cross talk. Just thinking out loud.

/Max

So everything at this point is tied back to either one of the transformer keps nuts or to the power cable ground.  I am wide open to any and all suggestions. 
I agree this latest noise sounds like a subharmonic of an oscilation.  I really need a scope for this kind of thing.


acheld, I am happy to answer any questions that I know answers to.  You can PM me and I’ll send you my email address.


Latole,


The newer Pro Jr uses snubber caps ont eh diodes.  The Rivera Era amps also use snupper caps on teh bridge rectifier.  This is clearly an issue Fender has.  I wonder if the flaw is actually the PT.


I followed the Hoffman layout for all the grounds.  Fender had a ground right next to V1.  Fender also had all the pots and the input jack 100% isolated from the front panel.  I need a plastic washer to get the input jack 100% isolated.  At the moment it is touching the nickel plated panel with the metal washer but I did tri it just hanging free as well and it made no difference.  I took photos of the original board before I sold it so I will look closer at that and see what I can find.  The ground screw ended up covered by the board due to not having anough clearance to mount the board in the original location I had picked.  I had to redrill all the chassis holes and move it an inch and although I can still see the ground screw, I can’t quite get a screw driver on it, but is not too hard to loosen the board.  I can certainly tie a Preamp ground in over there.

[Latole]

You say you've done everything right and checked, but there's still this problem.

I'm sorry, but this is the same story I've read dozens of times on forums and it always ends the same way;
The amateur builder made mistakes. Once corrected, after weeks of discussion and serious revision, the amp works very well.

I stop following.

[WimWalther]

The blog I read said the diode switching puts all this noise back into the secondary and it couples with the heater windings.  Does that sound right?

Yes, that's definitely one way that diode noise can propagate.

[proaudioguy]

You say you've done everything right and checked, but there's still this problem.

I'm sorry, but this is the same story I've read dozens of times on forums and it always ends the same way;
The amateur builder made mistakes. Once corrected, after weeks of discussion and serious revision, the amp works very well.

I stop following.

I say I built it exactly as Doug posted it.  I’m not sure Doug has ever built it that way since his personal amp is not built this way.  The board was revised after that, but yea it started out as identical.  I have built a Twin reverb from scratch, only issues with that turned out to be a bad reverb transformers.  Built a Princeton from a kit and that had zero issues.  I have heavily modified a deluxe reverb II and somewhat modified a Concert II, and I’ve recapped about 6 or 7 amps.  I have yet to put anything in the wrong spot or backward.  In every case any issues I ran into have turned out to be faulty parts.  I measured every part going into this.  There isn’t much there.  Now I’m just changing things just for trial and error because I don’t have a scope to actually look at what happens when I make changes.  That being said there is obviously a very microphonic capacitor in there.  Those 2 caps on that in were originally mica in Doug’s but he discontiued selling them so I had to substitute them and I still ordered the parts from him.  Perhaps these little yellow caps are not appropriate here.  Just remembered, I also changed a Marshall Lead into a Marshall Bass and added a LarMar, and it fired up first shot.  I am not a tube amp guy, but I am all about attention to detail for my day job.  I’m in a sevice industry as a full time FOH engineer.


In any case you are super rude and your comments are not in the leasst bit helpful so if you don’t want to help and you don’t want to follow, you should stfu.  I’ve been on this forum under 2 names (lost login for orignal) for 20 ish years (possibly longer), since I got my twin reverb turret boards from a member HERE in 2003 or 2004.  I’ve never run into anyone so rude.

[proaudioguy]

The blog I read said the diode switching puts all this noise back into the secondary and it couples with the heater windings.  Does that sound right?

Yes, that's definitely one way that diode noise can propagate.

This new mid volume parasitic oscilation only happened after I changed the speaker from the original ceramic emi fender label to a 27 year old Mojotone Alnico (which I used to have in this amp).  I’m going to move all the preamp grounds to the ground lug that is next to V1 in the amp and go from there.  There were some snubbers in the original circuit that Doug removed.  I added 1 of them back.  I’m also going to change the 10K to 33K to drop the corner frequency of the input Grid on V1 and see if that helps.  Changing the speaker did drop the level of the rizz a lot.  I’m guessing the speaker is just less sensitive.  Its still plenty loud.

[proaudioguy]

I think it sounds like one or several bad grounds. The high freq hiss/rizz might be inaudible oscillation or cross talk. Just thinking out loud.

/Max

Yea, I think there is definitely a new out of pass band oscilation going on and the sum and difference frequencies of the intermodulation are rearing their head, particularly the difference signal.

[proaudioguy]

I moved all th grounds on the RIGHT side of the amp to 1 Transformer lug.  I know that’s not best practice, but the 2 grounds over there reached.
I moved ALL the grounds on the left side of the amp to that ground screw next to V1.
I changed the resistor at the grid of V1A to 36K, instead of 10K.
I rearranged the POTS so the lugs are facing each other like inside a strat.  I eliminated one of the wires between the volume and tone and soldered the lugs to each other.  Many folks said the screw for the back cover getting NEAR the tone would cause a buzz so I figured I would get those lugs further from the chassis, even though I have not had the back cover on, and I have a shorter screw for that hole.


The amp is currently out of the cabinet.
There is a random noise occassionally, but I also have computer in my hand right next to the amp, so I can’t be convinced of that being a thing just yet.  The amp is dimed.


The ghost tones are totally gone.


The amp still has the rizz, but the amp is a little louder overall.  I’m thinking the oscillation was stealing a bit of power.


I’m hoping when the snubber caps get here that will clean up the buzz.

I put the amp back in the cabinet and the back cover back on and the ghost notes are back.

[proaudioguy]

Have volume and tone all the way down.  TOUCH speaker cone and make it MOVE and hum gets louder.  WHen its not moving, its not as loud.  It only gets louder while its moving.  Never saw that before in any amp ever.


BTW, this amp sounds substantially better playing through an EV Force 10 in my Super Champ.  Also, it has massive amounts of bass that the built in speaker is not reproducing.  Might help to taylor that response a bit?

[proaudioguy]

New video.  SO the ghost notes are caused by whatever touching the speaker does, which is adding more noise.  When the speaker moved the noise goes up.


If the speaker jack was wired backward and the feedback was wrong, wouldn’t that make it squeel audibly?  I tried it disconnected the other day.  The colors match what Hoffman did.

[Esquirefreak]

About the switch popping - You could try a X2 10nF cap across the PT primary winding. I use those in my builds and in some amps they made a big improvement as far as popping noises go.

/Max

[proaudioguy]

About the switch popping - You could try a X2 10nF cap across the PT primary winding. I use those in my builds and in some amps they made a big improvement as far as popping noises go.

/Max

I ordered 4 of the 1000V versions from Digi-key yesterday.  Thank you!