hum/buzz

[tubenit]

If you had an amp that had a slight hum/buzz with ALL the potentiometers turned down to zero whether a guitar or no guitar is plugged in................


What would be your starting place to try and diagnose and locate the problem?   Is there something "telling" specifically to a hum/buzz with the pots to zero? Once you start turning the volume pots up, the hum/buzz does get louder.


This happens whether amp is in the cab or not in the cab. I can't find anything mechanical that would explain this.  Voltages are good. Tried all new tubes known to be good,  different speaker, different guitars, different guitar cords, etc...............


Chopsticked everything, checked for leaky caps, reflowed solder joints, disconnected all relay switching, redid some heater wiring on preamp tubes, looked for ground loops and am following Hoffman's ground scheme and tried a couple of other grounding approaches also.  Checked all shielded wiring, flipped OT wires, disconnected the reverb, replaced a couple of filter caps, etc..... ........................   And I checked resistor values, checked for continuity, measured ohms across solder joints and more ..............  I tried bypassing and jumper over gain stages to try to pinpoint it (which usually works quite well) but no luck narrowing it down.


I am at a loss on this one.  My main specific question is there something unique and informative about the hum/buzz existing without anything plugged in and all pots to zero? 


I'm using a Hammond 1620 OT that has ultra-linear wires shrink tubed off and not connected to one another.  I've wondered if there might be some inherent noise with the O.T. not using the ultra linear wiring?   I should add, I hear no noise from the OT or PT itself meaning no vibration etc...   mechanically.

The noise is reasonably not at issue at volumes I normally play at.  Having said that, my other two amps are just about dead quiet at idle. 


Unless there is something revealing about the buzz/hum with all pots turned down, my best guess is this is an amp that has been completely rebuilt once (gutting and reusing the turret board) and modded several times & I'm thinking this may be an issue of one too many rebuilds and mods involved?


With respect, Tubenit

[sluckey]

What's the first thing we always want to know???

schematic? pics?

[tubenit]

When someone asks about resolving a hum/buzz problem, I typically ask for a schematic, hi-rez photos and voltages.  In fact, I usually won't help if those are not provided as it seems too often a waste of my time without that info.  I am affirming that it's a very reasonable and appropriate question to ask prior to helping.

Given this amp had a complete rebuild gutting the turret board and then reusing the turret board and then further mods beyond that, I think posting pictures would be somewhat futile in this case as too much of it would look questionable.

It certainly would be understandable to pass on trying to help on this thread without the pics. No problem.

Having said that, my primary focus and question is whether someone knows offhand if a hum/buzz with nothing plugged in (or guitar plugged in) with ALL the pots turned to zero would be informative of something else to try beyond the lengthy steps I've already taken?

For example, when someone states they've fired up a new build and it squealed, I know that "might" be that the OT wires need to be reversed.  So, I am looking for something informative of that type IF it exists to this specific question of a hum/buzz with all pots to zero.

Unless someone names something that I haven't already tried, I'm likely to just focus on enjoying the amp as it is & letting go of it being dead quiet at idle.......... OR perhaps gutting it someday and putting in a new turret board, tube sockets and pots. 


Respectfully,  Tubenit

[sluckey]

If it hums with all the pots on zero that kinda points to the PI and PA. To be sure, pull V1, V2, and V3. If still got hum, concentrate on PI and PA.

I think posting pictures would be somewhat futile in this case as too much of it would look questionable.

Questionable build quality could also be a factor.

[tubenit]

If it hums with all the pots on zero that kinda points to the PI and PA. To be sure, pull V1, V2, and V3. If still got hum, concentrate on PI and PA.

Steve, thank you for the response!  I appreciate it!

Yes, with V1, V2, & V3 pulled the hum is still there!  That has given me another idea of something else to look into.  Will check it out when I can and post back maybe on Monday?

IF an outlet had neutral and black reversed from what is proper ............. could that possibly cause a hum but the amp still play?


With respect, Tubenit

[sluckey]

The amp would still play but there might be some hum. Probably take less time to verify on your end than it took to type this message.   

[uki]

Tubenit, what about the transformers bolts, are the bolts really tightened?! I have seen transformers buzzing due to slightly lose bolts, the ones that hold the layers of the transformer, also the ones that hold the transformer to the chassis.

[pdf64]

Buzz usually indicates a heater problem. eg valve’s heater to cathode insulation imperfect, poorly balanced heater circuit, heater circuit not adequately AC referenced to 0V common.

Where the problem is due to a valve, suitable DC heater elevation can be very effective in resolving it.

[tubenit]

Bolts are tightened. Not a mechanical issue.


pdf64, thanks for that idea of elevating heaters.  Since I am using the 6.3v for my relays also ................... can I still elevate the heaters?  The relays are the ones Hoffman sells.


With respect, Tubenit

[pdf64]

The rectified supply etc for the relay would need to be isolated from chassis 0V common, as it will be sitting up at the DC elevation too. So it’s a bit trickier, but still feasible.
A stereo jack for the footswitch could help with this, ie by using the tip and ring contacts.

[tubenit]

The relays don’t use chassis ground but have sort of an “ground” within the relay structure itself.  Archives “relay switching” thread describes this. (Reply #3)


Foot switch for relays is isolated from chassis.


So I am understanding your response to indicate this should work? Appreciate your help! Thank you.?


With respect. Tubenit

[pdf64]

Yes, you should be fine to proceed on that basis.
Fingers crossed it helps!

[tubenit]

Thank you!  I appreciate the help and the response.  Will give it a try and report back.


With respect, Tubenit

[ac427v]

I Always agree with what Sluckey says. This time too. But if the hum gets louder when you turn up the volume then it is coming from an early preamp stage. So I think you have two hum problems.

Sorry

[Bieworm]

Hey Tubenit!

Surprised to see you having this kinda problem :)
On my latest build I had a buzz I couldn't get rid of. Turned out to be a misplaced grounding in the power amp, and the NFB wire needed to be shielded.
IDK if that is gonna be your issue, but it helps to take a schematic and separate the grounds visually (giving them dedicated colors for instance) and check if you created a ground loop at some point. Try moving ground connections in your PI, PA.
Also, some amps like the NFB wire to be shielded. I had 2 identical builds where one needed to be shielded, the other one didn't...

Also Sluckey's remark on tidyness and neat building is 100% correct. I found out the hard way to clean every turret als much as possible before connecting other parts to it. Even between testing certain values.. totally clean from solder.

[tubenit]

Progress!!    

It originally had a slight hum with buzz.  Still has a very slight hum.  The buzz is reduced to maybe 40% or less of what it was.  I would say it is very much within normal limits of amp idle noise and probably reasonably comparable to the original Fender Princeton Reverb I owned at idle.  I can live with this level of amp idle noise and enjoy the amp and probably almost never think about that it has any noise. If I had a ceiling fan on low, I think I could forget to turn the amp off and leave the room.

In other words, it is reasonably quiet and I'm happy with it. The most dramatic difference is when turning the pots up, it's now mostly just white noise and not an increase in "buzz".  For how loud I have the amp cranked, it's acceptably quiet at idle for me.

This is definitely a new one on me in problem solving.  Elevating heaters made no difference in noise. 

I would be quite embarrassed to tell you how many actual hrs I spent trying to resolve this.  This is probably the 5th time I worked on it. The list of things I tried was ridiculously long and towards the end I was really grasping for straws to say the least. 

The issue behind the noise was staring me in the face and I never considered it would even be an issue at all! It seemed such a non issue, it wasn't on my radar. Ugh!

The phase invertor tube socket is original to the amp from 13+ yrs ago or so and it's survived one or two rebuilds and numerous mods. One of the terminals to the pin of V4-6 was twisted in a 45 degree angle or so.  In other words, instead of the pin terminal (that you solder wires on to) being flat and standing straight up, it spiraled and stood up at a leaned over angle. 

I simply twisted that terminal back flat & made it stand straight up  and the buzz went away.  I have NO solid explanation for that making any difference at all. But the difference was rather dramatic.  My wild guess is the tube pin was not making very good contact?

I could leave it alone now and remain happy and consider this resolved.  However, I have a new tube socket so I think I am going to replace the old "worn out" socket with a new one later in the week  AND hope it stays as quiet as it currently is.

The fix was a 3 second solution I tried after countless hours of other efforts.  Quite a surprise to me.

THANKS for all your suggestions and helps you offered!  I am always grateful for your thoughtful and generous knowledge and experience.

And I will offer an apology now for NOT posting hi-rez pictures.  (especially to Sluckey)  Maybe one of you guys would've caught this from the git go?  Certainly is more validation for the need to post the pics when requesting help.  Wish I had done so.

I should add that Sluckey's advice to pull V1-V3 and which suggested the problem being the phase invertor or power amp gave me the idea to go ahead and twist that tab back to looking normal since it was part of the LTPI. However, I honestly expected that it would make no difference at all.

In the end, the only step to victory I probably contributed to the resolution was persistence.   

Best regards and with respect,  Tubenit

[tubenit]

I have another possible theory why straightening and flattening out this terminal resolved the issue.  I am wondering if there was a poor solder joint on this plate connection and by squashing and flattening the terminal, it gave the wire better contact?  Maybe nothing to do with the spiral and bent terminal but a cold solder joint?

If so, another good reason to go ahead and replace the socket altogether.

 ** UPDATE   I have the amp so quiet now that I'm nervous about replacing the socket at all.  It sounds great and is quite reasonably quiet at idle.  When I "tested" it
with my guitar after flattening and straightening the terminal,  I did not pay attention that my guitar was in the #2 position with a single coil and humbucker.  Switching to only a humbucker made the thing even quieter.  I think I'll just reflow that joint and sew the amp back up.  It sounds pretty amazing to me.


I'm thinking because this was V4-6 and a plate pin that a poor contact left it previously humming and buzzing especially when turned up.   


With respect, Tubenit

[SILVERGUN]

Glad you found it and thanks for sharing your experience
 

[Platefire]

Well congratulations on completing a long rough ride. Your patience and endurance has prevailed. I was going to say, have you tried plugging it in at another location. 
Happy Easter! Platefire