AB763 White Noise & Hum

[h4ck.b0x7]

How much white noise is acceptable when your amp is on with nothing plugged in and turned up to 10? Do you get any white noise? Regardless, what causes it?

Do you get any hum when you crank the reverb?

It's not crazy but there is some noise. For example, if I crank the amp and then roll down the volume on my guitar to bedroom levels I can hear some white noise in the background.

[h4ck.b0x7]

I should mention I did some poking around with a metal chopstick and it didn't seem to make any difference. It did hurt like a SOB when I got blasted though. Why do people use chopsticks if they get shocked?

[sluckey]

I should mention I did some poking around with a metal chopstick and it didn't seem to make any difference. It did hurt like a SOB when I got blasted though. Why do people use chopsticks if they get shocked?

Really! I don't think I've ever heard of a metal chopstick.

[h4ck.b0x7]

Sorry, I have a sick sense of humor. No I didn't use a metal chopstick, I used a regular wooden one. I had to go get sushi for lunch because I didn't have any around the house. Poking around didn't make any differences though.

[tubenit]

How much white noise is acceptable when your amp is on with nothing plugged in and turned up to 10? Do you get any white noise?

Yes, amps will have white noise when cranked to 10.  Regarding what is acceptable is up to your own standards or values?   Do you still enjoy your amp with the level of white noise that you have?  Is the noise level creating some level of emotional distress or frustration with it?

With respect, Tubenit

[h4ck.b0x7]

I hear people say they've built multiple Hoffman amps and they are all quite as a church mouse. It's ok if there is some noise when you crank it to 10 but there shouldn't be much at 6 with nothing plugged in. My 5F1 I built is practically silent when it is cranked with nothing plugged in. There is also some hum when you crank the reverb. I don't care so much about that as the reverb hardly ever goes over about 2. Give me a second and I will record a video and post it on Youtube.

[h4ck.b0x7]

It's not so much distress or frustration, it's more the fact that I just built it and I'm not sure what to expect. I don't think it is quite right though. Aside from the background noise the thing sounds PHENOMENAL. I will say though, if I turn the amp up to about 6 or 7 and then turn the volume down on my guitar to an acceptable bedroom level the white noise is overbearing. My Champ doesn't do that at all, you can crank it and turn down the guitar and it just cleans up.

[h4ck.b0x7]

[h4ck.b0x7]

I really wish I knew or had an idea how to track down what I F'd up so I could quit trying to diagnose this and just play my damn guitar.

I just realized that the tremolo works regardless of a pedal being plugged in.... That doesn't seem right.

[h4ck.b0x7]

Here is a picture, did I screw up something with ground?

[Willabe]

I posted this earlier today, I forget for who, but read this on grounding, it should help you;

http://www.valvewizard.co.uk/Grounding.html

[h4ck.b0x7]

I read it at work earlier today, you posted it on the Princeton thread I believe. Funny thing is I ordered that book yesterday and it will be here tomorrow, lol. Do you see a problem with the way things are grounded? Any ideas on the tremolo working without a footswitch?

[dajazzman]

You know I have a thread about my Princeton Reverb hiss and hum. It has almost as much hiss with the volume and treble all the way up. I can see it with a scope right on the plate of the first preamp tube. The hum signal is exactly the same signal I get right off the first filter cap, but the cap is a lot larger.

I can ground the first grid of v1-A and the hiss stops and I can ground the grid of V1- B and the hum and hiss stop. So it seems that the hiss is getting into the grid of V1-A which connect directly to the Input jacks and the hum is coming from the B+  right at the plate of V1- A.

When I plug my guitar in and turn the volume and treble to usable levels, I don't hear any noise. I've tried multiple tubes for the hiss, some are better and some are worse, but none stop it completely.

[dajazzman]

I can't get the footswitch to stop the Tremolo either. I basically just ground the jack and it stops. Maybe it's not the right footswitch that I have.

[h4ck.b0x7]

I've been reading your thread. How/where did you ground the grids?

As for the tremolo, if I put in the footswitch I can turn it on and off just fine. However, when I have the footswitch unplugged it still works. As far as I know in order for tremolo to work at all you have to short the tip to ground, usually people did it with a shorted rca cable. This doesn't seem right.

[h4ck.b0x7]

Sending pin 2 to ground gets rid of all of the sound. That's the first grid. When I say all of the sound, I mean guitar and everything.

[dajazzman]

Just a jumper from the grid pin on the socket to the chassis. I thought that about the tremolo also, but mine is on with nothing plug in and it stops when I short it. Looks like that's the way it should work from the schematic. I thought they were using the shorted plug to keep it off. Of course I could be thinking of it backwards, but that's the way mine works.

[dajazzman]

Yes grounding pin 2 will kill the guitar sound also. My hiss and hum is there with nothing plug in and the controls all the way up.

[h4ck.b0x7]

I know when it came to the tremolo in the past people used to keep a shorted RCA so they could keep it on without a footswitch. Normally the only way for it to work is to connect the antenna.

My hiss also disappears when pin 2 is grounded. Hmmm

[dajazzman]

I have a Deluxe Reverb reissue that an amp builder Installed a Hoffman board and wired it PTP. It actually has more hiss and hum that's sounds a little bit more raspy. So when I compare mine is actually more quiet, but somebody on my thread said his was dead quiet with the controls all the way up so I'm not sure now about mine.
 
Sounds like your Tremolo works the same, when nothing is plug in it's on and if you ground the jack it stops.

[h4ck.b0x7]

This is a Deluxe Reverb that I'm working on. People have said theirs are quiet as a church mouse so I'm trying to figure this out. I can tell something isn't right there is WAY too much hiss. My 5F1 Champ that I built is silent, except for the beautiful sound of the guitar.

[dajazzman]

I hear you, but I wonder if they have all the controls all the way up. It's a tube amp I would think there will always be some hiss and anytime you rectify AC to DC you bound to have some AC ripple that gets amplified, but I'm thinking like you what is normal.

[tubenit]

My rule of thumb for my personal amps is whether the amp is dead quiet at idle with guitar plugged in at the volumes I normally it at?  Is it quiet enough that with a very quiet ceiling fan on that I might not realize the amp is on at all?  And is it quieter then the manufactured amps that I previously owned? The original Princeton Reverb I owned was considerably louder at idle then the amps I've built for myself.

With respect, Tubenit

[sluckey]

I just realized that the tremolo works regardless of a pedal being plugged in.... That doesn't seem right.

That's normal operation for the Hoffman 6G16 bias vary tremolo circuit. A true AB763 trem circuit has to have a footswitch plugged in with the switch contacts closed to operate, but not so with Hoffman's circuit.

[sluckey]

I can't get the footswitch to stop the Tremolo either. I basically just ground the jack and it stops. Maybe it's not the right footswitch that I have.

Or a faulty switch. Nothing wrong inside the chassis.

[h4ck.b0x7]

Ok cool, nothing wrong with my tremolo. What about the hiss & hum? Possible ground loops in the way I did the buss bar? Would that introduce hiss though?

[sluckey]

I think your hiss and hum are normal.

[h4ck.b0x7]

Really, these are that noisy? Do you guys have any examples of what you get out of a cranked AB763 amp?

[h4ck.b0x7]

My rule of thumb for my personal amps is whether the amp is dead quiet at idle with guitar plugged in at the volumes I normally it at?  Is it quiet enough that with a very quiet ceiling fan on that I might not realize the amp is on at all?  And is it quieter then the manufactured amps that I previously owned? The original Princeton Reverb I owned was considerably louder at idle then the amps I've built for myself.

With respect, Tubenit

Is it quieter than the 2 DRRIs I've tried before I ripped out the PCBs in this one? Yes, however I'm not so sure that is a fair comparison.

If I turn it up to 2 and have the guitar plugged in with volume all the way down, could I hear it over a quiet fan? Heck, I could hear it over a movie I was watching last night. Would I think the amp is not on at all? HECK NO.

[HotBluePlates]

Did you ever check the input jacks themselves? Especially the switching contact?

https://youtu.be/4j0QO8vptcE

I echo the other assessments; the hiss/noise toward the start of your video wasn't apparent until the channel volume was up to 8-10. That's normal, and better than the original amps.

If you had an SPL meter, you could set the meter at a fixed point from the speaker, measure the SPL of the hiss, measure the SPL while playing the guitar, and have a signal-to-noise ratio. I think the S/N ratio of your amp would compare favorably to most guitar amps.

All tubes, resistors, etc contribute noise. It's just that some are noisier than others. And you'll never have perfect, zero-noise at any temperature above absolute zero.

There are some component choices and circuit design choices which can reduce nise towards a minimum. I'm going to experiment with some in my next build. I'm not sure any of it will be worth the effort, but that's also why I'm gonna try it to find out for myself.

... My 5F1 Champ that I built is silent, except for the beautiful sound of the guitar.

Your 5F1 has no more than 1/4 the power output; any "normal" hiss/noise will sound quieter just on that basis. The 5F1 also has fewer resistors (fewer noise sources), fewer tubes (again, fewer noise sources), and about half the cascaded gain stages. So it has less noise to start, and what noise there is gets amplified very much less (even though the 5F1 distortion may lead you to think of it as a "higher gain" amp).

[h4ck.b0x7]

Did you ever check the input jacks themselves? Especially the switching contact?

https://youtu.be/4j0QO8vptcE

I echo the other assessments; the hiss/noise toward the start of your video wasn't apparent until the channel volume was up to 8-10. That's normal, and better than the original amps.

If you had an SPL meter, you could set the meter at a fixed point from the speaker, measure the SPL of the hiss, measure the SPL while playing the guitar, and have a signal-to-noise ratio. I think the S/N ratio of your amp would compare favorably to most guitar amps.

All tubes, resistors, etc contribute noise. It's just that some are noisier than others. And you'll never have perfect, zero-noise at any temperature above absolute zero.

There are some component choices and circuit design choices which can reduce nise towards a minimum. I'm going to experiment with some in my next build. I'm not sure any of it will be worth the effort, but that's also why I'm gonna try it to find out for myself.

... My 5F1 Champ that I built is silent, except for the beautiful sound of the guitar.

Your 5F1 has no more than 1/4 the power output; any "normal" hiss/noise will sound quieter just on that basis. The 5F1 also has fewer resistors (fewer noise sources), fewer tubes (again, fewer noise sources), and about half the cascaded gain stages. So it has less noise to start, and what noise there is gets amplified very much less (even though the 5F1 distortion may lead you to think of it as a "higher gain" amp).

That was exactly what I was looking for. I'll see if I can get an SPL, there is probably one around here at work.

What can be done about the hum in the reverb? Right now I'm just using the shitty RCA cables that came in the amp to begin with. Does anybody know of a link that shows how to make nice cable that look more vintage? I have to get rid of these as well because of how far they stick out the back.

[dajazzman]

I think what I will do is removed the shielded cable from the Input jacks on V1 pin 2 and just put a 34k resistor to ground and see how much hiss is there compared with having the Input cable. I can see the hiss on the grid with a scope and I also see the hum which is exactly the same as the ripple on the "A" B+ on the plate of V1 just smaller. I thought about adding a another cap in parallel with the "D" B+ cap to see if that helps.

Otherwise listening to your amp, my DR that was rebuilt by an amp builder and my PR, they seem to all have some hiss when the volume and treble are all the way up.

[HotBluePlates]

That was exactly what I was looking for. I'll see if I can get an SPL, there is probably one around here at work. ...

So the signal-to-noise dB ratio won't do me a whole lot of good, because I have no "known quiet amp" standard against which to compare. But it's an idea for you.

The input jack question wasn't directly answered (or I misunderstood)... Are the input jacks new or reused? Have you verified good contact for all the shorting switches on the input jacks? If they were working correctly, I'd expect them to be shorting the input stage grids to ground with nothing plugged in, leaving only the little bit of resistance of the paralleled 68kΩ resistors.

... Does anybody know of a link that shows how to make nice cable that look more vintage? I have to get rid of these as well because of how far they stick out the back.

I don't know if the old crappy RCA connectors on the vintage Fenders are even still sold. I can tell you they are a huge pain to work with for making your own cable (I've done it, but it's been about 20 years).

... What can be done about the hum in the reverb? ...

If you're going to tackle that, you need to divide & conquer.

Pull the input tube out for both channels; you don't want any noise from before the reverb complicating your assessment of the noise level. Yank the 12AT7 reverb driver tube. Unplug both cables running to the reverb pan from the jacks on the amp. Work on getting just the reverb return stage, the mix gain stage, phase inverter & everything after quiet.

If/once that part is quiet, add the tank back in but not the driver tube. Rinse & repeat until you have everything back in the amp.

There's no sense in me guessing what might be the noise source blindly, unless you've gone and narrowed it to a particular segment of the reverb circuit. The cables & pan may be fine... I used a crap pair of RCA cables that came in the box of a $40 CD player to hook up reverb pans, which ran acceptably-quiet for years. So the best course is to verify rather than have "expert guesses". 

[h4ck.b0x7]

What do you mean by shorting switches? I used brand new Switchcraft jacks. I do know that with nothing plugged in the tips are grounded.

I have 4 of the old style RCA connectors. I thought about getting some nice Neutrik connectors and making them with shielded instrument cable.

As for the Reverb I'll start working backwards when I get home.

Is it normal for the bass knob to control the hum? I'm just finding it hard to believe that an absolute wash of noise is "normal" I don't expect it to be silent but damn.

[dajazzman]

My bass control does Increase the hum because it's coming from V1-A before all the tone controls. I'll try to make a sound clip so you can compare.   

[HotBluePlates]

What do you mean by shorting switches? I used brand new Switchcraft jacks. I do know that with nothing plugged in the tips are grounded. ...

If those are new Switchcraft 12A jacks, they're probably fine.

Those jacks have tip (hot) and sleeve (ground) lugs. Then there's a 3rd lug which goes to the contact the metal tip comes over and touches when no plug is in the jack. That "other lug" is a "normally-closed shorting switch" for the tip connection. This normally-closed switch opens on plug insertion.

I'm stepping through that description in a very deliberate manner, only because that normally-closed switch is used for other purposes in other amps. Sometimes in relation to effects loops, sometimes to change aspects of the input circuit in a handful of amps.

If that normally-closed switch either has poor contact with the tip when no plug is inserted, or if it has a poor solder connection to ground (either at the switch's lug or at the ground lug), then the switch doesn't ground unused jacks properly and you might get noise like you have.

I agree with Dajazzman's earlier suggested step to unhook the wire at the input tube grid, and temporarily connect a 34k-1MΩ resistor from the tube socket pin to ground. Use that to see if you can eliminate the noise. If you can, the noise is at the input jacks or their associated resistors.

[h4ck.b0x7]

Dajazzman, let me know what you get with that 34k resistor. I'm at work right now and have some obligations when I get off. It won't be until later tonight or tomorrow when I can get to it.

[dajazzman]

Ha! I'm at work also.  Probably  Saturday

[h4ck.b0x7]

What do you do? I'm a Software Engineer in the automotive industry.

[dajazzman]

I'm a  senior communication technician for a electrical utility company.

[g-man]

Deleted

[h4ck.b0x7]

Thanks G-Man, yeah you are right. I didn't have an extra grommet but it will be fixed. It's not a deal-breaker right now, there is some slack in the wire as well.

[Paul1453]

One thing you might want explore is the resistors.

The older carbon comp resistors are sometimes blamed for hiss and or pops.
I have read that the metal film or wire wound resistors do not have those issues.

I used the old carbon resistors in the C-E PS nodes of my recent 6V6 Plexi build,
and she is unbelievably quiet, so apparently not all carbon resistors are noisy.
Maybe only when they get older and degrade from heat and use.
The carbon resistors I used were all brand new.

I did use mostly all metal film 1% tolerance resistors in the other parts of my circuits. 

[h4ck.b0x7]

This thing is pretty much 100% metal film except for 1 carbon comp resistor and it wouldn't affect the normal channel, it is the 3.3M reverb mix resistor that is in conjuction with the cap to form a voltage divider.