Hiss/white noise in an amplifier

[Diverted]

In another thread I got help with an AB763 build that ended up going very well. Still I have some low volume white noise/static/hiss that I would like to root out. It's not bad but I would always like quieter.
The hiss is present as soon as tubes start conducting, and is there with volume and reverb down to zero. It does not vary as volume is turned up; it's a steady soft white noise at a constant level. No pops or flickers, just steady.
I think I can eliminate many of the usual causes of this. Are there any other strategies to get at the issue that aren't listed here?
I have tried:
1. Check tubes. I have swapped many tubes in and out of all stages the amp, tested them for gas and other issues. Regardless of what I'm using, the hiss remains.
2. I am using metal film resistors not carbon comps, so I don't think they're contributing.
3. I have checked and tightened all grounds in the amp and I believe they're all very solid. The earth ground is on its own; the power ground sees the HV center tap, artificial 6.3v center tap, bias cap, first two stages of filtering and speaker ground. A preamp buss ground tied in to the first input jack sees the second two stages of filtering and the cathode bypass caps.
4. I have checked and reflowed all solder joints in the amp, and chopsticked a ton. I don't believe solder joints are the issue.
5. I'm running shielded cable at the preamp grids, but even if I weren't the hiss does not go up or down and it's not hum. So I think it must be after the early input stages?

As I said it's not bad. The amp is loud and is going to be played loud, so it's not an issue as soon as I start playing. But if recording was going to be done on the amp, it'd be an issue.
Anything else I could be trying? Not just for this amp, but just to have the knowledge to help in the future.

Thank you.

[sluckey]

Are you using a 12AT7 for the PI tube?

Pull all little tubes except the PI. Still got hiss? If so, put a 47pF cap between pins 1 and 6  directly on the PI socket. Any better?

[Diverted]

Yes, it has an AT7 in the PI position.
At work now but looking forward to trying this when I get home later. Thanks.

[d95err]

Does the hiss vary with the reverb knob?

[Diverted]

Nope. Actually the reverb is pretty quiet though it gets a bit hummy as you crank it up. Sounds normal to me. This hiss is sort of like a background fuzzy monotone that "sits" under everything regardless of any of the controls' positions. It does not get louder as I adjust various pots.

[Diverted]

I went home for lunch and started pulling tubes.

No hiss with all small tubes but PI out. So I started going backwards and the hiss reappeared with V3 (Reverb recovery/preamp) back in. No change with V1 or V2 in or out. There is also no hiss with all tubes except V3 in place. So I think the issue is definitely around the V3 area. And I stand corrected; the hiss does get louder as I turn the reverb up.

Should I try separating the V3 cathodes which currently share a single cap/resistor?

Here’s a short vid: it’s actually not as loud as it sounds in the clip.

[sluckey]

I agree. I want you to measure the cathode resistor for V3. You can do this in circuit with power off. My monitor and eyes see an 82Ω resistor (gray-red-black). Should be 820Ω (gray=red-brown). My eyes could be lyin' to me! 

An 82Ω resistor would cause V3 to run hotter with increased gain and noise. If the resistor is indeed 82Ω then change it to 820Ω. Half watt is fine. Also change the other two blue resistors that look like the same. The one on the cathode of V1 pin 8 should be 1.5K and the one in the feedback circuit should be 820Ω.

[Diverted]

Thanks Sluckey. I agree it looks very black in that photo. But it's my camera. It's definitely brown blown up and the multimeter confirms it. 820s in all positions.

Would separating cathodes on V3 be something I should do?

[sluckey]

Ground the reverb return jack. Any better?

I don't think separating the cathodes will do anything for you, but you can certainly try it. Use two 1.5Ks. And while you're at it change the resistor on V1 pin 8 to a 1.5K.

Try a lower gain tube for V3. I have a 5751 in mine. Maybe even try a 12AT7, or 12AY7. Lower gain equals lower hiss. Remove the cathode bypass cap to lower the gain also.

[Diverted]

I will try those things. I did try a 5751 in that spot; have a few decent ones. It lessened but didn't knock out that hiss. I haven't tried a 12AY7 yet.
Thanks!

[Diverted]

Ground the reverb return jack. Any better?

No change

I don't think separating the cathodes will do anything for you, but you can certainly try it. Use two 1.5Ks. And while you're at it change the resistor on V1 pin 8 to a 1.5K.

Did all three, no change.

Try a lower gain tube for V3. I have a 5751 in mine. Maybe even try a 12AT7, or 12AY7. Lower gain equals lower hiss. Remove the cathode bypass cap to lower the gain also.

Did those. Slight reduction in hiss, but the issue remains.

I also went back through and re-flowed all solder joints from V3-V4, just to be sure, and also played around with the lead dress. Still not getting it. Boy, the more I hear this hiss the more I want to kill it! Thanks for the help so far.

[shooter]

Ground the reverb return jack. Any better?

No change

shouldn't that eliminate V3 

[Diverted]

Ground the reverb return jack. Any better?

No change

shouldn't that eliminate V3 

I’m sorry, yes you’re right I misread the note; I thought the suggestion was to run a ground jumper to the ground of that jack, presumably I thought to check the strength of the ground.

Grounding out the return itself does kill Reverb but the hiss remains.

[shooter]

try gator-clipping a .001 - .01uf cap across the 100k plate R on the recovery tube

[PRR]

Is your NFB loop connected correctly?

Is your speaker wickedly efficient?

Are you listening in a quiet room?

There's *always* hiss. Universal truth.

[Diverted]

I believe the loop is good and phasing on plates is correct. I’ll swap and see what happens.
Yes I am using a crazily efficient speaker. And I’ve put it in a few different AC circuits but all same.
Thanks!

[Diverted]

No dice on the cap across the V3a 100k load resistor. Tried three values from.001 to .01.
Still reading but it won’t be the end of the world if I can’t knock it out.

[sds1]

Here's something you can try:

"...the input grid stopper adds the most amount of Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!) Can we use a smaller resistor to reduce noise, but still keep the bandwidth the same? Yes, it's easy. We simply add a little extra capacitance to make up for the lower resistance. I recommend a 10k resistor, which will make the amp much less hissy, together with an extra 100pF to 470pF capacitor from grid to ground."



Source: http://www.valvewizard.co.uk/gridstopper.html

[Diverted]

Whoa. I will have to try this tomorrow. So, just a pair of 10ks to replace the 68s, and the 470pf cap?🤔🤔Thank you!

[sds1]

Whoa. I will have to try this tomorrow. So, just a pair of 10ks to replace the 68s, and the 470pf cap?🤔🤔Thank you!

It would be a pair of 22k's I believe. Looking forward to your results!

[Diverted]

Whoa. I will have to try this tomorrow. So, just a pair of 10ks to replace the 68s, and the 470pf cap?🤔🤔Thank you!

It would be a pair of 22k's I believe. Looking forward to your results!

Wife’s got me watching a movie and the amp suspense is killing me but it’ll have to wait till the afternoon. I’ll let you know how it goes.

[PRR]

> the input grid stopper

He says it hisses with Volume down. This suggests it is not the stuff before the volume control.

(But at this point it may be something unsuspectable.)

Ideally the prime hiss is the *guitar*. The 34k input R next.

[Diverted]

> the input grid stopper

He says it hisses with Volume down. This suggests it is not the stuff before the volume control.

(But at this point it may be something unsuspectable.)

It does, with volume at 0 and is more or less level though noise does slightly increase as it usually does as you turn it up. Interestingly when I max out the volume, the hiss drops to nothing at about 9.5. I don’t think it’ll be played wide open very often.

Also now that I think of it I don’t know how much luck I’ll have with that mod. The hiss occurs even with nothing plugged in, so the input to the grid is grounded.

[PRR]

> , the hiss drops to nothing at about 9.5.

That is truly strange. Does lead to a bad ground, shared component, or oscillation.

> luck I’ll have with that mod. It occurs even with nothing plugged in, so the input to the grid is grounded.

The 34k is still there, and should determine the output hiss with all knobs dimed. But we have to be really frikking high to care about the sound with guitar unplugged.

[Diverted]

But we have to be really frikking high to care about the sound with guitar unplugged.

[shooter]

we have to be really frikking high to care

 

no problem 

[Diverted]

This is a longshot, but what the heck.

This is the chassis I used for this amp. It is described as being made from "16 gauge galvanneal steel."
Nice solid chassis, but I am wondering if the galvanized coating is providing a dodgy ground that could be contributing to my hiss.
Any opinions? Admittedly I am grasping at straws and trying to think of unlikely things.

https://www.ebay.com/itm/Square-angled-Blank-Princeton-chassis/223683437665?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

[Diverted]

An update:

As suggested here, I swapped out the 68K input resistors for 22Ks, and ran a 500pf cap from V1A grid to ground. I didn't like it; it cut gain and did not affect the hiss. So I went back to stock configuration. Oh well.
Back to square one on the hiss I guess, I'll keep looking for a solution.

Here's a few pix, finally got the cabinet done. Thank you all for the help.

[vampwizzard]

it looks good at least!! if you continue to be patient you may be able to isolate the hiss. keep us posted!

[Willabe]

Where did you get those control knobs?!? 

Do they make them in several colors?

Very cool!   

[Diverted]

Where did you get those control knobs?!? 

Do they make them in several colors?

Very cool!   

I found them on an old radio parts website that sadly is no longer. I have a few ivory left, plus red and black.
The indicator lines needed to be filled in so I used red nail polish.
I may be able to help you out depending on the color/number you need.

[Diverted]

it looks good at least!! if you continue to be patient you may be able to isolate the hiss. keep us posted!

Don’t get me wrong! It’s not noticeable as soon as you can hear guitar volume. I just would love to have it whisper quiet.
I think I may start swapping out resistors and caps in V3 area one at a time. Who knows I may have a clunky part in there.

[vampwizzard]

Don’t get me wrong! It’s not noticeable as soon as you can hear guitar volume. I just would love to have it whisper quiet.
I think I may start swapping out resistors and caps in V3 area one at a time. Who knows I may have a clunky part in there.

Have you chop sticked the B+ lines to see if their placement is affecting things? Ive had interference from being too close in proximity to the signal path.

[Diverted]

Yes. I could open a sushi bar with all the chopsticks I’ve gone through.

[bmccowan]

Nice looking amp. Do you have a setup for a probe and listening amp? Easy to make. I have sometimes found the culprit component by starting at one end and going through the signal chain with controls set for maximum hiss. I was surprised once to find it was a brand new but bad pot.

[Diverted]

Nice looking amp. Do you have a setup for a probe and listening amp? Easy to make. I have sometimes found the culprit component by starting at one end and going through the signal chain with controls set for maximum hiss. I was surprised once to find it was a brand new but bad pot.

No I don't. I have an old Paco signal tracer but it's not working and I've been meaning to get to it one day. What is your system? Would love to hear about it, thanks.

[punkykatt]

https://el34world.com/Hoffman/tools.htm


scroll down to the last item to build a listening amp tool.

[bmccowan]

Mine is just like Doug's - I used a ballpoint pen housing to hold the capacitor. When I need a signal I use an iPad with a tone generator app. But for the hiss issue you will not need/want a signal.

[Diverted]

Mine is just like Doug's - I used a ballpoint pen housing to hold the capacitor. When I need a signal I use an iPad with a tone generator app. But for the hiss issue you will not need/want a signal.

That's true. Looks like a great tool to have anyway. I do have a signal tracer but it's an old one that's been hacked up and is not working. I think I may build one of these. Thanks!

[Diverted]

The questions are back😂
I gave the amp to my friend yesterday and it wasn’t to his liking or mine so I’m going to try to adjust.
Myself and a few friends have been playing it for a week and it has been very clean, a beautiful amp really.
My friend, who has always said he can never find amps clean enough for him, was getting this amp to break up heavily, with farty bass, at 4. Not at all what I was hoping but I guess I’m not surprised. He plays a hollow body with a very hot neck pickup and really, really attacks the strings. So it was flubbing out on him and was very boomy.

I’m thinking about what I can do to tighten the response to suit him better. Wondering about:
1. Adjusting coupling cap values downward?
2. Increase negative feedback?
3. Smaller cathode resistors? (First stage, elsewhere?)
4. 12AY7 inV1?

I know these changes will fundamentally alter the tone but if it stays as is, it’s going to sit in his closet I suspect.

Thanks!

[PRR]

> a very hot neck pickup and really, really attacks the strings. So it was flubbing out

Hide his vitamins and spinach.

http://images.hellokids.com/_uploads/_tiny_galerie/200901/popeye-the-sailor-plays-the-guitar-coloring-page_jy4.jpg

Turn down the guitar!!

I don't have your plan handy, but the hi/lo dual input scheme is FOR this situation. Also remove bypass cap on first stage.

Also: https://joemohrtoons.com/2010/11/23/canned-spinach-killing-popeye-cartoon/

[Diverted]

He flubs start to finish cuz he eats his spinach, no doubt.

He’s so boomy on it I’ll remove bypass cap in V1a, maybe reduce .047 coupling cap to reduce bass and go from there. I am sure it will be an improvement as he sounds nasty on it. A shame too as everyone else who has played it has sounded amazing.

Thank you!

[shooter]

everyone else who has played it has sounded amazing.

If he ain't paying, I'd side with majority rule 
 
this is where scopes come in handy, like PRR said, turn guitar DOWN, use AMP knobs n see if it stays clean.  There is no "universal fits all" amp in tubes

[Diverted]

Ha! I may tell him that.

Anyway, I removed V1a bypass cap and reduced v1b coupling cap to .005. He’s going to check it out tomorrow so we will see, and go from there.


The only other issue that has been cropping up since I put it all together and dropped in the spring is what I suspect is the speaker’s magnetic field pulling on the springs. When I turn reverb up I start getting a steadily increasing feedback; pulling the tank out of the cab eliminates it. I haven’t found a sweet spot yet in the cab where it’s totally eliminated but will keep trying; some are better than others. The speaker is very close to bottom of the cab.
Would some kind of shielding around tank work? Like lead flashing or something?

[PRR]

It's not speaker magnetic field. It is acoustic sound waves. The tank pickup is a bad microphone, small collection area. BUT put it close to a LOUDspeaker and it will pick-up enough to make feedback squeal. It is probably lower pitch than voice-mike squeal.

There is a heavy cloth bag sold for the purpose.

Neil Young drops wires through the stage and mounts his tank in the basement.

[Diverted]

That makes perfect sense except that it also happens with zero volume, no guitar plugged in. I can hold the tank in my hand outside the cab, turn reverb up past five and all is good. But as I slowly move tank closer to speaker, the feedback/ringing will begin again. I’ve checked all tubes for microphonics and I believe they’re all good.

Does Neil Young really do that? Cool.

[Williamblake]

Maybe your are having the tank too close to the power transformer?

[Diverted]

It’s actually very quiet hum-wise and so I don’t think that’s the issue though I could be wrong there. This sounds like more of a hollow pitch that increases, like a feedback.
I’ll keep playing with it. Someone suggested 1/32” lead flashing in another thread for someone having exact issue. I may try it as I want to add more lead weight to this heavy amp 😂

[punkykatt]

Do you have another tank to try?

[Diverted]

Do you have another tank to try?

No, not a short 9” tank like the one that’ll be in the cabinet. I do have some normal size ones but the issues don’t exist with them as they won’t fit and thus they’re well away from the speaker. Looks like I may order another! 😂