Everything was going ok when hiss appears in AB763 build.

[SourCream23]

Hey Guys,
I have build a AB763 based on Sluckey schematic (https://sluckeyamps.com/tdr/tdr.pdf). Thanks Sluckey, (Again)!
The amp was sounding great but today it started a louder hiss.
I opened it and checked everything.

When v1 and v2 are pulled, the hiss still there. When v3 is pulled, the hiss goes away.
When I put the 2 pin (grid) of v3 to ground, the amp stays dead quiet. No hiss.
When I put 470K (connected to reverb pot) to ground, the hiss still there.
So, Im pretty confident the problem is on that region.
I tried:
Change the 470K, 220K and 3.3M to metal resistors: NO change.
Change the tube to another two JJ´s 12ax7: NO change.
Resolder all connections in that area: No change.
Change all wires in that area: No change.

These topics below show the same problem but any presented any conclusion, I have read all them and cannot find a solution:

https://el34world.com/Forum/index.php?topic=24923.0
https://el34world.com/Forum/index.php?topic=25525.0
https://el34world.com/Forum/index.php?topic=26129.0
https://el34world.com/Forum/index.php?topic=23392.0

Any help will be apprecciated, I really dont know whats going on, is a pretty small part of circuit but I didnt find the solution yet.
Thank you guys!

[SourCream23]

Ok, few more tests.
I just removed the 3.3M and 470K of the circuit. Only ground - 220K - grid connected. The hiss still there.

[pdf64]

Is C8 still in place?

[SourCream23]

Is C8 still in place?

Hey pdf64, yes, with C8 the white noise remains.
I removed C8, R13 and R20 to test. With only 220k (R21) on grid the white noise still there.

[pdf64]

Maybe that stage's anode resistor R22 has gone noisy?
Dunno how muting the grid would stop the hiss in that case though 

Is the cathode bypass C12 definitely good?

[shooter]

When I put the 2 pin (grid) of v3 to ground, the amp stays dead quiet. No hiss.

Agree with pdf64


If v3 is in ground pin 7, still hiss?
IF V3 is out and NO power, ohm pin 3 at tube side to chassis, what's the value?

[SourCream23]

Maybe that stage's anode resistor R22 has gone noisy?
Dunno how muting the grid would stop the hiss in that case though 

Is the cathode bypass C12 definitely good?

Hey pdf64,

I have changed the R22 and C12 two times. Still hiss.

When I put the 2 pin (grid) of v3 to ground, the amp stays dead quiet. No hiss.

Agree with pdf64


If v3 is in ground pin 7, still hiss?
IF V3 is out and NO power, ohm pin 3 at tube side to chassis, what's the value?

Pin 7 in ground, still hiss. I just left a 220K resistor in grid in that part of circuit and I have the white noise.

Pin 3 without the tube is 815ohm to chassis.

I really have no idea what to test now. I changed everything of that subcircuit. Maybe the socket?

[SourCream23]

I wired everything again and recorded the noise.

https://imgur.com/a/8KOmwBD

In the video I turn the volume knob to maximum and go back to minimum. The noise floor is there. Before that issue the amp was very quiet.

[shooter]

my metered internet is in the red so your page will take a week to load! 


the amp IS quiet with the MV at zero?

[SourCream23]

The build has no master volume. Just the volume knob.
With volume in 0 I get the white noise.
With v1 and v2 pull out, still get the noise.
Grounding v3 pin 2 everything is dead quiet.

[SourCream23]

Ok, one more test.
I turned off the second triode of v3. Pins 6, 7 and 8. Still white noise.
I swapped the tube to another one 12ax7, no change.

Im start to thinking the problem could be a RFI interference.

When I turning the volume pot to maximum, the white noise gets more and more louder. See the video above. Maybe a RFI amplification plus the noise in pin 2 v3?

[shooter]

a scope will come in REAL handy about now, beg, borrow, steal 


you should be able to "change" the white noise level by moving signal cables.


if you have a guitar plugged in, start: guitar volume 0, amp volume 7, both TS's 5, turning up the guitar volume, when does the white noise "go away"?


Wait for conformation, I would put V3 on the C PS tap for troubleshooting, (assuming I left my scope in the other car)

[SourCream23]

Ok! I have a scope. I will try to trace the signal.
What you mean by "C PS tap"?

Thanks Shooter!

In my chassis I just have one hole where the dog house wires coming into chassis and to circuits. All gnds and B+ lines get into that hole. It could be a problem? Should I have more holes and separate the wires?

What you think guys?

Thanks a lot!

[shooter]

It could be a problem

yes Huston, that could be a problem 


both B+ and ground are the main "highways" for noise to ride.
if the sensitive grounds get next to the don't care grounds, they can talk, then tubes like the 1st gain stage pick it up and run with it.


how many ground wires are coming from the doghouse?

What you mean by "C PS tap"?

look at the schematic you posted in the OP, the PS (power supply) filtering sections are labeled A-D.
typically, you only want 2 gain stages per "TAP", or node.  There are 3.
**since it is a known good circuit, leave it alone for now**

the grounds that are associated with V1 (I count 4, plus the input jacks, which are already at chassis ground) all go to a point at the input jacks ground tab or a chassis screw real close to the input jack.  The chassis then acts as the "wire" back to the doghouse caps/ground.

[SourCream23]

how many ground wires are coming from the doghouse?

Two ground wires.
A preamp ground wire (3 preamp caps) and power amp ground wire (2 power caps)
They goes into a hole with A-D B+ lines.

Inside, one goes to right to power bolt (power caps) and the another one to left in preamp stage (preamp caps). The preamp ground bus is attached on input jack.
Do you think I need 3 ground wires to preamp? One to each B+ line?

I followed that ground scheme:
https://el34world.com/charts/ground

I did not wiring the back of pots. Preamp bus is going directly to input jack and it has a ground preamp wire attached on center.

[pdf64]

Im start to thinking the problem could be a RFI interference.

To try and eliminate that and ultrasonic oscillation as the cause, I suggest to try fitting a 10k grid stopper to V3 pin2 socket terminal, and a 100pF >500V cap (or thereabouts) between terminals 1 & 3.

[pdf64]

The noise seems a definite hiss, rather than a 100/120Hz hum.
Martin Manning has done a nice redesign of the 0V common layout  for black panel Fenders, which might be of interest https://ampgarage.com/forum/viewtopic.php?p=428877#p428877

[SourCream23]

Ok guys,

I drilled a hole to separate the "D" B+ line and preamp ground. Rerouted the wires, so the D line and preamp ground do not across other grounds wires. No change, hiss still there.

I have changed the tube socket V3, still hiss.

I have changed the C13 (0.1uf), no sucess.

I bypassed the v3, I connected the 0.022 after v1b to R23 220K (to simulate a normal channel) still hiss! So, the entire preamp is amplifing that noise.

Grouding v1 pin 2 has no effect on noise. So I believe that the input jack has no issue.

If I desconnect the v1 and v2 and left only v3, I have hiss too. The only way to noise get away is grounding pin 2 v3.

I installed a 10K grid stopper on pin 2 v3, no change.

I put the oscilloscope on B+ lines, all looking good and clean.

I started to think on pots, but it makes no sense because only v3 in the amp, the problem remains.

My head is burning out, I will rest a little and return to amp latter.

More any guesses guys?

I Really appreciate your time.

[SourCream23]

Im isolating everything before V3. I will concentrate only that stage. One time fixed, I will add the another one and so on.

[SourCream23]

Guys, my chassis is made of stainless steel, it could be the issue?

[acheld]

Guys, my chassis is made of stainless steel, it could be the issue?

No.

I'm not a fan of stainless steel chassis, but that's only because they are a pain (for me) to drill.   SS won't have any effect on hiss or noise.

[SourCream23]

No.

I'm not a fan of stainless steel chassis, but that's only because they are a pain (for me) to drill.   SS won't have any effect on hiss or noise.

Hi acheld, thanks for that!

I have changed all preamp filter caps and the resistors (10K's) of filter board. No change.

I have changed the volume pot, no change.

I have changed bypass cap and resistor of V1a and v1b, no change.

[shooter]

hook the scope to the speaker, post a screen shot.


hook the scope to V3's cathode, post a screen shot.
indicate the scope settings so we can see if its volts or microVolts

[tubeswell]

The only way to noise get away is grounding pin 2 v3.

The problem is either in the signal wire going to Pin 2 V3, or ‘before’ that. Try shielded cable on that wire, or look for a bad plate resistor on the preceding stage

[pdf64]

I’m getting outta ideas, and wondering whether we’re looking in the wrong place, eg perhaps the preamp hiss per se hasn’t changed 

Are you sure it’s not some coincidental external factor, eg use of a more sensitive speaker, a new transmitter in the local area?

Or had your ears syringed and your hearing became more sensitive?

Has the negative feedback loop been broken so the power amp’s now running open loop and so has more gain, especially at higher frequencies?

[shooter]

ya, that's why I asked for scope shots, If I can't see the problem, it doesn't exist 

[SourCream23]

hook the scope to the speaker, post a screen shot.


hook the scope to V3's cathode, post a screen shot.
indicate the scope settings so we can see if its volts or microVolts

Speaker Out at 200mV per division: Started on Volume at 0, and I turn up a little, no much.

https://imgur.com/psb2asK

V3 pin 1 at 100mV per division (volume at 0, no touch on that configuration)

https://imgur.com/ULESNgw

V3 pin 3 at 10mV per division (volume at 0, no touch on that configuration)

https://imgur.com/3atLwgA

V3 pin 2 grid at 20mV (volume at 0, no touch on that configuration)

https://imgur.com/PnKsp6w

It seems the grid collect the noise. But no matters what I do, it always get some level of noise on input. I tried many things, including leave the pin alone (a lot of hum here plus the white noise).

Just a wire connected on 220k resistor and ground gives the white noise too.

The problem is either in the signal wire going to Pin 2 V3, or ‘before’ that. Try shielded cable on that wire, or look for a bad plate resistor on the preceding stage

Thanks, I dont believe that a shield cable is the right solution but I can try.
I opened a twin reverb and put side by side of that amp (chassis side chassis on the same bench) the twin is dead quiet. No noise at all.

 

I’m getting outta ideas, and wondering whether we’re looking in the wrong place, eg perhaps the preamp hiss per se hasn’t changed 

Are you sure it’s not some coincidental external factor, eg use of a more sensitive speaker, a new transmitter in the local area?

Or had your ears syringed and your hearing became more sensitive?

Has the negative feedback loop been broken so the power amp’s now running open loop and so has more gain, especially at higher frequencies?

I really dont know where the problem is. I thinking on power amp, but the fact of grounding v3 solves everything it seems that power amp is Ok.

I opened a Twin and a Supersonic on the same bench to fix them on weekend, and no one has any type of hiss noise. So, even a local transmitter is close, these amps are died quiet.

The negative feedback resistor is ok too. Is the stock configuration for 8 ohm tap. 820ohm and 47ohm at the end.

That hiss is not normal because the amp at end of build was very quiet. I playing with it three, four days, and suddenly (next day when I turn on it) the noise appeared. And the noise is louder when you turn up the volume.

ya, that's why I asked for scope shots, If I can't see the problem, it doesn't exist 

Ahaha, I just uploaded your requirements, please, take a look. thanks pdf64.

I will stay the night with it again, so you guys have any idea, please tell me. I really dont know what happened here.

Really appreciate your time guys.

[shooter]

 
i'll look tomorrow when my internet provider gives me back my speed, it will take 15 minutes to just open the linked page today!!


WAG;


with everything up and running, what is your idle (NO signal) bias set at?

[SourCream23]

Ok Shooter.
24mV per tube. 6v6's.

[SourCream23]

Ok, last test of the night:

I change all connections of the v3a to v1a. So I could check if the problem is something mechanic or position related. No change.

At that point I disconnected everything before v1a.
I just have a 220K resistor connected on grid pin 2 like that:

https://imgur.com/a/S8NHn3d

The hiss is not louder, but still there.

[tubeswell]

As mentioned previously, a bad plate resistor or grid load resistor can produce hiss (or other undesirable noise). Use a 600V 0.01uF film cap wired to a couple of insulated gator clips to clip the plate connection of the plate resistor for the prior stage to ground (to bypass the plate resistor). If the hiss is still there, then you know its not coming from that plate resistor.  So then try bypassing another resistor to ground with the cap (e.g. the 220k grid load resistor for V3 to ground) to see if that kills the hiss. If either option kills the hiss, its likely to be that resistor which is bad, and so on.

[SourCream23]

Hey tubeswell.
I will try that right now. Thanks!

[SourCream23]

Hey guys, let's go:

15nF on plate resistor kills the noise!
15nF across 220K kills the noise!

Wow, one step further on that travel I guess. Maybe the noise is going into the grid and is amplified on the anode?
I've changed the 220K two times and problem persists.

[SourCream23]

Once the noise goes into the tube by grid (I guess), I removed the 15nF and installed a 500pF between pins 1 and 3. White noise appears again.
500pF across the plate resistor and ground makes no difference, white noise appears again.

So, at that point, the solution is shunt the white noise using capacitors?

Any ideas where the right place and cap values? The cap will affect the tone?

Thank you guys!

[pdf64]

I think the caps are just a fault finding technique, not a solution.

[SourCream23]

pdf64, it makes sense.

So, I look to chassis and cant imagine anything can be done at that point. Runout ideas.
I really appreciate the time of each guy here. Thanks!
 

[shooter]

looked at your speaker scope post;
I see ~ 200mV of white noise, but there appears to be coherent signals inside the grass.


take the scope to 50mV, adjust the time-base to see if you can see the stuff "running through the noise" 
use your trigger function to see if you can capture one.  a simple pic, posted here would be great.


the fact that the volume pot increases and decreases the noise tells me the noise is there at the pot, waiting to be let loose and amplified downstream.


got a signal source, phone with 1khz tone, sig-gen, set the input amplitude at 100mV, scope the speaker and snap a pic n post

[SourCream23]

Hey shooter,

Here a good pic in 200mV, speaker out.
Just the v3a connected with a 220k resistor on grid.

[shooter]

putting meters and scopes directly on the grid can cause unwanted interactions and dead-end rabbit holes


take one between C6 and C8
another one R23 and C14

[SourCream23]

Here we go:

[tubeswell]

Hey guys, let's go:

15nF on plate resistor kills the noise!
15nF across 220K kills the noise!

Wow, one step further on that travel I guess. Maybe the noise is going into the grid and is amplified on the anode?
I've changed the 220K two times and problem persists.

I'll call the plate resistor bad (or possibly a preceding plate (load) resistor? Have you tried the cap across any other preceding load resistors?)

[shooter]

It's the playoff and I'm starting to tail-gate before the big game so I'm out tonight


the output appears to be less volts than input for signal amplitude, indicating no voltage gain from the tube. 


connect the scope back to the RC junction and adjust the time-base to stretch the 2 points I have indicated to fill the screen, left arrow at left edge of scope, right arrow to right edge of scope.
post a pic.


power down and wait a few minutes for cap s to discharge, remove V3.  Ohm the plate Resistor at the pin side of socket to B+

[sluckey]

I have a 5751 low noise tube in that position. Much better results than most modern 12AX7s.

[SourCream23]

I have a 5751 low noise tube in that position. Much better results than most modern 12AX7s.

Hey Sluckey, I made the following test once:

I bypassed the v3. I connected the v1b directly to 220K resistor (c6 to r23) that going phase inverter. Everything quiet but when a increase de volume the hiss appears and becomes louder and louder. Like the white noise is amplified through all preamp.

With v3 go back, I have a floor white noise as I described above. When I increase the volume the white noise becomes louder and louder.

Hey shooter, I will provide your requirements now.

[sluckey]

Do you have a 100K Master Volume pot as shown on the schematic? If not you need to add a 47K resistor from right side of R13 to ground.

[SourCream23]

Do you have a 100K Master Volume pot as shown on the schematic? If not you need to add a 47K resistor from right side of R13 to ground.

No, I haven't. But I have the 47K to ground in that place.

I'll call the plate resistor bad (or possibly a preceding plate (load) resistor? Have you tried the cap across any other preceding load resistors?)

Hey tubeswell. Is a good call, but I have changed all plate resistors precending v3.
And, I did multiple tests with only v3b in circuit, I disabled v1 and v2. Just a stage going to phase inverter. At that stage the hiss appears like shown posts above.

Shooter, the pic is attached.
Ohm pin1 to B+ is 97.6K.

[SourCream23]

Hey guys! I took a day of rest and Im go back now.
I rewired the ENTIRE amplifier today. New wires.
No change, hiss still there. Lets go guys! Im not give up! Haaa!

[tubeswell]

Do the 600V cap bypass trick on all plates and report back about which one kills the hiss

[SourCream23]

Hello tubeswell, ok! I did. Lets go:

When I turn on the amp there is a floor white noise at volume 0. That noise is killed put the 15nF on the anode of v3a.

Ok.

When I increase the volume, the white noise gets worse. I can get rid that noise putting the capacitor on v1b or v3a (in v3a, the white noise goes away totally).
In v1b I can increase the volume without problems, but the floor white noise still there.

[tubeswell]

Hello tubeswell, ok! I did. Lets go:

When I turn on the amp there is a floor white noise at volume 0. That noise is killed put the 15nF on the anode of v3a.

Ok.

When I increase the volume, the white noise gets worse. I can get rid that noise putting the capacitor on v1b or v3a (in v3a, the white noise goes away totally).
In v1b I can increase the volume without problems, but the floor white noise still there.

Looks like V3a plate resistor is a problem (or another resistor in that part of the circuit)