AB763 idle noise

[6v6Pin1]

I built an ab763 one channel, with reverb, without vibrato amp using the layout here. It makes a lot of white noise when it's on.

With no guitar plugged in and all knobs at zero, I measure 54db with the meter right next to the speaker grill. As I increase volume and treble, the noise gets louder.

Does that sound like a normal amount of noise for this amp or something that is out of spec?
Thank you.

[shooter]

something that is out of spec?

like that!

should be near 0 audible with nothing plugged in
do you get guitar sounds through with the hiss?

[6v6Pin1]

do you get guitar sounds through with the hiss?

Yes, and I think it sounds good other than the hiss. But I would prefer it to be quiet. I just wasn't sure if that was possible.
Is the first troubleshooting step to change all the tubes and see if the hiss changes?

[sluckey]

I built an ab763 one channel, with reverb, without vibrato amp using the layout here. It makes a lot of white noise when it's on.

I have a very similar amp. With all knobs on zero it is dead quiet at the speaker.

Which layout did you use? I was not aware that Hoffman offered such an amp.

[shooter]

the first troubleshooting step

pull the 1st pre tube, then check
pull 2nd pre tube, check.

this should isolate to a narrower search

[6v6Pin1]

Which layout did you use? I was not aware that Hoffman offered such an amp.

I used this one, https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf, but I deleted the vibrato. It's certainly possible I didn't do this right. Should I draw a schematic of what I have?

[6v6Pin1]

pull the 1st pre tube, then check
pull 2nd pre tube, check.

With V1 and V2 removed, I am still getting 54db.

[6v6Pin1]

Which layout did you use? I was not aware that Hoffman offered such an amp.

I found this: http://sluckeyamps.com/tdr/tdr.pdf
My amp is very similar to this. I noticed a couple of differences with mine.
1. I do not have R?/100ka Master or R23/220k.
2. I do not have C23/47
3. The reverb recovery cathodes are not tied together. Each has it's own 1.5k/22uf to ground.
I will try these changes.

[sluckey]

pull V1, V2, and V3. Still noise?

[6v6Pin1]

pull V1, V2, and V3. Still noise?

Silent.

[PRR]

And yet the reverb knob has no effect?? (With V3 in.)

[6v6Pin1]

And yet the reverb knob has no effect?? (With V3 in.)

With V3 in, no guitar, all knobs at zero, I get 54db hiss.
Reverb knob on max, I get 60db hiss.

[sluckey]

Connect a gator clip lead between chassis and V3 pin 2. Still got noise? If so, connect that gator clip lead between chassis and V3 pin 7. Still got noise?

[6v6Pin1]

Connect a gator clip lead between chassis and V3 pin 2. Still got noise? If so, connect that gator clip lead between chassis and V3 pin 7. Still got noise?

Pin2 -> 54db
Pin3 -> 43db

[sluckey]

Please check the tube pins I asked for.

[6v6Pin1]

Please check the tube pins I asked for.

Sorry, typo.
Pin2 -> 54db
Pin7 -> 43db

[6v6Pin1]

Ok I am back.
https://sluckeyamps.com/tdr/tdr.pdf
Using this schematic, I was missing the master volume and R23/220k. I had c13 connected to c14.
To fix, I added a 47k to ground in place of the master volume and added R23/220k. This removed some noise.
After these changes, no guitar, all knobs at zero:
All tubes => 48db
Pull V1, V2 => 48db
Ground V3 Pin2 => 48db
Ground V3 Pin7 => 41db

Does this mean V3B is being fed noise, or VB3 is generating noise? Thank you for your help. I am trying to understand.

[6v6Pin1]

I disconnected everything from the V3B grid except the 470k resistor. Grounding the grid directly stops the hiss. Grounding the grid through the 470k brings back the hiss.
I tried a different tube and a different 470k and got the same behaviour.
There is 247v on the plate and 1.5v on the cathode.
What could cause this stage to be so noisy?

[sluckey]

There's a 220K resistor on V3-B grid in that TDR schematic.

[6v6Pin1]

Sorry. I'm talking about the other side of the tube. The amplifier after the reverb recovery. The 470k is connected to the reverb pot wiper.

[PRR]

Too hot/muggy to sit and think.

The combined cathode network for V3A/B is suspicious to my mind. Yes I know it works for Steve and that's normally good-enuff for me. But at this point, next cool day, cut the wire and solder 1.5k+10uFd under V3A cathode.

[shooter]

Too hot/muggy to sit and think.

Tstorm here but the rain can't get through the thick air 

[sluckey]

He used Hoffman's layout so the cathodes are already separated.

3. The reverb recovery cathodes are not tied together. Each has it's own 1.5k/22uf to ground.

[6v6Pin1]

I Replaced the grid-470k-ground with a grid=220k-ground.
I have sound level meter that I placed in front of the speaker.
470k metal film => 52db
220k carbon film => 50db
The noise disappears when the grid is connected directly to same ground.
Resistors add noise, but not his much, right?
Does the tube have too much gain?
Is the tube emphasizing high frequencies?
Can this noise get in through the cathode, plate, or heaters?
It seems like I have something fundamentally wrong with this side of the tube. This sub-circuit is pretty simple.

[sluckey]

Back in the '60s when the AB763 amp was invented, Leo chose to use a 7025 tube in that position. It's rumoured that Leo was very thrifty. He would not have used a more expensive low noise tube for that position if a regular 12AX7 was up to the task. So, my point is get several good 7025 tubes and roll them through that position and see if they make a difference. Noisy AB763 reverb recovery/mixer is a very common complaint. Nobody has come up with a solid solution.

[shooter]

are your drive and recovery tubes ground at the same point - ie, both at the preamp? 
if so try splitting off the recovery ground and moving it to the PA ground lug, or it's own chassis point.

[6v6Pin1]

are your drive and recovery tubes ground at the same point - ie, both at the preamp? 
if so try splitting off the recovery ground and moving it to the PA ground lug, or it's own chassis point.

Yes, they are both on the preamp ground now. I can try moving the recovery tube ground.

But here's what feels wrong to me. I had everything disconnected from the grid on the final tube stage before the PI. The grid was just a 470k resistor to ground. And this looks like it is causing all my noise.
Is that expected behaviour or do I have something not working right?
IE, if I have a standard 12ax7 amplification stage, and i put the signal through a 470k before it gets to the grid, will it be noisy? Is it just the physics and there is nothing that can be done? The dry signal goes through a 3.3M/10p and I don't notice any noise from that.

[shooter]

do I have something not working right

If you think about the 1,000's(?) that don't create the noise........