AB763 HISS on the Tremolo circuit

[CharacterZero]

Hi there,
I built a Weber 6a20 kit.  It sounds great except for the excessive hiss.  I removed tubes 1-3 and it was still there.  When removing tube 4 it disappeared.  When I touch my ground probe to pins 6 and 7 of V4, it totally goes away and I get local radio loud and clear.  This is also true if I touch the right tab on the tremolo intensity pot.  I have noticed that different people have different ways of connecting the wires from the boards to the pots of this amp.  I used the method of twisting the tremolo and reverb wires all together and then having them come off of the bundle at each pot.  Not sure if this was a bad idea to twist reverb and tremolo wires together.  One of the things that I don’t really like about this kit is the location of the dog house.  It seems to be moved over to the right (further away from the power section) which leads to longer lines of the filtered power.  I also twisted those wires as they go towards the A and B nodes which are traveling right along the same route as the reverb/tremolo wires (and the bus wire which terminates at the input jack is in the middle as well).    When chopsticking the whole amp, I don’t get any noticeable areas change or loud crackles.  However, the grid wire (v4 pin 7) and the red wire off of the reverb transformer, make a loud microphonic sound through the speaker when tapping it.  Is this normal?  Anyhow, I wondering if anyone has any tips on working with the tremolo noise issue.  Any suggestions would be greatly appreciated.


Other notes: 
current bias of JJ6v6S is 20 mA (almost the same of both) 356 on the plates.

V1
1: 144.7 VDC
2: 0.0 w/ radio
3: 1.17
6: 169.4 VDC
7: 0.00
8: 1.29 VDC

V2
1: 161.5 VDC
2: 0.0
3: 1.209
6: 158.7 VDC
7: 0.0
8: 1.29 VDC

V3
1: 351.3 VDC
2: 0.026 VDC w/ radio sounds
3: 7.4 VDC
6: 351.3 VDC
7: 0.026 w/ radio sounds
8: 7.41 VDC

V4
1: 166.1 VDC
2: 0.00 w/ radio
3: 1.26 slightly microphonic
6: 164.3 w/ slight radio
7: 0.00 w/ intense hum!!!
8: 1.26

V5 with tremolo on 5 approximate voltages bouncing back and forth
1: 110 to 304
2: -2.9 1.5
3: 0.7 to 2.1
6: 265 to 294
7: -11 to 8.9 ???  Difficult to read
8: 13.3 to 14.6

The only mods that I did to this amp were the Robrob Normal channel and the Rob Robinette Lead channel mod on the first section.

Used a capacitor on the roach to minimize ticking.

RCA Jacks are floating and have a single ground point where the reverb driver is also grounded.

I have tried different tubes with minimal change in the hiss. 

With V3 taken out, I still have the hiss.  The volume of the hiss increases as I increase the reverb pot with it out (which makes sense because the V4A is still there.  However, around 3.5-4 I start to get a loud oscillation (I suspect from the tank?  I disconnected the return RCA and it disappeared). 

The hiss does go up a little with the increase of both volume pots.

Thanks for your help,
Tyler

[CharacterZero]

Here are some photos: 

[Dave]

Look at the schematics for silverface "Whatever" Reverb. They put snubber capacitors in the reverb return for exactly the same reason. I would start there.


Specifically, a .002 in parallel with the 220k resistor to ground at the RCA connection.


Dave

[CharacterZero]

Thanks Dave for your advice.  I added a cap there (not sure why I had forgotten that as I had done it on another build :)

I also went in and resoldered a bunch of places and shortened some runs.  All of that had some smaller effects, but there was still a lot of hiss.  I then realized that the 0.1 uF capacitor coming off of V4 pin 6 was oriented backwards.  Voila!  That took the noise floor waaaayyyy down!  I knew that it had an impact, but wouldn't have thought that it would be that significant.  Anyway, I think that I still have one or two on the PI section backwards still.  Not sure that I am as clear on those.  Does anyone have any thoughts about that?

Also, I am getting a small hum on the reverb now.  Not sure if it is the room or the fact that the chassis is sitting outside of the cabinet???  I will continue to experiment.  However, I am still wondering if people typically twist the Tremelo and Reverb wires together or separately on their way to the pots?  Is there a best practice?

Thanks again for the support of this community, it really is amazing!
Tyler

[ac427v]

??? .1 cap installed  backwards??? Weber says this is their version of a Mallory 150 polyester coupling cap. How did you determine what was forwards or backwards. The Mallory it is modeled on is not marked.

[Dave]

Does the hum increase in volume as you turn up the reverb knob? If so, I always suspect a ground loop at the ground connection of the cathode of the reverb recovery stage. On Fender type amps, it is not uncommon for there to be a ground loop at that location. If that is it, you will have to remove the ground connection and reground it elsewhere. Some people say ground it where you ground the first stage, I say use and alligator clip and try several different spots until you find the one that the amp likes the most.

Yeah... I'm a little confused about those caps as well. I've never used those caps before, but if I had some and were going to use them, it would never occur to me that they are polarized. Not saying they're not, but I would never suspect it.

Dave

[CharacterZero]

Those are not Weber caps.  They are the bougie Sozo caps that I sprung mucho dinero for.  Thus I was frustrated when they sounded bad.  Now they sound better.  The line on them indicates the outer foil and the other side is the inner foil. Apparently, when they are backwards, they are like a giant antenna dumping lots of bad signal into the grid.
I didn’t totally understand their explanation for which direction to orient them from their description.

https://www.sozocapacitors.com/f-q/

Anyhow, I wanted to share my experience so that others might learn from it.

Dave,
I will have to do some more investigating when I get a chance.  I believe that you are right and have dealt with that before.
Thanks,
Tyler

[pdf64]

??? .1 cap installed  backwards??? Weber says this is their version of a Mallory 150 polyester coupling cap. How did you determine what was forwards or backwards. The Mallory it is modeled on is not marked.

See https://aikenamps.com/index.php/where-to-connect-the-outside-foil-on-capacitors

[sluckey]

However, I am still wondering if people typically twist the Tremelo and Reverb wires together or separately on their way to the pots? 

Run them separately. This is how Fender did the original AB763 DR...