Eliminating a slight bit of noise

[Diverted]

I'd asked a question yesterday about a new build w/respect to 6V6 screen running higher voltage than the plate. I ended up adjusting the first dropping resistor in the B+ rail from 1K to 2K. The screen is now just about at the same voltage as the plate.

I think it is squared away. Only issue I am having with the amp is slight and was there from the beginning, before I changed the dropping resistor so that's not part of the issue. I would be interested in hearing what others have to say.
The amp has a slight bit of buzz/noise even with volume at 0. The speaker sounds "live" as I can slightly hear juice running through it. I would like to do some recording with this one but want to see how I could knock out this noise. The reverb is quiet overall but does hum a bit as I turn it up.

The buzz/noise remains with of V1 or/and V2 pulled. It disappears with V3 out. I have tried different tubes in V3, no effect. No effect either from disconnecting the reverb tank.

Attached is one photo. There are more high res shots of the guts at the following link. Thanks!
https://imgur.com/a/76RJfJu

[Diverted]

VOLTAGES

V1A
P 202
C 1.6

V1B
P 201
C 1.6

V2
P 334
C 7

V3A
P 186
C 1.6

V3B
P 190
C 1.6

V4
P 339
S 339
C 20
R 504 ohms

B+ 367
B+1 358
B+2 338
B+3 297

[Latole]

I never built my amps with separate ground on main filter caps and they are all quiet.
 You may be right with separate ground, I don't know.

[sluckey]

It looks like your input jack is insulated from chassis? If so, where does the copper buss connect to chassis?

[Diverted]

Hi Sluckey,

No, the input jack is tied to the chassis.
The copper buss you see running along the top of the board is attached to isolated lugs on small terminal strips. That buss is connected to the input jack sleeve. All the other grounds branch off of that preamp buss apart from the first two filters, HV secondary CT, shield for the NFB wire and 6V6 cathode cap/resistor. The output jack is grounded at the jack, and the reverb input and output are grounded at the jacks as well.
Thanks!


One other note: Grounding out V3B grid kills the buzz dead. Grounding out V3A grid at the tank kills the reverb, but not the buzz. However turning up the reverb pot increases the buzz a little bit.

[Diverted]

Here's the schematic. There are some differences from what I actually built:
1. SS rectifier
2. I used a single pot tone stack/ no treble/bass.
3. Switches for NFB in/out and V1A cathode cap in/out.

[Diverted]

I never built my amps with separate ground on main filter caps and they are all quiet.
 You may be right with separate ground, I don't know.

Thanks, I wondered about that too. I disconnected grounds for filters 3 and 4 from the buss and routed them over to the main power ground with the first two caps. There was no difference in noise either way.

[sluckey]

Put your filter cap grounds back like they were. You had them right.

Now try this. Gonna move the cathode ground point for V3 pin 3.  Remove the jumper indicated in the attached pic. Now use a gator clip test lead to connect the cathode R/C to the reverb return jack ground lug (same point the 220K resistor is grounded). Any better?

[Diverted]

Thanks Sluckey. Unfortunately, no change.

Don't know if I mentioned it earlier but turning up the reverb makes the noise increase.

[Bieworm]

Reverb is prone to noise. It's very important where you put the components for that part of the circuit. Simply overheated 220k resistor can cause noise. It also has to be MF. I use 2W MF for those. Ground loops in that part of the circuit are easily achieved. 
I assume you played with the tank orientation and position? What does it do when you put the tank as far away from the amp as possible?
Also...I'm not a big fan of the bare wires on the tube socket that make the parallel connections of the triodes. You should really cover them with tubing.

[Diverted]

Thanks Bieworm. Yes, the uncovered wires ... I arched them progressively to keep them away from each other and since I only plan to use this myself, I wasn't too worried about it as I'm not going to bend them. But I agree overall; maybe I'll adjust them.

As for the 220K to V3A grid ... I've got a 2w in there, but not metal film. It's metal oxide I believe.
I did play around with the tank. As expected the noise increases the closer I put it to the chassis. I have it a few feet away on the bench, and it's oriented for quietest spot. It's pretty much parallel with the chassis, as it will be when I put it all together, but not underneath; to the side at the moment.

[Bieworm]

That 220k has very short leads.  Easily overheated that way. I would try another with MF,  longer leads and an alligator clip clipped on the piece of lead between the lug and the resistor body. Those 220k to ground on the reverb circuit often appear in troubleshooting noisy reverb discussions.
How about the cable to the tank or the rca connections? Are you sure about them being ok?

You could ommit the reverb circuit by disconnecting it .. just to narrow the problem down to the reverb section.

[Diverted]

Thanks. I have 100K meltal films, 1w. I'll run two in series for 200K@2w.
I've checked the cables and jacks, and tried a different set of cables to try to eliminate the one I was using. No change there w/respect to buzz.

The only thing that stops the buzz is pulling V3 or grounding V3B grid.
I'll report back on the 200K resistor fix. Thank you!

[PRR]

I hope I am quoting this right.

....The buzz/noise remains... Attached is one photo. .....

I really hesitated to point this out, because I'm not sure it is a problem, and it will be a pain to change.

But why does your PT CT come to a bolt, a hardware foot, a bolt, before jumpering to the First B+ Cap, which is really where it needs to go??

There are MASSIVE current spikes (buzz) in this path. I think looping it in and out of the chassis could be injecting buzz throughout the build.

And why is the output tube cathode network returned part-way along this path? There may be non-negligible buzz induced either directly up the power tube, or as differential between driver and power tube "grounds".

If there were slack in the red-yellow lead it would be easy to change; but there's not.

[PRR]

There are half-Amp spikes in the rectifier and first cap. If a bolted connection is 0.050 Ohms, and you have two, that is 0.1 Ohms, times 0.5 Amps, is 0.050V or 50mV of ripple across a couple inches of chassis. Small, but larger than a small guitar input. If even 1/1000th of that corrupts your first audio stage, you will hear it when "silent".

[Bieworm]

PRR ...you're the master!! You could be on to something.


Otoh it's easy to rule out the possible problem is in the reverb section. Disconnect it.
That will give you an idea if it's somewhere else you should look. Your V3 is also the PI. When you pull that tube there shouldn't be any sound anymore... so you say the buzz stops by pulling V3 doesn't per se mean it's the reverb.

[Diverted]

So, I think it might not have been in the reverb at all, but it was easiest to spot there.
I think it's in the first stage. I adjusted reverb leads, reflowed solder etc, chopsticked and earlier this afternoon got the amp to a point where there is ZERO noise at idle, with volume at 0. This is what was bugging me ... that I could hear juice/buzz in the speaker when it should be dead quiet.

Anyway, now the reverb is hum free right up to 10 on the pot. However, the issue persists. Though it's dead quiet at 0 volume, the buzz reappears as soon as I get the volume to about 3 or so on the dial.
It stays until I hit around 7 or 8 on the dial, and then vanishes above that to dimed.
There are two other issues coinciding with this:
1. the volume pot sweep is not smooth. I get some static/thumps etc. around halfway up the dial.
2. In addition, the tone has white noise associated it. I know a dimed amp is not going to be quiet, but it does seem moderately louder than what I'm used to.

I checked for DC across the 500pf tone cap thinking voltage might be getting into the pot. It was extremely low ( a few millivolts) but for good measure pulled it and put another one in anyway. And since I have a ton of them I just tried swapping out the 1M volume pot to try to deal with its issue. Neither change had any effect on the weird sweep of the volume pot and the white noise.

At this point, I'm pretty excited about he amp. About from those two issues, it sounds good and the reverb is really really nice ... very clean and hum-free which is awesome. If I could address these issues and quiet down that tone noise and volume pot, I'd be very happy!

Just wanted to say thanks again for all the help so far.

[Bieworm]

Regarding the vol pot issue... is it maybe some sort of oscillation? You could try to played with grid stoppers at the grid of V1B. Start with 1k5...10k...up to 470k. See if that helps... the higher the value, the more high freq will be lost. Remember the grid stopper has to be as close as possible to the grid lug.

[tubeswell]

Not sure how much you have changed from the original picture you posted, but the output tube cathode seems to be grounded at a separate ground from the reservoir cap ground and the screen cap ground seems to be going to the preamp ground (instead of being with the reservoir cap ground). If it were me, I’d keep all the output tube grounds (including the screen filter cap ground, cathode resistor ground and grid resistor ground) going to the same single ground return point as the reservoir cap ground and B+ cap ground and the rectifier (HT CT) ground. These are all ‘high current’ ground returns, and having them grounding at separate return points will be making ground loop hum.


Better yet, go with Merlin Blencowe’s ground buss scheme.


(Your report that the vol pot rotation has an effect on hum cancelling, indicates there is a ground loop issue)

[PRR]

...the output tube cathode seems to be grounded at a separate ground from the reservoir cap ground...

https://el34world.com/Forum/index.php?topic=27591.msg303145#msg303145
You said it better.

[shaun]

Man, that's some pretty wiring. Nice work.