Master volume and Presence

[Nikk]

Has anyone come across an issue with the AA electronics build like mine. By the way it is a great board and likely it is my error not the board or build that has caused the issue.
I am building the ODS 3rd gen based on the 124 .
The problem is that at certain settings of the master volume there is a weird almost radio tuner like noise but no signal is being picked up . whooshing but only at certain points also a little non harmonic distortion almost like 50 cycle hum but the amp is fairly quiet . just wondering if you all had any ideas I have tried screen to ground using a capacitor and increasing the input resistor value but nothing seems to make a difference . it does seem worse at different times perhaps the valves I dont know I am out of ideas and welcome any of yours. dialed into the right setting it is a fantastic sound I have used 2x8" celestions TF0818's bit of a punt but with some thought behind it worked out very well I think no loss of the bottom end really pleased I went for a design I saw on the tube which has a funneled vent in the back as said it is really impressive for sound and in a small cab .

IMG_20201224_105808.jpg

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[xtian]

The problem is that at certain settings of the master volume there is a weird almost radio tuner like noise but no signal is being picked up . whooshing but only at certain points also a little non harmonic distortion almost like 50 cycle hum

You are almost certainly hearing oscillation. Start by posting photos of your build, and perhaps we can solve by improving your lead dress.

[Nikk]

IMG_20210306_205647.jpg

This is not the most recent there have been a few minor changes since this was taken . I will have to pull it apart which I will do tomorrow and take some better photos.

Nikk

[xtian]

First thing to consider is where those wires to the MV and Presence are routed, and what other wires/traces/components they come near.

Come to think of it, an easy test is to disconnect your gNFB from OT secondary, and see if you have the same issue. You could have the OT primaries reversed!

[Richard1001]

You have the groundbus wire tied to the chassis at two points. At the input and also near the powersupply/transformer. This is not a good idea.
There should be only one point to ground near the input of the amp. Keep the poweramp and preampgrounds separate.

[10thTx]

This grounding has worked well for me. I know others have used different approaches also with success.

With respect, 10thtx

[xtian]

You have the groundbus wire tied to the chassis at two points.

Good catch! I missed that. (Will probably cause hum or buzz, but prob not the effects described in the OP.)

[Nikk]

This is a better image up to date also go easy on me please I know it is less than perfect.

IMG_20210525_173608583 (2).jpg

[Nikk]

Do you mean the connection at the PT end this does not go to ground at this point .

[Nikk]

I tried the OT primary 's but this just made a horrible noise . do you think the grounds i have at the PT end with the power cord ground should go to the main groundbus ?

IMG_20210525_173559025 (2).jpg

[Nikk]

Is there any advantage in trying to reroute the MV and Pres wires?

[talbany]

Is there any advantage in trying to reroute the MV and Pres wires?

Nikk
A couple of things
1. it looks like V2a grid resistor is being flown up into your filaments and cannot see what that is attached too?
That should have a grid resistor down low and be connected to your OD trimmer?
2. V2b's grid resistor is missing and should have shielded wire. it doesn't look like it to me
3.Your preamp buss bar is being used for 2 separate grounding points..That buss should be used for preamp only and again grounded down stream away from the power supply!
4. Your board is labeled wrong??. The 100k resistor coming from V2a's plate should feed the drive pot and the 150k should feed OD Vol pot. I's labeled backwards.
Tony

[Richard1001]

I tried the OT primary 's but this just made a horrible noise . do you think the grounds i have at the PT end with the power cord ground should go to the main groundbus ?

IMG_20210525_173559025 (2).jpg

There are some wires going to the groundbus which i think should have separate chassis ground.
Ofcourse when you just disconnect a wire from ground, there will be noise. You should have it connected, Just not on de busground.

You need at least 5 separate chassis ground points. The main power cord ground should have a separate chassis ground. Nothing else connects to that.

The main power filtercaps (caps for powertubes, screens and pi) go to another chassis ground. Close to the Transformer. Bias ground and filament ground can connect there to. If your Transformer had a shield, connect it here too.

The preamp should have a chassis ground close to the input. The bus is connected there. The preamp filtercaps are connected there too (V1, V2)

The output transformer ground is connected to ground by the output jacks. If they are isolated, at least connect a ground wire to chassis or direct to PI ground otherwise the feedback does not work (and présence)

The cathode of the tubes van be wired to chassis ground like you did.

Connecting Transformer wires to the preamp busground will induce noise. There is also a lot of HF noise on the mains voltage.

[Audiodog]

I see a lot of lead dress issues. First, separate the plates and grids. Some say it doesn’t matter, others say it matters a lot. I had oscillation symptoms that were directly related to this. There is no reason to have them touch, ever. Make sure there is space between them. Next, keep the wires from successive tubes separated from prior tubes. V2 and V3 lead dress are close, even though the cathodes don’t really matter. Make sure the NFB wire is not touching any other wires. Grid resistors are best at the tube as well, agree with Tony about V2. I do see it at the pot though. Shielded is needed on the OD pots.

Also, I would separate the shielded V1 wires. No reason to tightly tie them together when there is room to separate. See below for reason.

Eliminating possible lead dress issues will make diagnosis easier even if what I mentioned was not the cause.

[Richard1001]

Oh, and don't make a groundbus on the back of the amp, connecting the speaker ground, effectloopground and relay supply together like you did