Marshall 1969 Plexi Clone - Hum and Buzz

[QOTSA_Lover]

Hey Guys,

I am new to the forum and hope someone can help me with my Marshall JMP69 Clone.
I built this amp with a few "improvements" compared to my first build.
Basically I took a Suhr Sl67 layout as template.

1. 68k Inut resistor close to the V1 socket
2. PPIMV Master
3. Larry Style Grounding Scheme
4. Bias Resistors

The amp sounds great and is working. Every knobs works as it should. But I have a very annoying BUZZ/HUM even with nothing plugged into the amp.
I did a view tests:

Pulling V1 increases the hum
Pulling V2 and the amp is dead silent
With the tubes back in and volumes to zero the BUZZ goes away.

So I think my problem is anywhere in the preamp section?!?

I have a 68/69 Chassis Layout. The Preamp filter sits on the side of the chassis direct above the PT.
I noticed that I am able to change the BUZZ/HUM with the orientation of the Filter Cap.
My first guess: The PT is humming into the filter cap. So I took the filter cap out and did a view test with different positions.
The result: The BUZZ was back.

Finally I mounted the filter to the side again and tried different positions on that spot.
When mounted with the hot connections towards the tranny - BUZZ
When mounted with the grund connection towards the tranny - BUZZ is much better.

So my conclusion is, that there's still an issue with lead dress or sth like that. The orientation of the cap helps because I am cancelling out some frequencies - seems like a "phasing out" thing is happening there. But the problem is still there and has other reasons.


Maybe someone has an idea what's the reason for the BUZZ/HUM?


Thank you so much and stay well
Tom

[QOTSA_Lover]

Here some pics

[QOTSA_Lover]

Additional hint:

I am not shure but compared to some samples on YT it could be a 50cycle hum.

[sluckey]

I don't see a ground reference for the filament winding (filament winding center tap or artificial center tap). Do you have one?

[QOTSA_Lover]

Thank you...
yes it is there. I have 5 ground points along the frontside of the chassis.
The Heater is grounded on the first one with the input jacks and the V1 cathodes.

[sluckey]

The Heater is grounded on the first one with the input jacks and the V1 cathodes.

Try moving the heater ground to the same point the HT (B+) center tap connects to chassis. Any better?

[pdf64]

Are the shorting contacts on the input jacks wired up correctly?
To avoid damaging its heater-cathode insulation, the tube in V2 should be Chinese or vintage manufacture.

[Roe]

those TAD filter caps may fail. check it.

[QOTSA_Lover]

The Heater is grounded on the first one with the input jacks and the V1 cathodes.

Try moving the heater ground to the same point the HT (B+) center tap connects to chassis. Any better?

Thank you so much. I already tried this. No improvement...

[QOTSA_Lover]

Are the shorting contacts on the input jacks wired up correctly?
To avoid damaging its heater-cathode insulation, the tube in V2 should be Chinese or vintage manufacture.

Yes. Checked it. Seems to work as it should.
I tried NOS Tubes, Chinese, JJ... Ervything

[QOTSA_Lover]

those TAD filter caps may fail. check it.

Very Interesting. I will check this.
EDIT: Filter Caps are OK. Clipped new ones parallel and nothing changed for the better.

Clueless...

[_Steve]

Is it 60 or 120Hz hum? Not an expert here, but I believe that 120Hz usually points to filter caps or power section.

If you can see it on a scope, you could walk forwards from the input to where you start to see the hum. If you have the hum even on the input then you know its a grounding issue. I just had a bizarre signal issue and found it was crosstalk in the channel mixer using that method.

[Steveo]

I had to use a chopstick and start moving wires, my heater wires didnt like how they were situated around the ohm selector.

[QOTSA_Lover]

I had to use a chopstick and start moving wires, my heater wires didnt like how they were situated around the ohm selector.

Thank you for the tip. I already "chopsticked" the whole amp with no results. But I will check the heaters again.
I noticed that the shielded Belden COAX cable may be the culprit.
I don't know why.

With unscrewed input jacks I was able to move the two COAX wires more to the left side of the chassis. There are some spots where the buzz is better.
So I tried to attach the 68k input resistor to the input jacks and used a 22AWG standard wire from there to wire to pin 2/7.
The buzz decreased a little bit but it is still there.

[Steveo]

Hello, I noticed you have two resistors on v1a, i think pin 7 (Grid input) and also 2 on v1b maybe pin 2 (grid input) ? Why a pair of resistors for each? I noticed these 2 locations were very sensitive on my amp, so i just used one resistor soldered as close to the pin as i could. Im not sure if having two resistors in these spots is adding noise. Maybe?