Marshall 1969 Plexi Clone - Hum and Buzz

[QOTSA_Lover]

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?

Yeah you're right. Typical PLEXI input resistors. They are wired in parrallel so you get the typical 33k input. I don't think this spot is introducing hum and buzz but maybe a litte more hiss.

[sluckey]

I've never heard of a Marshall 1969 Plexi. Can you post a schematic?

[martin manning]

Maybe it's a 1969 JMP metal panel 1959?

[Roe]

plexis were made in 1969.

[QOTSA_Lover]

Late 69 Plexi Panel era just before they changed to the metal panel.

1987 Model JMP50
32+32uf Preamp Filter
V2a Cathody bypass Cap
NFB 47k @ speaker Jack tip
4.7nf Bright Cap
...

[Alnico]

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.

Do you still have any shielded cable in the amp? If so, is the shield grounded at both ends, or only one?

[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.

Do you still have any shielded cable in the amp? If so, is the shield grounded at both ends, or only one?

Thank you... Yes, but they are shielded only at one side to avoid ground-loops. Both input cables (68k to V1) are Belden COAX. The shield is connected only at one side at the jack's ground.

[sunnydaze]

I couldn't tell where the filter cap can is grounded, but if that's not grounded at (or near) the input jack, it can introduce hum - particularly if it shares or is close to the power cap ground.

Thanks
Mike

[QOTSA_Lover]

I couldn't tell where the filter cap can is grounded, but if that's not grounded at (or near) the input jack, it can introduce hum - particularly if it shares or is close to the power cap ground.

Thanks
Mike

Hey Mike, thanks for the input.
The preamp filter on the side of the chassis is grounded on the 2nd ground lug (near to the inputs).
The screen and PI filter is grounded at lug nr. 3
the mains filter at 5

[QOTSA_Lover]

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.

So how did you solve that problem?