The Amp Garage

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 ju...

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...

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 culp...

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
...

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 ha...

On my very first Plexi amp I built a few month ago I copied a real Marshall Plexi layout. The lead dress was done for the easiest wiring and I grounded the things with no knowledge anywhere to the chassis.... Guess what...?!? This amp is dead quiet :D My 2nd build was done with a “better” grounding ...

More pics

Thank you very much the replies. Hard decision how to wire the grounds this time. I am going to try a slightly different approach this time. I will use 4 ground points along the front. 1st Inputs and V1 cathodes, maybe heater 2nd preamp Filter, v2 cathodes, volume pots 3rd tone stack, speaker jacks,...

Roe wrote: ↑Sun Jun 14, 2020 2:42 pm the traditional marshall direction works well here

What do you mean? It's okay that way?
Or do you mean it's better to wire the grounds with the "classic" Marshall scheme? (Bus bar over the pots, filters direct to chassis, etc.)

Here’s a shot from the chassis

Hey Guys, I am working on a 2nd Plexi50. My first build has a strange BUZZ/HUM and I don't want to do the same mistakes again. 1. Larrys's Grounding Scheme: On my first build I drilled 5 holes along the frontside of the chassis. On the last grounding lug (5th) I attached the HV CT, the EL34 Cathodes...

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 culp...

Great Build!!!

Congrats!

Do you have any troube with hum/buzz/hiss on higher volume levels?
Is this a Larry Grounding Scheme?

Greetz
Tom

Roe wrote: ↑Thu Jun 11, 2020 5:07 pm 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...

pdf64 wrote: ↑Thu Jun 11, 2020 3:25 pm 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