Rebuilt plexi hums. Could use some input.

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Matthews Guitars
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Rebuilt plexi hums. Could use some input.

Post by Matthews Guitars » Sat Mar 13, 2021 9:13 pm

So I've almost completed a TOTAL rebuild of a '69 100 watt Plexi.

I started with an amp that, other than the fact that it was actually running when I got it, is a near total basket case, a true victim of one or more hacks with soldering irons, and apparently, a limitless supply of low quality drill bits.

It's a '69 chassis originally equipped with a non-grid style brown perf board (only required holes were drilled) and a secondary perf board for mains voltage selection, near the fuse holders.

Original parts still left in the chassis: The two boards themselves. ONE resistor was the only original part on the preamp board. Every other part was replaced or substituted. The choke was original. Neither transformers was.

Not less than 86 extra added holes were filled and welded up and reground, and the chassis was regalvanized.

I populated the original board with mostly NOS parts, replaced every tube socket, even the fuse holders are true NOS Belling-Lee screw types, and put in a DET-4145 power transformer and a new ****** output transformer.

As of now the amp is running but it hums. Too loud to be anything but a problem. I probably miswired something.

Aside from the hum the amp works great and sounds great.

The hum appears only when the amp is in operate mode, not standby. The hum does not disappear when you pull V2, the cathode follower, but it does disappear when you pull V3, the phase inverter.

No control, volume, tone, or presence, has any effect on the hum. The hum is always at the same volume.

Pic at URL below. Copy-paste to see.


https://www.marshallforum.com/attachmen ... jpg.86676/




Here's the board as it is right now. Working but humming.

Matthews Guitars
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Re: Rebuilt plexi hums. Could use some input.

Post by Matthews Guitars » Sun Mar 14, 2021 1:08 am

Update: Disconnecing the negative feedback lead reduces but does not eliminate the hum.

Incidentally, at this point the output transformer's outputs and the speaker jack connections are NOT grounded, out of an abundance of caution. I know that should be grounded but I don't want to ground the WRONG lead of the output transformer by accident. That M E R R E N transformer wasn't cheap! I don't want to blow it up!

danman
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Re: Rebuilt plexi hums. Could use some input.

Post by danman » Sun Mar 14, 2021 7:05 pm

Do you have the black wire from the OT secondary soldered to the sleeve connection on the output jack? If so, you will be fine running a wire from this terminal to a grounding point on the chassis. Being that this circuit employs NFB, the OT's secondary will need to be referenced to ground for proper operation and this may help to eliminate the hum. I'm not sure which color wires are used on the primary side but if you are reading correct plate voltage on pin 3 of all sockets you most likely have that wired up properly.

Have you checked the bias to be sure that the tubes are not drawing excessive current? This can also be a source of hum especially when the tubes are redplating. Does the amp play properly and sound correct other than the humming? One other cause of noise in new builds can be lighting fixtures over your workbench. Have you installed the chassis in a shielded headbox or placed a metal shield over the open chassis when testing? Many times a noisy amp will quiet right down when properly shielded. Can you post some pics and voltage readings?

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neikeel
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Re: Rebuilt plexi hums. Could use some input.

Post by neikeel » Fri Mar 19, 2021 6:34 pm

It's sorted

PI/screen wiring was wrong way round....
Neil

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