Getting a hum from my 1968 plexi clone

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screamindemon
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Getting a hum from my 1968 plexi clone

Post by screamindemon » Mon Jun 14, 2021 10:11 pm

Hey guys,
I bought this amp used it is A Metro kit someone built. I have had it for some time and it has been modded by me with a couple different circuits but it has always hummed since day one. So i think it is in the power section. What is the best way to diagnose this problem. I would love to finally resolve this. Amp sounds good otherwise .It's a 100 watt with lay down transformer.

Appreciate any help given, thanks

Can't figure out how to post a picture but here is a link.

https://imgur.com/Zqq4mOA

screamindemon
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Re: Getting a hum from my 1968 plexi clone

Post by screamindemon » Mon Jun 14, 2021 11:02 pm

Here is a video of the hum.

https://youtu.be/tVMnhyvibqE

revolver1
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Re: Getting a hum from my 1968 plexi clone

Post by revolver1 » Tue Jun 29, 2021 5:01 am

Hi screamindeamon, sorry this is a bit of a late response.

Learn to drain your filter caps before anything. I take a short lead with croc clips on each end, clip one end to the chassis and the other to one of the V1 plate resistors on the circuit board (these are the two 100k resistors joined together with a wire coming off of each to the V1 socket. Flip your stand by switch on. You can use a voltage meter set to DC start high range and switch range down as the voltage falls. Read between chassis ground and the filter cap positive terminals for varification of voltage present. Leave the lead attached while working on the amp. Don't forget to remove lead when you have finished and reset the stand by switch.

Does it still do it when the amp is in the head shell, is there a a shielding plate in it?

Looks like you have a few gain mods in there, the increased gain could well be a source for noise.

Does it have a grounding scheme that separates the different sections of the amp across the chassis? They call it a Larry grounding scheme around here or star grounding. Do a search and it should bring up a diagram illustrating the suggested grounding points for the different sections of the amp. A poor grounding scheme is a likely source for noise. Make sure all grounding lugs are giving a very low continuity reading to your power cord earth pin and that they are good and tight with star washers fitted.

Have you tried swapping the preamp valves around just to rule out a dodgie valve? You can sometimes get away with a slightly microphonic valve in V3 but not in V1 or V2.

Lastly I'd look at lead dress (how the wiring is routed). If wires have to cross each other ensure they do it at 90' to each other. Also separate out the wires on your preamp sockets. They look a bit bunched up. They want to peel away from the socket keep them evenly spaced and don't cross them. If they have to cross for instance crossing the heater wires, cross at 90'. Pay close attention to signal (green wires) and separate from wires taking voltage. Try keep the wiring flat to the chassis it helps with shielding.

Wiring that carries AC are generally more prone to noise than those carrying DC. If wires are run close together with say a feed and a return they are best twisted together to cancel out the EMF (magnetic field) radiating from each wire.

Don't know but the bias pot should be over with the bias circuit not on top of the phase inverter. Check against lay out diagrams (see bottom of board index page for downloads).

Straighten out wires coming off the input Jack's and controls pots as in try not to let them cross.

Most probable cause is the gain mods which to a degree will be an unavoidable consequence and probably fixed with say a Boss NS2 noise gate but best cures are lead dress and a well implemented grounding scheme.

Hope this may help, good luck and let us know how you get on.

screamindemon
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Re: Getting a hum from my 1968 plexi clone

Post by screamindemon » Fri Jul 02, 2021 6:26 pm

Thanks for the reply.
This amp has always hummed since I got it. Originally it was supposed to built by rockstah with a mod 5 I believe.
since it has always done it and did not change after the preamp gain mods I don't think it is that. I believe it is something in the power section or grounding. I will check the things you noted. Yeah the bias pot was always just hanging there, I don't see a good way to mount it. Where could i find layout diagrams for this I looked but did not see them.

I did try changing preamp tubes also.

Thanks for you help.

revolver1
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Re: Getting a hum from my 1968 plexi clone

Post by revolver1 » Sat Jul 03, 2021 4:35 am

I seem to recall reading along time ago that rockstah had stopped bothering with the Larry/ star grounding scheme so that could be it. I personally wouldn't build an amp with out it especially with gain mods, it really does make a difference to the noise floor. Biggest improvement is grounding the power transformer center tap up by the inputs but really it's the whole scheme, every bit helps.

I think you will have to accept a certain amount of noise with gain mods but you should be able to get it a bit quieter than it is. Failing that as I said it's a noise gate that can also be quite cool for high gain tones, if you set it up with the threshold just before it starts chopping the note it kinda adds a subtle cocked wah thing to it.

O.K. I just looked in the lay out section and the only thing of use to you for the bias pot is the full 100w kit build instruction down load, see it down the bottom of the board index. Other than that go into the 50w section or the do it yourself amp builds and look for an amp build with pics for reference. You'll see a few of us have done it in slightly different ways but its essentially the same thing. It looks to me that rockstah picked up the ground at the phase inverter, I'm honestly not sure if it will make a difference but it doesn't seem like a great way to do it to me. I'd be inclined to keep the sections seperated.

All you need to do is this, the pot has 3 legs one of the outer legs and the middle leg both go to ground and the other outer leg goes to the resistor. Re route the resistor between the two bias capacitors at the end of your board and pick up your ground at the two turrets that are linked together with the bias capacitors on the side of the board closest to the controls.

So you could solder one outer leg to the turret and the middle lug to the wire linking the turrets and the other outer leg to the resistor routed between the caps. Position it so you can easily adjust the pot with a screw driver, I like it facing kind of up and towards the front of the chassis that way you won't have your hand draped over the live circuit board when biasing. Check the bias after you do it.

Also look at the way others have spread out their pre amp wiring.

:thumbsup:

revolver1
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Re: Getting a hum from my 1968 plexi clone

Post by revolver1 » Sat Jul 03, 2021 4:53 am

O.K, regarding the bias circuit I just had another look at your amp gut shot, it's a bit fuzzy, I was wrong he hasn't grounded it to the phase inverter but it looks like he's added another resistor to the other side of the bias pot going to the ground at the bias caps. Maybe he had trouble getting the bias range where the wanted it.

Any way I'd go with a 47k resistor before a 22k phier pot to ground as I detailed above. Should be fine. :thumbsup:

screamindemon
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Re: Getting a hum from my 1968 plexi clone

Post by screamindemon » Sat Jul 31, 2021 6:07 pm

Still unsure how to Mount this bias pot.
I am working on the amp today, so I will look around the forum again for some clues.

Thanks

danman
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Re: Getting a hum from my 1968 plexi clone

Post by danman » Sun Aug 01, 2021 6:41 pm

That type of pot is meant to be soldered onto the turrets. If your amp doesn't have the necessary turrets on the board, you can add a few to the board or switch to a chassis mount style pot. Here is a link for some turrets that are easy to add.
https://tubedepot.com/products/bifurcat ... poxy-board

Here is a second link to a bunch of the old Metroamp layouts. You will notice some of the layouts use a vertical mount pot and others use a horizontal type like the one found in your amp. Either one will work the same but they do mount to the board slightly different. If you don't mind drilling a hole in the chassis and mounting a standard potentiometer for bias, you can take a look at some of the Ceriatone layouts for wiring ideas.

https://drive.google.com/drive/folders/ ... 549g-K1iDA

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