Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on August 09, 2023, 11:11:25 pm

Title: 2553 PT position
Post by: Leevi on August 09, 2023, 11:11:25 pm

A Marshall 2553 is causing hum even if you remove the power tubes.It looks like the PT is not in an optimal position which probably causes the hum.Everything looks very original in the chassis and the both transformers are produced by Drake.I found some pictures where the PT installation differs from the model where the issue is.Pls. see the pictures

/Leevi

Title: Re: 2553 PT position
Post by: kagliostro on August 10, 2023, 01:06:28 am

Ciao Risto

Seems that the difference is the fact that the PT in one version is lying down

Is the other the version with the problem ?

Franco

Title: Re: 2553 PT position
Post by: Leevi on August 10, 2023, 02:26:29 am

Ciao Franco,
The problem exists in the version with lying down PT.I wonder since everything seems to be from the factory production i.e. no mods done.

/Leevi

Title: Re: 2553 PT position
Post by: kagliostro on August 10, 2023, 06:30:17 am

Can you find an end bell and add it to the PT so you can mount it the other way ?

Franco

Title: Re: 2553 PT position
Post by: Leevi on August 10, 2023, 06:38:59 am

I can mount it vertically without end bell.The connectors will stay unprotected but it does not matter since the cabinet is protected anyway.
/Leevi

Title: Re: 2553 PT position
Post by: kagliostro on August 10, 2023, 10:17:15 am

I was thinking to add the bell because it is a shield to the EMF

An alternative you can try is to dismount the PT, close the squared hole with an iron sheet

put the necessary holes for the wires and mounting bolts on this sheet, then mount the PT lying Down but on the Upper Side of the chassis

Franco

Title: Re: 2553 PT position
Post by: Leevi on August 10, 2023, 11:01:28 am

It's already mounted and the result is complete silence.
The bell is not an option since the connecrtors are pretty massive.Pls. see the picture.
/Leevi

Title: Re: 2553 PT position
Post by: kagliostro on August 10, 2023, 02:06:37 pm

No Humm ..... PERFECT 👍👍👍

I understand the difficulty to add one other bell

You can stop here if you want

But, if you want to cover that part of the PT an alternative can be to build an aluminium shield (1.5mm or bettter 2mm)

I'll try to draw my idea for that and I'll post it later (if the draw is good)

Franco

Title: Re: 2553 PT position
Post by: kagliostro on August 10, 2023, 02:26:36 pm

Here I'm (unfortunately I havent a 3D software, only 2D

This is the shield I would try in place of the missing End Bell



(https://i.imgur.com/o3bYRKe.jpg)

You must cut and drill it in a 1.5mm or 2mm aluminium sheet, then you have to bend the 3 parts at 90 degrees (when the sides are bent the shield will have the same measures of the laminations), position the shield with the missing side on the bottom and fix it to the PT bolts using treaded joint nuts and screws


Better to use 2 layers of some adhesive plastic sheet to electrically insulate the inside of the shield

(https://i.imgur.com/i5tMddz.jpg)

Franco