Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mayburyds on July 21, 2014, 06:18:04 am

Title: VRM Problem
Post by: mayburyds on July 21, 2014, 06:18:04 am

Hi,



Got a VRM kit from Trinity amps for my Tube Doctor 5E3 kit.


The VRM seems to do it's job OK in reducing the volume but when I start to turn the pot about a third of the way around I start to get a humming noise over and above the normal amp hum. As I continue to turn the pot to about two thirds of the way around the hum turns into a buzzing noise. After two thirds to full on the buzz goes away and the amp sounds like it normally does.


The amp works fine with no extraneous hums or buzzes without the VRM kit installed.


I have spoken to Trinity amps and the wiring on the VRM is OK and the where I installed the VRM is OK.


To connect the VRM to the amp I disconnected the B+ wire on pin 8 on the rectifier and reconnected it to the B+ output on the VRM board. I then connected a new wire from pin 8 on the rectifier to the VRM B+ input. No other changes to the amp itself.


I have taken the VRM put for the mo.


Anyone any ideas what might be causing the problem?

Title: Re: VRM Problem
Post by: sluckey on July 21, 2014, 06:23:32 am

Put a filter cap (20-40µF) directly on pin 8 of the rectifier and ground. This will smooth the B+ input to the VRM and may help.

Title: Re: VRM Problem
Post by: mayburyds on July 21, 2014, 07:29:21 am

Thanks for the reply.


If it helps this is the noise I'm getting -


https://soundcloud.com/maybury1974/amp-noise-1


Cheers.

Title: Re: VRM Problem
Post by: mayburyds on July 23, 2014, 05:33:37 am

Sluckey,


Sorted!


Put a TAD 16uf 475v cap in as that is what the other filter caps are and problem is solved.


Many thanks again!


Cheers.