Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on October 12, 2016, 04:11:08 am

Title: Adding one gain stage. O.T. Need to swap the primary wires?
Post by: chocopower on October 12, 2016, 04:11:08 am

i´m adding a EF86 to an old 60´s amp.


Original amp was:


gain stage------cathodyne------push pull EL84. 


Amp HAD N.F.B Loop. Not now.


Should i swap primary leads to avoid oscillation?




Thanks in advance.

Title: Re: Adding one gain stage. O.T. Need to swap the primary wires?
Post by: John on October 12, 2016, 06:47:44 am

I'm assuming you mean swap *secondary* leads. But anyway, no. It's the NFB that causes the squeal when the blue/brown wires are hooked up "wrong". If no NFB, there should be no oscillation either way.


That said, if you're adding a gain stage, you might want to keep that NFB ?

Title: Re: Adding one gain stage. O.T. Need to swap the primary wires?
Post by: chocopower on October 12, 2016, 10:11:26 am

Amp had just one 12ax7 and 2 EL84.


P.I. was a cathodyne, so, only 1/2 12ax7 was "working"


One more gain stage was mandatory

Title: Re: Adding one gain stage. O.T. Need to swap the primary wires?
Post by: HotBluePlates on October 15, 2016, 12:06:19 pm

You would only need to swap the wires if the new, inverting gain stage is inside the feedback loop.

I'm guessing you may have already figured this out by now in the amp itself...