Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on March 27, 2018, 08:01:32 pm

Title: about heaters hum
Post by: Colas LeGrippa on March 27, 2018, 08:01:32 pm

Hola compadres !


My friend brought me an old ''Ford'' amplifier for a cap job. It's a SE single 6V6, 5Y3 and 2 X 12ax7, one being for the trem circuit. I added a grid stopper at the power tube, one ohm resistor for the bias, a cathode resistor bypass cap, a cap at the cathode of the preamp tube, replaced the three power supply caps. Incredible: now this thing got balls !!!!
The problem is the heaters. One side of the heater circuit is grounded , the other side carries  6.3VAC to pin 2 of the 6V6 then to pins 4-5 of the others. ( pin 7 of 6V6 and 9 of the 12ax7 tied to ground). Hummmmmmmmm that's the problem. Would it be possible to move the grounded heater pins to the cathode of the power tube ( 12-15V ) to get rid of the hum ? I have never elevated that kind of heater circuit before.


Second question: which old SE amp uses a trem circuit in doug's library ?


Thanx


Colas

Title: Re: about heaters hum
Post by: Colas LeGrippa on March 27, 2018, 08:16:13 pm

...FOR THE TREM  I have found the Vibro champ which is almost exactly a copy of the Ford ampifier I have ( Ford being the copy of Fender I guess....but who knows ???? ), with a 5Y3, 6V6 and two 12ax7.


Colas

Title: Re: about heaters hum
Post by: PRR on March 27, 2018, 08:43:24 pm

There has to be a heater return path. On cheap crap, they used the chassis. You can see the "other" heater socket lug bent over and soldered to case. What about the other 6V lead out of the transformer?? Is it there? Or did they use the extra-cheap trick of not even bringing that lead out, just wrap it to PT frame and rely on PT bolts to complete the circuit?

Title: Re: about heaters hum
Post by: Colas LeGrippa on March 28, 2018, 11:26:01 am

Hola PRR  here's the heaters supply, thanx.

Title: Re: about heaters hum
Post by: sluckey on March 28, 2018, 11:36:43 am

I would remove the chassis connection from all tube filament pins and the indicator lamp. Now run a wire to connect all the stuff you just disconnected. At this point your heaters and lamp should light up.

Now connect a 100Ω half watt resistor between pins 2 and 8 on the 6V6 socket. And connect another 100Ω half watt resistor between pins 7 and 8 on the 6V6 socket. Now your filaments are elevated to the 6V6 cathode voltage.

Title: Re: about heaters hum
Post by: Colas LeGrippa on March 28, 2018, 11:53:59 am

That's what I thought without being 100%sure. Thanx.


Colas

Title: Re: about heaters hum
Post by: PRR on March 28, 2018, 02:39:03 pm

.

Title: Re: about heaters hum
Post by: Colas LeGrippa on March 28, 2018, 10:31:26 pm

Thanx for the precision PRR but I had understood. Sluckey sure knows how to explain things to kids, I'd bet he had a couple... :icon_biggrin: Job done, humfree.


Thanx bros.