Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MZANNETOS on May 30, 2014, 10:58:23 am

Title: Issue to lower voltages on my new build
Post by: MZANNETOS on May 30, 2014, 10:58:23 am

I built a cathode 4 x EL84 cathode biased amp and used a PT that I had available
Issue is that the PT is giving me 350-0-350 where I only need 290-0-290
So I am getting some high voltages
I tried the zener diodes in series with the cathode going to ground from the CT, but it seems for this amp it does not work
I am using 5W 15V zener diodes
And have them in series on a terminal board
What I noticed is that if I measure from the first diode I do see the drops and if I keep going will see subsequent drops of voltage until I get to the last one connected to ground. And then I get the same original reading of 350-0-350
Would anybody have had the same issue?

Title: Re: Issue to lower voltages on my new build
Post by: HotBluePlates on May 30, 2014, 06:39:45 pm

Quote from: MZANNETOS on May 30, 2014, 10:58:23 am

I built a cathode 4 x EL84 cathode biased amp and used a PT that I had available
Issue is that the PT is giving me 350-0-350 where I only need 290-0-290

The 350-0-350v shows up when the tubes are installed (meaning the power supply has a load on it)? This implies B+ as high as ~495vdc (peak voltage from a rectifier with no voltage drop). If so, then you know the real answer: that the transformer just has inappropriate voltages, and a different transformer is the best option.

But let's say you don't have the spare cash to buy iron, and need to use what you have. Could you use 6V6's instead? Technically, the B+ will be above rated maximum voltage, but we know Fender's been doing it for decades.

Quote from: MZANNETOS on May 30, 2014, 10:58:23 am

I tried the zener diodes in series with the cathode going to ground from the CT, but it seems for this amp it does not work. I am using 5W 15V zener diodes. ... What I noticed is that if I measure from the first diode I do see the drops and if I keep going will see subsequent drops of voltage until I get to the last one connected to ground. And then I get the same original reading of 350-0-350.

Did you remember to install the "-" end of the zeners to ground, and the "+" to the power transformer center tap. That puts the center-tap negative of the ground by the total zener voltage drop.

But again, if you have no current drawn, the zeners won't pass enough current to go into breakdown and drop voltage.