5f1 build - retrospective + questions

[martinman]

Hi guys,

I completed a 5f1 champ a couple years ago with the help of the forum. I went the Weber route which in hindsight was a good choice. During the build however I was pulling my hair out due to a number of challenges mostly due to component sourcing. This was my 4th amp build, and once again, I was reminded that this isn't a paint by numbers hobby.

I went with an "upgraded" Heyboer transformer which provided a 375-0-375 secondary. At the time I didn't realize, but it is a bit hot for the 5f1. I played around with bias resistors and eventually ended up at the right spot (w/ 680ohm bias resistor). Plate voltage is an astounding 433V...

Question #1: The heyboer is above the spec - should be more like 330-0-330 for the 5f1. I don't think it's hurting anything being up around 375. I wanted to verify that I can run other power octal tubes -- 6l6, EL34, KT66, KT88 without a problem? I would modify the bias resistor in each case -- or better yet -- install a bias pot.

Question #2: 5y3 vs 5ar4 rectifier. I have the 5ar4 in there now -- i suppose this is contributing to high plate voltages on the 6v6. If I drop down to 5y3 is it true that I can expect about a 10% drop in voltage?

Thanks again!

[hylaphone]

What is the current rating on your power transformer? That will determine which tubes may be run sanely.
As for the 5Y3, it should drop ~60V.
Here is a handy chart for quick reference:
http://www.300guitars.com/articles/r...ge-drop-chart/

[martinman]

What is the current rating on your power transformer? That will determine which tubes may be run sanely.

Specs on the OT:
7K SE primary X 4 ohm secondary

PT:
HV is 370-0-370 @.060mA
6.3v @ 1.5 Amp and 5v @ 2.5 Amp

[Dave H]

I went with an "upgraded" Heyboer transformer which provided a 375-0-375 secondary. At the time I didn't realize, but it is a bit hot for the 5f1. I played around with bias resistors and eventually ended up at the right spot (w/ 680ohm bias resistor). Plate voltage is an astounding 433V...

A good way to reduce the B+ voltage is to add a pi filter. It's just an extra resistor and capacitor. It is also great for reducing the hum single ended amps have. It's a better option than increasing the cathode bias resistor as that moves the bias away from its optimum point for class A.