Reduce filament with resistor

[sbirkenstock]

Hi everybody,

I did reduce my filament in a BF Vibroverb style amp from 6.6 to 6.35 volts with one .1 Ohm 5 watt resistor.
Works fine!
I read somewhere, that you should put a resistor on both sides of the filament supply.
Is this required? Any advantage to that?

Best regards,

Stephan

[Phil_S]

No need to do both legs. I've done this, too. I think you are done.

[matt h]

(deleted)

[gui_tarzan]

I have two small transformers that I was going to try to use (one or the other) in a tube overdrive pedal project but they both put out around 8v AC on the heater leads. I don't know how to properly lower that voltage which I'm sure is high because of the line voltage. They're both NOS from the 1950s, never been hooked up to anything.

[matt h]

(deleted)

[Phil_S]

I have two small transformers that I was going to try to use (one or the other) in a tube overdrive pedal project but they both put out around 8v AC on the heater leads. I don't know how to properly lower that voltage which I'm sure is high because of the line voltage. They're both NOS from the 1950s, never been hooked up to anything.

Are we talking about just one 12AX7? The math is the same for whatever, but here's how: 8.0-6.3 = 1.7Vdrop needed. Heaters need .3A and that is the load on that winding. Ohms Law: R=V/I; 1.7/.3 = 5.66 Ohms. Try a 5 Ohm 5W resistor and see what happens. 5W is more than you need, but these get hot. You will probably be fine with a 1W. Getting to 5.66 will take some figgurin'. OTOH, I like the 12V idea, too.

[johnnyreece]

I have two small transformers that I was going to try to use (one or the other) in a tube overdrive pedal project but they both put out around 8v AC on the heater leads. I don't know how to properly lower that voltage which I'm sure is high because of the line voltage. They're both NOS from the 1950s, never been hooked up to anything.

Maybe rectify it and run DC heaters at 12v (okay, not quite 12...but closer)? I also have very little clue what I'm talking about, so take that with a grain of salt.

[gui_tarzan]

I'll have to make a ground tap as well, neither the B+ or the heater leads have a tap. Yeah, Phil, a single 12AX7.

[Phil_S]

FWB gives you a CT for the B+, here: https://tubeamparchive.com/viewtopic.ph ... 8eb65f71ac unless you won't/can't do the FWB

I'm guessing 8V is without a load and am thinking you don't get to 12V rectified. Probably closer to 11V.

[jaysg]

Aren't properly selected diodes better?

[rp]

OP FYI - I recalled this post from our brethren at Hoffman:

http://el34world.com/Forum/index.php?PH ... #msg128989