ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Variable power resistors - cathode bias?

3/4/1998 9:34 AM
Whit	Variable power resistors - cathode bias? Hi Y'all, Was leafing thru the Mouser catalog last night and saw these 10W variable power resistors (Ohmite brand I think). They look like big ceramic ones with a center terminal that slides back and forth between the ends. Any comments on the suitability of these as cathode biasing resistors, allowing for bias adjustments without soldering? The power rating and adjustability seem like good things, but I'm wondering if they might adversly affect the sound, or perhaps drift off on value if vibrated or bounced around. All comments humbly appreciated... ... Whit

3/4/1998 9:34 AM

Whit

Variable power resistors - cathode bias?

Hi Y'all,

Was leafing thru the Mouser catalog last
night and saw these 10W variable power
resistors (Ohmite brand I think).
They look like big ceramic ones with a
center terminal that slides back and forth
between the ends.

Any comments on the suitability of these
as cathode biasing resistors, allowing
for bias adjustments without soldering?
The power rating and adjustability seem
like good things, but I'm wondering if
they might adversly affect the sound, or
perhaps drift off on value if vibrated
or bounced around.

All comments humbly appreciated...

... Whit

3/4/1998 12:34 PM
Doc	These wirewound resistors are excellent for zeroing in on the right operating current as a cathode bias resistor in power stages. I use them all the time. It seems that you can't get the right fixed value you need..only a little higher/lower, but never the right one. I install one with a resistance value about double what I think I'll need. Tie the slider to one end of the fixed element. When you're setting the bias, loosen the sliders clamping screw just enough so that you can push the slider across the winding, but leave it tight enough to make good electrical contact. Lock down the screw when you're finished. Doc

3/4/1998 12:34 PM

Doc

These wirewound resistors are excellent for zeroing in on the right operating current as a cathode bias resistor in power stages. I use them all the time. It seems that you can't get the right fixed value you need..only a little higher/lower, but never the right one.

I install one with a resistance value about double what I think I'll need. Tie the slider to one end of the fixed element. When you're setting the bias, loosen the sliders clamping screw just enough so that you can push the slider across the winding, but leave it tight enough to make good electrical contact. Lock down the screw when you're finished.

Doc

3/4/1998 12:50 PM
Whit	Doc, Excellent, thanks...! I was hopin' they were what they appeared to be: an elegant (and safe) method for setting bias. ... Whit

3/4/1998 1:22 PM
J Fletcher	I rebuilt my Princeton Rev for the umpteenth time a couple of weeks ago,cathode biased 6L6's,plate voltage=360,with a 270ohm cathode resistor,and an old Ampeg transformer with a primary impedance=6600ohms.Man,does it sound great.Sounds way louder than I expected,to boot.Puts out about 20 watts.In fact,I decided to build a 4 6L6 version with 2 12's and see what it would be like on stage.Idles pretty hot compared to a fixed bias amp,so I was considering a fan.I am considering the idea of using the cathode current to drive a fan of suitable voltage to help cool things down.Hate to waste that power that the resistor consumes.I've seen schematics of amps which use this current to heat the filaments of 12ax7's.Ever try that?Seems that the voltage isn't regulated well enough for that purpose,but might be ok for a fan.I've noticed that cathode biased amps have lots of idle current,usually greater than 50ma per tube,where do you set yours?Do you"ear ball"it in?How about screen voltages?Transformer impedance?I've looked in the RCA Tube manual for their recommendations,but was interested in real world(ie. rock and roll)experiences.....

3/4/1998 1:22 PM

J Fletcher

I rebuilt my Princeton Rev for the umpteenth time a couple of weeks ago,cathode biased 6L6's,plate voltage=360,with a 270ohm cathode resistor,and an old Ampeg transformer with a primary impedance=6600ohms.Man,does it sound great.Sounds way louder than I expected,to boot.Puts out about 20 watts.In fact,I decided to build a 4 6L6 version with 2 12's and see what it would be like on stage.Idles pretty hot compared to a fixed bias amp,so I was considering a fan.I am considering the idea of using the cathode current to drive a fan of suitable voltage to help cool things down.Hate to waste that power that the resistor consumes.I've seen schematics of amps which use this current to heat the filaments of 12ax7's.Ever try that?Seems that the voltage isn't regulated well enough for that purpose,but might be ok for a fan.I've noticed that cathode biased amps have lots of idle current,usually greater than 50ma per tube,where do you set yours?Do you"ear ball"it in?How about screen voltages?Transformer impedance?I've looked in the RCA Tube manual for their recommendations,but was interested in real world(ie. rock and roll)experiences.....

3/4/1998 2:24 PM
Whit	Hi Jerry, Been a while since I've run into you, I guess I should get out more. Anyways, that's a real interesting idea running the fan off the cathode current. Might even speed up and slow down based on the current, and hence the need for cooling. Might not, can't say for sure. My experience with idle current is mostly single tube class A SE amps (Champs), but I usually have around 44ma of idle current with 6L6's. It isn't ideal (>50ma is closer to the sweet spot methinks), but it's a good compromise for swapping between 6L6 and 6V6 depending on my mood. In other words, with about 400vdc on the plates, and the usual 470 Ohm cathode resistor, my amps are quite happy digesting either L's or V's. Sound about the same too, with the L's giving more tight bass and larger "grunt". As for the screen, it's either standard Fender (a few volts less than the plate via the 1K resistor from the power supply) or I run it "triode" connected. Of course, having a rather tame B+ allows all of this swapping and switching since the maximums (for 6L6 and 6V6) are respected. I'm hoping to push the envelope in one of my "Champs", with 6L6 on board, using the variable power resistor that started this thread. I use the Champ OT that Angela sells, and it's a roughly 5K primary (should actually measure it someday!), and is very forgiving of all I've thrown at it. And as you know, I'm definately on the higher gain side of the ballpark than what you like, so put the above in that context Talk to you again, hope to run into you someday in our lunch time travels... ... Whit

3/4/1998 2:24 PM

Whit

Hi Jerry,

Been a while since I've run into you, I
guess I should get out more. :-)

Anyways, that's a real interesting idea
running the fan off the cathode current.
Might even speed up and slow down based on
the current, and hence the need for cooling.
Might not, can't say for sure.

My experience with idle current is mostly
single tube class A SE amps (Champs), but
I usually have around 44ma of idle current
with 6L6's. It isn't ideal (>50ma is closer
to the sweet spot methinks), but it's a good
compromise for swapping between 6L6 and 6V6
depending on my mood.

In other words, with about 400vdc on the
plates, and the usual 470 Ohm cathode resistor,
my amps are quite happy digesting either L's
or V's. Sound about the same too, with the
L's giving more tight bass and larger "grunt".

As for the screen, it's either standard
Fender (a few volts less than the plate via
the 1K resistor from the power supply) or
I run it "triode" connected.

Of course, having a rather tame B+ allows
all of this swapping and switching since
the maximums (for 6L6 and 6V6) are respected.

I'm hoping to push the envelope in one
of my "Champs", with 6L6 on board, using
the variable power resistor that started
this thread.

I use the Champ OT that Angela sells,
and it's a roughly 5K primary (should
actually measure it someday!), and is
very forgiving of all I've thrown at it.

And as you know, I'm definately on the
higher gain side of the ballpark than
what you like, so put the above in that
context :-)

Talk to you again, hope to run into you
someday in our lunch time travels...

... Whit

3/4/1998 6:03 PM
J Fletcher	Hi Whit...I've been a fixed bias kind of guy in the past,but my Princeton sounds so good that I'm going to try this out.Let me know if you're placing any orders to Angela in the next while,and maybe I can go in on the order and we'll split the shipping etc.Another idea I was thinking of trying was using 2 output transformers,one for each push-pull pair,and having vibrato that modulates each pair 180 degrees out of phase with each other,so that as the signal gets louder in one speaker,it get's quieter in the other.I think the old Rhodes piano had something like this.Kind of a ping-pong effect....see ya

3/4/1998 6:03 PM

J Fletcher

Hi Whit...I've been a fixed bias kind of guy in the past,but my Princeton sounds so good that I'm going to try this out.Let me know if you're placing any orders to Angela in the next while,and maybe I can go in on the order and we'll split the shipping etc.Another idea I was thinking of trying was using 2 output transformers,one for each push-pull pair,and having vibrato that modulates each pair 180 degrees out of phase with each other,so that as the signal gets louder in one speaker,it get's quieter in the other.I think the old Rhodes piano had something like this.Kind of a ping-pong effect....see ya

3/5/1998 8:52 AM
Bruce	What a cool idea J!! Spatial disorientation! Whew! That would be bad on a pair of external speakers. Bruce

Page 1 of 2

Next>

Last Page>>