Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tnmgcarbide on May 20, 2020, 07:43:20 pm

Title: how much power does a fixed bias trimpot need to handle
Post by: tnmgcarbide on May 20, 2020, 07:43:20 pm

i've seen them from teeny , 1/8 w trimmers  to massive Clarostats
 and Allen-Bradleys........ how much power do they pass in a
50, or 100 watt tube amp ?  the mil-spec components sure look cool inside the chassis,  but  $10 toward a  better  celestion or jensen goes a lot further.  what's  the ideal trimpot in a 6l6 or el34
 design?

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: tubeswell on May 20, 2020, 09:00:49 pm

Its pretty easy to work out if you look at a schematic that has voltages marked on the bias supply.

Power = Voltage x current

and

Current = Voltage/Resistance

So if you know the voltage and resistance, then

Power = Voltage x (Voltage/Resistance) = Voltage x Voltage / Resistance = Voltage^2/Resistance

Usually, you rate the power rating of a component to handle at least 2 x the maximum power 'seen' by the component.

So the power rating of any component is 2 x (Voltage^2/Resistance)

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: tnmgcarbide on May 21, 2020, 01:54:02 pm

thanks for the update to  ohm's law .

my question was more : would a bigger resistor be beneficial in the circuit, or a waste of money.



cheers, it was a good answer, but  not practical.

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: mresistor on May 21, 2020, 02:46:15 pm

   there is very little current in the bias ckt as it is basically a "potential" on the grid.. I wouldn't worry about it..   trim pot or regular pot  or a super grade pot..   Doug uses little piher trim pots..  why would he
use that part if it wasn't acceptable?


btw   I think tubeswells answer was about as practical as one can get in electronics and a given circuit.

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: shooter on May 21, 2020, 02:48:13 pm

does a Yugo and a Bentley both get you to the grocery store?
since you already know they both meet ohm's requirements, I'd call it dealers choice  :icon_biggrin:

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: mresistor on May 21, 2020, 03:02:38 pm

Quote from: shooter on May 21, 2020, 02:48:13 pm

does a Yugo and a Bentley both get you to the grocery store?

Yes  but from first hand experience the yugo may get you there .. but due to terrible brakes you may over shoot the store..

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: shooter on May 21, 2020, 03:14:11 pm

 :laugh:
The only one I "played with" was in a field and needed drug out for scrap, it broke into smaller pieces, but I got it ready for the scrapers  :icon_biggrin:

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: mresistor on May 21, 2020, 05:39:07 pm

well  to the Yugos credit   I might have been thinking of a Lada..     

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: tnmgcarbide on May 21, 2020, 06:01:03 pm

don't forget the lubricant . try some 2-26 from crc . i hate to be dry
when i get the reach around.

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: 66Strat on May 21, 2020, 06:12:04 pm

Quote from: shooter on May 21, 2020, 03:14:11 pm

:laugh:
The only one I "played with" was in a field and needed drug out for scrap, it broke into smaller pieces, but I got it ready for the scrapers  :icon_biggrin:

I had a field car when I was 15. My best friend and I laid out a 1/4 oval track at the farm and practiced our dirt track skills. :icon_biggrin:

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: Dave on May 21, 2020, 06:15:29 pm

Mresistor, not that I doubt anyone's connection to the foreign car market, but I bet you could count the number of people on this forum who know what a Lada is on one hand.
Not that they are missing much. I rented one years ago and some guy pulled out in front of me so I sounded the horn which was mounted in the trunk. I almost crashed anyway.

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: shooter on May 21, 2020, 06:53:18 pm

Quote

practiced our dirt track skills

I built a go-cart track for the kids, ~~400' X 50' oval_ish
fwd 10 years and I was running my jeeps on it, then the WRX on it, dialing my eye to hand, the record, 37mph on the back straight WITHOUT leaving track  :icon_biggrin:

***Manual transmissions ONLY ***   :laugh:

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: mresistor on May 21, 2020, 08:14:46 pm

Quote from: Dave on May 21, 2020, 06:15:29 pm

Mresistor, not that I doubt anyone's connection to the foreign car market, but I bet you could count the number of people on this forum who know what a Lada is on one hand.
Not that they are missing much. I rented one years ago and some guy pulled out in front of me so I sounded the horn which was mounted in the trunk. I almost crashed anyway.

The one I was in was in Holland in the 80's  and the person that owned it said it had Breznev brakes..     LOL    it was a soviet tin can..   they were using the car to smuggle bibles into the USSR>
https://en.wikipedia.org/wiki/Lada (https://en.wikipedia.org/wiki/Lada)

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: tnmgcarbide on May 21, 2020, 08:58:32 pm

m
useless
 
how's this...

does a push-pull 6aq5 need in   p-p a different OT inductance  than an el84 p-p?   10k vs 7k ? or the reverse ?

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: Latole on May 22, 2020, 03:10:32 am

  I always put multi tour Cernet pot on fixed bias amp.

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: shooter on May 22, 2020, 09:45:44 am

Quote

different OT inductance 

looking at the datasheets, something ~ 8K works for either

Title: Re: how much power does a fixed bias trimpot need to handle
Post by: tnmgcarbide on May 22, 2020, 10:56:38 am

thanks .

 i know the only truth is to test in the circuit itself, but i
 don't  need to reinvent the candle... thomas edistone

you guys re great . thanks to all who post here , i'm happy to
get any and all information .

cheers