Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on July 24, 2023, 08:00:25 pm

Title: 1/4 watt resistor crisis
Post by: joesatch on July 24, 2023, 08:00:25 pm

ran out of 1 watts dont have any 1/2 watt. working on a tonestack circuit so the voltages should be pretty low. can i use 1/4 watt resistors?  specifically 1/4 watt into preamp grid?

Title: Re: 1/4 watt resistor crisis
Post by: bmccowan on July 24, 2023, 08:48:57 pm

First - my Mainer answer; don't matter to me.
Next - I suspect if you do the math - it's fine. Bunches of reissue PCB based amps use them.
Next - I wouldn't, given that I could get the resistors the project deserves in a few days for cheap money. You work on a lot of amps - why not just order up a whole kit of all values. Its so nice to have what you need on hand.

Title: Re: 1/4 watt resistor crisis
Post by: kagliostro on July 25, 2023, 12:19:27 am

The bigger the resistor the low the intrinsec noise

I use metal film 2W resistors and bigger where required

I can see them better, they have a better look on the Board and I can handle in an easier way than the invisible 1/2 - 1/4 ones

Franco

Title: Re: 1/4 watt resistor crisis
Post by: dwinstonwood on July 25, 2023, 04:29:47 am

I just bought a bunch of 600mW (0.6 Watt), 1% Vishay's in all of the typical guitar amp values. I had always searched for and bought 1/2 Watt's in the past.
The Vishay MBx/SMA - Professional series has a huge range of values.
I even bought some 90.9K's, a weird value that can balance LTP outputs much closer than the usual 82K (if that's what you want).
They averaged about 26 cents a piece @ 10 or more, so a paid a bit more for them.

Just go to your favorite online electronics supplier and apply these filters:

Metal Film-Through Hole
Vishay
600 mW
1% Tolerance
MBx/SMA Series

Title: Re: 1/4 watt resistor crisis
Post by: pdf64 on July 25, 2023, 06:58:39 am

With resistors, watch out for their voltage rating. Many types have a 250V limit, standard sub 1W is 350V.

Title: Re: 1/4 watt resistor crisis
Post by: dwinstonwood on July 25, 2023, 08:30:56 am

Thanks pdf64.

The Vishay 600mV series I referenced above do have a 350V rating:

https://www.mouser.com/ProductDetail/Vishay-Beyschlag/MBB02070C9092FC100?qs=LkqrmUh5PII7wRxfAHYqwA%3D%3D

Title: Re: 1/4 watt resistor crisis
Post by: dbishopbliss on July 25, 2023, 10:57:55 am

Quote from: dwinstonwood on July 25, 2023, 04:29:47 am

I even bought some 90.9K's, a weird value that can balance LTP outputs much closer than the usual 82K (if that's what you want).

What change (improvement???) do you notice by having the outputs more balanced?

Title: Re: 1/4 watt resistor crisis
Post by: dwinstonwood on July 25, 2023, 02:34:07 pm

Quote from: dbishopbliss on July 25, 2023, 10:57:55 am

What change (improvement???) do you notice by having the outputs more balanced?

I would say none!  :icon_biggrin:

I was playing with this calculator, and 90.0K gets darn close, so, why not? I bought some.
https://www.ampbooks.com/mobile/amplifier-calculators/long-tailed-pair/calculator/
But, I don't think my ears would notice a difference.  :dontknow:

Title: Re: 1/4 watt resistor crisis
Post by: joesatch on July 25, 2023, 03:03:09 pm

i found a hack. so i am good!

Title: Re: 1/4 watt resistor crisis
Post by: PRR on July 25, 2023, 10:44:45 pm

WHAT "tonestack"?

But for argument/example, the usual Fender stack played LOUD may have 100Vrms across the first 100k resistor.

100V^2= 10,000

10,000/100,000 is 1/10 Watt