PI plate resistors values

[dude]

Amp l'm building will have approx 360 volts to PI plate resistors 100k and 82k, my math to figure the value isn't working, 360/82,000,  x 360 = 1.5 watts, so .5 watt PI R's will blow, need 2 watts, seem, l'm off some where. Lots of amps have .5 watts here? Will .5 wall carbon comps handle the voltage. Forgive my ignorance.

[pdf64]

Schematic, or at least a rough description of the intended circuit, is necessary.

[2deaf]

You're not dropping the entire 360V across those resistors.

[tubeswell]

You measure the power dissipation across the resistor (which in the case of a plate resistor, is from the HT node to the plate)

[cboysen]

Amp l'm building will have approx 360 volts to PI plate resistors 100k and 82k, my math to figure the value isn't working, 360/82,000,  x 360 = 1.5 watts, so .5 watt PI R's will blow, need 2 watts, seem, l'm off some where. Lots of amps have .5 watts here? Will .5 wall carbon comps handle the voltage. Forgive my ignorance.

Lets assume, for easy numbers, that your 12ax7s one triode draws about 1.22mA of current through the plate, that also means 1.22mA through the plate resistor. 82K*1.22mA = 100V. That is a drop of 100V across the 82K resistor. In other words, 360V (when referenced to ground) on one side, and 260V(when referenced to ground) on the other.

100V*1.22mA = 0.122W

Thus 0.5W Resistors are more than adequate, as long as the current consumption isn't bigger than a couple of miliamps.

Your math is just fine, just remember that the power dissipated is a product of the voltage across the resistor, times the current through it. Also, you should consider looking at load lines, it would explain a great deal regarding exactly this topic.

Kind regards
Christian

edit: 2deaf and tubestwell beat me to it..

[PRR]

If all 360V is in the resistor, there is no place for signal.

Normally an amplifier is a "fair fight" between the resistor and the TUBE. Each takes about half of that 360V.

You may find that 1/2W is "OK on paper" but won't last forever. That's for 90-day warranty amplifiers.

Yes, if the tube goes Dead-Short then all 360V is on the resistor. If the player is going to leave a "dead" amp plugged in, then you may want to use 2W resistors to minimize smoke and panic and repair labor.

[dude]

OK, thanks makes sense, I'm not dropping all the 360v. The amp is a clone to a BF super reverb, PT is 600v @200 mA, 50uf cap, two 6L6GC's @100mA, projected B+ 410v range. Choke, 1K first rail then to PI plates, estimated voltage 400v to PI resistors, 82K and 100K, estimated after plates R, 215/220v, dropping 180v. Looks like a .5w plate R is close to max (around .4w), maybe I need 1 watts but all the BF Super Reverbs schematic's I see have .5 and "seem ok". 
I need to do some more reading. 

[shooter]

maybe I need 1 watts

fwiw;
"common" practice for me and I believe others, use 1W metal film for all plate R's.  Helps with gain stage noises. the "new" ones are about the same physical size as the old carbon type

[dude]

I hear ya shooter, l did the math wrong second time around, it's about .27 watts, dropping 180v so .5 is well below. I did notice that a lot of builders use 1 watt plate R's, the 6g3 Deluxe brown tweed back in the day used 220k, 1 watt on the preamp first stage, always wondered why, probably because of noise, like you say, math says over kill but Leo used 1 watts in that amp, now l know why. I'm building this amp with all carbon comps, except the two power rails, wanted to use 1 watt carbon comps in the PI plates but hard to find them. Didn't want to pay $8 shipping for two 1 watt resistors even though l can't find a supplier

[shooter]

fwiw;
I typically pick up the "10 pack" for a buck.  for ~$20 including shipping I have all the commons, all the required now, and a handful of oddballs for next time. 

https://www.parts-express.com

[sluckey]

After reading this thread I decided to check my two 6L6 AB763 amps. Both amps were running the 82K side at .24W and the 100K side at .29W.

I just blindly followed Fender's lead and used 1/2W resistors. But if I had known the dissipation was that high I would have used 1W resistors.

[cboysen]

After reading this thread I decided to check my two 6L6 AB763 amps. Both amps were running the 82K side at .24W and the 100K side at .29W.

I just blindly followed Fender's lead and used 1/2W resistors. But if I had known the dissipation was that high I would have used 1W resistors.

Hi Steve, I'm sure there is a perfectly valid reason for your assesment, but would you care to elaborate?

Kind regards
Christian

[sluckey]

The 82K resistor was dissipating .24W and the 100K was dissipating .29W. IMO, for a safety factor, a resistor should have a power rating that's twice the actual power dissipation. The 82K was just below that safety margin and the 100K was slightly over the line. So, IMO a 1 watt resistor would have been more appropriate for the 100K and may as well use a 1W for the 82K as well.

Not really anything to lose sleepover. Leo didn't... 

[cboysen]

The 82K resistor was dissipating .24W and the 100K was dissipating .29W. IMO, for a safety factor, a resistor should have a power rating that's twice the actual power dissipation. The 82K was just below that safety margin and the 100K was slightly over the line. So, IMO a 1 watt resistor would have been more appropriate for the 100K and may as well use a 1W for the 82K as well.

Not really anything to lose sleepover. Leo didn't... 

I see. I just assumed that the company would rate the resistors according to a life time of use at that given dissipation, like you see on other equipment, but I guess RMS and Peak values may be far apart, and could ruin a resistor if running it close.

Although, as spoken like a true snake oil salesman: The electron scattering inside the pits of the carbon comp resistor, is said to be the reason for added noise or 'warmth', with more scattering the larger the voltage drop is across the resistor, and the closer you run the resistor to its max capacity. So larger resistor, less noise/'warmth'. If this 'warmth' is the reason for using carbon comp resistors in the first place, maybe just in this case, you could argue that 0.5W CC would sound better than 1W CC;)

Kind regards
Christian

[PRR]

> IMO a 1 watt resistor would have been more appropriate

Remember these 420V preamp B+ *-Face models derive from Tweeds with 250V preamps. 1/2W resistors live forever at half of 250V. Yeah, if a tube Shorts they slow-cook in an hour; but hey, that's a tube problem! And 1/2W resistors are too cheap to account.

As preamp voltages rose, nobody was very concerned at first. And then CBS took over and nobody was concerned at all. Since those dark days, I thought it had become Standard Practice for "quality" amps to run 1W even 2W plate resistors... but maybe not.