First stage voltage drop resistor - how many watts?

[Twosteps]

Hi everyone, I've successfully rebuilt my 5E9A tremolux to pretty much original spec and really love how it sounds, but then came across one question, how many watts should step down really be? In the orig schem it said 1/2w unless noted otherwise and that's what I did and the clean and the trem that I got was just awesome but kept reading about it and it seems like everyone uses the 2w resistors for that job so I changed it - the first one to 2w. While I could feel more power some how and it operates very cool(the 1/2w gets warm) it lowers the headroom somehow, lost some highs, sag more I'm not sure I like it but now that safety comes in to play I'm not sure about putting a 1/2w back either. What do/would you guys use?

[tubenit]

I use either 2W, 3W or rare occasions 5W dropping resistors.  I don't use 1/2 watt ever.

it lowers the headroom somehow, lost some highs, sag more

You're saying changing resistor wattage changes those dynamics?   


With respect, Tubenit

[Twosteps]

Yeah, I know I'll be getting those... but they measure very close one is CC 2.75k 1/2w another one is Metal Oxide 2.69k 2w. I'm sure the difference is in either the type or watt. The tonal difference is too big not to mention

[Twosteps]

Oh, also would you mind sharing the reason why? I'm using the calculation here

https://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.html

and it seems like it needs .33w max, am I missing something? Thanks!

[sluckey]

What numbers did you use to calculate .33w?

The 5E9A layout shows 25 volts across a 2500Ω resistor. That calculates to .25 watts actual dissipation. Double that for safety margin, so a 2500Ω 1/2W resistor would be fine. However, in the modern world many people prefer to use metal oxide flame proof resistors in the power supply. Those are commonly found in 2 or 3 watt sizes and are quite affordable.

While I could feel more power some how and it operates very cool(the 1/2w gets warm) it lowers the headroom somehow, lost some highs, sag more I'm not sure I like it but now that safety comes in to play I'm not sure about putting a 1/2w back either.

I don't see how changing the wattage of that resistor could have any impact on any of that. But if you don't like the sound, put the half watter back in. It worked just fine for Fender.

[Twosteps]

I put in 2.7k tp drop 30v that's the real measurement I get with the 2w one. I don't wanna believe in something else besides numbers either but when I hear it... maybe it's the type 

[jjasilli]

I use either 2W, 3W or rare occasions 5W dropping resistors.  I don't use 1/2 watt ever.

it lowers the headroom somehow, lost some highs, sag more

You're saying changing resistor wattage changes those dynamics?   

With respect, Tubenit

Could be.  As a resistor gets warm > hot, its resistance increases, lowering downstream voltage.


The solution is Ohm's Law; and the Power Formula.


Using a calculator is OK, but unnecessary, and seems to detract from basic understanding.  You can use the tube charts to find the current drawn through the resistor; measure the current draw directly; or calculate it based on the voltage drop using Ohm's Law.  Then use the Power Formula to find the Watts dissipated by the resistor.  Then the double the Watts to find the Wattage Rating for the resistor.  This is for safety and to lessen the drift upwards in OHMS of the resistor as it heats up in actual use.

[2deaf]

I don't actually see a note about 1/2W unless noted otherwise on the schematics I have.

1/2W for the 2.5K dropping resistor is fine at idle, but the screens just might draw three times as much current once that amp gets to rockin' and rolling.  It might have 18.8mA across 2.5K for a voltage drop of 47V and a dissipation of 0.88W.

[PRR]

I do not know why the sound would change. Sonic effect of that R will be quite swamped by the large caps on each end. Something else changed.

Here (attached) is why you use a FAT resistor in the first dropper position. While the *running* power may be part-Watt, the *START-UP* power is huge, though only for a brief instant. At turn-on the big PT and rectifier bring the 1st cap up to 400V almost instantly. The 2500 has another big cap at its far end, and it must charge to nearly 400V. The peak power is 67 Watts!! Falling to 13 Watts at 1/20th of a second, 3W at 1/10th sec.

What happens with a small resistor is, every turn-on a teeny bit of carbon (whatever) is burnt-off. The resistor still works. But over thousands of turn-ons it loses enough stuff to drift off value, or even fail completely.

Footnotes:

Yes, I got the running current wrong. This has little effect on the starting current.

This plot is for bang-start. 5Y3 will start-up much slower. Using a half-second ramp-up, the peak power is only 1.8 Watts, though over 1 Watt for most of 0.3 seconds. Indeed a 1/2W part may survive many millions of such starts.

[DummyLoad]

https://www.mouser.com/datasheet/2/303/res_40-1228678.pdf


series 47 for the WV and power handling overload rating.


--pete

[Twosteps]

Thanks a lot for the reply everyone 

Re PRR; Thanks so much for a very clear answer! I'm using 5U4 though so should I be worry if I were to go back and use 1/2w r, it's a bang start isn't it? I'm using it with 300-0-300 and getting around 370v, no particular reason, I just tried all combinations between 5Y3 & 5U4 and 300-0-300 & 330-0-330 and like how this one sounds the most.

Re DummyLoad; 7w WW? Any reason for that one? That got me curious, my friend has a 3k 3w that I could use... BTW love your protozilla!

[PRR]

5U4 5Y3 are both thermionic (hot cathode) rectifiers. Power does not come up INSTANTLY like Silicon rectifiers.

[Twosteps]

Got it, thanks again!

[DummyLoad]

use 7W WW part with SS rectifier for the pulse power rating.

i prefer higher wattage resistors so that if there is a problem downstream, resistor doesn't burn up. 3W part with tube rectifier will run cooler, & cooler running resistors generate less thermal noise.

--pete