Express 1K 5W vs 1K 25W

[dariez75]

Hi guys,
recently I built a TW Express using all correct value stuff. After 2 days of playing I've noted a strange fuzzy sound when I played with lowest guitar volume pot. The sound was sinusoidal. After spent lot of time with oscilloscope, I did multimeter test with a friend of mine while he played. What we found was a bad 1K 25W resistor in the power supplier board. What we did was swap resistor with 1K 5W resistor. The amp works fine now, no more problems when I play. Well, are so important 25W power for this resistor on not? Could Ken have chosen this kinda resistor because of his great stock in house and wanted to use it? Are there some real (math) technical reasons?
Thanks for answers!

[strelok]

5W is probably fine. The 25W resistor is fairly well known for being overrated. Ken liked to use overrated parts to ensure longevity and reliability in his amps, especially in the power supply. Both the power and output transformers are much larger than they absolutely needed to be, and aside from the reliability issue, contribute much to the sound of the amp.

As for the the 25W resistor I have no idea why Ken used such a large resistor in that spot. Possibly for reliability, what was easily available at the time, something I haven't thought of or some combination of the three.

The most plausible answer I can come up with is that if one of the screens shorted out, you might want a higher wattage supply resistor than the screen resistor which is usually 5W. So that if/when that does happen the screen resistor which acts like a fuse of sorts melts before the power supply resistor does and saves on potentially having to replace both of them.

I'm not about to say that the way Ken did it was wrong or unnecessary, but I can show you the basic math to calculate how big it should be at the very least. How overkill you want to go with it is up to you, and since its a clone the 25W resistor is one more thing closer to making it like an original.

Since P = EI we need to figure out the voltage drop and current through the resistor. For the preamp we can figure about 1ma per triode, so about 6ma total. Then we need to know the screens, I'll be exceedingly generous with the calculations here. The absolute max on most data sheets for screen dissipation on an el34 is about 8W. In reality it will never be close to this. If the screen voltage is about 400v (again its a bit lower in reality) then:

I = P/E
P= 8w
E = 400v
then:
I = .02A = 20ma

So for two tubes its 40ma plus the preamp which gives us 46ma. Again this is much more than what will really be present in the amp.

Now for the voltage drop. We know the resistor is 1K and since
E= IR
then
.046A*1000R= 46V

Now we can figure out how much power the resistor will dissipate since we know the current through it and the voltage drop across it:

P = EI
then
46V * .02A = 0.92W

So 5W seems to be more than enough, though there may be other factors involved as I mentioned earlier. Larger resistors spread the heat being dissipated over larger areas and reduce the stress placed on them. How much of a difference that makes I don't know. I certainly wouldn't go below 5W as a 1W or 2W would probably get pretty warm.

All that being said. I'd be more curious to know why the 25W failed in the first place. That's quite surprising. I'd suspect maybe something else, bad solder joint maybe? Did you put a meter across it and read the resistance value just to verify? If something did cause it to fail and it wasn't just bad from the factory or just a solder joint, I would be concerned that something else might be going on.

[martin manning]

This resistor has been discussed many times. Here's one thread: https://tubeamparchive.com/viewtopic.php?t=2379

[Littlewyan]

That 25W resistor is completely necessary. When you start playing through the amp the screen current sky rockets and the resistor dissipates at least 10W of heat. I have a 25W cement resistor in my Express and after 20minutes of playing that gets very hot.

[dariez75]

Thanks for lot of replies guys. Yesterday I've played for 1 hour with THD hot plate 8 Ohm attenuator, after turning off the amp and discharged the circuits, I try to touch the 5W film resistor and this one was warm but not hot. With a friend of mine and his oscilloscope we did a complete analysis of the amp and nothing seems to be wrong or failed. We are almost sure I bought a bad resistor. Now I try to find on mouser a brand new 25W resistor, anyone who knows mouser's code?

[Littlewyan]

280-CR25-1.0K-RC

There you go mate

[dariez75]

Thanks

[strelok]

That 25W resistor is completely necessary. When you start playing through the amp the screen current sky rockets and the resistor dissipates at least 10W of heat. I have a 25W cement resistor in my Express and after 20minutes of playing that gets very hot.

I've made a fool of myself once again lol.

Totally forgot to take into account the fact that as the resistor drops voltage it allows the screen to draw more current without exceeding its dissipation limits and thereby causing it to drop more voltage. At least that's the conclusion I came to from reading the other thread. That or I just made a fool of myself again.

Glad the amp is working fine now though. What was wrong with the original 25W out of curiousity?

[Littlewyan]

The screens always draw more current as you start playing and working the power valves, but because you've got a 1K resistor there it will dissipate a lot of heat. Plus I think due to the high impedance of the OT the Express draws more screen current than normal.

[strelok]

Yeah, these are all things I should know lol. I've been doing this for too long to be overlooking simple factors like the fact that if you draw more current through a resistor the voltage drop is going to increase as well.

[dariez75]

The oldest 25W resistor had a strange behavior, it doesn't drop voltage in constant way but in "sinusoidal way". Using a multimeter we seen the drop voltage was inconstant. For instance, before resistor I've got 377 Volt, after the resistor 376 thus 340, thus 250 until to 20 volt. I wasn't able to play guitar. The sound goes fuzzy going upside down until mute.