input grid stopper and grid leak resistor wattage?

[Dingleberry]

Hi.

Some time ago I run out of 68k 1/2W and 1W resistors. When sourcing my stashes I found a bag full of nos 68k 1/4W CC resistors that I once bought.
My rule of thumb has been that never use 1/4W resistors anywhere in a tube amp and always play safe and overrate, but the situation got me thinking. Did some math and could not find any reason not to try. Works just fine, definately no added noise or other issues.

I also found a bag full of nos 220k 1/4W CC resistors and have never used them in a tube amp. What about power tube grid leaks? There's not supposed to be neither significant current nor voltage across them, am I right? Haven't dare to try because they just look so tiny like they don't belong in a tube amp;)

Would I be able to use them in for example 5E3 circuit as grid l eak (6V6 control grid to ground) with no worries under normal operating conditions?

Many commercial product line tube amps seem to use 1/4W resistors as preamp cathode resistors and they seem to work fine there with no worries.

I have always used minimum 1/2W, mostly 1W resistors in all signal path, but the lack of some values got me thinking have I used them just because I'm used to do it without no significant theoretical/technical point of view.

-T

[frankdrebin]

no big voltage or current,go ahead.

[pops]

1/2 watt resistors were the norm in the day as they were more common and therefore less expensive. Not necessarily needed for the power, just the expense for the bean counters. Today's electronics 1/8-1/4 are more common and therefore less expensive so used buy companies when they can.

[surfsup]

The "noise" produced by smaller power-rated resistors is supposedly more than the larger and higher-power resistors of the same resistance. Supposedly. The 1/4 watt should work fine as input resistor but if the noise statement is true, that first resistor's noise will be amplified across the entire amp since it is at the input, added to the guitar's signal. So some builders use larger watt values here to try to minimize hiss at higher output levels of the amp.

[jjman]

There are different types of "noise" from resistors but I don't know their names. One is related to the dc that is flowing thru them when they are used as plate resistors. We generally call this "hiss." That noise is lower if the wattage rating is higher. Since there is no dc curent flowing in grid stoppers (and usually no ac current btw,) I suspect lower wattage resistors in that application are ok. Traditionally grid/leak/bias resistors also do not have dc current on them. (Yes they have dc voltage on them.) Grid leak resistors used on a grounded-cathode-grid-leak-bias setup carry dc current albeit very little. These are typically 2.2-4.7meg and I've only seen them on inputs and never on output tubes.

Grid/bias resistors on output tubes carry all of the ac current/signal coming from the PI, to/from ground/bias. I wonder if this approaches 0.25 watts ac?

[Dingleberry]

[/quote]Grid/bias resistors on output tubes carry all of the ac current/signal coming from the PI, to/from ground/bias. I wonder if this approaches 0.25 watts ac?

I found this:
http://hyperphysics.phy-astr.gsu.edu/hb ... cohml.html

If I got that right we are talking about few to something like 50 milliwatts even in very extreme p-p signal levels. For example if the output swing could be 150V p-p (106,5V rms.) into 220k input z, it will be about 44mW.
250mW rating should definately be enough.

-T

[tubeswell]

1/4W resistors are fine here. However, as a RoT, 1/2W or 1W resistor make for less noisy grid leak resistors.

[Dingleberry]

1/4W resistors are fine here. However, as a RoT, 1/2W or 1W resistor make for less noisy grid leak resistors.

Got to try and see if I hear any increase in noise. I've got some CC, CF and MF 220K's half watters and watters. Weekend is coming so I might even make a test box and listen is there really an audible increase in noise in real world situation. I've got some very nice and robust make before break 3x4 pole rotary switches which will suit perfectly fot that purpose.
I'll post my observations when I get into it.

-T

[pdf64]

I can't envisage that any conceivable degree of noise that a resistor could add at the power tube grid would be audible - there's just not enough gain after that point.

A couple more aspects to this -
Bigger sized resistors tend to be more robust, useful for high vibration and point to point construction, in which the component body has little support.

Higher wattage resistors tend to have a higher voltage rating - 1/4 watt types tend to have a limiting element voltage of 250V or even less.

It makes life easier in a production (or any) environment to specify a suitable default resistor type, rather than having several types of the same resistor value; simpler documentation, stock and production controls.
Pete

[thejaf]

Since I have a large stash of 1/4watt CF, I've used them anywhere I feel safe, such as input grid leaks and stoppers (1meg to ground, 68K to grid, grid stoppers after gain/volume, etc). I would not, however, use them in the power tube grids just because they carry your bias reference.