6v6 Plexi resistor value query

[Mehfuzhoss]

Hello,

I am building a 6v6 plexi. the layout is calling for 2 x 1 ohm resistor that connects pin 8 of the 6v6 tubes. if if use 2 x 100 ohms instead ; how much will it vary tone wise and also is it safe to do so!

please advise

Thanks

[jordan86]

Those resistors are not there for tone. The signal does not pass through them. They are there to measure the bias. You measure millivolts across that resistor which tells you your milliamps of current. It would be best to stick with the 1 ohm value. Get a 1 ohm, 1% tolerance metal film. 1/4w or 1/2w.

With a fixed bias circuit I think 100 ohms, or even 50 ohms will cause you problems. 

[PRR]

Classic Martini Recipe
2 ounces dry gin
1 ounce dry vermouth
1 dash orange bitters

OK, and you change the "1 dash bitters" to 100 dashes. That might change the "tone".

[d95err]

1 ohm is selected to make as little impact on tone or the bias as possible, while still producing enough voltage drop to be measurable.

You can probably go up to 10 ohms without much impact (although I would use 1 or 2W resistors).

If you go higher, the amp will merge towards cathode bias, which will affect the tone (subjectively better or worse).

Some amps are actually designed to use a mix of fixed and cathode bias.

[jordan86]

You’d be better of replacing them with a plain jumper wire if you don’t have any 1 ohm resistors.

[tubeswell]

Apart from ideally needing 1 ohm resistors there, you also ideally want 1% tolerance rating (i.e. 99% certainty that it certified as 1 ohm resistance), because the purpose of those resistors is to measure the mV drop across each one to work out the tube current. (1V/1R = 1A, so 1mV/1R = 1mA and so on). If you end up with a '1 ohm' resistor that measures 1.5 ohms, then you'll get a bogus current calculation.

[PRR]

> If you end up with a '1 ohm' resistor that measures 1.5 ohms

A 1 Ohm *will* "measure" 1.5 Ohm on an ordinary bench meter. They don't do well with small numbers.

I can't see a damm reason we care much more than 10%. Maybe 5%. Which is about 3% in Gm.

However 1 Ohm is beyond the traditional range of 10% 1/2W parts. So you are probably "forced" to a low-% 1W part.

[dude]

Most meters can’t read 1 ohm resistors, here’s a way to check the value, measure a 100 ohm resistor, say it reads 101.5 ohms. Add your 1 ohm resistor in series, hopefully you’ll get 102.5 ohms, therefore your resistor is a 1 ohm. I usually buy at least 15 1 ohm 1% then do the test above. If my results are within 1/10, l’ll use them, 1.1 ohm is close enough for me.

[tubeswell]

If I measure 0.7 ohms between the meter leads and 1.7 ohms across the resistor, I’m happy. If it’s a 1% tolerance resistor I’m happy. I did say ‘ideally’ in my previous post. You guys are critical today.

[PRR]

Sorry.

[sluckey]

Many high end meters have a "Relative" button. I short my probes together and push the relative button to set my display to zero. Then I measure the resistor. With analog VOMs and VTVMs you always did a similar procedure before measuring any resistance. Short the probes together, set the zero knob for zero on the meter scale, proceed with measurement. If you change resistance scale you must repeat this procedure.

In reality, I trust a precision 1Ω 1% resistor to be in tolerance and don't think too much about it. 

[jordan86]

I accidentally bought some 5% tolerance 1 ohm’s a while back. Just in a hurry and not paying attention. I soon discovered, as others have said that you can’t measure 1 ohm on a MM. So I did the in series trick. They were close enough that I felt ok about using two of them. I think they measured 1.1 ohm?!?

I wired them up and then cross referenced their readings to the manual bias measurements. It was like .3ma off. Close enough for me. My ears don’t hear the difference from 22.2 to 22.5ma. And close enough to get a safe measurement to me.

I really try to be a perfectionist as long as it’s not too inconvenient. :)

[EL34]

I just measured a metal film - 1 ohm - 1% tolerance


One meter said it was .8 ohms
The other meter said it was 1.3 ohms


Many times just touching the two leads together will give you a small measurement.
If your meter has a zero out function, that is the way to go

So meters are not reliable in general for this application

Maybe a $500+ fluke can do it, not sure
I only have cheapo Chinese meters here.

[tubeswell]

I just measured a metal film - 1 ohm - 1% tolerance

One meter said it was .8 ohms
The other meter said it was 1.3 ohms

Whaddaya expect - cheap asian rim meters 

FWIW, is there a handy link thread on measuring 1 ohm resistors? (I quickly looked but couldn't find)

[PRR]

Since typical casual contact resistance is a fraction of an Ohm, sub-Ohm measurements need to be done "4 wire Kelvin". Which is a hassle.
https://en.wikipedia.org/wiki/Four-terminal_sensing