Another noob question about checking cathode bias..

[powmat]

Greetings and apologies in advance for a (probably) dumb question...

So describing myself as a complete 'Noob' is not entirely accurate.

I started dismantling electronics and electrical devices when I was about 8 years old ( circa 1965). I have been mercilessly abusing electric guitars since Jr High School - and the various amplifiers they were plugged into.

I completed a 2 yr associate degree in "Electronics Technology" in 1978 - when tubes were still a big part of the curriculum, have worked as a service tech at various times doing electronics troubleshooting and repair.

So I should probably be able to answer this question...but...

I recently completed a Weber 6A14 kit ( BFPR). All went well, it sounds absolutely friggin wonderful. As some other builders have reported, I also did NOT measure my cathode bias voltage initially. I simply set the bias pot at about mid-point - did the initial power-up with no-tubes and 'light-bulb' limiter, then again with the cheap tubes from Weber. No smoke, sounds great. Tremolo works great.

After a respectful break-in period of a maybe a week, I ordered some JJ 6V6 and pre-amp tubes, as well as a cannabis rex.

Now that I have installed my new JJs I do want to measure my cathode bias voltage.

Having read all the many posts of the different bias voltage measurement methods, ( and also having shocked myself a number of times), the technique of measuring the voltage across a 1 ohm 1% resistor between cathode and ground to calculate the bias voltage - appeals to my sense of safety.

I have ordered some 5W resistors, banana jacks, etc. and I plan to wire up those test points as detailed many places here in the forum

But here ( finally) is my question:

A 1 ohm resistor is almost just a piece of wire. Almost no resistance at all. So in essence - I am wiring an (almost) dead short between the high-voltage on the cathode ( pin 3 ) and ground..

That seems like a bad thing - so to speak.

Can anybody speak to that? Clearly I am missing something, because it is obviously a very popular method, safe, relatively accurate, etc.

Thanks again.

Matt Powell

[LeftyStrat]

The cathode is pin 8. Pin 3 is the plate (anode). Pin 8 is already tied to ground, your just adding a small resistance between pin 8 and ground to be able to measure the voltage and easily calculate the current.

[Prairie Dawg]

You measure the voltage drop across the 1 ohm resistor in mV and call it mA and that gives you your cathode current. Check it against the weber bias calculator. Remember the original Princeton Reverb did not have an adjustable bias.

There's no high voltage on the cathode to speak of. Remember to turn your vibrato speed and depth all the way down because the vibrato is bias modulation.

[tubeswell]

JJ 6V6 ...tubes, ....

So in essence - I am wiring an (almost) dead short between the high-voltage on the cathode ( pin 3 ) and ground..

No. The cathode on a 6V6 is Pin 8. You wire the 1R between Pin 8 and the ground.

(Pin 3 is the plate, and that idles at over 400 VDC. You DO NOT want to put a 1R between that and the ground!)

[powmat]

As Homer Simpson would say "doh!"...

Thanks all, for another helpful, patient reply.

I feel pretty stupid for asking that, now that I understand, but I totally get it now.

The current is already flowing to ground via pin 8. Adding this tiny resistor creates a "shunt" path, allowing a small percentage of that current to flow through the 1ohm resistor, where the voltage drop across it can be safely meaured.

And thanks also for pointing out my mistake regarding pin 8 instead of pin 3. I would like to think I would have caught that when I sat down o do the work. Or maybe you prevented my wife from becoming a widow MUCH too early...and for that I thank you!

Many Regards,

Matt

[pdf64]

Adding this tiny resistor creates a "shunt" path, allowing a small percentage of that current to flow through the 1ohm resistor

The 1 ohm resistor is in series, rather than shunt (ie parallel); all the current through the tube flows through the 1 ohm resistor, as it is in series with the path to ground.
The 1 ohm resistor doesn't need to be high power. 1/2 watt is more than ample for any guitar amp tube, though it may blow in the event of a tube short.
0.5 watt have the benefit of being readily available in 1% tolerance and flame retardant package.
Higher power resistors are mostly only 5% tolerance, so the measurement may be a little less accurate, but not a big deal.
Pete

[powmat]

Thanks pdf64,

Wow. Okay - now I really feel like a dummy. Series... Okay.

Thanks for your patience.

[Andy Le Blanc]

It does not matter where you put the 1r ( in series), it could be place any where on the HT side of the power tubes, but the safest place for measurement is on the ground end of the cathode.

I always install them when I can, if the amp comes back for work it makes for an easy check up. The 1r makes the reading possible and the math easy,

But make sure you don't forget to move the decimal... because its V x A = W or... E x I = P

the measurement over the 1r gives you mv/ma, which you must convert to amps, move the decimal three places to the left, so say if you measure 30 millivolts over the 1r

you get .03 A,.... 400 (volts) x .03 (amps) = 12 (watts)

and as far as tolerance goes tubes are a 20% device, so any measure thats tighter than that is pretty good, keep your goals conservative per the "Design-Maximum Values" for the given tube type in your receiving tube manual and you'll be fine...

[tubeswell]

It does not matter where you put the 1r ( in series), it could be place any where on the HT side of the power tubes, ...

But what we're saying is don't put the 1R between the plate and the ground

[powmat]

Thanks again guys ( Andy and Tubeswell),

I am all about easy, safe, and convenient. Emphasis on safe.

Andy: Good point about tubes being "20% devices". So even though some posters have questioned the accuracy of the "1R voltage drop" method ( for lack of a better term for it) - it does seem the accuracy is in a useful and reasonable tolerance range given the variance in tubes, etc.

Tubeswell: Thanks again for reinforcing this point. Rest assured that I do (now) understand. I am no longer thinking that I should be using the 1R resistor to ground the high voltage (Plate voltage). Not sure why I thought that initially. Seems quite insane now actually,....., but it certainly bears restating!

Thank you again for the support.