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Old 11-19-2012, 12:46 PM   #1
macgregger
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Cathode resistor

Hey guys,
I'm hoping the amp gurus can chime in on this question.
I'm looking to mod my amp a little bit to make bias readings a little bit easier on my jtm45 clone. I plan on installing a one ohm resistor from the cathode to ground.
What I'm looking to clarify is:
A) material type of resistor. Metal film, carbon film, carbon comp, etc. Does it matter?
B) power rating. Bigger equals better? What would be better? 1/2w, 1w, 2w?
And
C) tolerance. I know lower is better, but can I get away with using a 5% resistor without seeing any massive negative drift?
I'm only asking this as I would like to avoid placing an online order for two specialized resistors if I can get something closer at my local shop that's maybe not as accurate.

Thanks
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Old 11-19-2012, 01:40 PM   #2
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Material doesn't matter.
Bigger is better, you certainly need more than 1/2 a watt. I'd use 2 or 3.
I'd go with a 1 or 2% resistor, but you can probably get away with 5. If you're concerned, you can always measure the resistance and correct your numbers accordingly.
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Old 11-19-2012, 07:40 PM   #3
Phoenix V
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Material does matter as they all have different drift co-efficients. You don't want your in series resistor having a overall direct effect on the bias, you just want it as a means to measure the bias point.

Your cathode sense resistor(s) will get warm both from self heating and also from the surrounding components. You want a part that has a low temperature and vibration drift co-efficient (in that as the temp of the part varies or the part vibrates, given as it's in an amp) then the nominal resistor value stays within a defined tolerance band.

Metal film is the way to go in this case as they have low drift co-efficients, usually 1% nominal tolerance and come in 1/2, 1 and 2 watt ranges. A larger surface area helps to dissipate heat better.

Carbon comp and carbon films have poor temp and vibration drift co-efficients, not really optimal.

Wirewounds have abysmal drift co-efficients and shouldn't be used for measuring your cathode bias given there are better alternatives readily available, as above.

So my choice would be MF > CF > CC > WW
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Last edited by Phoenix V : 11-19-2012 at 07:43 PM.
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Old 11-22-2012, 10:11 AM   #4
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thanks for the input guys.
my other option, which will take a little more work but will probably be better in the long run, is to install a panel dc ammeter in the chassis. i would use an on-on-on dpdt toggle switch where the middle position is passthrough, up is power tube A and down is power tube B. This will produce a more accurate reading in my mind, plus it will look cooler.
before:

after:


here are the switches i was thinking about using:
http://www.digikey.ca/product-detai...N1132-ND/259886
http://www.smallbearelec.com/servlet/Detail?no=900
whaddya think?
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Old 11-24-2012, 02:57 AM   #5
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While I see what you're trying to do, I'm not sure this method is wise. Voltage sense measurement is safer than direct current measurement, which has to break the circuit to insert the meter.

Breaking and making the cathode to ground directly can cause problems in that if the switch is break before make the tube cathode to ground is severed for a split second before the meter is then connected in circuit.

Also, what if the switch one day just fails altogether. Both tubes could lose cathode ground altogether.

It's better to always have a path to ground for the cathodes. So inserting your inline low value resistor (remember you don't want to markedly alter the cathode bias, you just want to sense the bias current as a voltage) between each cathode and ground means the switch then becomes non critical and you just measure across the in line resistors. If the switch or meter fails completely, the cathodes still have a path to ground.

Select MF high quality resistors for the sense. If you select something low like 1 ohm 1% 2watt metal film then the voltage measured across it is the same as the bias current through it (not counting resistor tolerance and drift).

Something like this might be a bit better. Switch is a 3 position double pole, centre off. A 100mv FSD voltmeter should do the trick. I think the standard bias current for a JTM45 is around the 40 - 50ma range.

And the obligatory warning that tube amps contain lethal voltages. If you don't feel comfortable doing this yourself, DON'T

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Last edited by Phoenix V : 11-24-2012 at 06:33 AM.
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Old 11-24-2012, 10:35 PM   #6
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thanks phoenix,
i do want to do whats right. i built this amp my self, and i feel i know most of the ins and outs of it.
that way does look a lot better. my only problem is going to be sourcing that resistor. digikey doesnt have any that match those parameters. Arrow electronics does, but it's showing a six week lead time....
i picked up some 5% tolerance carbon comp resistors at the local shop (as this was all they had), but they measured 1.3 ohms. thats going to throw the (direct) current reading off by 10 mA and means more math for me.
whats the point of the trim pot? you think i can omit that?
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Old 11-25-2012, 01:41 AM   #7
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Quote:
Originally Posted by macgregger
thanks phoenix,
i do want to do whats right. i built this amp my self, and i feel i know most of the ins and outs of it.
that way does look a lot better. my only problem is going to be sourcing that resistor. digikey doesnt have any that match those parameters. Arrow electronics does, but it's showing a six week lead time....
i picked up some 5% tolerance carbon comp resistors at the local shop (as this was all they had), but they measured 1.3 ohms. thats going to throw the (direct) current reading off by 10 mA and means more math for me.
whats the point of the trim pot? you think i can omit that?


If you only have access to 5% resistors then so be it. Make sure they are still 2 watt.

You'de be wise to buy say a band of 10 of them (still cheap) and pick the two that measures closest to 1.0 ohm. Also remember to check what ohm reading you get when you cross your multimeter probes to each other. It won't be zero ohm, unless it has a zeroing function which you can activate. Whatever figure you get will be the error figure you will need to cancel out when selecting your sense resistors. You measure 1.3 ohm across the resistor, but what is your meter error? Your 1.3 ohm resistor may well be a 1.1 or 1.0 once you subtract the meter error. Do the same error check with your low mv DC volt range. Short the probes and check what value you get. If its non zero, remember to subtract that from any measurements you take.

The trim pot can help correct the error from the sense resistor tolerance and the error of your new bias meter (as it also has a tolerance band). So, you measure the sense voltage first with your handheld digital meter. Then check it with your bias meter. Guaranteed they will not show the same value. Then adjust the trim pot until the bias meter shows the same value as you had with the handheld meter.

You can omit it if you like, but that introduces even more maths for you having to adjust the meter to correct for both its tolerance and the resistor tolerance.

Good luck with this hey. One day I'd love to do the same with all my tube amps. Time work and family never let me self indulge in such activities
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Last edited by Phoenix V : 11-25-2012 at 01:54 AM.
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