More questions on the basics (ohm's law) and designing the power supply.

[Gtr0]

Hi

I have more questions that cover probably the barest of basics that have me scratching my head.

For some months I have been obsessing on the power supply. Mr. Aiken suggests metal oxide, 1% tolerance, 5 watt, minimum rating 750V.

Just about every 5W resistor I see is rated for 500V AC. I assume without a rating explicitly stated that it would be the same for DC(?) Often 10+ watt resistors are capable of 1000V, but are rather large (perhaps with exception to Mills). - I just purchased some Ohmite Audio Gold resistors that are 10W/1000V, but they are pretty big, close to 2 inches in length. Which can be a problem for layout.

So I started looking into calculations to figure what I would require in my specific case for a specific amplifier.

Here's where I start scratching my head.

If voltage feeding the B+ node for the pi is 500VDC (right after the choke, before the decoupling resistor) and has an RC filter of 10k/50uF, my calculations tell me that the resistor need to be rated at 25 watts. What am I missing here? Here was my process...

500 (VDC) / 10,000 (Ω) = .05 A
500 (VDC) x .05 A = 25 watts

to double check this, I use an app on my iPhone called EE Toolkit that has an ohm's law calculator... if you feed in V and R, it will calculate I and P for you and it also says 25 watts.

But since I used 5W Mills resistors in my last few builds and the amp didn't blow up, I figure I must me missing something here. Can someone fill in the blanks?

Much appreciated!

[pdf64]

The 500V is across the cap, not the dropper resistor feeding it.
The steady state Vdc across the resistor will depend on the current being drawn by the circuits it is feeding, eg phase splitter and preamp.
The instantaneous voltage across the resistor, eg at start up, may be a lot higher than the steady state voltage, hence the benefit of the high voltage rating. It’s only going to be needed momentarily, until the cap charges up.
Specifically, in 0.5 sec, the cap will charge up to ~350V, and will then follow the curve to its eventual steady state voltage, see http://referencedesigner.com/rfcal/cal_05.php

[Dave H]

If voltage feeding the B+ node for the pi is 500VDC (right after the choke, before the decoupling resistor) and has an RC filter of 10k/50uF, my calculations tell me that the resistor need to be rated at 25 watts. What am I missing here?

To apply Ohm's law you need to use the voltage across the resistor not the B+ voltage. For the resistor feeding the PI and preamp tubes assume 1mA per dual triode. Four 12AX7s would be 8mA total. The voltage drop across the resistor is therefore 8mA x 10k = 80V. Power in the resistor is V^2/R, i.e. 80^2/10k = 0.64W

[Enzo]

500vAC means a sine wave that peaks at 707v at alternating polarities.


Keep in mind that Fender et al never used special extra high voltage rated resistors.

[Gtr0]

Ooooh yeea. Sometimes I feel so... unimaginative.

I know such is the case for plate resistors, and yet never applied it to the node resistors that come directly before it. Thanks, sometimes I just need a little hand holding ;-)

I will hold on to that!!

[Helmholtz]

A resistor (like most other electronic parts) only sees the voltage difference between its terminals. And this is what is limited in the datasheet.
It won't know that it actually floats on 500V. But it is always a good idea to mount such power supply resistors not too closely to other components or the chassis (# creepage and clearance)

[g1]

Also, should you really need a high voltage resistor and the available ones are too cumbersome, you can put resistors in series to add the voltage ratings. For example, two 5K 500V in series would give you a 10K at 1KV.

[SoulFetish]

Hi

I have more questions that cover probably the barest of basics that have me scratching my head.

For some months I have been obsessing on the power supply. Mr. Aiken suggests metal oxide, 1% tolerance, 5 watt, minimum rating 750V.

Just about every 5W resistor I see is rated for 500V AC. I assume without a rating explicitly stated that it would be the same for DC(?) Often 10+ watt resistors are capable of 1000V, but are rather large (perhaps with exception to Mills). - I just purchased some Ohmite Audio Gold resistors that are 10W/1000V, but they are pretty big, close to 2 inches in length. Which can be a problem for layout.

So I started looking into calculations to figure what I would require in my specific case for a specific amplifier.

Here's where I start scratching my head.

If voltage feeding the B+ node for the pi is 500VDC (right after the choke, before the decoupling resistor) and has an RC filter of 10k/50uF, my calculations tell me that the resistor need to be rated at 25 watts. What am I missing here? Here was my process...

500 (VDC) / 10,000 (Ω) = .05 A
500 (VDC) x .05 A = 25 watts

to double check this, I use an app on my iPhone called EE Toolkit that has an ohm's law calculator... if you feed in V and R, it will calculate I and P for you and it also says 25 watts.

But since I used 5W Mills resistors in my last few builds and the amp didn't blow up, I figure I must me missing something here. Can someone fill in the blanks?

Much appreciated!

The reason he recommends 5W/750V MOS (metal oxide) resistors, is to protect the resistor in the event of a tube or capacitor failing short.
If this happens, the resistor will have the full power supply voltage across it. Metal Oxide resistors usually have a better surge ratings than cement wire wound, or vitreous enamel resistors of the same power rating. The idea being that, the resistor should survive the "short" long enough to blow the appropriate fuse and save you the trouble and cost of replacing them.

edit: KOA Speer series SPR (power film) and MOS (Metal Oxide) are available in 3W/700V & 5W/800V ratings.
SPR
MOS

[Gtr0]

A resistor (like most other electronic parts) only sees the voltage difference between its terminals. And this is what is limited in the datasheet.
It won't know that it actually floats on 500V. But it is always a good idea to mount such power supply resistors not too closely to other components or the chassis (# creepage and clearance)

Good note! Thanks.

Also, should you really need a high voltage resistor and the available ones are too cumbersome, you can put resistors in series to add the voltage ratings. For example, two 5K 500V in series would give you a 10K at 1KV.

Right - I ALWAYS forget that.

The reason he recommends 5W/750V MOS (metal oxide) resistors, is to protect the resistor in the event of a tube or capacitor failing short.
If this happens, the resistor will have the full power supply voltage across it. Metal Oxide resistors usually have a better surge ratings than cement wire wound, or vitreous enamel resistors of the same power rating. The idea being that, the resistor should survive the "short" long enough to blow the appropriate fuse and save you the trouble and cost of replacing them.

edit: KOA Speer series SPR (power film) and MOS (Metal Oxide) are available in 3W/700V & 5W/800V ratings.
SPR
MOS

Nice, thanks for the info!

Thanks everyone... I find it to be both a learning experience and fun sorting this out and learning about it and especially the different thoughts and ideas of others!

[Helmholtz]

Metal Oxide resistors usually have a better surge ratings than cement wire wound, or vitreous enamel resistors of the same power rating.

Do you have data to support this? I thought that wire wound resistors had the highest surge power ratings.

[SoulFetish]

Do you have data to support this? I thought that wire wound resistors had the highest surge power ratings.

I'm looking at some literature now. I'm glad you asked, because I may be mistaken about this....
*After about 15-20minutes of scanning over through some manufacturers data and white papers, it clear that metal oxide has very good overload characteristics, and can handle short duration surges of several hundred times their power rating. But, I can't find any data to support that they are better suited to handle current surges than wirewound resistors, and my initial research seems to indicate that wirewound is better in many applications. But, with so many different resistor technologies specifically suited to so many different power applications, simply saying either is "better suited to handle.." as a blanket statement was an oversimplification and inaccurate on my part.
Now that my lawyer is done talking, I guess that was the long way of saying that I...
I was wrong . (ah, the 3 most freeing word strung together in the English language.)

[Helmholtz]

I think the main problem with wirewound resistors is that they are only available up to around 10k. High resistance values are hard to produce as they require many turns of very thin wire.
IIRC power surge capability generally increases in the following order: carbon film < metal film < carbon compound < metal oxide < wire wound.

[eschertron]

Instead of the terms "better suited" or "best...", perhaps "well-suited" takes care of any quantification quibbles, and allows a strong endorsement of M.O. resistor properties. Besides, being cheap and small and well-suited to electrical demands probably does allow them to be called "better suited" to the task.

[g1]

I think the main problem with wirewound resistors is that they are only available up to around 10k. High resistance values are hard to produce as they require many turns of very thin wire.

I've run into the occasional vintage amp (not sure country of origin?) that used those old wirewound for everything. The amount of 'open' higher value resistors seemed higher than with other resistor types.

[Gtr0]

I think the main problem with wirewound resistors is that they are only available up to around 10k. High resistance values are hard to produce as they require many turns of very thin wire.
IIRC power surge capability generally increases in the following order: carbon film < metal film < carbon compound < metal oxide < wire wound.

Yes, usually I find them max out at 6k8, 8k2 or 10k... once in a while I can find higher values... just picked up some Audio Gold (Ohmite wirewound) 15k, 18, and 22k.... but they are large tubular... almost 2 inches (just under 5 cm). Other than that, I think I found some 5W Royal Ohm, the brick ones, in higher values... but I probably wouldn't use those unless I was in a pinch on a circuit where it is less than 500V unloaded... which is rare in my small workshop.