Results 1 to 15 of 15

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

  1. #1
    Supporting Member
    Join Date
    May 2018
    Location
    Belgium
    Posts
    144
    Thumbs Up/Down
    Received: 50/0
    Given: 55/0
    Rep Power
    2

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

    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!

    0 Not allowed! Not allowed!
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

  2. #2
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,759
    Thumbs Up/Down
    Received: 510/1
    Given: 468/2
    Rep Power
    18
    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. Itís 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

    3 Not allowed! Not allowed!
    Last edited by pdf64; 10-28-2019 at 10:51 AM.

  3. #3
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,228
    Thumbs Up/Down
    Received: 536/4
    Given: 285/0
    Rep Power
    16
    Quote Originally Posted by Gtr0 View Post
    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

    2 Not allowed! Not allowed!

  4. #4
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    31,996
    Thumbs Up/Down
    Received: 1,804/7
    Given: 0/0
    Rep Power
    54
    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.

    1 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  5. #5
    Supporting Member
    Join Date
    May 2018
    Location
    Belgium
    Posts
    144
    Thumbs Up/Down
    Received: 50/0
    Given: 55/0
    Rep Power
    2
    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!!

    0 Not allowed! Not allowed!
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

  6. #6
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    2,237
    Thumbs Up/Down
    Received: 1,703/1
    Given: 969/2
    Rep Power
    5
    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)

    1 Not allowed! Not allowed!
    Last edited by Helmholtz; 10-29-2019 at 10:18 AM.
    - Own Opinions Only -

  7. #7
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,728
    Thumbs Up/Down
    Received: 1,603/23
    Given: 4,088/11
    Rep Power
    22
    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.

    1 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  8. #8
    Senior Member SoulFetish's Avatar
    Join Date
    Jan 2016
    Location
    Massachusetts
    Posts
    1,204
    Thumbs Up/Down
    Received: 406/1
    Given: 346/0
    Rep Power
    6
    Quote Originally Posted by Gtr0 View Post
    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

    2 Not allowed! Not allowed!
    Last edited by SoulFetish; 10-29-2019 at 04:41 AM.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  9. #9
    Supporting Member
    Join Date
    May 2018
    Location
    Belgium
    Posts
    144
    Thumbs Up/Down
    Received: 50/0
    Given: 55/0
    Rep Power
    2
    Quote Originally Posted by Helmholtz View Post
    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.

    Quote Originally Posted by g1 View Post
    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.

    Quote Originally Posted by SoulFetish View Post
    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!

    0 Not allowed! Not allowed!
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

  10. #10
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    2,237
    Thumbs Up/Down
    Received: 1,703/1
    Given: 969/2
    Rep Power
    5
    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.

    0 Not allowed! Not allowed!
    - Own Opinions Only -

  11. #11
    Senior Member SoulFetish's Avatar
    Join Date
    Jan 2016
    Location
    Massachusetts
    Posts
    1,204
    Thumbs Up/Down
    Received: 406/1
    Given: 346/0
    Rep Power
    6
    Quote Originally Posted by Helmholtz View Post
    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.)

    2 Not allowed! Not allowed!
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  12. #12
    Supporting Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    2,237
    Thumbs Up/Down
    Received: 1,703/1
    Given: 969/2
    Rep Power
    5
    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.

    0 Not allowed! Not allowed!
    - Own Opinions Only -

  13. #13
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    2,240
    Thumbs Up/Down
    Received: 421/0
    Given: 1,137/1
    Rep Power
    11
    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.

    1 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


  14. #14
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,728
    Thumbs Up/Down
    Received: 1,603/23
    Given: 4,088/11
    Rep Power
    22
    Quote Originally Posted by Helmholtz View Post
    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.

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  15. #15
    Supporting Member
    Join Date
    May 2018
    Location
    Belgium
    Posts
    144
    Thumbs Up/Down
    Received: 50/0
    Given: 55/0
    Rep Power
    2
    Quote Originally Posted by Helmholtz View Post
    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.

    0 Not allowed! Not allowed!
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Amp power supply questions.
    By OldSolder in forum Theory & Design
    Replies: 16
    Last Post: 03-11-2018, 01:25 AM
  2. 1959 Fender Tween Twin 5F8-A Power Supply Questions
    By keithb7 in forum Maintenance, Troubleshooting & Repair
    Replies: 13
    Last Post: 06-25-2016, 07:27 AM
  3. 5F2-A Power Supply questions
    By MikeJH in forum Tweed Builders
    Replies: 10
    Last Post: 03-28-2009, 01:49 AM
  4. Vintage Tranny and power supply Questions
    By stingray_65 in forum Theory & Design
    Replies: 4
    Last Post: 02-03-2009, 03:27 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •