Bias Measurements While Playing Question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by matt1969, May 8, 2015.

  1. matt1969

    matt1969 Member

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    I was just troubleshooting my 1974 100 watt superlead as to why my Power transformer gets so hot. I mentioned this in a previous thread....

    Anyway, as I was checking the bias under idol & playing conditions, I noticed that the bias rises as I turn the volume(s) up. At idol, with all controls to zero, by bias is set to ~ 36mA but when I plug a guitar in and play, as I turn the volume or both up, the bias goes up to over 200mA. Is this normal?

    The tubes do not red plate & the amp sounds fine.

    Is this a problem & why my tranny might be getting hot? I can't find anything else wrong with the amp.
    Thanks.
     
  2. Blue Strat

    Blue Strat Member

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    When playing, bias current will definitely rise to at least 2X over the idle setting. 200mA seems high but could be normal. Also note that as bias probes heat up they're more likely to give you incorrect readings.
     
  3. donnyjaguar

    donnyjaguar Member

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    What Mike says and consider that your meter may be misinterpreting the complex DC waveform while playing. You're prolly getting more a peak reading than an average I would guess.

    Can you (safely) measure the resistance in the primary of your output transformer? This can be done with the amp off and discharged without removing any components.
     
  4. matt1969

    matt1969 Member

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    yes I can. can you tell me how? where do I put my leads? I measured the center tap of the OT to pins 3 on the output tubes & i get 14.2 ohms.
     
  5. EFK

    EFK Supporting Member

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    Does your bias current reading rise when you turn up the volume but *are not* actually playing anything? That would be a problem. While playing, yes, as noted it will bounce around.
     
  6. matt1969

    matt1969 Member

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    Just when I play it rises way up.
     
  7. Kyle B

    Kyle B Supporting Member

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    That's how it should work.

    "Bias" is the no-signal DC current flowing through the tube. Your signal is AC .. it sits ON TOP OF the nominal DC bias. The DC bias never "changes". The current through the tube will fluctuate rapidly while playing, and this can not be measured with a voltmeter set to DC ('cuz it ISN'T DC).

    Your meter almost certainly is giving you bogus numbers.

    How hot is the transformer actually getting??? Can you touch it and keep your hand there for several seconds? Or can you fry an egg on it????

    If I were convinced a power transformer is getting "too hot", I'd break the heater & B+ connections (not at the same time), and insert an ammeter to measure the actual current it's delivering. That's really the only way you can be certain of what it's putting out.

    It could be operating "normally". Consider that, when this was made, wall voltages were about 10% lower than they are now. Higher input voltages means everything inside the amp (including the PT) are being driving 10% harder.

    Getting a variac or some other means of lowering wall voltage may help your heat issue.
     
  8. mark norwine

    mark norwine Member

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    Kyle,

    I swear....I think you & I were separated at birth!

    I'm reading through the thread, ready to post that a.) "it's only 'bias current' at idle [quiescent]" with AC on top, and b.) plate current going up while amplifying is normal....and what do I find?

    That you've already pointed / answered this already...
     
  9. Kyle B

    Kyle B Supporting Member

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    Careful Mark - you'll scare people away with big words like "quiescent" LOL

    I kid of course... this forum is a technically minded audience.

    But reminds me of an event earlier this week. We were in a big "pow wow" here at work, talking about sticking a small projector in a product. The marketing folks are questioning why it can't get brighter... So I say "It's Physics -- there's only so many photons you can jam through the system..." Marketing woman hears "photon", gets up and walks out of the room LOL

    She came right back in - it was only for comic relief... but the point was taken.

    I was tempted to say "Hysteresis" just to see what would happen....
     
  10. GT100

    GT100 Member

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    The one I like to use (when appropriate) is "High (or low) Thermal Inertia".

    Lloyd



    Sent from my iPad using Tapatalk
     
  11. matt1969

    matt1969 Member

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    Hot where I can only touch it for a second. Id' say I could fry an egg.

    It is a replacement PT made for 120V. Every once in a while, it will only get warm under the same conditions: same settings, same room/outlet same speaker cable- same everything.

    If it never got just warm every once in a while but always got hot, then I wouldn't be so concerned but since it will only get warm now and then, I believe something must be wrong.
     
  12. Kyle B

    Kyle B Supporting Member

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    Your wall outlet voltage is NOT a constant. Depends on how many people in your area are running their A/C (or whatever) at the time. The more power being pushed through the electric companies wires, the more voltage drop they experience, and the lower your outlet voltage would be. Presumably your transformer would be "warm" at these times as well.

    You can set your meter to AC VOLTS, stick it in the wall socket, and see this effect for yourself. In my area, I find about 10V higher at night than during the day. If you find it's also true for your area, you could consider to log a few days worth -- See if there is indeed a correlation between wall voltage and transformer temperature (of course, if you're gonna do this, get a "real" thermometer... Touching it isn't quite precise enough to prove anything ;) )

    Note the temperature in your room matters as well. 10 degrees drift in room temperature pretty much assures you'll have 10 degrees of drift in transformer temperature as well (one of the few things in the world of physics which operates intuitively!)
     
  13. Kyle B

    Kyle B Supporting Member

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    Yes, "Inertia" is usually a winner too :)

    But it's only 3 syllables. Ya get more points for 4 or 5 syllable words!!
     
  14. pdf64

    pdf64 Member

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    Surely a lower line voltage will tend to reduce the amp's power dissipation? Rather the amp is more likely to run hot when the line voltage is high.
     
  15. matt1969

    matt1969 Member

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    This is VERY interesting! This could be all there is to it.

    I should lower the voltage little by little with my variac and see were the transformer stays just warm and see if the voltage is low within range of what my wall voltage might drop to under heavy load conditions.
     
  16. Blue Strat

    Blue Strat Member

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    ^^^ Laughing at engineers who still haven't learned to communicate with lay people. ;)
    It's not so much about communicating correct information as it is communicating useful information (to the target audience). Or, as I love to say, being right is way over rated. ;)
     
  17. Kyle B

    Kyle B Supporting Member

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    Absolutely

    Sorry - That was a Friday Afternoon brain fart. Need more coffee. I know what I was thinking, but what I wrote was indeed misleading. :)

    A more complete version of what I meant to say --- When the line voltage is lower, the amp would run at what the OP calls "warm". When higher it would get "hot".

    That is, if my theory is correct :)
     
  18. Kyle B

    Kyle B Supporting Member

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    Oh yeah, I'm with ya on that one!

    I just honestly didn't think the word "photon" was THAT esoteric of a concept...

    It's really weird sometimes .. Out of 800 people in my company, there are only 9 others to whom I can actually converse freely and they'll understand me....

    Sometimes I try a little word association to determine if I'm talking to a "real" electronics guy or not. Goes like this:

    "Say the first word that comes into your head when I say .... BETA"

    If they say "Transistor gain", I know I've found a kindred spirit. If they say 'fish', I know I gotta dumb it down. If they say 'pre-release software', I can at least talk computer-y stuff to 'em. If they say "Radioactive decay", I realize I'm probably in over my head LOL
     
  19. Blue Strat

    Blue Strat Member

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    ^^^ You realize that the sales and marketing folks that you "know more than" are more highly compensated, on average, than engineers...right? ;)
     
  20. Kyle B

    Kyle B Supporting Member

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    Baseball players make millions to catch a ball.

    Nobody ever said life was fair ;)

    I've said many times, in my next life I'm coming back as a Marketeer. What a gravy gig! When things go right, you get to take all the credit. When they go horribly wrong, you just blame engineering....
     

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