Understanding load on triode and impedance matching
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 9th June 2019, 11:05 PM #11 itsikhefez   diyAudio Member   Join Date: Aug 2015 So a 45 triode is capable of 4Vrms with a 5K:8R OPT and capable of 8Vrms with a 5K:32R OPT? That seems a bit low, isn't it ? These formulas are so basic, I should probably just go read a book... Last edited by itsikhefez; 9th June 2019 at 11:07 PM.
rayma
diyAudio Member

Join Date: Apr 2011
Quote:
 Originally Posted by itsikhefez So a 45 triode is capable of 4Vrms with a 5K:8R OPT and capable of 8Vrms with a 5K:32R OPT?
The output power is the same in both cases, so if P = 8W = V^2 / R,
then for an 8 ohm load, 8W = V^2 / 8, and V = sqrt (8 x 8) = 8Vrms.
Also for a 32 ohm load 8W = V^2 / 32, and V = sqrt (8 x 32) = 16Vrms.

I think that 8W into any headphone should be more than plenty.

Last edited by rayma; 9th June 2019 at 11:15 PM.

 9th June 2019, 11:22 PM #13 itsikhefez   diyAudio Member   Join Date: Aug 2015 A 45 is capable of 2W, not 8W. 8 ohm load would be then 5.65Vrms, and 32ohm would 11.3Vrms... 2W is still way more than enough. Thanks again for working me through the calculations
rayma
diyAudio Member

Join Date: Apr 2011
Quote:
 Originally Posted by itsikhefez A 45 is capable of 2W into 8ohms, not 8W. 8 ohm load would be then 5.65Vrms, and 32ohm would 11.3Vrms...
The 8W must be for a different tube in that circuit.

If 2W = V^2 / 8, then V = sqrt (2 x 8) = 4Vrms for an 8 ohm load.
If 2W = V^2 / 32, then V = sqrt (2 x 32) = 8Vrms for a 32 ohm load.

The 2W is still likely to be an order of magnitude larger than you'd ever actually use.

Last edited by rayma; 9th June 2019 at 11:29 PM.

 9th June 2019, 11:32 PM #15 itsikhefez   diyAudio Member   Join Date: Aug 2015 Ah oops. My original calc was correct then. The 8W would be for the 300B
 10th June 2019, 12:12 AM #16 cerrem   diyAudio Member     Join Date: Jun 2005 Location: San Diego, CA Keep in mind that there really is no " mis-matching" or "matching: with respect to loading a triode... Matching is for transmission line applications... The tube does not give a S##T what load you put on it...as long as the load swings in a safe region over the average of a cycle... When a particular transformer is at a specific plate load, usually for minimal distortion ....then the inductance, leakage and winding capacitance will determine the bandwidth for that given plate load.... Once the reflected load changes, then the bandwidth will move .... That is usually why you stick with the intended plate load of a transformer...unless you intentionally change the reflected load and have re-calculated the new bandwidth to be acceptable..
nigelwright7557
diyAudio Member

Join Date: Apr 2008
Location: Carlisle, England
Quote:
 Originally Posted by itsikhefez George and another source recommended the resistor approach and 2 other sources mentioned that loading with the resistor is not necessarily required. I'm trying to understand the theory behind both approaches.
The main thing with a valve amp is not have an open circuit output.
Even loading a 8R output with a 220R resistor should prevent any problems.
So 300R headphones should be fine and so is 8 ohms.
__________________

 10th June 2019, 03:50 AM #18 johnsurnamerobinson diyAudio Member   Join Date: Nov 2018 Nigelwright7557 is quite correct one should never run a transformer output unloaded since headphones are liable to be unplugged the permanantly connected resistor could save your o/p transformer.
itsikhefez
diyAudio Member

Join Date: Aug 2015
Quote:
 Originally Posted by cerrem Keep in mind that there really is no " mis-matching" or "matching: with respect to loading a triode... Matching is for transmission line applications... The tube does not give a S##T what load you put on it...as long as the load swings in a safe region over the average of a cycle... When a particular transformer is at a specific plate load, usually for minimal distortion ....then the inductance, leakage and winding capacitance will determine the bandwidth for that given plate load.... Once the reflected load changes, then the bandwidth will move .... That is usually why you stick with the intended plate load of a transformer...unless you intentionally change the reflected load and have re-calculated the new bandwidth to be acceptable..
Sorry but I'm missing something in your explanation here. You're saying that it doesn't matter if the reflected load on the tube is 5K or 100K? I forgot where I saw it but one resource mentioned that ideal load was 3*Rp (which is around 5K for the 45), and in another, there was a graph that showed the distortion/power at a given impedance (I couldn't find such a graph for the 45). Based on the graph, there was an "ideal" load to put on the tube for lowest distortion.
Or is that what you are saying?

Quote:
 Originally Posted by johnsurnamerobinson Nigelwright7557 is quite correct one should never run a transformer output unloaded since headphones are liable to be unplugged the permanantly connected resistor could save your o/p transformer.
That makes sense. How do speaker amplifiers protect themselves? Can those without a dummy load get damaged if turned on without speakers?

 10th June 2019, 05:37 AM #20 PRR   diyAudio Member     Join Date: Jun 2003 Location: Maine USA Tube amps can usually *idle* without a load. OVER-drive, clipping, without a load causes large kick-backs which damage transformer insulation. Best power in a transformer coupled triode is often near 2*rp (depending on ratings. Best distortion is at *infinite* load, but that is zero power. Do not worry about loading in 10X or 20X rp if you get enough power output.

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