Got some money, going to buy OPTs for PP GU-50
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 23rd November 2020, 06:36 PM #31 nerdorama   diyAudio Member   Join Date: Aug 2005 Location: Spokane Here's a post on this from years ago by Mike at Magnequest regarding his rule of thumb for calculating an acceptable bias current for an unknown transformer. RE: Looking for a output trannie match, Scott LK72 - mqracing - Tube DIY Asylum
 23rd November 2020, 06:53 PM #32 TG   diyAudio Member   Join Date: May 2002 Location: Kiev, Ukraine Correction to my previous post: 56mA is the cathode current, not the plate one (forgot about g2). Plate current is 44mA, g2 current is 12mA, DCR is 250R (each half), so extra 3.1W of heat at 90mA and 22.5V drop. Not that great indeed.
 23rd November 2020, 11:00 PM #33 Schmitz77   diyAudio Member   Join Date: Apr 2015 There is a saying here in germoney: whoever buys cheap buys twice
rongon
diyAudio Member

Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
 Originally Posted by nerdorama Here's a post on this from years ago by Mike at Magnequest regarding his rule of thumb for calculating an acceptable bias current for an unknown transformer. RE: Looking for a output trannie match, Scott LK72 - mqracing - Tube DIY Asylum
Quote:
 calculate the voltage drop--- 90 ohms times 150 mils would be a voltage drop of 13.5 volts. 90 ohms times 75 mils would be a voltage drop of 6.75 volts as a rule of thumb--- your probably not going to have a problem if you keep this voltage drop below 20 volts (in the context of an ot with tube friendly impedances and power supply voltages and etc). Remember it is a rule of thumb meaning someone, somewhere will be able to find a case where it would get you in trouble--- but they should be rare cases indeed. generally---- my rule of thumb looks at anything less than a 20 volt drop as not being much to worry about--- again--- excluding the unusual or weird case that someone might dream up. other rule of thumb is to calculate how many watts are dissapated across the winding---- 90 ohms and 75 mils would dissapate about one half of one watt of power going up in heat. Not much at all. formula is current squared times resistance
Hmmm...

So with Ip = 0.09A per winding, each winding (half of the primary) needs to have DCR of no more than about 220 ohms.

220R(0.09A) = 19.8V dropped across the winding

The power dissipated would be

0.09squared(220R) = 1.782 watts (hot!)

So it looks like 220 ohms per primary half would be marginal. Better to aim for 150 ohms or less.

Is this achievable? I don't know... I need to measure some OPTs.
--

A pair of 8k:VC OPTs meant for Push-Pull EL84 measure about 200R per primary winding, or almost 400R from plate to plate.
I guess those would be marginal for Ip = 90mA per tube.

--

Last edited by rongon; 24th November 2020 at 10:56 PM.

 24th November 2020, 11:53 PM #35 rongon   diyAudio Member     Join Date: Jul 2009 Location: Across the river from Rip's big old tree... Another push-pull OPT I have, with a 5k primary rated for 30W and 75mA per tube max, measures 85R per primary winding (170R plate to plate). Specs for the Dynaco MK-III Output Transformer A-431: 4.3K Push-Pull with 33% Screen Taps to 4, 8, & 16 ohms 60 Watts Frequency Response: 10 Hz to 100 kHz +/- 1 dB @ 1 Watt Primary DC Resistance: 142 ohms Primary Inductance: 40H @ 120 Hz Hmmm, only 142 ohms. I think that's plate-to-plate too. So it does appear that a 50 or 60 watt OPT will work best for my application. The thing is, I remember what the Dyna A431 OPTs sounded like. I tried using them for a pair of 300Bs and 2A3s. They sound kind of big and dopey compared to something like the Dynaco A470 (ST70) or the OPTs from a Fisher 800C (why did I sell those off??). My suspicion is that big honkin' OPTs like the Dyna A431 have that big transformer thickness to their sound. Unfortunately, that's what's going to be required to run GU-50s safely with 90mA quiescent Ip. Am I right, or am I on the wrong track here? Transcendar's 30W 6.6k OPT specs are: 6.6K Push-Pull to 4, 8, &16 ohms Frequency Response: 10Hz to 100 kHz +/- 1 dB @ 1 Watt Primary DC Resistance: 235 ohms Primary Inductance: 65 H @ 120 Hz Weight: 4 lbs So 235 ohms plate-to-plate and 4 lbs each. Those are a lot lighter in weight than Edcor's 30W OPTs (6 lbs each). I wonder what that means... It looks like if I want some headroom for high current swings, I may need a pair of big boy OPTs. Perhaps 50W rated. -- Last edited by rongon; Yesterday at 12:02 AM.

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