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Post by StrangeC on Aug 29, 2008 22:48:40 GMT -7
Looking to bias these in a Blackfase style amp. Plate voltage is about 425v according to my Bias Rite. The data I could find on these tubes is 35w ea. So if I figure correctly; (35/425) * .7 = 57.6mA. That seems high. First time figuring the numbers- help me out.
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Post by bustertheboy on Aug 30, 2008 15:16:32 GMT -7
Hi StrangeC my blackface vibrolux '65 was 35 watts in total running at about 438v, biased at 35mA. Not 35 w per tube- I've never seen a two 6L6 amp running like that, especially at a low voltage like that. Brett ps- update- I just checked Dan Torres book "Inside Tube Amps" and he gives these figures Bias current about 26-28 ma per 6L6 tube. Plate dissipation is about 13 watts per tube- total 26 watts. Rated output is an (optimistic) 40 watts. With 35 mA you'd achieve this figure.
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Post by hdahs143 on Aug 30, 2008 15:56:42 GMT -7
This should cover the bases for you! By the way, those tubes are probably rated at 25 watts max safe plate dissipation. 35 watts per tube is up in the 6550 area. www.webervst.com/tubes/calcbias.htmHarold
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Post by bustertheboy on Aug 30, 2008 17:41:28 GMT -7
But do keep in mind the voltage you're running at or you're expecting the impossible. Some early black face 6L6 type amps ran at the sort of voltage you're talking about (420v), but many ran up to 460 v- in Gerald Weber's book, full of schematics, there's Tremoluxes up to 460v, and Supers from 425- 480v. Those high figures are when you get higher wattage.
Brett
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Post by StrangeC on Aug 31, 2008 8:54:38 GMT -7
The amp is a Gries 35. Dave Gries says it has about 435v on the plates- I measure 425v with my Bias Rite. Problem there?
Regarding the NOS Philips 7581A tubes- They are 35w tubes and can take up to 500v on the plates. They're probably more like a KT-66 than a 6L6GC.
I believe I have the bias formula correct: Tube Watt/Plate Voltage x 70% (or 60 or 50...)
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Post by hdahs143 on Aug 31, 2008 12:34:38 GMT -7
You are correct the 7581a is a 35 watt tube. The 6L6WGB which was listed in the thread title is a 25 watt tube.
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Post by bustertheboy on Aug 31, 2008 13:43:47 GMT -7
You are correct the 7581a is a 35 watt tube. The 6L6WGB which was listed in the thread title is a 25 watt tube. So it's not a 6L6- all is clear now Brett
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