763 heater current

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beezerboy
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763 heater current

Post by beezerboy »

I'm thinking I want to try a pair of KT66 tubes but they draw more heater current than the 6L6GCs. does anyone know how much current the 763 heater circuit uses? I've got the MojoTone power tranny & it specs 4A available. ya, I know, I could measure it but hoping someone know off hand.
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martin manning
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Re: 763 heater current

Post by martin manning »

By "763" I assume you mean AB763 circuit with 2x 6L6? No need to measure, just add up the heater current: 0.9A x number of 6L6, + 0.3A x number of 12A_7's. KT66 use 1.3A so the bottom line is you'll need an additional 0.8A.
beezerboy
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Re: 763 heater current

Post by beezerboy »

actually.... I guess what I really wanted to know was current requirements per tube. I left out the Tremolo circuit when I built it, so no V5. it has 2 ea 12ax7, 2 ea 12AT7. so that should be 1.2A to run the 12s. the spec on my MoJo Tone transformer says it makes 4A for the heater windings, so that leaves 2.8A to play with. if two KTs draw 2.6A, I've got 200 mA to spare.... good enough??

and thanks for the reply
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martin manning
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Re: 763 heater current

Post by martin manning »

Yes, sounds good.
Stevem
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Re: 763 heater current

Post by Stevem »

It still pays to confirm that you have no less then 6.1 volts with all the tubes in.
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Roe
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Re: 763 heater current

Post by Roe »

beezerboy wrote: Tue Oct 24, 2017 12:00 am actually.... I guess what I really wanted to know was current requirements per tube. I left out the Tremolo circuit when I built it, so no V5. it has 2 ea 12ax7, 2 ea 12AT7. so that should be 1.2A to run the 12s. the spec on my MoJo Tone transformer says it makes 4A for the heater windings, so that leaves 2.8A to play with. if two KTs draw 2.6A, I've got 200 mA to spare.... good enough??

and thanks for the reply
the new MIC 66s draw less current. the russian ones are closer to 1.3A
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dbeasley
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Re: 763 heater current

Post by dbeasley »

Worst case scenario you could measure heater voltage with the KT66s in the amp. If you get a voltage drop from 6.1-6 3vAC then chances are the heater winding can't handle it.

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tubeswell
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Re: 763 heater current

Post by tubeswell »

beezerboy wrote: Tue Oct 24, 2017 12:00 am actually.... I guess what I really wanted to know was current requirements per tube. I left out the Tremolo circuit when I built it, so no V5. it has 2 ea 12ax7, 2 ea 12AT7. so that should be 1.2A to run the 12s. the spec on my MoJo Tone transformer says it makes 4A for the heater windings, so that leaves 2.8A to play with. if two KTs draw 2.6A, I've got 200 mA to spare.... good enough??
Don't forget to add the pilot lamp current draw. Some bulbs draw 150mA, some are 300mA, some are 500mA
He who dies with the most tubes... wins
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