Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on September 14, 2017, 12:14:45 pm
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I’m sketching up a 1watter, borrowing from others hard work. I’m using an inter-stage tranny as my PI, my working sketch has it cap coupled to the basically “standard” PA design, (lifted from a moonlight V3), grid leak, grid stop, caps, (ignore pin numbers on tube).
I pulled up a schematic of a GA-5T that uses a tranny PI, and noticed there are no R’s or C’s in the PA section.
I get why I don’t need caps, what I’m not sure of are the grid R’s, especially once you push it into clipping, I believe the tranny is rated at 50mA, I’m thinking the grid will smoke before the tranny, but I’d prefer if it’s designed so neither smoke.
Thanks as always for the insight.
dave
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You doubt the wisdom of the GA's designer??
Transformer grid return is better than resistor.
Unless you NEED grid-blocking distortion. (One player lives there.)
Where did you get a grid transformer?
50mA sounds more like an output transformer than grid iron.
What are you driving this with? A 6L6 @ 50mA sure could burn the grids of 6SN7. A 12AX7 sure can't keep them warm.
You can get a Whole Watt out of a 50L6 or an undernourished 6AQ5. No phase splitter.
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You doubt the wisdom of the GA's designer??
Only if It was designed on Monday :icon_biggrin:
Where did you get a grid transformer?
It's from USA transformers, I mis-quoted DC spec, actual 10mA
I will be using a 12__7 as driver, the PA section I lifted shows 7.5vdc at Rk, so I shouldn't need much drive AC.
here's the "specs" of the tranny, Ive used it twice, but both have been AC coupled, this time I'm gonna try it like Gibson did before they abandoned it :BangHead:
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http://triodeelectronics.com/unintrui.html (http://triodeelectronics.com/unintrui.html)
http://www.triodeel.com/pdf/uis-1.pdf (http://www.triodeel.com/pdf/uis-1.pdf)
http://site.triodestore.com/TriodeUIStestdata.jpg (http://site.triodestore.com/TriodeUIStestdata.jpg)
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Thanks, I had everything but the data sheet. I think there's a typo on power level, in my world 1M.W. is mega, not milli :icon_biggrin:
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I was musing over the PI driver, thinking I would use a 12__7, but looking at Gibson's voltages it shows 8.7V with Rk of 1K, which is 8.7mA. Since the tranny is rated 10mA this seems "too close for comfort".
Would a 12__7 bias much different than the 6C4?
I'm thinking 2K for Rk would keep me in "the safe" zone, assuming it also get me enough swing to drive the PA?
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> Would a 12__7 bias much different than the 6C4?
12A_7 covers a 5:1 range of fatness. Pick one.
6C4 is half a 12AU7.
7mA is not too much for a 10mA transformer. Transformers do want substantial drive.
Driver transformer and 6SN7 final is an odd combination. You either do your design homework, or you nail something up and try it.
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You either do your design homework, or you nail something up and try it.
One big improvement I've learned here, it's easier to calculate the smoke than re-order parts :laugh:
I will be building the PA on my hybrid board, I don't have the tranny, so i'll get the PA working n if expectation's are met, i'll bite on the tranny
thanks for the insights