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Old 18th September 2013, 07:35 PM   #1
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Default Did I get this load calculation right?

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So I've discovered that the 6x5 recto won't supply enough current for 2 6y6's so I've chosen the 6ax5 instead. It draws twice the heater current though, putting me right up to the 4a rating of the 6.3v supply of the PT.



The 6ax5gt will supply up to 150mA of DC current.

Let me see if the math works out. Correct me if I've skipped anything.

6y6 plate diss. = 12.5w
let's use max voltage of 200v
that's 62.5mA (X2) = 125mA

6y6 screen diss. = 1.75W
Let's say 200v
that's 8.75mA (X2) = 17.5mA

12at7 plate diss. = 2.5w
@ 300v
that's 8.3mA (X2) = 16.6mA


Total is 159.1mA

With the output tubes biased down to about 55mA, that would put the total at around 144mA and 11W per tube. Sound reasonable?



Here's a photo of the test model as it currently sits. All that is active right now are the heaters, bulb, and available unloaded DC. I'll add more to it as components get selected and testing them along the way. This is a work in progress to help me learn all these confusing aspects of electronics. The schematic is a skeleton of a 5C3 power section and will be altered as needed as I work my way from the power supply to the input jack. Bit by bloody bit I swear I'll learn these things yet!

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Last edited by Sea Biscuit; 18th September 2013 at 09:19 PM.
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Old 18th September 2013, 07:53 PM   #2
kevinkr is offline kevinkr  United States
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Why not use something like a 5Y3, 5V4 or similar, you appear to have the filament windings.
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Old 18th September 2013, 08:24 PM   #3
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Quote:
Originally Posted by kevinkr View Post
Why not use something like a 5Y3, 5V4 or similar, you appear to have the filament windings.
Doesn't really matter which schematic's "skeleton" is chosen. I don'y even really care if it sounds good or not. The main idea here is to study the component selections from power to input, step by step, one by one, and learning as many details as I can along the way, broaden my very narrow understanding of tube amps. I've built 5 working amps in the past, all direct copies of popular amps, and mostly I just learned how to assemble. I learned a little bit about the concepts but not much. This sort of approach should yield many opportunities to pick up knowledge. Or at least I'm hoping it will...

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Old 18th September 2013, 08:36 PM   #4
kevinkr is offline kevinkr  United States
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Right, I get your point, but you can apply the same logic to selection of a rectifier to see whether or not a specific device is the good or even a reasonable choice for the application. Studying the data sheets knowing transformer constraints and target load current and voltage would provide an answer.
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Old 18th September 2013, 08:45 PM   #5
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Right. The transformer ratings are on the drawing and the rectifier's current output capabilities are the same as the HT current capabilities. My question at the moment is did I leave anything out of the total calculation for use with a 12at7 and 2 6y6's? I think I have it right but looking for some clarification.
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Old 18th September 2013, 08:53 PM   #6
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It appears that you can exceed the rated maximum output current of the rectifier under some circumstances, your calculation seems to indicate that you are continuously over - I have not checked the other assumptions to see if they are valid.

Generally I like 25 - 50 % margin on rectifiers although I know the Dyna ST-70 broke this rule and got away with it because of the good foreign 5AR4 (usually mullard) fitted.

I would choose another rectifier based on your calculations if they were solidly derived.

The saving grace is you probably will not run the outputs at the rated dissipation.
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Old 18th September 2013, 09:03 PM   #7
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Using the tube chart data, I calculated the expected current draw using plate & screen dissipation of the 6y6's, and plate dissipation of the 12at7. Is there anything else to factor in?

And yeah, according to the calculations (max voltage and dissipation from tube sheets), at full bias I would be over the current rating at idle, but biased down from 12.5w to 11w plate dissipation, it brought the idle current to 144. Could bias down another watt or two for some wiggle if need be.

Does it sound like I'm understanding this correctly?

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Old 18th September 2013, 09:40 PM   #8
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I think you are on the right track.. You will learn a lot during this process I think..
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Old 18th September 2013, 09:44 PM   #9
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One comment I would make is to place the driver a bit closer to the output tubes, and be aware that oscillation with a breadboard like this is possible. (grid and screen stoppers on the sockets should help)

One other comment is safety related - cover all exposed high voltage points. In general be careful, there will be plenty of zap points if you do not think about that in advance.
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Old 18th September 2013, 09:49 PM   #10
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In reviewing the transformer I realized the secondary voltages are far too high, and the HV secondary current rating is low unless it is given as the DC rating. Given the low target plate voltages it seems like something in the vicinity 450VCT would be a better choice.
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