Did I get this load calculation right?
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 18th September 2013, 07:35 PM #1 Sea Biscuit   diyAudio Member   Join Date: Sep 2013 Location: USA Did I get this load calculation right? 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! Last edited by Sea Biscuit; 18th September 2013 at 09:19 PM.
 18th September 2013, 07:53 PM #2 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts Why not use something like a 5Y3, 5V4 or similar, you appear to have the filament windings. __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
Sea Biscuit
diyAudio Member

Join Date: Sep 2013
Location: USA
Quote:
 Originally Posted by kevinkr 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...

Last edited by Sea Biscuit; 18th September 2013 at 08:26 PM.

 18th September 2013, 08:36 PM #4 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts 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. __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
 18th September 2013, 08:45 PM #5 Sea Biscuit   diyAudio Member   Join Date: Sep 2013 Location: USA 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.
 18th September 2013, 08:53 PM #6 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts 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. __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
 18th September 2013, 09:03 PM #7 Sea Biscuit   diyAudio Member   Join Date: Sep 2013 Location: USA 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? Last edited by Sea Biscuit; 18th September 2013 at 09:10 PM.
 18th September 2013, 09:40 PM #8 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts I think you are on the right track.. You will learn a lot during this process I think.. __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
 18th September 2013, 09:44 PM #9 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts 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. __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
 18th September 2013, 09:49 PM #10 kevinkr   diyAudio Moderator     Join Date: Sep 2004 Location: Boston, Massachusetts 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. __________________ "To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

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