I’m cornfused by different responses to parallel output tubes, so you get to take a quiz!

Discussion in 'Shock Brother's DIY Amps' started by andrewRneumann, Jul 5, 2020.

What color should Joe connect to his speaker jack?

1. andrewRneumannTele-Meister

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Joe has an 8-Ohm speaker he wants to use with the following output transformer. He’s running 2 parallel output tubes in single-ended operation. He really likes the sound that one tube makes when using a 5,000-Ohm load, so he’d like each tube in the parallel set to “see” 5,000-Ohms as a load.

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2. FenderLoverPoster Extraordinaire

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Does your question ask what you really want to know? Each tube will 'see' the other tube, so the question is a little confusing.

Keep this in mind: Z=Va^2/Pa

and check this out:
http://www.valvewizard.co.uk/se.html

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3. rangercasterPoster Extraordinaire

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Try them all and see what sounds best
...

But ...

When smoke comes out of your amp ...
You made the wrong choice ...

4. tubegeekFriend of Leo's

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It's all there in black and white for you.

There will be twice the current into the load through two tubes. For the transformer to interact with twice the current and behave the same, the impedance will be 1/2 the original.

V=I*Z

I*Z = (2*I)*(1/2*Z)

Power will not stay the same, unsurprisingly.

P=V*I

V*2*I= 2*P

Two tubes will deliver twice the power into the load.

5. ReverendRevolverTele-Afflicted

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".... when joe allows the magic smoke to leave Joe's amp, he has chosen incorrectly...."

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6. andrewRneumannTele-Meister

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Could it be said that for a typical pentode the anode resistance of the parallel tube is so high compared to the impedance of the speaker load that it could safely be ignored?

7. tweedy_woodpeckerTele-Meister

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I took me a while to get a grip on this topic. What really helped me was drawing a few load loadlines to understand what really is going on in different configurations and calculating the load for various turn ratios and vice versa. Also figuring out the concept of power matching versus current or voltage matching helped me a lot.
And remember: Your 5k are just for a certain frequency, usually 1kHZ. Impedance is another concept that takes a while to understand.

If Joe likes the yellow wire for a single tube (5k:8Ohm at 1kHz) using the white wire will give him the same impedance matching since the resistance of the two plates in parallel is only half that of a single plate.
Does Joe know which turns ratios the different coloured wires give him?

8. andrewRneumannTele-Meister

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I think Joe told me it was the square root of the primary/secondary impedance ratio. Was he right?

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9. tubegeekFriend of Leo's

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Naah, that answer's way off. It's ten, raised to the power of one half the log of the impedance ratio!

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10. rangercasterPoster Extraordinaire

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I'm surprised orange hasn't gotten a single vote ...

But it's early ...

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11. rangercasterPoster Extraordinaire

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Perhaps ...
Perhaps not ...

I'm on the fence ..

12. elpicoTele-Afflicted

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It's a bit weird that the first response wasn't "white." with the thread just ending there.

Don't you love it when you ask a simple question and people post everything but the answer?

How much 5000/2 is has nothing to do with democracy. If 60% of people you poll think 5000/2 = 4000 does that make it right? 5000/2 = 2500. It's not a matter of opinion.

Last edited: Jul 6, 2020
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13. aleskiTDPRI Member

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Cornfused sounds delicious.

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14. andrewRneumannTele-Meister

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If I had put “black” as an option I think the servers may have gone berserk.

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15. andrewRneumannTele-Meister

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Thank you everyone who has been gracious enough to take the poll! I also appreciate people playing along and helping me truly understand this. I indeed had the green wire hooked up on the MC10, and this was after I consulted multiple forums and books.

I will now reveal the anatomy of my, err um Joe’s, cornfusion.

As you can see, Joe clearly divided all the impedances by 2. That’s what nearly every thread Joe searched on the internet said to do. (Joe’s new to this, so application of concepts using correlation is not his strong suit, yet. There may be others like him.) This makes it look like green will provide the desired 5k:8 ratio.

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16. tubegeekFriend of Leo's

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Math is a superpower. It can be used for good... or evil.

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17. hemingwayPoster Extraordinaire

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It's now my word of the day.

I'm also beefwildered.

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18. tubegeekFriend of Leo's

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"Cobsmacked"

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19. tweedy_woodpeckerTele-Meister

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Now Joe got me cornfused

Why did Joe divide the impedances in the datasheet?

Using two tubes will not change the OT impedances. You just need to divide the impedance for two tubes in parallel (5k:2=2.5k) and then choose the appropriate tap thus matching the load.

So with 8 Ohms you want white for two tubes (2.5k) or yellow for one 6V6 tube (5k).

Green with an 8Ohm speaker will always reflect 10k not 5k. Your red numbers would be right for a 2Ohm load on the secondary.

Last edited: Jul 6, 2020
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20. King FanPoster Extraordinaire

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“Bamused”

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