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'Output transformer taps/impedance for SEL'
Author:GeorgeG (registered user: 61 posts )
Date: Mon, Jan 09th, 2017 @ 17:49 ( . )

The layout for the SEL shows the 125ESE output transformer taps as: White=16 ohms, Yellow=8 ohms, Green=4 ohms, Orange= NC (all 5K taps) but the schematic shows White=8 ohms, Yellow = 4 ohms, Green/Orange = NC (all 2.5K taps). Which one is correct?

The Extreme docs which is also an Octal power tube with a 125ESE transformer shows White=16 ohms, Yellow=8 ohms, Green=4 ohms, Orange= NC (all 5K taps) for both layout and schematic. I don't know why they are different given the identical output tube and transformer.

Thanks.

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'Output transformer taps/impedance for SEL'
Author:Dave C (registered user: 202 posts )
Date: Mon, Jan 09th, 2017 @ 20:47 ( . )

Which version of the the P1eX document are you looking at? From what I can tell the 10.10.04 version is consistent between layout and schematic (5K at the primaries).

In reality, you can hook it up to reflect either 5K or 2.5K at the primaries. My personal preference is 2.5K with a single ended EL34 ....sounds better to me.


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'Output transformer taps/impedance for SEL'
Author:GeorgeG (registered user: 61 posts )
Date: Mon, Jan 09th, 2017 @ 23:00 ( . )

Yes the P1 Extreme is consistent between the layout and the schematic but the SEL documents, which is my build is not consistent. It sounds like either way - 5K or 2.5K will work so either the SEL layout or schematic are both right?

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'Output transformer taps/impedance for SEL'
Author:SX4Amps (registered user: 193 posts )
Date: Tue, Jan 10th, 2017 @ 00:33 ( . )

If you are building the SEL with a KT88, the OT it will be wired different than the P1-Extreme. The the white on the SEL will be 8 ohm and the yellow will be 4 ohm because the preferred primary impedance for the KT88 is 2500 ohm.

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'Output transformer taps/impedance for SEL'
Author:GeorgeG (registered user: 61 posts )
Date: Tue, Jan 10th, 2017 @ 11:16 ( . )

My build is an SEL but the tubes I use are the KT77, KT66 and EL34.

How do I determine the prefered load on the tubes so that I know which OT tap to use for a given tube and speaker impedance?

Thanks.

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'Output transformer taps/impedance for SEL'
Author:SX4Amps (registered user: 193 posts )
Date: Tue, Jan 10th, 2017 @ 11:25 ( . )

Then you probably should go with the same hookup for the P1-Ex (5000ohm) Here is a link to the Hammond 125SE. Sorry I should have probably posted this before. [link]

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'Output transformer taps/impedance for SEL'
Author:GeorgeG (registered user: 61 posts )
Date: Tue, Jan 10th, 2017 @ 11:51 ( . )

Thanks. How do I determine what OT tap/impedance that a given power tube needs to see? IOW, how do I know that the KT88 and KT77 should be wired to a different tap on the OT?

Thanks.

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'Output transformer taps/impedance for SEL'
Author:SX4Amps (registered user: 193 posts )
Date: Tue, Jan 10th, 2017 @ 12:06 ( . )

I do not know too much about the KT66 and KT77 but I think they are the same specs as the EL34. Maybe someone else can chime in. From what I know, the KT88 likes to see 2500 ohm primary impedance and the EL34 likes to see around 3200-3600 ohm primary so you use the 5000 ohm taps for the EL34. Primary impedance depends in the speaker load impedance. I may not explain this as well as "Uncle Doug." Here is a good video that may help you conceptualize it a little better. [link]

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'Output transformer taps/impedance for SEL'
Author:GeorgeG (registered user: 61 posts )
Date: Tue, Jan 10th, 2017 @ 12:36 ( . )

I found this tutorial on my main question of determining what impedance to present to a given power tube which may help those that follow me:

[link]

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'Output transformer taps/impedance for SEL'
Author:GeorgeG (registered user: 61 posts )
Date: Tue, Jan 10th, 2017 @ 12:58 ( . )

The short form version is that the appropriate impedance to present to the output tube via the OT is not a single number. It depends on the anode voltage which can vary widely across designs, the maximum power dissipation design point for a given tube and the operating point that you desire within the operation limits for that tube which will change its tone and tube life. The target is (Va)2/Pa so for the SEL design shown here with a measured Va of 346V (my actual is higher) then for a 25W EL34/KT77 etc you get 5K load impedance (4789) and for a 35W KT88 you get 3.5K (3420). Higher load impedance runs colder and is more conservative. Therefore for a 25W tube the layout (white wire to 16ohm speaker) is appropriate and for a 35W tube either the layout or schematic can work as they are on either side of the operating target with the layout being more conservative. This according to the link above anyways and my knowledge is a sum total of 30 minutes old so feel free to read further.

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'Output transformer taps/impedance for SEL'
Author:GeorgeG (registered user: 61 posts )
Date: Tue, Jan 10th, 2017 @ 13:04 ( . )

One more observation: my SEL using a KT88 with the more conservative load impedance wiring (white wire to 16ohms) was the cleanest sounding output tube with the most clean headroom. I tested the EL34, KT66, KT77 (my favorite), 6l6GC, and KT88. My pre-amp tube comparisons had at least as much affect on tone as did my power tube selection with my overall favorite being the Mullard 12AX7/4004 as used in George Metropolous designs, depending on guitar pick up used (single vs HB coils).

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'Output transformer taps/impedance for SEL'
Author:SX4Amps (registered user: 193 posts )
Date: Tue, Jan 10th, 2017 @ 13:36 ( . )

Correct, I have looked at that many times and always wondered why there are primary OT impedances that are used as a standard when they don't consider plate voltages. I'm still learning as well ;)

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