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Old 13th March 2004, 07:19 PM   #1
Fuling is offline Fuling  Sweden
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Default Output transformer issues...

Hi.

As Ive mentioned earlier on this forum Im collecting parts for a 813 SE project.
The choise of OPT was (is) a big question here, since this will be a big and very expensive project and I dont want any junk in that important position.
Finally I decided to use Hammond 1628SEs, but since they seem to have some serious problems in the upper frequency range theyre out of question.
(See the thread "Whats wrong with Hammond 1628SE?")

Sad, since 813 would give more than 25 fairly linear watts with into that load if theyre pushed a bit into class A2.

The second most logical choice would be a pair of Lundahl C-core OPTs. The problem here is the low primary inductance, 23H for a 5,6k unit is a bit light I think.
The question is: How much would I suffer from the low inductance?
The plate resistance of a triode wired 813 is about 1,8k and my coarse calculations makes me believe that Id need about twice the inductance to get full frequency response.

813 builders out there, what are you using?
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Old 13th March 2004, 08:03 PM   #2
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Default Lundahl 1688

Hi,

You might want to look into the new Lundahl 1688 designed for the 845. 9.2k or 5.5k with 70H at 70mA. The datasheet is on their website.

Michael
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Old 13th March 2004, 08:25 PM   #3
Fuling is offline Fuling  Sweden
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I cant seem to find it on their homepage
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Old 13th March 2004, 08:35 PM   #4
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Default Might work

http://www.lundahl.se/pdfs/datash/1688.pdf
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Old 13th March 2004, 08:51 PM   #5
Fuling is offline Fuling  Sweden
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Thank you, but I found it when I had a closer look at their homepage.

Ill mail Lundahl and ask if they can make a pair with slightly bigger airgaps, since Ive found a sweet point at 900V 80mA that gives almost 27W (in light A2 operation).

Going from 70 to 80 ma shouldnt dump the primary inductance too much, dont you think? From 70 to 50-60H Id guess.

BTW: Thanks again for pointing me towards this transformer, I had no idea they had it.
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Old 14th March 2004, 06:17 PM   #6
Fuling is offline Fuling  Sweden
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I just posted a mail to Lundahl, will check for an answer tomorrow.

One thing that confuses me is the connections of the secondaries.
The transformer is wound as follows: 50+50: 1+2+2+1+1+2+2+1

To get 9,2k:8 we want a ~34:1 ratio, 100:3 would be good enough I think.
I would do it this way: All "2" windings connected in parallel and all "1" windings in parallel, then those two banks of windings in series = 100:3

Accordning to the Lundahl datasheet some sec. windings shall be connected in a very strange configuration, some of them in antiphase. Why?
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Old 14th March 2004, 07:18 PM   #7
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Default 23H is a bit low.

Yes, 23H is a bit low. XL will equal the reflected load at 39 Hz. This will make the load line both elliptical and lower than 5.6Kohm.

If the 23 H is measured at very low voltage drive, expect it to increase a factor of 2 or 3 as the voltage on the primary increases. If this is the inductance called out in the transformer catalog, it usually won't be the inductance at low drive, but it will be the inductance at a higher drive level. Unfortunately, marketing usually spec's the inductance at it's maximum level (at a high drive level.)

8H per kohm of reflected load is a rule I've heard more than once (or 45H for 5.6K.) 8H/kohm gives us a -3 dB point of 20 Hz. You can correct for low primary inductance a little by adding more bias current. Add enough current so that the load line biased up for a 2.6K load.
2.46K = 1/(1/Xl)^2 + (1/RL)^2 = 1/[1/(2*PI*20*23H)^2 + 1/5.6k^2]

Besides the primary inductance, keep an eye on the leakage inductance and total parasitic capacitance on the primary side. Usually, not always, the secondary side's capacitance is low enough not to be an issue.
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Old 14th March 2004, 08:22 PM   #8
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Just for your information, you might check out Plitron / Amplimo's audio transformers here .

I hope this helps, and have fun!

Best,

George Ferguson
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Old 15th March 2004, 01:50 AM   #9
EC8010 is offline EC8010  United Kingdom
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If the primary inductance is inadequate, audio current from the output valve will be diverted from the load into that inductance. In other words, distortion will rise at low frequencies. As a rough rule, the reactance of the primary inductance ought to be equal to the reflected load resistance at the lowest frequency of interest. Thus, I would suggest that for 20Hz, you would like to see 45H. (Which is exactly what VoltSecond suggests.)
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Old 15th March 2004, 10:15 AM   #10
Fuling is offline Fuling  Sweden
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I got an answer from Lundahl today, they can make a pair of LL1688 gapped for 85mA without extra cost for the modification.
I asked for 80-85mA airgap but 80 was too close to 70 according to Mr Lundahl himself.
I guess the inductance will drop to maybe 50H from the original 70, but I think that will be good enough for practical purposes.
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