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Old 19th October 2013, 02:37 AM   #1
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Default 70v transformers explored.

You guys have helped me in the past to understand the various taps on a line transformer. I have a transformer like the one shown below.

In a push-pull amp, I understand to use the 8k tap for top tube and common terminal for bottom tube. B+ goes in between on the 2k tap.

Is it possible to use the 4k and 1k (or 2k 500ohm) taps for screens?
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Old 19th October 2013, 03:31 AM   #2
TheGimp is offline TheGimp  United States
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It will work, but you will have a missmatch. The feedback won't be the same for both tubes.

Al lof the 70V transformers I tried had lousy frequency response. They seem to be designed for PA operation with limited BW. A measure of primary inductance and leakage inductance was consistent with that.

I suggest doing a frequency sweep before spending a lot of time building anything with them.
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Old 19th October 2013, 03:47 AM   #3
DUG is offline DUG  Canada
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Look at the "Voltage or turns ratio to output" column.

If 500 is 8X then 1000 is 11.313X and 4000 is 22.627X

If you use the 2K connections the 22x tap is 6.62 turns from the Center Tap but the 11x is only 4.68 turns from ct.

Not really balanced.



You could try it and let us know what you think of the sound.

However:

The 8000R side windings at 70V is designed for about 9mA. (0.625W)

Are you using this to build a 1.25W push-pull amp?
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Old 19th October 2013, 04:22 AM   #4
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I have built a couple of surprisingly nice sounding pp amps with the 7010, mine were branded International.

6eb8

6bm8
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Old 19th October 2013, 04:24 AM   #5
AJT is offline AJT  Philippines
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looks like your center tap is the 2k ohm tap
outer plate leads, com and 8k...

to confirn, input about 6vac on the secondary 8ohm tap and common,
then measure primary voltages on taps....
you can then confirm the other taps suitable for ultra linear or not..
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Old 19th October 2013, 08:17 AM   #6
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Imagine the chart shows the primary 'turns' for the column marked voltage or turns ratio.

For the 2kPP setup, the 4kohm tap is 6 turns away from the CT tap (ie. 2kohm tap), and the 1kohm tap is 5 turns away from the CT. Normally in a UL setup, the UL taps are the same number of turns away from the CT - if the turns to each UL tap aren't the same then the screens are being driven to different voltage levels - so not a perfect match.

Same situation for the 1kPP setup.
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Old 19th October 2013, 11:49 AM   #7
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Quote:
Originally Posted by AJT View Post
looks like your center tap is the 2k ohm tap
outer plate leads, com and 8k...

to confirn, input about 6vac on the secondary 8ohm tap and common,
then measure primary voltages on taps....
you can then confirm the other taps suitable for ultra linear or not..
Thanks. I'll give that a shot and see what I get.

I'm just messing around and seeing if it'd work. The transformers I'm working with are a little dull in the high-end but bass is great. I'm working with a lot of old parts here and I don't even know if my tubes are all that great either.

I'm using a concertina PI and just for fun I placed the concertina's output caps across the 70v transformer's outer windings just to hear it. It sounded surprisingly good but I left the B+ on the CT. Funny thing was, ALL noise was eliminated that way. When I removed B+ all the noise came back. I can't figure that one out.

I have a negative feedback question too. Doesn't global-nfb put the cathode's DC across the opt's secondary and the speaker? Or is the resistor in series with the nfb minimize any effects of a small DC current on the secondary?
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Old 19th October 2013, 11:57 AM   #8
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The dc bias on a cathode is only a volt or two, and that bias voltage is divided to a large level to the lower cathode resistor used for the common feedback function. And then that dc voltage is divided again by the series feedback resistor and the speaker resistance. So effectively there is almost zero influence on the speaker.
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Old 19th October 2013, 11:58 AM   #9
AJT is offline AJT  Philippines
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using the 2k ohm tap, you should be getting symetrical voltage readings from taps intended for plates and screen taps...

regarding the nfb, the currents will be too small to matter if that means taking the first stage cathode as feedback node...
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Old 19th October 2013, 12:55 PM   #10
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I think the chart in the first post may need clarification. When using the 8K and Common with the 2K as center this would yield 8K plate to plate. In the case of a AB PP amp, 8K/4 for a 2K loadline. In class A pp then 8K/2 for 4K load line.

There is no symmetrical configuration for UL, see 8K plate to plate example. Using number of turns specified on he Speco 7010 transformer spec sheet.
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