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Old 10th July 2011, 04:10 PM   #1
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Default interstage tansformer impedence

Anyone?

I have a pair of inter-stage transformers from Motorola consoles. They were used as phase splitters to drive four 6V6 output tubes in PPP.
I haven't been able to find any information on the transformers. And I'm not certain what was used to drive them as I only have the schematic for the power amp.

My question is this. If the transformer has a 1:1 ratio and is simply used to invert half the signal. Does the impedance have to be matched between the driver preamp tube and the output tube? I was thinking of using them between a 6SL7 or 6922 preamp tube and a pair of 6L6GC output tubes.

Kevin

Last edited by mr2racer; 10th July 2011 at 04:25 PM. Reason: incomplete entry
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Old 10th July 2011, 06:17 PM   #2
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Quote:
Originally Posted by mr2racer View Post
My question is this. If the transformer has a 1:1 ratio and is simply used to invert half the signal. Does the impedance have to be matched between the driver preamp tube and the output tube?
No. From the theory of mutual inductance, a lightly loaded (Zsec >> (wM)^2) secondary has almost no effect on the primary circuit.

Quote:
I was thinking of using them between a 6SL7 or 6922 preamp tube and a pair of 6L6GC output tubes.
It's not possible to use a 6SL7 that has rp= 44K with an IST. You need a triode with a considerably lower rp, and run it "hot" to get that rp down. Some thing like the large signal half of a 6DR7, 6BX7, or something like that. Your low frequencies start to fall off when Xl(pri)= rp.

You also need to find out how much DC those IST splitters were meant to carry as well.
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Old 10th July 2011, 07:06 PM   #3
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this is a subject I don't understand, I do hope someone can clarify for me.

the transformer secondary will reflect to the primary the impedance presented to it.

this ignoring a zobel, will be the output tubes impedance, the plate-cathode?

effectively, its pretty high, near infinity, or whatever it may be, certainly 100s k or even meghoms.

that means the preceding valve will be loaded by if its 1:1, the same value neglecting losses. and presumably the other way around, the primary will match to the secondary what its loaded with, remember tx's don't work in isolation??

ie the 2dary will reflect its load to the primary, and vice versa?

so what does that mean for the load on the driver valve? near infinity?

what happens when we start say a 1:2 ratio?

I don't understand this area at all......can someone help explain in simple terms...

fwiw I think tx's are fascinating animals...
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Old 10th July 2011, 07:12 PM   #4
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The inductance of the primary (it is a coil) is in parallel with the reflected impedance from the secondary. In a parallel combination the lower of the two dominates so once frequency gets low enough for the primary inductance to become dominant then it determines the response (in league with the driver tubes effective plate resistance).
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Old 10th July 2011, 07:17 PM   #5
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A comparatively easy way to deal with the issues Miles raised and get the high gain a 6SL7 section offers is to buffer the common cathode wired triode with a DC coupled ZVN0545A source follower. Cap. couple the follower to the IST. Now, the trafo is being fed from a low impedance and no "standing" DC is present. You have to experiment with the value of the coupling cap., to get "flat" freq. resp., as the cap. interacts with the inductance of the trafo.
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Old 10th July 2011, 07:19 PM   #6
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you might find some here Lundahl guidelines

and maybe study Lundahl spec sheets

you say it have been driving a PP output stage
means you have two secondaries (1+1)
do you have one or two primaries
1:1 with one primary should write 2:1+1
1:1 with two primaries should write 1+1:1+1
at least thats my understanding of it
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Old 10th July 2011, 07:29 PM   #7
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The main issue you will have to deal with is interwinding capacitance which will sap the top end away. As Miles pointed out the only way to overcome this is to drive it with a super low impedance driver. This will also tend to help low end response.
Remember these things were only required to work out to about 15K so they will sound rolled off normally.
I have used a paralleled 5687 running at about 30mA in such a situation - it worked but only just. Something like an ECC99 will do the job in one bottle. Forget about using them as plate loads - go parafeed and use a blocking cap of about 4uf. There is a real possibility of a low frequency hump so test carefully.

Modern interstage transformers are very expensive because they attempt to overcome these intrinsic limitations by design. They can be as expensive as top end output transformers.

Shoog

Last edited by Shoog; 10th July 2011 at 07:32 PM.
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Old 10th July 2011, 09:25 PM   #8
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In my professional opinion, I think it would be better to spec the transformer:

1st measure the DC resistance on both primary and secondary.

record your results.

with a signal generator, inject 100 mV of signal measure this at the primary and secondary with an oscilloscope, record results.

insert ac ammeter between the transformer primary and ground. inject 100mV of signal on the other end of the primary,
record your results.

once you have done that, we can discuss further designing your circuit properly.
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Old 10th July 2011, 09:41 PM   #9
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I thought this might help.
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Old 10th July 2011, 10:29 PM   #10
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Thanks, DavesNotHere. I'll make those measurements and post them.

I found a schematic for the tuner/preamp section of the console. I'm not sure but I think the tubes used to drive the amp is a pair of 6J5's in series.
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