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Old 21st June 2009, 04:25 PM   #1
jetbat is offline jetbat  United States
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Default Interstage transformer questions

Hi. I have some questions about interstage transformer and tube interaction. I have been searching around but have not found the information I need.

It seems the interstage transformer replaces the plate resistor in the tube circuit.

How does this affect the gain of the tube and how do you figure this out?

Do different interstage transformers cause different gains?

How would changes in the voltage to the preamp tubes affect the gain when using the interstage compared to a plate resistor?

Would the cathode resistor/capacitor values need to be changed when switching from plate resistor to interstage or for different interstage trans?

Thanks for any help
Scott B.
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Old 21st June 2009, 04:37 PM   #2
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Default Re: Interstage transformer questions

Quote:
Originally posted by jetbat
Hi. I have some questions about interstage transformer and tube interaction. I have been searching around but have not found the information I need.
It seems the interstage transformer replaces the plate resistor in the tube circuit.

Yes

How does this affect the gain of the tube and how do you figure this out?

A rough guesstimate would be the u-Factor of the VT times the PRI : SEC turns ratio.

Do different interstage transformers cause different gains?

They will if the PRI : SEC turns ratio differs.

How would changes in the voltage to the preamp tubes affect the gain when using the interstage compared to a plate resistor?

The same way for resistive loading: higher plate currents mean higher g(m)'s and higher gain.

Would the cathode resistor/capacitor values need to be changed when switching from plate resistor to interstage or for different interstage trans?

You're better off using fixed bias and grounding the cathode directly. An RC network means capacitive currents in the cathode, which means capacitive currents at the plate. This can cause resonance effects in the IT, and some really nasty distortion. You can avoid the resonance distortion by not bypassing the cathode bias resistor, but that makes for reduced gain, and an increased effective r(p) which will cost you low frequency performance.

I had that very problem with the first amp I designed back in high school. An IT stage just would not work right with cathode bias and a bypassed cathode resistor. Removing that capacitor fixed the distortion problem, but the low frequency performance went all to hell. I redesigned the first stage to include a low frequency compensation network to get back the bass. It really was a brute force solution, but WTH, I was still in high school.
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Old 21st June 2009, 05:52 PM   #3
jetbat is offline jetbat  United States
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Thanks for the response, but what I am tying to do is understand what is going on between the interstage trans and the tube. I know I can raise the gain of the tube by raising the value of the plate resistor, but with the interstage, I get lost.

I found this bit of info;

"Either resistance or inductance can be the load of vacuum tube circuit since they both have reactance to AC signal. The different is that reactance (from resistor) has DC resistance up to 10 times or more of the same reactance from Inductor."

So raising the inductance of the transformer will raise the gain of the tube? And with the interstage the resistance no longer controls the gain?

And I found this;
" Inductor offers highest gain obtainable from that gain stage while the total gain of gain stage with resistor will depend on resistance. The higher the resistance, the higher the gain with higher power loss and higher output impedance."

How does that work? Does that mean since the plate sees a higher DC voltage, the tube has higher gain?

Also would changing the cathode resistor value affect the gain of a tube with a interstage trans?

I'm just trying to figure out what is happening in the circuit.
Thanks
Scott B.
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Old 21st June 2009, 07:54 PM   #4
ChrisA is offline ChrisA  United States
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Think of "impedance" not voltage. If you think this way then a resistor and transformers are the same thing. It is easy to know the impedance of a 100K plate resistor it is 100K Ohms. But a transformer's secondary impedance depends in the impedance of the circuit connected to the transformer's primary. If the turns ratio is 1:1 the transformer secondary is the same is whatever is connected across the primary. But if not 1:1 the impedance ratio is equal to the SQUARE of the turns ratio. Voltage is "transformed" by the turn ratio but impedance is transformed by the square of the turns ratio.

If you don't know the turns ratio of a given transformers you will have to measure it. Find a low voltage A/C source (12V A/C power cube, aka "wall wort") and measure the voltage ratio, then square that to get the impedance ratio.

EDIT.... Wait a minute. Does this circuit you are looking at supply PLATE current with transformer? That's not likely. You are going to have to post the schematic. But still the above is true
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Old 21st June 2009, 10:15 PM   #5
jetbat is offline jetbat  United States
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If I understand correctly, yes the plate current passes through the interstage. Attached a interstage schematic.
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File Type: png interstage.png (12.3 KB, 705 views)
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Old 22nd June 2009, 12:49 AM   #6
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"Either resistance or inductance can be the load of vacuum tube circuit since they both have reactance to AC signal. The different is that reactance (from resistor) has DC resistance up to 10 times or more of the same reactance from Inductor."

Yes, that is quite true, and one of the reasons to use xfmr coupling: the DC resistance of the primary can be ignored. Since you have inductive reactance loading the plate, you can have the gain and headroom without the need to resort to excessive DC voltages since you're not dropping DC across the primary.

So raising the inductance of the transformer will raise the gain of the tube? And with the interstage the resistance no longer controls the gain?

Yes, you got it.

And I found this;
" Inductor offers highest gain obtainable from that gain stage while the total gain of gain stage with resistor will depend on resistance. The higher the resistance, the higher the gain with higher power loss and higher output impedance."

How does that work? Does that mean since the plate sees a higher DC voltage, the tube has higher gain?


Gain depends on two things: g(m) and the plate load. Within limits, the gain will increase with a larger plate resistor, the limit in the extreme being an active (constant current source) plate load that will give nearly u-Factor in gain. The g(m) depends on the plate current, rising with a rising plate current, which also serves to drive down the r(p) (therefore making the u-Factor nearly constant). What Vpk does for you is give headroom that allows a greater output voltage swing before you hit either the saturation point or plate current cutoff -- either of which will cause distortion.

Also would changing the cathode resistor value affect the gain of a tube with a interstage trans?

Yes, it will since cathode degeneration decreases g(m) and with it, the gain.
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Old 22nd June 2009, 05:29 PM   #7
jetbat is offline jetbat  United States
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Thanks, now I am starting to understand.

Since inductance controls the gain and you donít have the negative effects of resistance, you probably won't have too much inductance. But what would be the lower limit before gain starts to be reduced? I have charts showing the gain of a stage with different value plate, grid leak, and cathode resistors. But I have not seen any for inductance. Do you know a way to determine what value of inductance would give you a specific amount gain?

Also how would you figure out the impedance on the secondary windings? Would it be just the tubes internal plate resistance or would the inductance be figured into the equation?

Thanks
Scott
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Old 22nd June 2009, 06:00 PM   #8
ChrisA is offline ChrisA  United States
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Quote:
Also how would you figure out the impedance on the secondary windings? Would it be just the tubes internal plate resistance or would the inductance be figured into the equation?

Thanks
Scott [/B]
See above. The secondary winding impedance is the impedance on the primary times the square of the turns ratio. This applies to all transformers.
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Old 22nd June 2009, 09:57 PM   #9
jetbat is offline jetbat  United States
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So since there is no plate resistor, only the tubes internal resistance would be factored into the equation along with the turns ratio? The inductance only affects the gain and will not affect the output impedance. Is that correct?

Thanks
Scott

attached is another schematic of the circuit.
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File Type: jpg interstage2.jpg (40.7 KB, 618 views)
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Old 22nd June 2009, 11:08 PM   #10
ChrisA is offline ChrisA  United States
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Quote:
....So since there is no plate resistor, only the tubes internal resistance would be factored into the equation along with the turns ratio?
"only the tubes internal resistance" plus a 2.2K cathode resistor.

(The power supply, tube and resistor are in series across the primary.)
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