What's happening in this plate circuit?

[Groover]

On the Musicmaster Bass amp the 2nd preamp stage feeds the primary of the interstage transformer/phase splitter. There is a 15K resistor in parallel with the transformer primary where the 100k plate resistor would be in a typical Fender triode circuit.

I am wondering:
a) about how the DC and AC components of the signal divide between the resistor and the primary - (I don't really understand how the primary coil reacts to DC and AC. As I recall an inductor passes DC easier than AC)
b) why this R is so much lower compared to typical transformerless stage.

[imaradiostar]

I'll hazard a guess- I bet it's to make the resistor the major load for the triode and swamp any of the shortcomings of the transformer. The load of the interstage TF is probably very reactive. By loading the triode with a resistor the transformer just goes along for the ride.

I'll wait for Steve or RG or someone else who knows better to weigh in with a real answer.

jamie

[Steve Conner]

As far as I understand it, the 15k resistor is there to damp resonances in the interstage transformer, flattening its frequency response. A 12AX7 by itself has a plate impedance of about 50k, so the resistor needs to be smaller than that to achieve any increased damping.

Most of the AC current flows in the resistor because it's performing a kind of swamping function, but I expect most of the DC would flow in the transformer primary. It depends on how the DCR of the winding compares to the resistance of the damping resistor.

If the transformer was better quality, it might not need a damping resistor at all, but Fender probably found that they could get by with a cheaper transformer, slug it, and still have enough drive for the 6V6 grids in spite of the signal wasted by the resistor.

If you have a MM Bass and want to experiment, you can take the resistor out, replace it with a series RC, a pot, whatever, and see how it affects the tone. It might sound bad, but it shouldn't damage anything.

Which I guess is something like what Jamie said...

[Bruce / Mission Amps]

Yeah, I think Jamie and Steve have it... it's probably some kind of a swamping resistor used to lower the "Q" of the OT and circuit.

Q:
From Wikipedia

In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is,[1] or equivalently, characterizes a resonator's bandwidth relative to its center frequency.[2] Higher Q indicates a lower rate of energy loss relative to the stored energy of the oscillator; the oscillations die out more slowly. A pendulum suspended from a high-quality bearing, oscillating in air, has a high Q, while a pendulum immersed in oil has a low one. Oscillators with high quality factors have low damping so that they ring longer.

[imaradiostar]

I just had a brilliant thought- I have a MMB interstage transformer sitting at home and I work at a lab where I can easily put it on an HP signal analyzer and test to see if there is any particular non-linearity in the audio range.

I can then tack a 15k resistor across it and feed it from a 40k resistor (ax7 plate load) and see how different it is.

Someone remind me Tuesday night so I remember to bring in to work with me Wednesday (the next day I work). We'll find out if this is true or not!

jamie

[Enzo]

Well, if you do such tests, remember to put some DC through the primary to mimic this circuit.

[imaradiostar]

I think the equipment in question is only capable of 10 VDC offset- might have to come up with a scheme for this test.

jamie

[imaradiostar]

nevermind, I'm dumb- 10 volts should be plenty.

jt

[octal]

I built a similar circuit using a 12AU7 driving a transformer PI. The damping resistor really cleaned up the squarewave response (less ringing, flatter.) I liked the sound better that way- the cleans were definitely cleaner- my guitarist buddy did not.


Nathan

[Groover]

So I am reading MerlinB's book now and getting a handle on triode gain stages and so...

How do I calculate the load line etc. for the triode feeding the primary of this Interstage transformer? Should I treat it like an SE output stage and ignore the 15K resistor? (There is probably a formula for calculating the impedance of the IT primary with the 1k5r in parallel, probably involving units of inductance for the IT primary, the value of which I don't know. )

[imaradiostar]

I never mentioned- I spent a little time checking out the musicmaster interstage TF on an analyzer at work. The frequency response was pretty dismal!

I would guess your calculations will be based on the triode plate r, parallel resistor r, and the reflected primary Z all in parallel.

This relationship may fall apart at the lower frequencies if the transformer isn't sufficiently large or doesn't have enough inductance to pass the low frequencies.

jamie

[Groover]

How do I determine (or even estimate) the primary's reflected Z ?

By the way, with respect to DC at quiescence there is only about 2 volts dropping across the primary||1k5.

[Wilder Amplification]

How do I determine (or even estimate) the primary's reflected Z ?

By the way, with respect to DC at quiescence there is only about 2 volts dropping across the primary||1k5.

You run a signal generator into the primary and measure the voltage out on the secondary.

Then divide the primary voltage by the secondary voltage...this will give you the voltage/turns ratio.

Then you multiply that by itself to get the impedance ratio.

Then multiply the impedance ratio by the ohms tap rating. If you measured secondary voltage at the 16 ohm tap for instance, you'd multiply the impedance ratio by 16.

Let's say for instance on a push-pull OT you run 120VAC across the primary plate wires, and on the 16 ohm tap you measure 10 volts -

120VAC / 10VAC = turns/voltage ratio = 12

12 x 12 = Z ratio = 144

144 x 16 ohms = 2.3K reflected Zplate-plate

If you wanna know the reflected impedance on a push-pull OT from plate - center tap, this would be roughly 1/4 the Zplate-plate. In Class A mode it actually appears as 1/2 the Zplate-plate, but once the other tube goes into cutoff, it transitions to Class B mode and becomes 1/4 Zplate-plate.

[Groover]

1) Is a test tone from a cable tester at 1kHz and/or 440Hz at +4dBu enough signal to test the transformer? I can measure AC volts at the primary and then at the secondary.

2) Since this is a interstage transformer I don't know what the secondary impedance is. The center tapped secondary is tied directly to the output tube grids and the CT goes to ground.

[Wilder Amplification]

1) Is a test tone from a cable tester at 1kHz and/or 440Hz at +4dBu enough signal to test the transformer? I can measure AC volts at the primary and then at the secondary.

2) Since this is a interstage transformer I don't know what the secondary impedance is. The center tapped secondary is tied directly to the output tube grids and the CT goes to ground.

Ah yes...the "po' man's" phase inverter.

In the case of interstage x-formers it doesn't matter. You're driving a super high impedance load with it so hardly any primary current will flow, which is the reason for having the plate resistor in parallel with the primary (to get plate current to flow so that a voltage drop can be generated across a load, which then gets fed into the interstage x-former primary). You're more concerned with the VOLTAGE ratio itself, not the impedance ratio. That's just done by simply injecting a voltage into the primary without the resistor on it, then measuring the primary and secondary voltages, then dividing the primary voltage by the secondary voltage.

More than likely it's a 1:1 interstage iso-transformer though so Vpri = Vsec in that case.