unity gain?

[shooter]

Is this configured as unity gain?  Seems like there would be equal signal at the cathode or anode.  It’s the 2nd half of a 6sn7, the 1st  half is normal V1 kinda configuration.  A second question (SE AMP), does the amount of times you flip phases matter?

Thanks

Dave


 

[sluckey]

Is this configured as unity gain?  Seems like there would be equal signal at the cathode or anode.

Yes, it's unity gain. Signal is equal on the cathode and anode. Actually, the gain is more like .8 to .9, but it's easier for many people to just think of this as unity.

A second question (SE AMP), does the amount of times you flip phases matter?

no

[HotBluePlates]

... (SE AMP), does the amount of times you flip phases matter? ...

Only if you're making a feedback loop.

[jjasilli]

Also if your amp has reverb or more than one channel, which get used at the same time.  When the separate signal paths get mixed they will be either in or out of phase, 180°.

Just for illustrative kicks & giggles, applicable to the hi-fi arena, caps put signal a little out of phase.  Hence the desire for the simplest circuits, and maybe interstage trannies instead of caps in the signal path.

[HotBluePlates]

Is this configured as unity gain?  Seems like there would be equal signal at the cathode or anode.

If you're interested, here's the logic behind Sluckey's confirmation of (slightly less than) unity gain:

1. Any signal input to the tube results in a plate current change. Current though a resistance causes a voltage drop across the resistance which is proportional to both the amount of current and resistance. Both the plate and cathode circuits have equal resistances, so any signal current will cause an equal voltage output appearing at the plate and cathode.

2. The tube doesn't know absolute voltage values with respect to input signal, it feels only a difference of voltage between its cathode and grid. The cathode output voltage due to signal current is in-phase with the grid input voltage, so as grid voltage increases cathode voltage also increases leaving little net voltage change between grid and cathode. So if there is a signal current causing a big voltage change at the cathode, there must be an even bigger grid voltage change because the cathode voltage offsets the net grid-to-cathode voltage.

3. All this means for a (for example) 40v output at the plate there is also a 40v output at the cathode due to equal resistances, and there must be a more-than-40v signal at the grid to not be overwhelmed by the cathode voltage. Therefore, there is a gain slightly less than unity to each output.

Extra side-note: 4. Your circuit is fixed-biased. If it was cathode biased instead, the input impedance due to the bootstrapping effect would be very much more than the value of the resistor between grid and cathode load. That's because the "ground side" of the grid resistor has a voltage change with output signal. That allows the use of a very much smaller coupling cap and a maximizing of the gain of the stage before this split-load inverter.

[PRR]

> If it was cathode biased instead, the input impedance due to the bootstrapping effect would be very much more

He could instead change the 2.2Meg:470K to 22Meg:4.7Meg and get a large input impedance.

This appears to violate the "1Meg max" on some tubes. In fact it is quite legal. The spec-sheet assumes normal operation. Then a 1V change of G1 voltage due to G1 current in 1meg would throw the tube full on or off. But here the grid is nominally 50V, and even a 10V change due to G1 current won't matter much.

[shooter]

I think i'm gaining knowledge!, the proplem is retaining it!!  The amp i'm working on does use feedback taken from the spkr side of the OT.  I have three phase shifts till the OT. so this would give me the negitive feedback (180 out from input assuming i connect to the proper point on the ot)??   thanks all for the insite.

dave

[jjasilli]

The NFB from the OT, called global NFB, usually feeds into the PI circuit.  So, NFB "polarity" should not be a concern for the signal channels  in your preamp.

[sluckey]

The total number of phase reversals from input to output does not matter. You're only concerned with the phase reversals between the sample point (usually OT secondary) and the injection point. This is referred to as the loop. And the actual number of reversals inside the loop are not important. An even number of reversals is equivalent to no phase reversal and an odd number of reversals is equivalent to one phase reversal.

Figuring out the proper phase for a loop is easy for the tubes. But you will rarely know if you have a phase reversal inside the OT. Most manufacturers do not offer phase data with their transformers. So, without knowing the phase relationship between primary and secondary of the OT, picking the right wire for NFB becomes a 50/50 coin toss. It's best to plan to be wrong. Connect the full length primary wires as you expect to be correct. If you get it wrong, reverse them. Only when you are sure of correct phasing should you trim the leads for neatness.