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A number of the old Valco amps used what I believe is a floating paraphase. I know it's definitely not a balanced paraphrase w. the shared R to ground. Anyway, I see that some of them utilize different values of coupling cap to the tubes. For example, the cap off the first/input side may be as low as .01uF and then the cap off the second side might be .02 uF. I suspect this was to try to minimize distortion as the signal was fed to the second triode, possibly to cut blocking distortion. What I'm wondering, though, is whether or not this is affecting the operation in some way of the power section, as the coupling caps feeding each power tube (in a 2-tube power amp) are different. Additionally, if the cathode resistor for the paraphrase tube is a shared resistor for both sides, do the two couplers need to be of equal value likewise or can this differing cap value layout be utilized anyway (I have only seen it in inverters wherein the the cathodes are split and of different R values). ???
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The "first" coupling cap defines the bass roll off, as all the signal for both phases goes through it. Of course, the two phases will have slightly different time constants and I suppose the inverted phase will have a two knee roll off characteristic... the higher frequency knee determined by the "first" coupling cap, then the second knee determined by the larger cap going to the "bottom" output tube. So the slope of the roll off will be 6dB/ octave for the the frequencies below the first knee and 12dB/octave below the second knee.
Practically, I doubt it will make much of a difference as the knee frequencies are probably set pretty low compared to a guitar's lowest output freq. I suppose the amp would behave a bit like an SE amp at frequencies below the second knee.
Did that make any sense?
Nathan -
Kind of! I'm slowly digesting it. I understand the concept of the rolloff and the knees, don;t quite get the reference to SE operation....Comment
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The reference to SE was simply a way of stating that the balance of the PI will vary below the second "knee." As the drive in a PP amp becomes more and more imbalanced, it approaches SE operation. (i.e. if only one tube is driven and the other is just sitting there at the idle bias condition.) Hope that clears things up.
You could measure the outputs of the PI with a meter or 'scope, run a sine wave generator in and observe the balance (or lack thereof) as you reduce the input frequency.
NathanComment
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Most of the old Valco stuff that used a paraphase inverter was using the non-balanced version rather than the floating version, which is balanced. For an example of a floating paraphase inverter, take a look at the Vox AC100 schematic. You can also get some great info about both on Merlin's Valvewizard site if you haven't been there already. The nature of the non-balanced inverter caused some of the grunginess of the Valvo sound. Some people will stick a pot in that circuit to adjust one of the triodes in balance with the other and make the circuit behave in a more balanced manner, but the amount of balance will change from tube to tube.
GregComment
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Thanks - I think I see it more clearly now. I know it's unbalanced, I think I got the floating paraphase and the 'unbalanced' type mixed up. I'm pretty sure I've seen the unbalanced type called something specific previously, I just can't remember where or what it was called.Comment
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Another name for the floating paraphase is the see-saw.
GregComment
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