Floating paraphase stage

[ReadyTeddy]

We use the paraphase PI in most of our amps, and it works very well (in my opinion).

The issue I had, that confused my for some time, was unpleasant distortion on the decay of the signal when the amp was overdriven.

After some investigation, this was due to the inverted signal drive line going into grid conduction before the non-inverted (or should that be re-inverted). Once the phase inverter was rebalanced to avoid this the problem disappeared.

I think the paraphase (and indeed floating paraphase) splitter went out of favour due to hi-fi driving the state-of-the-art and the paraphase inverters lost out to the concertina (in terms of balance), and the LTP (in terms of distortion cancellation).

My guess is that up to clipping in the PI and/or grid conduction by the power valves there will be little audible difference between the various PIs, however their behaviour under clipping or at grid conduction will vary.

Concerns about audible effects of phase shifts at HF in the PI in a valve guitar amp are in my view pointless.

OTOH, phase shift between the 2 outputs of the PI can cause both output tubes to conduct at the same time, which doesn't seem like a desirable characteristic. Also, given all the topologies that avoid phase shift between the 2 PI outputs, why use a topology that has this issue?

[jpfamps]

OTOH, phase shift between the 2 outputs of the PI can cause both output tubes to conduct at the same time, which doesn't seem like a desirable characteristic. Also, given all the topologies that avoid phase shift between the 2 PI outputs, why use a topology that has this issue?

Agreed.

However, given the limited bandwidth of a guitar amp, and the evidence also presented above, the phase shift at HF doesn't seem (to me) to be a practical concern.

[ReadyTeddy]

Agreed.

However, given the limited bandwidth of a guitar amp, and the evidence also presented above, the phase shift at HF doesn't seem (to me) to be a practical concern.

True enough, but the topology in question relies on the transconductance of the PI's 2nd stage to be some particular value or the match (or the desired mismatch) between the 2 PI outputs will change. All in all, I would say that the topology is poor-especially given that the alternatives, give or take a resistor or two, cost no more money in parts.

[jpfamps]

True enough, but the topology in question relies on the transconductance of the PI's 2nd stage to be some particular value or the match (or the desired mismatch) between the 2 PI outputs will change. All in all, I would say that the topology is poor-especially given that the alternatives, give or take a resistor or two, cost no more money in parts.

Absolutely. That's why this inverter quickly fell out of favour (note to self: this Forum has a UK English spell checker!).

However, most version of the paraphase employ some form of negative feedback on the second stage, which will mitigate to some extent against valve ageing.

[printer2]

True enough, but the topology in question relies on the transconductance of the PI's 2nd stage to be some particular value or the match (or the desired mismatch) between the 2 PI outputs will change. All in all, I would say that the topology is poor-especially given that the alternatives, give or take a resistor or two, cost no more money in parts.

I found I liked the sound of the paraphase better than when the amp was switched to cathodyne. Thickened up the sound of a more or less clean signal. Kind of fell apart when cranked though. Some pretty good sounding amps have used the paraphase, just have to use it in its sweet spot.

[ReadyTeddy]

I found I liked the sound of the paraphase better than when the amp was switched to cathodyne. Thickened up the sound of a more or less clean signal. Kind of fell apart when cranked though. Some pretty good sounding amps have used the paraphase, just have to use it in its sweet spot.

My comments referred to the PI configuration in which the 2nd tube is just a separate amplifier whose gain is adjusted to be one by means of a voltage divider at the grid connection. This is not a paraphase. The paraphase used negative feedback on the second tube to keep its gain at the correct value despite differences in tubes' transconductance.

[pdf64]

Unless you can cite a good reason, you may be entering the fray too late to reasonably redefine the terminology that has been used from the start.
I suggest that you use the terminology as defined in How to design valve guitar amplifiers
Pete

[kg]

what exactly is the question/discussion here?

[printer2]

My comments referred to the PI configuration in which the 2nd tube is just a separate amplifier whose gain is adjusted to be one by means of a voltage divider at the grid connection. This is not a paraphase. The paraphase used negative feedback on the second tube to keep its gain at the correct value despite differences in tubes' transconductance.

RADIOTRON DESIGNER'S HANDBOOK

Phase inverter (Fig. 12.30A)
This is a popular arrangement with twin triode valves, either general purpose or
high-mu. It is not self-balancing, and requires individual adjustment for accurate
balance both during manufacture and after the valve has been replaced. It is slightly
out of balance at very low frequencies owing to the two coupling condensers operating
in the lower channel, but C2 may be made larger than C1 if desired. It gives a gain
(to each channel) equal to the normal gain of one valve.
If it is preferred to avoid individual balancing, the value of R2 is given by
R2 = (RI + R/ A where A is the voltage gain of valve V2• If RI and R2 both have
± 10% tolerances, the maximum possible out of balance will be nearly 20% due
to the resistprs alone, plusvaIve voltage gain tolerances.
Separate cathode resistors, each by-passed, are helpful in reducing valve gain
tolerances, but require independent cathodes. If a common cathode resistor is used,
it may be unbypassed, thus introducing negative feedback for out-of-balance voltages.
The hum level is quite low.
N.B. This circuit was originally named Paraphase, but the latter name covers a
large number of different circuit arrangements and cannot therefore be used to distinguish
one from another.

Self-balancing phase inverter (Fig. 12.30B)
In this circuit VI and Vi are pentodes, and any unbalanced voltage appears across
the common plate resistance R 3 and is fed to both suppressors through the blocking
condenser C 3, thus causing degeneration in the valve producing the larger signal
output, and regeneration in the other. Ref. C19.
Cv) Self-balancing paraphase inverter
(A) Floating par~phase (Fig. 12.31)
* This circuit is, to a considerable extent, self-balancing thereby avoiding any necessity
for individual adjustment except in cases where a very high accuracy in balancing
is required.

Seems many call the circuit a paraphase, That is what I know it by.

[potatofarmer]

what exactly is the question/discussion here?

Are you getting vaklempt? I'll give you a topic: paraphase inverters, neither paraphase, nor inverters. Talk amongst yourselves.

[ReadyTeddy]

Unless you can cite a good reason, you may be entering the fray too late to reasonably redefine the terminology that has been used from the start.
I suggest that you use the terminology as defined in How to design valve guitar amplifiers
Pete

My mistake--I thought the therm paraphase referred to what is actually called the floating or self balancing paraphase. Whatever the name, the original paraphase is a poor design, not worthy of emulation, IMO.

[ReadyTeddy]

Unless you can cite a good reason, you may be entering the fray too late to reasonably redefine the terminology that has been used from the start.
I suggest that you use the terminology as defined in How to design valve guitar amplifiers
Pete

oops--double post. The website did something strange when I clicked on submit.