Phase reversal of ONE of the two inputs

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junior marbles
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Phase reversal of ONE of the two inputs

Post by junior marbles »

On my 1974x I noticed today that the polarity/phase is not consistent but reversed between Normal and Tremolo inputs, as measured with a phase checker.

If I strictly stick to one or the other of the inputs, I can make sure that polarity is aligned and not phase reversed, from guitar to speaker (either at the speaker connection or OT primary), but how can I have the same polarity on BOTH channels?
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Re: Phase reversal of ONE of the two inputs

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junior marbles wrote:
Sat 12/04/21 3:48 am
On my 1974x I noticed today that the polarity/phase is not consistent but reversed between Normal and Tremolo inputs, as measured with a phase checker.

If I strictly stick to one or the other of the inputs, I can make sure that polarity is aligned and not phase reversed, from guitar to speaker (either at the speaker connection or OT primary), but how can I have the same polarity on BOTH channels?
If it's a real 1974x I wouldn't do this. For the value loss.
But..
You can join the output from both channels with mixer resistors at the 1st half of the PI.
But you're a triode short for 1 channel and they're still out of phase. Maybe you could connect the tone stack of 1 channel at the cathode instead of the plate..
If you look up the schematic of a 1987 plexi you'll see what I mean with the mixer resistors.
I'm building a 5F4 at the moment and I converted it to a 1 one input amp, with 2 volume knobs so the channels are mixed all the time.

But ask yourself.. what are you trying to achieve?better you leave that amp along and build something else
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junior marbles
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Re: Phase reversal of ONE of the two inputs

Post by junior marbles »

What I hear you say is: there is no easy way to fix this?

How come phase is reversed on one of two ECC83 channel anyway?
I cannot see what in the circuits would account for that.
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Re: Phase reversal of ONE of the two inputs

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junior marbles wrote:
Sat 12/04/21 12:02 pm
What I hear you say is: there is no easy way to fix this?

How come phase is reversed on one of two ECC83 channel anyway?
I cannot see what in the circuits would account for that.
No, not easy indeed.
Watch this Uncle Doug video.. he explains that phase stuff very clear.
https://youtu.be/U6By31V9fDo
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junior marbles
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Re: Phase reversal of ONE of the two inputs

Post by junior marbles »

I get the phase inverter functions for the power tubes.
Maybe I was not clear: when I plug into Normal (V1) I get a different polarity at the speaker compared to plugging into Tremolo (V3). So one of two preamp sections sends a polarity-inverted signal to the phase inverters, the other does not. Where is that reversal taking place? Circuits for both preamp sections look identical in regards to phase. What am I missing?
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Re: Phase reversal of ONE of the two inputs

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junior marbles wrote:
Sat 12/04/21 11:55 pm
I get the phase inverter functions for the power tubes.
Maybe I was not clear: when I plug into Normal (V1) I get a different polarity at the speaker compared to plugging into Tremolo (V3). So one of two preamp sections sends a polarity-inverted signal to the phase inverters, the other does not. Where is that reversal taking place? Circuits for both preamp sections look identical in regards to phase. What am I missing?
Did you watch the video? I was aware you didn't mean the phase inverter.
Look: when you enter a signal to a grid of a triode, you can take the amplified signal from the plate or the cathode. When you tap from plate the phase gets reversed, when you tap from cathode the phase remains the same.
This happens every time the signal enters a grid.
Mostly we take the amplified signal from the plate and couple it to the next stage. A cathode follower is the opposite of that.
Now look at your schematic and see how many times the signal/phase gets reversed in each channel. If that's an equal number you can parallel the channels.
Given the tremolo oscillation triode doesn't amplify this does NOT count as a phase reverser.
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Re: Phase reversal of ONE of the two inputs

Post by junior marbles »

My issue is not with the phase inverter V2 but with the two preamp channels V1 and V3. Signal flow of "Normal" input through V1 is self evident. On "Tremolo" input, one half of V3 serves as tremolo oscillator, the other half is a standard preamp circuit (and has my favorite sound).

For reasons that are not clear to me, plugging a signal into the Tremolo input connected to preamp tube V3 ends up with a polarity reversal at the speaker, compared to when plugging into the Normal input which is connected to V1. Both, V1 and V3 seem to have the same grind/plate/cathode layout, signal flow and traditional phase reversal between grid and plate.

Where does the polarity reversal of the signal between input and output take place in one channel and not the other?
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Re: Phase reversal of ONE of the two inputs

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junior marbles wrote:
Sun 12/05/21 3:12 pm
My issue is not with the phase inverter V2 but with the two preamp channels V1 and V3. Signal flow of "Normal" input through V1 is self evident. On "Tremolo" input, one half of V3 serves as tremolo oscillator, the other half is a standard preamp circuit (and has my favorite sound).

For reasons that are not clear to me, plugging a signal into the Tremolo input connected to preamp tube V3 ends up with a polarity reversal at the speaker, compared to when plugging into the Normal input which is connected to V1. Both, V1 and V3 seem to have the same grind/plate/cathode layout, signal flow and traditional phase reversal between grid and plate.

Where does the polarity reversal of the signal between input and output take place in one channel and not the other?
Each channel enters an opposite side of the PI. And the PI is coupled on the output tubes the same way / direction. So that translates to each plate, coupled at the output transformer. I think that's what you measure at the secondary...
IF you'd want to bridge the channels you'd have to join both with mixer resistors right before the 1st grid of the PI. The second grid should be coupled to ground by a 0.1uf cap at the conjunction of the 2nd grid and the 470k resistor.
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