Paraphase inverter question

[catalin gramada]

Regarding bias filtering, please consider that inserting a series resistor before the filter cap in the k Ohm range might be much more effective than doubling capacitance.

Roger that, Thank You

[vintagekiki]

Since you are expecting a hi-fi amplifier performance from a guitar amplifier, I'm afraid you will have to do a lot of work on grounding and shielding and revision the parts, as oldtimer built-in parts are a reflection and spirit of his time.

EDIT 190908
When the amplifier is brought with the measuring instruments "under the thread", the question arises ... what's left of the sound ...

Tip
Try for comparison purposes to perform identical measurements on another similar guitar amplifier.

[catalin gramada]

OT output with grids disconected from PI (4.7mV ripple)




Grids not coupled to PI (there are identical superimposed )





Sorry I have to learn what differential mode scope meant. Please to explain for a truck driver way of understanding. Thanks

[catalin gramada]

Do You think the ripple is injected into the PI through feedback loop, than returned to power grids with different phase shifts the difference will be amplified instead to be cancelled, please ?

[Helmholtz]

Wow, so disconnecting the PI significantly increases ripple voltage at the grids without NFB??

What about differential measurements? Point is that the phase shift causes the differential voltage between the grids to be larger than the difference between peak values.

[Helmholtz]

Do You think the ripple is injected into the PI through feedback loop, than returned to power grids with different phase shifts the difference will be amplified instead to be cancelled, please ?

No, there is no such indication as ripple is stronger without NFB.

[vintagekiki]

A seemingly stupid question.
Is there ripple when all tubes from the amplifier are removed and the amplifier is on?

[Dave H]

I couldn't understand why it didn't work as I described (badly) in post #55 (outputs out of phase) so I decided to simulate it by injecting a 50Hz sine wave where the bias supply connects. The first plot below is as the schematic Catalin posted. It shows a small phase shift. I then changed the capacitors to 4u7 to push the effects of their phase shift well below 50Hz and produced the second plot which now has the outputs 180* out of phase.

[catalin gramada]

A seemingly stupid question.
Is there ripple when all tubes from the amplifier are removed and the amplifier is on?

Are You asking about OT output with tubes removed ? Yes it is . This is what I get:

[Helmholtz]

Well, that is direct PT to OT leakage field coupling.

[catalin gramada]

Well, that is direct PT to OT leakage field coupling.

Of course it is, but do You think is relevant for my issue, please ? cause I don't.

[Helmholtz]

but do You think is relevant for my issue

No.

[catalin gramada]

https://m.youtube.com/watch?v=Fx_DuHrTt6g

[Dave H]

Sorry I have to learn what differential mode scope meant. Please to explain for a truck driver way of understanding. Thanks

A scope is usually used to measure a signal wrt ground. Differential mode is where both channels are used to measure the difference signal between two points not wrt ground. Set the scope channels to A+B mode with one channel inverted and connect the A and B probes across the points to be measured (the earth clips are not used). Make sure channels A and B are set to the same voltage range including the fine pot. If you want to be really fussy connect both probes to the same signal and adjust the fine pots for the best common mode rejection.

[Dave H]

Wow, so disconnecting the PI significantly increases ripple voltage at the grids without NFB??

The PI outputs are low impedance compared to the 220k resistors from grids to bias supply so there's a potential divider which reduces the ripple at the grids when the PI is connected.