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Old 12th Feb 2021, 1:31 pm   #41
Levente
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Default Re: Vintage tube mixer input impedance

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I see.. but then there is no use of that section of the cap... would not be better if the resistor is connected there as the first filter/voltage drop and the going to the second, third, and 4th down the line?
The reservoir will have the most ripple voltage (hum) across it.

Lawrence.
that is why is not used? but why is connected to the 6x4 rectifier... sorry I don't understand
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Old 12th Feb 2021, 1:46 pm   #42
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Default Re: Vintage tube mixer input impedance

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that is why is not used? but why is connected to the 6x4 rectifier... sorry I don't understand
Without the reservoir capacitor the waveform at the rectifiers cathode would look like a series of half sine waves.....not good.

The energy in the reservoir capacitor is what keeps the stages working when the rectifier isn't conducting, the rectifier only conducts for a fraction of the +ve half cycle of its supply voltage.

Lawrence.
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Old 12th Feb 2021, 2:24 pm   #43
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Default Re: Vintage tube mixer input impedance

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that is why is not used? but why is connected to the 6x4 rectifier... sorry I don't understand
Without the reservoir capacitor the waveform at the rectifiers cathode would look like a series of half sine waves.....not good.

The energy in the reservoir capacitor is what keeps the stages working when the rectifier isn't conducting, the rectifier only conducts for a fraction of the +ve half cycle of its supply voltage.

Lawrence.
Thanks Lawrence, so it needs to be connected and what I have here is incorrect as that part of the cap technically is not in use at all..
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Old 12th Feb 2021, 2:29 pm   #44
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Default Re: Vintage tube mixer input impedance

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Thanks Lawrence, so it needs to be connected and what I have here is incorrect as that part of the cap technically is not in use at all..
The capacitor is in use, it's the reservoir capacitor.

Lawrence.
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Old 12th Feb 2021, 2:36 pm   #45
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Default Re: Vintage tube mixer input impedance

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Thanks Lawrence, so it needs to be connected and what I have here is incorrect as that part of the cap technically is not in use at all..
The capacitor is in use, it's the reservoir capacitor.

Lawrence.
ok... i am really confused... i donno anymore
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Old 12th Feb 2021, 2:54 pm   #46
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Default Re: Vintage tube mixer input impedance

Often (but not always) with a filter resistor connected to the cathode and the reservoir capacitor that junction would be used to feed a power output stage, not a stage that would feed into another stage of amplification, that's because of the relatively high level of ripple voltage present across the reservoir capacitor.

Lawrence.
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Old 12th Feb 2021, 3:03 pm   #47
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Default Re: Vintage tube mixer input impedance

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Often (but not always) with a filter resistor connected to the cathode and the reservoir capacitor that junction would be used to feed a power output stage, not a stage that would feed into another stage of amplification, that's because of the relatively high level of ripple voltage present across the reservoir capacitor.

Lawrence.
Thanks Lawerence..
This I understand... but that capacitor pin which is only connected to the cathode of the 6x4 and nothing else... that is where I am lost at and getting confused which pin of the cap is the reservoir cap is it that one? but that is not connected to the 4k resistor here at all.. i am really sorry i think I do your head in with this...

...the reservoir cap is only used here to store energy straight from the rectifier and nothing else... ?

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Old 12th Feb 2021, 3:26 pm   #48
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Default Re: Vintage tube mixer input impedance

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Thanks Lawerence..
This I understand... but that capacitor pin which is only connected to the cathode of the 6x4 and nothing else... that is where I am lost at and getting confused which pin of the cap is the reservoir cap is it that one? but that is not connected to the 4k resistor here at all.. i am really sorry i think I do your head in with this...
Photo in post#33:

Again it looks to me that circuit wise the 4k resistor is connected between two of the capacitors in the can, one of those capacitors being connected to a 4.7k resistor and a circuit feed and the other capacitor being connected to the cathode of the rectifier.

The reservoir capacitor is the one that's connected to the cathode, the 4k (as connected) is a filter resistor and the other capacitor it's connected to is a filter capacitor.

Lawrence.
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Old 12th Feb 2021, 3:44 pm   #49
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Default Re: Vintage tube mixer input impedance

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Thanks Lawerence..
This I understand... but that capacitor pin which is only connected to the cathode of the 6x4 and nothing else... that is where I am lost at and getting confused which pin of the cap is the reservoir cap is it that one? but that is not connected to the 4k resistor here at all.. i am really sorry i think I do your head in with this...
Photo in post#33:

Again it looks to me that circuit wise the 4k resistor is connected between two of the capacitors in the can, one of those capacitors being connected to a 4.7k resistor and a circuit feed and the other capacitor being connected to the cathode of the rectifier.

The reservoir capacitor is the one that's connected to the cathode, the 4k (as connected) is a filter resistor and the other capacitor it's connected to is a filter capacitor.

Lawrence.
I quickly drew what I see... mainly the "minus" and the "triangle" pins on the cap is what I dont get... the "-" pin of the cap is just there as it is...

the 4 k resistor is only connected to the triangle pin and the rectifier
the "-" pin of the cap is only connected to the rectifier (same where the 4k is)

the triangle pin is connected to the OT and the 4,7k resistor and then the rest i did not display as it is not important for now...
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Old 12th Feb 2021, 3:50 pm   #50
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Default Re: Vintage tube mixer input impedance

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...the reservoir cap is only used here to store energy straight from the rectifier and nothing else... ?
When the rectifier conducts it supplies the valve circuits as well as charging up the reservoir capacitor, the reservoir capacitor needs to be charged up because when the rectifier isn't conducting the charge in the reservoir capacitor is all that's available to feed the valve circuits and in doing so its charge falls to a lower level so it has to be replaced, the difference between the maximum voltage and the minimum voltage across the reservoir capacitor is called the ripple voltage and it's frequency will be 100Hz for a full wave rectifier fed from a 50Hz AC supply and it will be 50Hz for a half wave rectifier fed from the same.

Plenty of tutorials etc on the internet about this.

Lawrence.
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Old 12th Feb 2021, 4:14 pm   #51
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Default Re: Vintage tube mixer input impedance

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...the reservoir cap is only used here to store energy straight from the rectifier and nothing else... ?
When the rectifier conducts it supplies the valve circuits as well as charging up the reservoir capacitor, the reservoir capacitor needs to be charged up because when the rectifier isn't conducting the charge in the reservoir capacitor is all that's available to feed the valve circuits and in doing so its charge falls to a lower level so it has to be replaced, the difference between the maximum voltage and the minimum voltage across the reservoir capacitor is called the ripple voltage and it's frequency will be 100Hz for a full wave rectifier fed from a 50Hz AC supply and it will be 50Hz for a half wave rectifier fed from the same.

Plenty of tutorials etc on the internet about this.



Lawrence.

Thank you, understood, also looking at it on line of course. Considering the above explained, in my mixer, the reservoir cap is not connected properly, left out of the circuit right? according to the drawing i made earlier...

Levente
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Old 12th Feb 2021, 4:27 pm   #52
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Default Re: Vintage tube mixer input impedance

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Thank you, understood, also looking at it on line of course. Considering the above explained, in my mixer, the reservoir cap is not connected properly, left out of the circuit right? according to the drawing i made earlier...
The reservoir capacitor is always connected to the cathode of the rectifier unless there happens to be a surge limiter connected between the cathode of the rectifier and the reservoir capacitor.

In your layout connection sketch and photos the cathode of the rectifier is connected to a capacitor, that capacitor is the reservoir capacitor.

Lawrence.
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Old 12th Feb 2021, 4:30 pm   #53
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Default Re: Vintage tube mixer input impedance

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Thank you, understood, also looking at it on line of course. Considering the above explained, in my mixer, the reservoir cap is not connected properly, left out of the circuit right? according to the drawing i made earlier...
The reservoir capacitor is always connected to the cathode of the rectifier unless there happens to be a surge limiter connected between the cathode of the rectifier and the reservoir capacitor.

In your layout connection sketch and photos the cathode of the rectifier is connected to a capacitor, that capacitor is the reservoir capacitor.

Lawrence.
Thanks Lawrence, i leave it as it is then.. i was about to solder the resistor to the reservoir cap pin but i will leave everything as it is now... thanks again

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Old 12th Feb 2021, 4:35 pm   #54
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Default Re: Vintage tube mixer input impedance

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Thanks Lawrence, i leave it as it is then.. i was about to solder the resistor to the reservoir cap pin but i will leave everything as it is now... thanks again
You could solder it to the reservoir tag to make it look more conventional, electrically there's nothing to be gained.

Lawrence.
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Old 17th Feb 2021, 5:28 pm   #55
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Is there a resistor connected between Pin 7 and ground?

Lawrence.
Nope... there is none. Only this brown waxy little drop on the pic is at pin7 which goes to the middle tap of the volume/gain pot, and the mica cap from pin7 goes to a ground point. nothing else is connected to pin 7.
The mystery component must be resistive then, try and measure its resistance.

Lawrence.
Dear Lawrence, hope you are well!

Finally I did manage to measure the mystery component pictured earlier:

Measuring only 7.4 Ohms and just out of curiosity measured the capacitance and reads about 92uF. I wonder what this could be? A capacitor or a resistor...

Surprisingly there are only a few out of spec's parts found... that very first burning 4k reads 5.4k, the second filter can capacitor (20uf+20uf) is dead. No capacitance can be measured.

There is also one resistor, the plate resistor of 12au7, which supposed to be 50k according to the color code but measures only 36k.

Can a resistor drift downwards and not upwards? I attached a pic from that resistor in question.

Thanks a lot again for your input and great help as always I simply can not thank enough for this forum and you...
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Old 17th Feb 2021, 5:35 pm   #56
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Default Re: Vintage tube mixer input impedance

The grid should have a DC path to ground, my 1st guess would be that the mystery component is an inductor.

A 50k resistor going down to 36k would be unusual in my experience, did you measure it out of circuit?

Lawrence.
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Old 17th Feb 2021, 6:02 pm   #57
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The grid should have a DC path to ground, my 1st guess would be that the mystery component is an inductor.

A 50k resistor going down to 36k would be unusual in my experience, did you measure it out of circuit?

Lawrence.
Ok.. thank you. I will not touch that thing.

With regards the resistor, i removed it from circuit and measuring 36 kOhm on the dot. Not sure... the color coding calls for 50k unless mislabeled in the factory (?).
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Old 17th Feb 2021, 6:07 pm   #58
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Default Re: Vintage tube mixer input impedance

That's a possibility.

Usually resistors of that kind of value go high rather than low.

Lawrence.
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Old 17th Feb 2021, 6:18 pm   #59
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Default Re: Vintage tube mixer input impedance

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That's a possibility.

Usually resistors of that kind of value go high rather than low.

Lawrence.
I see... should I keep this or swap this with a new one? but which value... 36k or 50k?
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Old 17th Feb 2021, 6:25 pm   #60
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Default Re: Vintage tube mixer input impedance

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That's a possibility.

Usually resistors of that kind of value go high rather than low.

Lawrence.
I see... should I keep this or swap this with a new one? but which value... 36k or 50k?
I would fit a 47k and take it from there.

If you have the time try and reverse engineer a schematic for your mixer.

Lawrence.
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