v72 Gain Pot

[abbey road d enfer]

The additional gain was needed as Abbey Road wanted to use a 200 ohm input impedance so the input transformer was wound differently which I guess somehow lowered the ratio which makes no sense to me as you would think it would go up.

Are you positive about it?
I would think that it's the output xfmr that was modified for a 200r output impedance, since EMI had chosen 200r as their nominal interconnect impedance.

 

[CJ]

That makes more sense,

 

[andre tchmil]

It's quite easy. With 0dB attenuation, the connection is direct to the primary, and the shunt resistor does not exist.
Your concern is relevant only to a pot.

View attachment 129522

But this way the impedance seen by the microphone varies a lot for let's say 30db stepped attenuation

 

[abbey road d enfer]

But this way the impedance seen by the microphone varies a lot for let's say 30db stepped attenuation

Then you do it differently.

Then it's the transformer that sees a varying source impedance. U-pad is not constant-impedance on both sides.
Does it really matter?

 

[CJ]

They tweaked that circuit for the best gain vs noise specs so the best way to use it is to vary your mic, mic distance or amplifier volume unless you are using it for something else,

 

[mik]

You need to take a look at the V78 schematic. If I remember correctly, the V78 is the V72 with a pot for adjust the gain. I made a clone of that schematic about twenty years ago with great success. Naturally, the pot will crackle a bit at the end of the range due to the DC, but it’s not much... And the bandwidth remains unchanged. I recommend following this approach.

 

[tinn]

The V77 is similar. It has a different input transformer and variable gain.
As the min gain is even higher the W77 Pad was used for hi levels.
Might be some inspiration.

 

[firerunner]

Tinn I think that's a great option. I'm working on a homebrew V72 using the pad from the W77 and the gain control and v1 cathode bias/feedback setup from the V77, using a variable pot instead of the 40-50-60db fixed steps. Will report back when it's all running, need track down a short in the power supply first

I think a simplified version of the V76 gain control could be worth exploring, swinging the cathode bypass cap between v2's feedback and ground, if I'm reading the schematic properly. Seems like the W77 laid the ground work as the V76 gain control is a pad and gain control in one switch.

 

[telewulf]

Hi mik, tinn & firerunner,

in order to better understand the differences between V72 / V77 / V78, I drew the following pics. It´s very easy to see the differences when you flick through them.

Hello. I have a pair of v72s that were racked a long time ago, and I am going through them trying to get them to match in level better.

@ pine box: I know it is been almost one and a half years, so every further word might be obsolete, but there is one thought in my mind:
The V72 is designed to produce +34 dB very accurately and in 31 years of using them as summing amps I encountered not a mere decibel difference in levels. So if you find it nessesary to readjust levels between yours, there might be some technical fault, wouldn´t you think? If I were in your shoes I would check which one of your V72s does not produce precisely +34 dB and try to repair it instead of deviating from the original schematic. Just my humble opinion ...

best wishes from Bremen,
telewulf

 

[firerunner]

Hi mik, tinn & firerunner,

in order to better understand the differences between V72 / V77 / V78, I drew the following pics. It´s very easy to see the differences when you flick through them.



@ pine box: I know it is been almost one and a half years, so every further word might be obsolete, but there is one thought in my mind:
The V72 is designed to produce +34 dB very accurately and in 31 years of using them as summing amps I encountered not a mere decibel difference in levels. So if you find it nessesary to readjust levels between yours, there might be some technical fault, wouldn´t you think? If I were in your shoes I would check which one of your V72s does not produce precisely +34 dB and try to repair it instead of deviating from the original schematic. Just my humble opinion ...

best wishes from Bremen,
telewulf

Thanks for taking the time to draw those up telewulf. I'm curious what role R19 plays with the sound.

 

[mik]

Thanks for taking the time to draw those up telewulf. I'm curious what role R19 plays with the source

R19 works as a global negative feedback. In this case, there are two: one represented by R19, and the other by the network
R20-R18

 

[telewulf]

There must be a whole bunch of V72 experts round here and I´m sorry to admit that I can´t claim to be one of them, but nevertheless, here are my thoughts on the feedback matter:

- In order to get negative feedback, you must have phase inversion.
- Every tube offers two signal outputs:
-> The voltage at the anode is inverted against the signal fed to the grid.
-> The voltage at the cathode has the same phase as the one fed to the grid.
(hence the name "cathode follower")

So if you link the two cathodes - and R19 does that - you definitely get negative feedback.

But then I loot at the combination R18+R20||C39||C40 ... Well firstly let´s call the phase being fed to the grid of tube 1 "0°"
=> phase at the anode is 180°
=> phase at grid 2 is 180° as well
=> phase at anode 2 is 0°
This signal, taken from anode 2 at a phase of 0° is then fed back to the cathode of tube 1, which itself is at 0° as well.

To my untutored eyes ... well ... crack my if I´m wrong ... wouldn´t that be POSITIVE feedback???

 

[ruffrecords]

It is the other way round. R19 provides positive feedback. R18/20 provide negative.

I think you are confusing the action of the cathode when used as an input. A +ve input to the grid increases grid/cathode voltage. A positive input to the cathode decreases grid/cathode voltage.

Positive feedback is often used to increase the open loop gain of an amplifier.

Cheers

Ian

 

[abbey road d enfer]

There must be a whole bunch of V72 experts round here and I´m sorry to admit that I can´t claim to be one of them, but nevertheless, here are my thoughts on the feedback matter:

- In order to get negative feedback, you must have phase inversion.
- Every tube offers two signal outputs:
-> The voltage at the anode is inverted against the signal fed to the grid.
-> The voltage at the cathode has the same phase as the one fed to the grid.
(hence the name "cathode follower")

So if you link the two cathodes - and R19 does that - you definitely get negative feedback.

But then I loot at the combination R18+R20||C39||C40 ... Well firstly let´s call the phase being fed to the grid of tube 1 "0°"
=> phase at the anode is 180°
=> phase at grid 2 is 180° as well
=> phase at anode 2 is 0°
This signal, taken from anode 2 at a phase of 0° is then fed back to the cathode of tube 1, which itself is at 0° as well.

Think of it.
What happens if you inject a signal in the cathode of the first tube?
It's anode goes in the same direction and the anode of the second tube in the other direction. Injecting it in the cathode creates negative FB.
In other words, the 1st tube grid is the non inverting input of the two-stage combo, when the cathode is its inverting input.

 

[telewulf]

Think of it.

I did, honestly! ... still am!

Well, I must admit, this takes some getting used to. But since I always found your (ruffrecords as well as abbey road d enfer) remarks to be very knowledgable and helpful, it will certainly be worth while to try harder to get my thoughts around it!
It still goes to show, with a tube it´s not as stragiht forward as with a 5534 ...

best wishes from Bremen,
Wulf

 

[telewulf]

Raaaa! Got it now! - Sorry, took me some time, but hey - my brain is more than half a century old
I always regarded the cathode just as an output, but when you inject a signal into it, it is like injecting the reverse into the grid - THANK YOU!

 

[ZHOUJUN]

关于V72的增益控制,在输入和输出中添加电位器限制会引起阻抗匹配问题，改变R20可以控制几个DBS的增益，也许可以添加一个可调输入变压器来调整输入电平而不改变输入阻抗。