v72 Gain Pot

[pinebox]

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. The previous tech has modified them by removing the 1.25k resistor in position 12 and soldering in a 1k pot. I am trying to understand the benefit of doing this instead of using a pot as an input pad before the signal gets to the pre. Since the v72 was designed as a fixed gain and this position has high DC voltage I think I will return this unit to stock and put a trimmer on the input, unless I am advised against that?

 

[My3gger]

Do you have it in rack with correct power transformer, phantom, etc?V72 is not that easy to modify for reasons well discussed here, they require experiences to get it right without damaging something.
This rack could be the way, depending on how is gain control done and if they can replace phase switch with fixed input pad (my preference, haven't seen their circuit either)
https://www.vintagetools.de/VT-RACK...be-preamps/VTCR-custom-rack-2x-V72-empty.htmlGood tubes should get gain close enough, tighter match can be done down the line. I fiddled with same 1k25 resistor by adding parallel resistor but later dropped it to have original circuit. Hope it helps.

 

[pinebox]

Do you have it in rack with correct power transformer, phantom, etc?V72 is not that easy to modify for reasons well discussed here, they require experiences to get it right without damaging something.
This rack could be the way, depending on how is gain control done and if they can replace phase switch with fixed input pad (my preference, haven't seen their circuit either)
https://www.vintagetools.de/VT-RACK...be-preamps/VTCR-custom-rack-2x-V72-empty.htmlGood tubes should get gain close enough, tighter match can be done down the line. I fiddled with same 1k25 resistor by adding parallel resistor but later dropped it to have original circuit. Hope it helps.

The rack is of good quality, has all the features one might want, but my friend is using it across his mix bus so matching gain is a priority. From the video it seems like they did something similar to what my intention is, just attenuating the input signal. I think I will restore these to factory spec and go that route, thank you!

 

[My3gger]

They say gain pads, it means gain control not input or output level control (pads). Using them for mix bus will require experienced technician check the tubes and other other things like input transformers.

 

[pinebox]

They say gain pads, it means gain control not input or output level control (pads). Using them for mix bus will require experienced technician check the tubes and other other things like input transformers.

Yes, if you watch the video it is a proprietary gain circuit he has added to the enclosure. Actually changing the gain of the cassettes would not be possible with the plug and play style product offered.

 

[My3gger]

Let us know what you find out, they will be happy to explain the circuit.

 

[inkster]

Having literally worked on hundred's of V72's(Dan Alexander hired me to go through them when he first began importing them in the early 90's).
Any mods to the circuit have fairly dramatic effect on the frequency response, the short wire from the input xfrmr to the grid of the 1st tube is short for a reason and lengthening it for a DI or gain switch really can rob the module of high end and the nature of the parafeed output also means that any mods to the stage between the 2 tubes needs careful consideration otherwise response suffers...thus I typically recommend using the module stock with possibly an external pad on the front end and if needed a external output attenuator on the output.
Avoid phantom power or you can kill the input xfrmr.
The modules sound great stock.

 

[inkster]

If you look at the documents available on the REDD 47 you will see a similar sort of control(fine gain) as the one you described installed in your modules, which works but again can affect the response of the module, many designers shy away from using a pot or trimmer as they can be noisy(not just when turning) compared to fixed value resistors.

 

[inkster]

I also have quite a few original parts for the V7X modules including xfrmrs if anyone needs something

 

[pinebox]

Thanks, good to know I am on the right track, I will also decommission the 48v while I am at it.

 

[inkster]

Thanks, good to know I am on the right track, I will also decommission the 48v while I am at it.

If you take a good look at a V76 schematic you'll see they used a very involved switch combining pad networks and feedback resistors to control gain and assure good freq response.

 

[Bo Deadly]

I'm not familiar with these but the circuit does have both negative and positive feedback paths so it could be sensitive to meddling. My instinct would be not to insert series resistance on the input (which would be the case with a pad) or mod R12. If you're only looking to trim a few dB, maybe fiddle with R20 which looks like it's for adjusting the gain a little (it's in series with another resistor and it looks like it's even labelled "adjusted"?). Maybe if you swapped that out with a 10K pot wired in rheostat orientation, that should give you some decent gain adjustment I would think. Or just tack a parallel resistor on top of R20. But that will bring the gain down, not up. So you would want to restore the modded one to 1.25k (and use the original resistor that was removed if you still have it) and then tack a 50K across R20 of the other unit and see where you're at. Iteratively try different resistors until the levels match.

 

[My3gger]

Having literally worked on hundred's of V72's(Dan Alexander hired me to go through them when he first began importing them in the early 90's).
Any mods to the circuit have fairly dramatic effect on the frequency response, the short wire from the input xfrmr to the grid of the 1st tube is short for a reason and lengthening it for a DI or gain switch really can rob the module of high end and the nature of the parafeed output also means that any mods to the stage between the 2 tubes needs careful consideration otherwise response suffers...thus I typically recommend using the module stock with possibly an external pad on the front end and if needed a external output attenuator on the output.
Avoid phantom power or you can kill the input xfrmr.
The modules sound great stock.

Some used pots between secondary-grid connection as a level (not gain) control with all the problems it causes, even shielded wire might cause some changes if long enough. DI input into tube grid can be made perfectly fine with use of relay powered by the heater supply, don't know if shielded reed relay would be better at this position. I'm writing about copies because originals are very tight inside to do this kind of work.
Phantom with a bit of delay should do fine in original V72s, i can't see it causing sudden voltage rise as described by Oliver Archut hitting tiny trafo wires.

 

[Michael Tibes]

On Phantom, I don't know whether this information was from Oliver Archut or where I read or saw it a long time ago, but it was mentioned that there was actually arcing happening in a V72 input transformer with 48V phantom applied. I never used 48V directly at these modules any more as I did before, I always used caps in between if I really needed phantom.

If this is a new build with new iron it might be less or not at all dangerous.

Michael

 

[abbey road d enfer]

The previous tech has modified them by removing the 1.25k resistor in position 12 and soldering in a 1k pot. I am trying to understand the benefit of doing this instead of using a pot as an input pad before the signal gets to the pre.

I think the idea was that providing adjustable NFB was a reasonable way of achieving variable gain. However, a reasonable implemebntation would have been to use a 2K pot, since wih 1K, gain is always higher than stock. With 2K, that leaves the possibility to adjust in both directions, + or -.
Using an input pad has a few drawbacks:
Since the noise is constant, the signal-to-noise ratio decreases as attenuation increases.
It's almost impossible to design an input pad for microphone input with less than 20dB of attenuation.
Most pads introduce some alteration of the frequency response.

Since the v72 was designed as a fixed gain and this position has high DC voltage

This resistor is connected to ground and there's only about 1V or so across it.