Understanding V72s and changing gain

[My3gger]

Hi,

while i'm waiting for some trafos from out member, i would like to understand this preamp better and squeeze a little more gain out of it. Schematic of V72S is attached, original gain is 40dB, as i will use 1:10 input trafo this is 6dB less, so 34dB.
This is enough for most situations, but there are some cases where i need ~10dB more, it would be nice if i could change it with a pot or switch.
First i would like to know more about how feedback works here. There are two resistors and caps in // from V2 anode to V1 cathode. This looks like frequency compensated NFB to me. There is also 50k resistor between V1 and V2 cathode which also looks like NFB to me, but i'm really not sure which because of voltage dividers in V1 and V2 cathodes and also because it lacks capacitor i'm used to see.
Oliver from TAB wrote that this preamp has DC and AC NFB along with positive feedback. I'm looking and looking at this circuit for days and can't understand it. Normally there is anode to cathode NFB with resistor and cap (or anode to anode like in Gates SA70), so i guess it's AC NFB. But here it's very different, i'm already lost at V2 anode to V2 cathode feedback which looks like negative and as i said compansated. But what about this DC/AC thing and positive feedback? There is also this voltage divider to V1 anode and G2. I think i understand it but 8u cap looks very high compared to any other circuit i've seen or built. Is it anode decoupling and just powers this two nodes?

I know i'm asking many things, i would be very happy if someone tells me about feedbacks for the start.
Google showed this classic "V-72 gain mod" from Recording.org (Oliver said it will form a filter beside variable gain), and another from Drip which i wouldn't trust since i've seen what they done to this and some other preamps.

 

[PRR]

> There are two resistors and caps in // from V2 anode to V1 cathode. This looks like frequency compensated NFB to me.

Do the math. Looks like 85K resistance and 90pFd capacitance. 20,815Hz. Above that, the cap dominates; below that, only the resistor matters.

> 50k resistor between V1 and V2 cathode which also looks like NFB to me

Look again. Keep track of phase. This is Positive feedback. You find a similar thing in the Dynaco tube preamp; also in several exotic servo amps, and at least one vacuum opamp.

> There is also this voltage divider to V1 anode and G2.

The 50K 8uFd? That's a plain B+ filter decoupler. Reduces the AC ripple from the power source, and knocks-down big signal sneaking back from V2 plate.

Serious gain change will be *tricky*. It's been designed to do what it does, not what some guy wants a half-century later.

Clip a 270 ohm resistor across the "580"(?) resistor under V1. That *may* jack-up gain 3X without major ill effect. If I'm wrong, and it screams, rip it off again.

Or be professional, like the original users. There was usually a makeup amp between mike-amp and tape deck final recording stage. Usually a fader between mike amp and makeup amp.

 

[gemini86]

Messing with the feedback R too much causes some odd phase shifts. Mostly in the low end. But you could probably bump it up to 100-120K (clip a 220K across it) ...see how it goes.

 

[My3gger]

Thanks a lot for helping me understand it, now we also have translated manual. I was looking at Dynaco too, but couldn't find information about how phase determines if FB is pos. or neg., so wasn't sure. I really like what they did, it's much more advanced than any other preamp with 2 tubes.
Fixed gain is not really a problem for me but other people who use them seem to complain when we go straight into converters.
I will check a few options for variable gain, maybe i could do it with simple trimmer if i can find one for front plate.
Gemini86, your idea is probably worth checking too, i just have to recalculate capacitance for same response.

My input transformer doesn't have a CT where i could put a that 1u cap for HPF. Does this cap work with transformer DCR or in some other way? I can't see how it would form a filter with inductance here... It is probably good to limit lows right before the input, or not? Many other preamps have coupling caps for this (or is it just "side effect"?), so i might do the same.

 

[gemini86]

I've read in other forums/threads that the cap on the primary is for radio interference. Many people pull them out for better response. I honestly don't know for sure though.

 

[My3gger]

Check translated manual, it clearly states it is a high pass filter. It seems like we have V series picnic lately 

 

[My3gger]

Clip a 270 ohm resistor across the "580"(?) resistor under V1. That *may* jack-up gain 3X without major ill effect. If I'm wrong, and it screams, rip it off again.

That looks like 680 ohm resistor, lower side of divider has 560R. 270R in // with 680R is 193R, so 500R pot wired as a rheostat with 180R series resistor would make gain variable. It could set it back to original resistance with pot fully open.
I looked at V78, 50k resistor from positive feedback goes directly into cathode of V1, beneath this point is 1k pot with 100R series resistor to ground. FB conditions are biasing are quite different here. My plan for V72s looks better to me because it can be set back to original plan.
There is some voltage in cathode which would probably make pot scratchy, wouldn't it? I was reading 10 years old CJ's post at another forum where he proposed MBB switch with slightly different arrangement in V72, can't find it anymore. Iirc it was about cap coupling NFB which would upset it badly. But switch seems like a good idea instead of 500R pot, what do you guys think?

 

[CJ]

almost done hacking an output transformer for Kid Squid, then i can mess with this V72 on the bench,

in the mean time, enjoy this> 

http://paleoelectronics.com/RDH4/CHAPTR07.PDF

 

[My3gger]

Oh no...only this part is like a few years of diy. Checked it some time ago, now i can study it a little.
And yes, mess with V72 along all those 1948T and 349T.  ;D

 

[DaveP]

It's very informative to study all the V series pre's V76, V77 and V78.
Check out the different specs and the way they achieve them, some are very elegant engineering.
The transformers and the feedback mechanisms change and they affect the noise performance too.
Its also interesting to see what they changed from earlier to later versions
Here are the rest of the translations:-
V76

 

[DaveP]

V77

 

[DaveP]

V78

 

[CJ]

wow, cool stuf Dave!  thanks!   

 

[My3gger]

Yeah, thanks a lot for taking time, translation really helps understanding it. 

 

[gemini86]

Anyone have a good schematic of the v77? Only one on the web (for free) is very low quality and mostly unreadable.

 

[DaveP]

Rodney,
PM me your email and I'll send it to you, too big to post
best
DaveP

 

[MatthisD]

Gain controller for v77

 

[gemini86]

Thanks dave and Matthis.

Very interesting stuff. The gain is changed in the amp with a cathode bypass cap on the input. 40dB with no bypass, 50dB with 100µF and 500Ω in series and 60dB with the 100µF to ground.

The W77 is mostly an input attenuator, with the last 3 steps changing the amps gain with cathode bypass.

I wonder why the V72 has all that strange feedback? The difference between V72 and V77 is a higher input transformer, they removed the 4MΩ from V2 plate to V1 plate, removed the 50kΩ feedback from V2 cathode to V1 cathode and removed the DC from the neg feedback loop via a 2µF blocking cap.

Seems like they removed the complexities of the V72 from the V77 and gave it more gain (as well as a way to change the gain).

I can't build a decent EF804S model without proper screen current information, so I can't accurately simulate any of this, but swapping the tube for ef86 shoes that the response is quite similar to V72. Low freq phase shift is a bit more drastic though. Almost 40 degrees at 20HZ

 

[DaveP]

I can't build a decent EF804S model without proper screen current information, so I can't accurately simulate any of this, but swapping the tube for ef86 shoes that the response is quite similar to V72.

If you take the screen current to be one fifth of plate current you will be able to simulate it pretty well
best
Dave

 

[gemini86]

I'll give that a try.