V76s build - work in progress

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Ilya

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Maybe a problem in you circuit or test setup? Have you posted the schematic you are using? Did you remove or bypass the frequency filtering components?
This was extensively discussed earlier in this thread. No, I'm sure my setup is fine. The schematic is as per original, and yes I did remove all filtering components.

The Jensen 1:10 works very well in the v76. I told Ollie this and he started to offer a 1:10 option on his 76.
Thanks, I'll see if I can get hold of Jensen for a reasonable cost.
 

dmp

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This was extensively discussed earlier in this thread. No, I'm sure my setup is fine. The schematic is as per original, and yes I did remove all filtering components.

Maybe I'm being thick but I missed it. Take the CMMI-10C. If you reproduce test circuit 1 or 2 from the datasheet, it should be flat out well beyond 20kHz
Now put that transformer into the v76, the secondary is connected to the grid, which has very high impedance. Somehow you are now getting -16dB at 20kHz? I don't get it.

I built mine with the Jensen 115k-E90 and it is fantastic. The Jensen 10k61-1M on the output. I'll see if I can find a frequency sweep from when I built it, or I can sweep it again this week and post.
 

Ilya

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Maybe I'm being thick but I missed it.
No, you didn't. I mixed up forum threads. Here's the thread I'm talking about. Basically, the winding capacitance is the culprit of this roll-off.

Now put that transformer into the v76, the secondary is connected to the grid, which has very high impedance. Somehow you are now getting -16dB at 20kHz? I don't get it.
I'm not talking about CMMI-10C. On the contrary, it's down only 0.5 dB at 20 kHz which is fine. With parallel primaries this increases to ~4 dB which is also tolerable (frankly, I should've skipped the lowZ option altogether).
-16dB happens with Sowter. Please read my post with graphs on the previous page once again and you will see that.
 

dmp

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Ah, ok, lots to read through there. Interesting discussion
I misread the CMMI-10C graph - it does do well out to 20kHz in 1:10
Surprising info about the Sowter
 

dmp

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I checked the frequency response of my v76 build using RMAA. I ran my interface in and adjusted the send level with and without the pad at 58 dB gain.
 

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Ilya

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Moscow
I checked the frequency response of my v76 build using RMAA. I ran my interface in and adjusted the send level with and without the pad at 58 dB gain.
Thanks for taking your time to do the graphs.

I've finalized the build and here are some quirks that I found (and solved):

1. HF roll-off. As stated earlier, Sowter 1004 is not suitable for 200R input impedances. The solution was to switch to Cinemag 9994A (CMMI-10x will also suffice).
2. LF roll-off. This was buggering me as I was getting approximately -3dB at 20Hz which is not ideal. The solution was to increase the interstage cap. In the original schematic (V76S, 1:30 input tx) the coupling cap between V2 and V3 tubes is 35nF. This value proved to be too low and cut the LF too early. In contrast, in the V76M circuit (measuring version, 1:10 input tx) this cap is 10 times larger (0.3uF). And this is the correct value for extended LF response.
3. Oscillation. For some reason the input stage oscillated at approximately 300 kHz. I was able to solve this problem by inserting the resistor in series with the anode choke. But this solution didn't work for the second unit. So I put a small (100p) cap across the feedback resistors and this killed the oscillation. I removed the additional resistor from the anode choke and there was no trace of oscillation either. It's still a mystery to me as to where the HF "bleeds" and forms a positive feedback loop. I suspect that this is probably a layout issue. Anyway, I didn't measure any signal degradation with this 100p compensation cap, so I'm leaving it in circuit for time being.
4. My "virtual inductor" option for HPF (gyrator) proved to be a great and affordable solution. It worked as expected.
Here's the real inductor measurements (for all 3 switch positions):
1674587228681.png
And this is the gyrator measurements:
1674587318659.png

Now, I have a plan for next steps.
First, I'm going to write up a full building guide and put it on the site. All schematics will be available as well.
Second, I'm going to look into a scaled down version of this circuit. Since chokes are rather expensive components, I'm going to try to substitute them with CCS. Will see how this works out - need to do quite a bit of reading about this since I have not that much experience with tube circuits ATM. I'm going to strip this version of most of the options - I'll leave the tube DI option, pad and phantom. I might leave the HPF and high shelf in place, but this depends on the price of these options. My goal is to make bare bones V76 and see how much that would cost in the end.

If you guys have any input on the issues that I had during the build, I'd gladly discuss them.
 

dmp

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It's still a mystery to me as to where the HF "bleeds" and forms a positive feedback loop. I suspect that this is probably a layout issue.
Try shielding the chokes. I folded sheet metal into a can, wrapped it with adhesive backed mu-metal.
 

Ilya

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702
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Moscow
Try shielding the chokes. I folded sheet metal into a can, wrapped it with adhesive backed mu-metal.
Both chokes are from Sowter. The choke for the first stage is in mu-metal can and the can is grounded. The output stage choke is not shielded, but I doubt that it has any influence on the oscillation issue.
 

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