Eros Phono finished! Plus question about gain

[dave-tx]

Thanks to all for the suggestions.  I'll get the scope on the intermediate nodes tomorrow evening and report back. 

I double-checked the 0.030 and 0.010 caps and the 10.5k resistor, all are in the correct location and solder joints appear to be solid. 

Since the identical lack of gain is present in both channels, does that tend to point to a common component to both channels (assuming, for now, that it's not a swapped component in both channels)?  Is it possible that the 6922 isn't functioning properly?  I have no experience at all with the behavior of a misbehaving tube (would that even manifest in both channels?) and have no idea how to test the tube itself, other than visually verifying that the heater glows.

Grid and plate stoppers all appear attached well and measure to the correct resistances, and the 100uF caps are oriented as per the manual.

Would there be any value in trying to bypass the RIAA equalization to identify it or eliminate it from the equation?

Thanks for the help, everyone.  I'm glad this is a puzzler (so far), thinking about it is definitely a learning experience!

-David

[dave-tx]

Quick update with measurements at the intermediate points mentioned by Paul.

Measurements were approximately the same for both channels.  Measured visually, peak-to-peak, with an o'scope.  20mV input sine wave.

Measurements are at 60Hz, 500Hz, 1kHz, 10kHz, 20kHz

T19/T27: 20mV for all frequencies
T4/T12: 600mV, 220mV, 200mV, 50mV, 30mV
T9/T10: 800mV, 300mV, 200mV, 200mV, 150mV
C3/C8: 600mV, 220mV, 200mV, 50mV, 40mV

[grufti]

I can't make any sense of the values that you measured without digging into the schematic and I won't be able to do that before Saturday. I hope you get help from either one of the Paul's much sooner. Something is fishy, but what? I bet you didn't need me to tell you that.

In response to Grainger: what really matters at this point are the ratios between the voltages. As long as all values are either RMS or peak-to-peak everything is pretty straightforward.

[dave-tx]

Is there any value in disconnecting the EF86 plate from the RIAA filter, leaving only the 75kohm load resistor and checking the gain at the plate there?  Or will it be too hard to know what the correct gain should be without the RIAA filter in place?

Similarly, is there any way to isolate the 2nd stage 6922 to test it on its own?

Thanks,
David

[Paul Joppa]

Bingo!

The 100uF is not connected. Perhaps it is in the wrong holes? - there are several in a row.

I determined this because there are large AC voltages on the 6922 cathodes (C3/C8), which should be bypassed to ground by that capacitor.

With respect to questions about the tubes, at least in this circuit, the fact that the DC voltages are right indicates that their gain parameters (transconductance and/or mu) are within specs.

[dave-tx]

Thanks, Paul!  I won't be able to check until late tonight (or early Saturday morning, depending on how many beers the evening brings), but I'm betting you're right about the wrong hole. 

Will update...

-David

[Grainger49]

Dave,

You are a true Texan.  I remember fond nights at Up In Smoke, a BBQ joint North of Fort Worth.  Great place, great wings and very fresh Lone Star beer!

[dave-tx]

Not sure if my neighbors would consider me a true Texan (I'm originally from Wilmington/Philadelphia), but I've been here long enough to love good BBQ!

Took a quick look at Page 29 in the manual, and although I have the 100uF cap in the right place, I am missing the 1" red wire that connects the cap to breg...on both boards, of course.  Essentially, as Paul stated, the cap is unconnected.

Will add this in the morning and report back. 

Thanks,
David

[dave-tx]

Updated output voltages...All look good relative to each other, particularly considering margin of error on visual scope measurements.

60Hz: 40v
500Hz: 11v
1kHz: 7v
10kHz: 1.7v
20kHz:1v

And of course, the only test that really matters...Put it back in the system and delight ensued.  I have a feeling I won't be getting much done this weekend other than listening.  And I'm okay with that.

Thank you very much, everybody, for patience and help in finding this mistake.
-David

[galyons]

Good news!!! Enjoy.  Let us know how you like it as it burns in.

Cheers,
Geary

[grufti]

Two 1" wires, small cause, big effect. I'm glad you found it. Have some fun listening!

[dave-tx]

Impressions so far...WOW.  This thing sounds amazing.  I've been mostly listening to new music that I've been accumulating while building the Eros, so it's hard to do much A:B comparisons, but I did go back to a couple of old favorites and have been stunned at the sound.

Only question that remains is how long can I hold off before ordering the BeePre... :-)

-David