voltage measurement problems after output upgrade BeePre 2

[Paul Birkeland]

Well all your tubes are working properly.  When I have run into issues like this, usually it ends up being input/output wiring issues that are causing the problems. 

It may help to post some build photos and we could look at what you have built.

[denteom]

Nice to know the voltages are fine.  I don't think it is the input. Have checked that before during the initial build and the upgrade one, have already fixed the problem, already re-enforced, added additional solder to the inputs and was listening to the unit prior to the second upgrade and it was fine.  I think it is the output since we de-solder and resolder again with added xlr.  Hope these photos can tell what the problem is.

[Paul Birkeland]

With the preamp running, play a 60 Hz tone into it using an 1/8" TRS to RCA adapter and an old phone or a computer as a source with the volume control turned all the way up.  Turn the level controls to maximum, then set your meter for AC volts and measure the AC voltage at:

1.  Pin 3 on each 4 pin socket
2.  The center post of each 9 pin socket
3.  The center post of each output RCA jack

That will give a good idea of where the signal isn't getting through.

[denteom]

Does the trs in one end headphone and the other end in stereo that is 2 RCA or the more common, 2 ends in TRS which commonly used in battery powered portable speakers? Then I plug it in the inputs? Will the small TRS be easily be plugged to the line in (no need for adapters)? If  it ends in a single TRS where do I plug it on the red side or on the (white or black side of the line-in?)

[denteom]

Also can can the problem be also due to poor contact on the nine pin tubes? Not sure how hard should I push.  The pins are so tiny unlike the one on the 300B which is easier to push through the socket.  They should build sockets like the ones on PC computer, with zero insertion force. hahahah...

[Toobuzz]

Plug something like this into a computer or old phone (the 1/8” trs side) and run RCA cables from this adapter to you BeePre2 input.

ANDTOBO 3.5 mm TRS to Dual RCA F (2-Pack), 3.5mm Stereo Male to Dual RCA Female Audio Breakout Adapter https://a.co/d/2NiSSZ8

Please unplug all outputs from the BeePre. Max out the volume on both the computer/phone and BeePre2, play this, and take the measurements that PB has indicated.

[denteom]

I got 0 ACV on all the pins that you told me to check using the red as test lead and the black one clipped on number 4 terminal. That's weird, what could be the problem? I plugged the pre amp yesterday to my speakers with switching of the 6CM7 tubes location.  Still same problem very soft volume and sounding like mono (not stereo) despite everything on the pre amp is maxed out.

[Paul Birkeland]

Could you post a picture of your meter or the make and model?  If it is one that only has a 200V and 700V scale for AC, then you may need a different meter. 

Another test you can try is playing the tone through the cable with the volume on your electronic device maxed out, then just use your meter across the AC plugs to check the AC voltage there.

[denteom]

This is antique multi-meter from dad, still working fine.  How low AC volt do I have to turn it down to?  BTW it is now switch to DC cause I rechecked again the DCV so far they are still within the parameters. I did placed it on 1000, 250, 50 ACV (I think I tried 10 also). Below the photo is the TRS adapter with the RCA.  I plugged it to my phone, then tried it on the headphone out of the Macbook air as well.

[Paul Birkeland]

Yeah that analog meter is really good for certain things, but not for chasing signals that start out at less than 1V.

If you're in the US, I would grab one of these:
https://www.harborfreight.com/electrical/electrician-s-tools/multimeters-testers/11-function-digital-multimeter-with-audible-continuity-61593.html

On the 2V AC scale, you can resolve these signal voltages pretty easily.

[denteom]

Ok I'll try it again at 10. If still can't get a signal, I have a friend who has an oscilloscope.  Not sure if he has a less than 2VAC.  What should I expect from the voltage test?

[Paul Birkeland]

If you measure the AC voltage on the RCA plugs before plugging them into the BeePre, that would be your starting signal (probably around 0.3V-0.5V).  That same voltage should appear on pin 3 of each 4 pin socket (you can leave the preamp off for that test).

With the preamp running and the volume controls turned all the way up, you would expect somewhere around 1.5V or so on the center post of each 9 pin socket, and you would expect to see nearly that same voltage at each RCA output jack.

[denteom]

Finally found the problem. It was the 220 ohm 1/4 watt resistor connected to C7 and the center pin of socket C.  I forgot to trim off the excess wire after soldering it to C7.
My last problem is the fine volume control 0 db. it looses contact on the left channel, I have to tap it, press on it or wiggle it.  The other fine channels are ok.  I've sprayed contact cleaner to the switch and switch it several times but the performance is kinda erratic.  Not sure if this problem is also causing the ever so slight softer sound on the left channel. How do I resolve that problem?

[Paul Birkeland]

The fine switch doesn't really work that way.  The input/output are connected by wire, not the switch, so what you're more than likely dealing with is one of the empty terminals on that switch making contact with the metal hardware on the switch itself.

[denteom]

So how do I resolve that problem on the 0 db? Switching from -1.5 to 0 db, yes there is sometimes a slight loss in sound in between.  Which does not happen when I switch between the other fine channel switch, transitioning on the other fine channels is ok. It's only between the -1.5 to 0.  The -1.5 db and lower all work fine.