S.E.X. 2.1 Voltage Question

[Adrian]

I just completed the build and carefully checked each wiring point, visually checked solder connections (I know that may not mean anything), took resistance readings, and voltage readings.

All resistances are almost exactly per check sheet values.
Tube filaments light equally.
All voltages are also right on expected values (my line voltage is 123.1VAC) EXCEPT:
A/B 7 ...  -2.53/-2.52VDC vs. -3.15 (20% less)
A/B 8 ...  3.63/3.62 VDC vs. 3.15 (15.2% more)
These values place 6.15VDC across the filaments (nominal value of 6.3VDC).
C1 ... -2.57VDC vs. -3.15 (18.4% less)
C2 ... 3.66VDC vs. 3.15 (16.2% more)
C4 ... -2.56VDC vs. -3.15 (18.7% less)
C5 ... 3.86VDC vs. 3.15 (22.5% more)
C-Terminal voltages are consistent with the tube filament voltages.

I don't thnk these individual readings are a concern because the filament voltage is only 2.4% below nominal.
Is this correct or should I be concerned?

Also, I am now carefully checking all solder connections (re-solder if doubtful) because I have a bee-buzzing noise of constant volume regardless of pot setting coming out of the left channel with the phone jack fully inserted and half-volume buzzing out of both channels with plug inserted half-way.   

[mcandmar]

Those heater voltages are normal so i wouldn't worry about those.

Try swapping the tubes around and see if the buzzing noise moves channel, that will confirm if the issue is a noisy tube or something else.

[Adrian]

mcandmar:  Thanks for the quick response.
I reflowed all the common/ground connections and components going to them, repowered, and still have the same buzz.  Waiting for the tubes to cool down and I will swap them as suggested.

[Adrian]

Swapped tubes and the buzz also swapped channels - have to get different tubes now.

[Paul Birkeland]

You can also put some time on them.  We have found that 50-100 hours of use on a tube that has been sitting in a box on a shelf for 50 years can help a lot.

-PB

[Adrian]

PB:  Are you suggesting that I turn on the amp and let it sit for 2 - 4 days and then see what happens?

[Paul Birkeland]

Yes.

[Adrian]

OK
Got another problem after I connected the input.
I only have right channel sound when I put the phone plug fully into the jack.
I know only one channel is coming through because I played an analog test album that separates the left and right channel outputs and ONLY the right channel plays regardless which socket the tubes are in. (I did swap the inputs and can verify that only the right channel plays.)

ALSO, the LEFT Channel Plate Choke (the one connected to "B") runs hot and the RIGHT Channel Plate Choke (the one connected to "A") is cool - strange!

Does this point to a problem with the phone jack or the left input ... or maybe neither?
Why does the Plate Choke on the Channel that doesn't play (LEFT) get hot and the Plate Choke for the Channel that plays (RIGHT) is cool? Should the Plate Chokes get hot? - as hot as the main tranny?

[mcandmar]

Sounds like you have a short in the left channel, somewhere between the output transformers and the headphone socket.  Double check the wiring, if you cant see anything post up a few pics..

[Adrian]

Thank you, sir.  I'm looking into it now.

[Paul Birkeland]

ALSO, the LEFT Channel Plate Choke (the one connected to "B") runs hot and the RIGHT Channel Plate Choke (the one connected to "A") is cool - strange!

This won't happen unless there is something extremely amiss in the voltage checks.

[Adrian]

PB:  I checked all the wiring several times and see no evidence of shorting.

I did not check the voltages after I connected the inputs and discovered this problem and this should have seemed obvious if one channel is drawing excessive current (me bad!).

Also, on the left channel I noticed a bulge at the top of the 1000 microF cap (gain adjustment?) in parallel with the 680 ohm cathode bias resistor.  If this cap is blown (bulges usually mean that), then perhaps the bias resistor is not doing its job of maintaing the proper -V bias at the grid causing the valve to conduct at maximum current (way outside its linear range).  This might cause the plate choke to draw excessive current.  Just my thoughts on it.  But, I don't like the look of the cathode bias cap.  I'll try to get a picture of this.

Going to check the resistances and voltages.

[mcandmar]

Oh thats not good, if its bulged its done for, bin it.  You definitely need to start from scratch with some resistance measurements to find the issue before powering up again.

[Adrian]

mcandmar:  I agree.  Upon resistance checks I found infinite resistance at B1 and upon trying to get this reading the grid stopper (220 ohm) broke at the body - just split apart.  Probably overheated it.  I was afraid that would happen.  After I soldered in these four grid stoppers I checked their resistances and all of them went up (229 - 252).  The now broken one read 235 after installing.

There is no reason for me to procede with any futher diagnosis.
I will remove the offending parts (grid stopper, cathode bias resistor and cap).

I've never order parts like this before so off into the unknown.

[Adrian]

Thinking back, I probably damaged the bias cap upon initial voltage checks.
I did touch the +VM probe to something near the left channel and terminal strip 21-25 and got a little smoke!  I continued on with the checks and all were good in the end.  I probably should have started over and checked all of the voltages again.
It is true: you learn alot when you screw up.