[microbus67]

Just finished the Cornet2 build. Initially the test point voltages were +/- 10% spec and well balanced R -> L.
After about an hour of play(excellent sound, coming from a stone-silent background) there was a scratchy, crackling sound from the left speaker, then no sound on the left, with normal sound remaining on the right.
Left it on all night(to allow any weakened/diseased parts to exhibit their illness more clearly!).
Measurements now are as follows:
H+: 6.38
B+: 361
R222L: 280.6,           R222R: 159.1
R211L: 329.5/307.8,   R211R: 327.4/298.0
R212L: 280.3,            R212R: 151.4
R204L: 152.8,            R204R: 146.6
Tube complement:
Sovtek 5AR4(slower warmup and better Bass/Midrange detail as compared to the original Tung-Sol 5Y3GT).
Amperex 12AU7
New-issue Tung-Sol 12AX7s
?Any suggestions Re: most likely point/s of pathology that would explain loss of a channel and these observed voltage anomalies?
It's probably something extremely obvious(like the 1.8K "Brick Wall" R223 I originally installed!), but tube replacement had no effect(except for very slight changes in voltages- the overall function and pattern of imbalance remained).
Thanks for any help!   

[MusicMtnMonkey]

Total newbie kit builder here, you may have to wait for more experienced help, don't do anything rash from my advice or take it too seriously....

Looking at your number I noticed that R212L and R212R then R222L and R222R are Very far apart in numbers.  If they are really the left and right channel of the same valued resistor (shouldn't they have the same value), I'd guess you need to start here to find the problem...  My guess is you installed resistors of the same value in the left channel and resistors of the same value right channel Instead of installing the same values at spot R212 and R222 for the respective channels...

[tubesforever]

Here is my take on some things to look at first.

First off, you can perform all these first checks with the power off.  As the B+ heads down from the rectifier tube it gets dropped down at  R220, then R221 and then by R216, R211, R209 and R202. 

On your measurements you are showing us values at R222 which comes after the final signal cap.  I would need to know if R219 is showing 155 volts.  This would indicate the function of the R220 and 221 step downs.  With the power up then R 219 should be 155v and R 216 should be approximately 327 volts.

As your first step I would recommend testing the values of R220, 221, 202, 211, 209 and 202. 

Second, I am not sure why you have a dead channel unless one of the tubes is bad, or one of the resistors in the direct circuit pathway is blown.  Jim uses a series of 220v resistors between each component juncture in the direct circuit pathway so if something does go wrong the issue stops at that resistor.   

I want to thank Jim for taking extraordinary care and thinking of safety first.  Safe circuit design is evident in every aspect of his board.     

The resistors in the direct signal pathway are R201, R204, R208, R212, R214 and R219.  You should check these and see if one has sacrificed itself for the team so to speak.   

If you find a dead resistor make sure to let us know its place on the board.   

Speculating a bit, let's imagine your build was perfect and a new tube failed.  If the voltage rose, it might take out a direct signal resistor. 

Do you have fresh or known tubes you can swap in if the resistors all check out on the B+ line?  You might be the silly victim of just a random tube failure.  Don't get over anxious that you might have something wrong with your workmanship.    Remember it worked perfectly for a time before the failure occured.   A resistor or two swapped out and a fresh tube might see you playing music for decades without further incident.

In summary, with the power off check all your B+ resistors. 

Then check the direct signal path resistors.  If these all check out, power it up and check the voltage values at every test point.  Then power down, swap out some different tubes and fire it up again.  Check your values. 

Finally, the H+ is high IMO and could contribute over time to early tube failure.  With a tube expecting 6 volts on the heater I feel pretty comfortable between 5.75 and 6.20 volts.  I have friends running their tubes at 6.3 but 6.38 is seriously pushing the envelope.  Just add some resistance to R223.  If you are at 1.8 now, try a 2.0 ohm resistor.  6v would be a lot nicer on your tubes than 6.38v from my seat.

Interestingly enough as the rectifier burns in over time the voltages might change ever so slightly .  Mine came down a bit and I ended up soldering the original R223 value back in place.

I hope this helps.

[microbus67]

Terrifically helpful and encouraging!
Thanks very much indeed.
I'll run through these diagnostic checks in the morning and let you know what I find.
I have, by the way, subbed in fresh tubes all around, with no change in the malfunction, so I too suspect a failed resistor, or an inadvertant bridge.
Also, I'll drop that H+ into a more humane range ; )
Thanks again for your kind help.

[el34]

You shouldn't see much of any DC voltage at R222 on either channel,
so that jumps out at me as a problem... Those are on the output side
of the last coupling cap.  With the power off, measure resistance
from each R222 to ground, and check resistance across each
C208 (output coupling cap) should be very large or infinite.

[microbus67]

Update @ 11:00...
Checked all enumerated resistor values----> within 5%
Thanks!
Thought more about the "Bad Tube" concept.....
?When a dual triode acts bad but multiple replacements act the same(statistically improbable), what is the implied defect?
The socket.
I "re-adjusted" the contacts in the proximal X7 socket(the one that has felt a little wonky from the start) and got the following, symmetrical voltages:
B+ 372.5
R219L: 172.4   R219R: 172.1
R211L: 341.7   R211R: 341.8
R212L: 163.9   R212R: 164.1
R204L: 161.2   R204R: 163.1
I lowered H+ to a leisurely 5.82, so all filaments can now live the La Dolce Vita...
Haven't actually listened through the resurrected unit yet... worked all day instead on a pair of replacement(Linkwitz "Thor") sub-woofers(#1 Son took my beloved Hsu's off to OSU with his Voigt pipes and his EL84 PP.
I'm hopeful, without expectation....if it's still broke, I'll fix it some more : )
Thanks again for your kind help

[hagtech]

Ok, I'm a little late here.  But a scratchy sound indicates a bad connection.  Broken solder joint, loose contact, failing tube.

jh

[microbus67]

Thanks Mr. Hagerman,
This is in accordance with my observations.
I've obtained new sockets(somewhat sticky/dodgy from the start) and will replace the suspect component, hoping that this will bring measured values/function into line.
Until I can find the time to make the replacement I'll continue to enjoy the superb(backup) Bugle.
Thanks!
Mark Franklin   

[microbus67]

Finally came back to the ailing C2 with a few months more experience from a few more amp builds.
Adjusted H+ to 6.25, tightened up the socket pins, and re-heated a couple  cold B+ resistor joints.
Up, running, and sounding more amazing with each side.
Thanks again to all for your kind help!