Channel imbalance ...from a shorted rectifier?

[atomix]

I

[Paul Joppa]

Just to be sure, swap the inputs L to R - make sure it's in the Crack and not in the preamp ... :^)

If the voltages are right, then the tubes and the power supply are almost certainly fine. It's barely imaginable that tubes could result in such a large difference, but swapping tubes with the same effect makes it pretty certain the tubes have the same gain.

The volume control resistances you quoted were for a fairly high volume setting, but volume control balance usually appear at very low volume settings. Where were you set when observing the level difference?

Som people add a series resistor between the input jack and the volume control, to reduce the gain. Any chance you have such resistors, and they might not be matched? You can measure the resistance at the RCA input jack on each chennel, should be 100K on each channel if it is unmodified.

[atomix]

Paul, thanks for the quick reply!

I have tried swapping the inputs (and hoping!), but the problem persists.  The preamp still works fine with other amps.

As for the volume pot measurements, yes those were from fairly high settings, probably 10 o'clock all the way around to 5 o'clock maxed out.  I have heard about the channel imbalance at low volume levels, but the Beyers seem to require settings pretty high in the range.  The channel imbalance doesn't change within the 10 o'clock to 5 o'clock settings (using the preamp to lower gain just for this experiment).

But ah ha, you found a modification to the amp without even looking!  There is a 90k 2% resistor shunted to ground on each of the input jack's channels.  This should form a nice little voltage divider with the pot and roughly halve gain, correct?  Even still, the input resistance still measures fairly consistent, 83.2k (L) to 82.3kohms (R) -- should I be concerned about ~1% there, even though it's dividing the right channel voltage a little more?  I'm not thinking of the whole circuit here I confess...a little level mismatch at the input seems like the worst place to have it!

Dumb tube question -- can the biases (i.e. DC voltages) measure fine, and I'd still have transconductance / small signal gain problems?    I can't get off the 'hard' fix...
Thanks again!

[Paul Joppa]

"...90K resistor shunted to ground" would be in parallel with the 100K pot, and would produce an input resistance of about 45K. Since you measured twice that, I conclude that either the input circuitry is not wired the way you think it is (inconceivable!) or your meter is not working (worn battery?)

Yes that was a Princess Bride reference - still working on my first coffee this AM  :^)

[atomix]

Paul, appreciate the touch of humor in all this!

It's more than possible that I misread the resistor this morning, or I may have not carefully traced what it was connected to.  I'm pretty confident in the meter...the user is usually the problem. 

Doesn't seem like you think a 1kOhm difference would be a big deal though, right?

[atomix]

Well  darn, I've got to stop trying to debug this early in the AM & in poor light.  Really misread things--those resistors were on the headphone output jack (as spec'd in the manual), and are the 2.49kohm resistors.  I misread brown for black & kept on reading, without measuring...

No resistors in the input path.  The 83kOhms is just the TKD volume pot in the path before the 12AU7. 

So, I think I'm back to

[Paul Birkeland]

You can download a 60Hz test tone and play this into the amp with the volume control up about half way.

Then measure the AC voltage on terminals 1 and 5, then let us know what these are. (Black probe to ground, red probe to 1 or 5)

[atomix]

Paul B, downloaded and played a 60Hz tone from http://www.ronelmm.com/tones/, and my measurements sure want to convince me I'm crazy!...

T1 -- 1.061 VAC
T5 -- 1.061 VAC
T6 -- .583 VAC (output jack, right) -- confirmed at the jack's terminal on the back side
T10 -- .591 VAC (output, left)

I also took a quick measurement before the output coupling caps on the 6080 and was within 25mV (1.325 to 1.35 VAC I think)

From these numbers, I would think the left & right would be the same volume.  BUT, with the test tone it's clearer than ever to my ears that the left channel is louder.  I checked this with my Objective 2 with an A-B comparison about 4 times because I couldn't believe it.   

[Grainger49]

Is it possible that the headsets are imbalanced?

[Paul Birkeland]

I think I'd take a long, close look at the headphone jack.  I'd perform the measurements again, checking the voltage at the jack with headphones plugged in.

I'd also try turning the volume control up about half way (you can decrease the source signal to compensate since your cans will be plugged in).

This should give you a clearer picture of what's up.

I am also curious about your 1/4" to 1/8" converter, something may be amiss there...

[atomix]

Uh oh.  Found the problem it seems...there is a wild imbalance on the Beyer DT880 "600 Ohms."
578 ohms on the right to 47 ohms on the left.  Purchased used, eBay, unfortunately.   Will have to see what can be done there.

My 18 Ohm ear buds are balanced sounding through the Crack.

Appreciate all the debug help!  Wish the problem was the amp, it would be easier to fix likely!

[Paul Birkeland]

It sounds like somebody replaced a driver with the incorrect part, maybe Beyer can sell you a 600 ohm driver.