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August 21, 2019, 04:49:08 AM

Author Topic: Channel imbalance - tubes?  (Read 153 times)

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Offline ejb14

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Channel imbalance - tubes?
« on: May 12, 2019, 10:21:34 AM »
Hello -
My S.E.X 3.0 amp has been working pretty well, though I have always noticed a slight channel imbalance.

Before I install the C4S, I decided to check things out again - I re-ran the resistance and voltage checks - all is good within spec. I can provide those if needed.

Assuming my issue is the tubes - my question is this - is it reasonable or expected for tubes to vary as much as 13-15%?  This test was with the tubes supplied with the kit.

I checked the voltages at the tip and ring with a 60hz tone played from my DAC (a Schitt Modi, which has perfect channel balance voltages based on the voltage at the RCA jacks)

I swapped the tubes and the the voltage imbalance swapped sides as well.

The good news is that the volume pot appears to be near spot on throughout the range.

Here are the voltages

1st tube orientation:
Volume Position  Middle Lug Vol Pot       Tip      Ring
3 o'clock             0.036/0.036            0.191    0.167
12 o'clock           0.098/0.098            0.554    0.481
9 o'clock             0.417/0.419            2.348    2.059
Max                    0.730/0.732            4.130    3.609

Second tube orientation (tubes swapped):
Volume Position  Middle Lug Vol Pot       Tip      Ring
3 o'clock             0.036/0.036            0.175    0.201
12 o'clock           0.098/0.098            0.480    0.540
9 o'clock             0.416/0.421            2.048    2.359
Max                    0.730/0.731           3.618     4.140


Offline Paul Joppa

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Re: Channel imbalance - tubes?
« Reply #1 on: May 12, 2019, 12:22:32 PM »
Yes, it appears to be the tubes. Parameter variations of +/- 25% or so are considered to be within spec for tubes (transistors have much more variation). I don't have any solid numbers, but similar variations can occur between speakers, especially if they are not positioned symmetrically in the room.

Of course you can get a few spare tubes and match them up.

Cheaper would be to splice in a resistor between the input jack and the volume control to reduce gain on the higher-gain side - looks like about 15K ohms would do the job with the tubes you have measured.
Paul Joppa

Offline ejb14

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Re: Channel imbalance - tubes?
« Reply #2 on: May 12, 2019, 02:17:46 PM »
Thank you Paul!!

Makes perfect sense - add extra resistance on the high side.