Bernieclub,
The output voltages were measured at maximum volume setting, where the volume pots should not make much difference. But to make sure, I measured the resistance between input and middle of the pot at maximum volume, and it is 2 Ohms for both channels.
JC,
I measured the DC voltages with no input signal, and minimum volume setting of course.
Now I swapped the tubes and measured again.
The DC voltages did follow the tubes, with exception of the cathode voltage at the pre stage (pin 6)
Of course I did not measure the exact same voltages, and by "followed" I mean only
that the absolute value of the difference was roughly the same,
but the direction of the difference became the opposite.
The AC output voltages also followed the swapped tubes.
Grainger,
Yes, that is correct. First I measured DC voltages with no input signal.
Black figures are for left channel, red figures for right channel.
All values except for the output voltage are DC.
Then I connected the source, and set volume to maximum and measured the output voltage in AC.
OK, so can we conclude now, that the difference is only caused by the tubes?
If so, what is best way to handle it?
Before we go to the possible options let me mention something,
that might be of importance:
There are two visual differences between the 2 stock tubes.
1. the first has no marking at the bottom, the second has the following in red: "Zenith, made in USA"
2. when switched on, and looked from the top, the first one has a wire across that glows, while the second does not have this.
The second one with the marking gives the higher gain.
And now for the possible options:
1. order a matched pair (if available)?
2. order some more tubes and try replacing them until there is a good match?
3. mod any values to achieve the same gain with the existing tubes?
4. .....?
