Finished my SPP build with Edcor XPWR008 power transformer and voltage at my 7189 power tubes are 11.7,11.9,11.4,11.9 Thought I had order a matched quad could this be why one channel is louder? My heater voltage is at 6.8 what size resistor is needed to reduce this voltage to 6.1?
Look elsewhere for your channel imbalance. Have you measured
the amplifier gain, to be sure the amplifier is the problem?
The output tube filament current total is: 760mA x 4 = 3.04A
To drop 0.5V, use R = 0.5V / 3.04 A = 0.16R
Suggest paralleling two 0.33R resistors, each of 2W or 3W rating.
the amplifier gain, to be sure the amplifier is the problem?
The output tube filament current total is: 760mA x 4 = 3.04A
To drop 0.5V, use R = 0.5V / 3.04 A = 0.16R
Suggest paralleling two 0.33R resistors, each of 2W or 3W rating.
Got back to tacking some measurements Plate voltages coming into plate at pin 6 at the 12at7 one measures 82.3dc and the other channel measures 95.9dc. Feedback is not connected, B+ is 300v on the other side of 75k resistor
Sorry forgot to mention just acquired an oscilloscope learning how to use it. where else should I check?
Can you post the output section of the amplifier as well? That is cut off.
If there is a channel imbalance in this amp, after testing the tubes, look at C100 first.
If you cannot measure it, just replace it in both channels. Use a 1000uF, 16V or 25V part.
Also check R104 for value.
Measure the DC voltages with respect to ground at:
6BQ5 cathodes
6BQ5 grids (needs a high impedance meter)
12AT7 plates
12AT7 cathodes
If there is a channel imbalance in this amp, after testing the tubes, look at C100 first.
If you cannot measure it, just replace it in both channels. Use a 1000uF, 16V or 25V part.
Also check R104 for value.
Measure the DC voltages with respect to ground at:
6BQ5 cathodes
6BQ5 grids (needs a high impedance meter)
12AT7 plates
12AT7 cathodes
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Pin 2 voltages on the power tubes varied a lot so I guessed at average. Voltages taken with shorting pins on input
If you have swapped all the tubes between channels, and the same amp channel still has lower gain,
then try replacing the C100 in both channels per post #9.
Also check R104 for value.
then try replacing the C100 in both channels per post #9.
Also check R104 for value.
Thanks for the advise I have swapped tubes around guess I will pull the board out but not today Michigan is all blue sky and warming time for that walk
Maybe swapping the wires for the OPTs might be worth a try to eliminate them as culprits? Do you have a volume control? Some of those solder joints look like they could benefit from a bit more solder; maybe there is a poor joint on one channel so just reflowing the solder and adding a bit could be worth a shot.
Thanks for all the help. Cannot swap outputs my wires are to short could I check ac between screen and plate? resistors r106 and r206 measure correct for resistance and are fed by the same b+ voltage but the voltage on the other side is 77.8 vs 91.3 and was measured touching wire thus avoiding a bad solder joint before I replace is there any other possible. Note all resistors measure correctly in circuit
Back chasing the channel imbalance. I have swapped output transformers, installed new caps at c100,200, swapped resistor 106,206 And moving/swapping tubes to mes it seems in the first stage due to dc difference at R106/206 here are some AC voltages taken on occilascope
