Sounds like the one pair of output tubes is near the end of life. The difference between the pairs is large enough that it would be difficult to bias properly with a single VR7 adjuster. But with each push-pull pair balanced the amp should sound quite good. Does it? If it does and you watch the power transformer for overheating (as well as red plates on the high- current pair of EL34s) you could enjoy them until the worn-out pair begins to sound unpleasant to you. (But the amp is not performing at its best, because the currents are quite mismatched*)
But it appears from what you say that the amp is running quite well and is stable, even with the mismatched pairs of EL34s, so I suggest that you could pop in a good matched quad now, and see what currents (test point to pin 3 = 3 V difference) you get with you bias max at around -50 volts. I fully expect that you would be close with both channels and balance between V1(3) and V2(4). Good luck.
If it were my amp I would definitely install those 10 ohm precision resistors between each EL34 cathode and ground so that you know exactly what the currents are, and therefore being able to balance them properly. Until then, I would calculate the required voltage drop over each winding (test point to pin 3) at the desired plate current for each tube, based on the actual resistance. Initially I would set iurrent at 40 ma. So, for example, the side where you measured 66 Ohm you would adjust the drop to be V=I*R= 0.04 * 66 = 2.64 V, and the 76 Ohm side would be .04*76=3.04V (*this shows how mismatched the current are if you set both sides at 3V)
If you use this method I suggest you remeasure the windings in the primaries carefully and note them for future reference. The 66 and 76 Ohms you reported sounds low to me (lower than any of my output transformers for EL34) And it is more unbalanced. Wonder who made the OTs for AirTight - expensive Japanse output transformers are typically close in specs.
But it appears from what you say that the amp is running quite well and is stable, even with the mismatched pairs of EL34s, so I suggest that you could pop in a good matched quad now, and see what currents (test point to pin 3 = 3 V difference) you get with you bias max at around -50 volts. I fully expect that you would be close with both channels and balance between V1(3) and V2(4). Good luck.
If it were my amp I would definitely install those 10 ohm precision resistors between each EL34 cathode and ground so that you know exactly what the currents are, and therefore being able to balance them properly. Until then, I would calculate the required voltage drop over each winding (test point to pin 3) at the desired plate current for each tube, based on the actual resistance. Initially I would set iurrent at 40 ma. So, for example, the side where you measured 66 Ohm you would adjust the drop to be V=I*R= 0.04 * 66 = 2.64 V, and the 76 Ohm side would be .04*76=3.04V (*this shows how mismatched the current are if you set both sides at 3V)
If you use this method I suggest you remeasure the windings in the primaries carefully and note them for future reference. The 66 and 76 Ohms you reported sounds low to me (lower than any of my output transformers for EL34) And it is more unbalanced. Wonder who made the OTs for AirTight - expensive Japanse output transformers are typically close in specs.
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ok, my amp is wired wrong. Or someone changed something in it.
As you can see, my wiring is a bit different from other amps I have seen on the internet. At the same time, using the schematic provided here by other posters, isn't pin 4 supposed to connect to SG1 or SG2 (I assumed stands for screen grid 1/2) through a 100-ohm resistor? On mine, pin 4 is hooked up to P1/P2 (again, I assumed it to be plate 1 / plate 2) along with pin 3 on each tube. I will go back and check everything compare to the schematic provide and will report any discrepancies. Good practice since I am currently on the chapter of learning how to read circuit anyway.
Should I rewire it to match the schematic? I have the tools and skills to do soldering, it's the knowledge and background training that I am sorely lacking.
As you can see, my wiring is a bit different from other amps I have seen on the internet. At the same time, using the schematic provided here by other posters, isn't pin 4 supposed to connect to SG1 or SG2 (I assumed stands for screen grid 1/2) through a 100-ohm resistor? On mine, pin 4 is hooked up to P1/P2 (again, I assumed it to be plate 1 / plate 2) along with pin 3 on each tube. I will go back and check everything compare to the schematic provide and will report any discrepancies. Good practice since I am currently on the chapter of learning how to read circuit anyway.
Should I rewire it to match the schematic? I have the tools and skills to do soldering, it's the knowledge and background training that I am sorely lacking.
You are correct, looks like the screen taps on the OT are not connected. Apparently someone triode-connected the amp, i.e. connected the EL34 screens to the anode, with or without a dropping resistor (usually 100 ohms). Follow the wires and see if you can verify that.
Find out what is going on before making any changes,
Find out what is going on before making any changes,
amazing midrange... just use one gz34.
Currently it is what come with the amp. The GZ34 / 12ax7 was Air Tight selected (EH for 12ax7 and chinese GZ34). The ecc82 is a weird brand that I don't recognize. I am afraid to put in new tube for the 12ax7 and 12au7 since it require messing with the AC balance thing. I have lot of vintage tubes I can put in and I have already replaced the chinese GZ34 (the clinking noise they made when warming up just giving me anxiety).
Hello, with the negative lead on the test point, and the positive lead on pin 3, I get - voltage. Is that right ok? Does it have to be positive voltage?
After soldering the screen grid to proper taps, I tried to put in the Air Tight Platinum selected tubes. Unfortunately, they behave erratically. The voltage drop went as high as -12v, and I can't even adjust the bias voltage to get it lower than -4V. Tube at the V2 red plating after like 10 seconds of turning on.
The vintage EL34 fat bottle tested at 30 38 40 40 on my TV7. I paired the 40/40 and the 30/38 together. No problem adjusting the voltage on V1 and V2 here. Accurate down to 3.04 and 2.64 (V1 is 77 ohm and V2 is 67ohm). Unfortunately, V3 and V4 voltage drops are only 1.6V and 1.2V. I can't get it to go up without the other channel going up. V3 and V4 channel has a hum. All voltage is negative.
Tomorrow I will check all the components in the power supply section again just to be sure. Thank you everyone for helping me so far.
You are measuring a difference in voltage, or drop across each leg of the OT primary winding. If your multimeter is happy with negative voltage you could do it this way, but typically you measure by putting the red lead on the highest potential.
Sorry this happened. At this point I think it is best taking the amp to a good tech, before those good Tamradio OTs get damaged.