Hi
This amplifier works alright, but if I increase the volume above 3, it distorts. I replaced the power tubes, but still the same. I took some readings on the output ultralinear transformer . The right hand side winding has a DC reading of 54.5ohms and dissipates 30mA at 450V plate voltage. The left hand side winding has a DC reading of 32ohms and dissipates 45mA at 450V plate voltage. The bias voltage reads -52V and it is a fixed bias with a bias balance pot. I guess that the distortion is due to the inbalance in output power between the 2 tubes caused by the large difference in ouput impedance. I also guess that the left hand side winding has partially shorted (reading 32ohms DC resistance). Can this be the reason for the distortion or something else can be the cause. Please assist. Attached pdf.
Thanks
This amplifier works alright, but if I increase the volume above 3, it distorts. I replaced the power tubes, but still the same. I took some readings on the output ultralinear transformer . The right hand side winding has a DC reading of 54.5ohms and dissipates 30mA at 450V plate voltage. The left hand side winding has a DC reading of 32ohms and dissipates 45mA at 450V plate voltage. The bias voltage reads -52V and it is a fixed bias with a bias balance pot. I guess that the distortion is due to the inbalance in output power between the 2 tubes caused by the large difference in ouput impedance. I also guess that the left hand side winding has partially shorted (reading 32ohms DC resistance). Can this be the reason for the distortion or something else can be the cause. Please assist. Attached pdf.
Thanks
You are not measuring impedance, you are measuring resistance. Transformers run on turns ratios. And when we are talking impedances of 4000-6000 ohms or thereabouts, the difference of a few ohms at DC won't matter.
HAving said that, it is possible the transformer is defective. One quick check would be to check the UL taps. One side has 54 ohms and the other 32 ohms? DO the UL taps on each side measure resistances of similar percentage of those respective readings?
Now:
Are both screen resistors healthy? Close to 470 ohms? Are both plates of the phase inverter sitting about the same at DC, and with a test signal are they seeing similar signals at the plates? and then at the power tube grids?
What does your distortion look like on a scope?
HAving said that, it is possible the transformer is defective. One quick check would be to check the UL taps. One side has 54 ohms and the other 32 ohms? DO the UL taps on each side measure resistances of similar percentage of those respective readings?
Now:
Are both screen resistors healthy? Close to 470 ohms? Are both plates of the phase inverter sitting about the same at DC, and with a test signal are they seeing similar signals at the plates? and then at the power tube grids?
What does your distortion look like on a scope?
ALL amps distort above a certain level.
But ... but ... but ... on THREE?
Shouldn´t it be on 9 or 10?
Well, the "number" on the volume knob means little, it´s just a reference number and nothing else.
"Usually" Musical Instrument amplifiers are set up to reach maximum power (so reaching clipping) about 6 or 7 , he idea being that if you have a weak Bass pickup , or very old rusty strings, you are a light touch player, etc., you still have some extra gain available to compensate.
If you have a strong pickups Bass , or are a heavy handed tapper slapper, it may start distorting on, say, 5, and if you use an Active Bass or maybe a 5 string one or some boost pedal instead of the passive Jazz Bass or Precision it was designed for, maybe 3 is all you can rise the volume before clipping, yet the amplifier is working fine.
That´s why I told you the "number" by itself does not mean that much.
To know whether the amplifier works properly or not, measure power output at clipping: set all tone controls to "5" or "6", inject 50 or 100mV 1kHz tone into the Normal channel input , connect a 4 ohm 100W resistor to "normal 4 ohm sopeaker out", scope it, and slowly rise volume until both sinewave peaks start clipping.
I say both because it will hardly be symmetrical so one will clip first, keep rising until both do.
Then do the Math : V^2 / 4 ohm = RMS Watts.
I would expect 50/60W RMS with fresh tubes, maybe 40W RMS with well worn ones.
Any lower, replace tubes with new ones and remeasure.
If you have catastrophic low power, say 40W with new tubes or less than 30W with old ones, or have wild waveforms, post scope screen pictures.
But ... but ... but ... on THREE?
Shouldn´t it be on 9 or 10?
Well, the "number" on the volume knob means little, it´s just a reference number and nothing else.
"Usually" Musical Instrument amplifiers are set up to reach maximum power (so reaching clipping) about 6 or 7 , he idea being that if you have a weak Bass pickup , or very old rusty strings, you are a light touch player, etc., you still have some extra gain available to compensate.
If you have a strong pickups Bass , or are a heavy handed tapper slapper, it may start distorting on, say, 5, and if you use an Active Bass or maybe a 5 string one or some boost pedal instead of the passive Jazz Bass or Precision it was designed for, maybe 3 is all you can rise the volume before clipping, yet the amplifier is working fine.
That´s why I told you the "number" by itself does not mean that much.
To know whether the amplifier works properly or not, measure power output at clipping: set all tone controls to "5" or "6", inject 50 or 100mV 1kHz tone into the Normal channel input , connect a 4 ohm 100W resistor to "normal 4 ohm sopeaker out", scope it, and slowly rise volume until both sinewave peaks start clipping.
I say both because it will hardly be symmetrical so one will clip first, keep rising until both do.
Then do the Math : V^2 / 4 ohm = RMS Watts.
I would expect 50/60W RMS with fresh tubes, maybe 40W RMS with well worn ones.
Any lower, replace tubes with new ones and remeasure.
If you have catastrophic low power, say 40W with new tubes or less than 30W with old ones, or have wild waveforms, post scope screen pictures.
I saw this somewhere---seems like a good test:
The Super-Secret Transformer Tester
When we open the DC loop, the voltage on all windings will increase until something conducts current. If all of the windings are open (as they should be), the NE-2 lamp will break over and flash when the voltage across it hits 75-90V. But if there is a shorted turn inside the transformer, the iron’s stored magnetism will be entirely spent on trying to make the current keep flowing in that turn of wire. The neon light will not flash. Watch the lamp. If it flashes, there is no internal or external short. If it does not flash, there is a short. It’s that easy. Caveat: If there is a load on any winding, those loads will eat the inductive kickback and the neon will not light up.
The Super-Secret Transformer Tester
When we open the DC loop, the voltage on all windings will increase until something conducts current. If all of the windings are open (as they should be), the NE-2 lamp will break over and flash when the voltage across it hits 75-90V. But if there is a shorted turn inside the transformer, the iron’s stored magnetism will be entirely spent on trying to make the current keep flowing in that turn of wire. The neon light will not flash. Watch the lamp. If it flashes, there is no internal or external short. If it does not flash, there is a short. It’s that easy. Caveat: If there is a load on any winding, those loads will eat the inductive kickback and the neon will not light up.
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The reason I ask is that do you think one of the valves went short circuit you took out. It is unusual to have unequal resistances in the OPT and not over the whole winding. Does either valve you took out look like the plate is pitted. If you take a photo of both and post it. With a modern solid state rectifier and no fuses on the HT its quite possible to fry the OPT.
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> It is unusual to have unequal resistances in the OPT
This is fairly "usual" and there is a logical explanation. The simplest winding is one side, then the other side over that. They should have *same* number of turns. But the outside turns are longer. More copper resistance. But DCR should be <10% of nominal audio impedance so it hardly matters.
The next step is interleaf: part of one side, part of other side, repeat 2 or 4 times with some secondary turns between. This reduces but usually does not quite eliminate the DCR difference.
Yes, a truly balanced construction is possible but typically wants a special bobbin which wants to increase leakage inductance. At lower impedances Western Electric could do some superbly balanced line transformers. But this far past what any small audio needs.
This is fairly "usual" and there is a logical explanation. The simplest winding is one side, then the other side over that. They should have *same* number of turns. But the outside turns are longer. More copper resistance. But DCR should be <10% of nominal audio impedance so it hardly matters.
The next step is interleaf: part of one side, part of other side, repeat 2 or 4 times with some secondary turns between. This reduces but usually does not quite eliminate the DCR difference.
Yes, a truly balanced construction is possible but typically wants a special bobbin which wants to increase leakage inductance. At lower impedances Western Electric could do some superbly balanced line transformers. But this far past what any small audio needs.
Certainly there will be some difference in the DC readings of the two halves of the transformer---due to, as mentioned, different methods of winding. But I don't think this accounts for the ~68% difference that the OP is reporting---that can't be normal. Need to do the test for transformer shorts.
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