Experimenting With Three Methods For Adjusting AC Balance In Vacuum Tube Amplifiers
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- เผยแพร่เมื่อ 11 ก.ค. 2024
- The old books suggested (1) set the balance pot in the middle (2) use a signal generator and oscilloscope and adjust for symmetrical clipping top and bottom of the sine wave (3) use a distortion analyzer and adjust for lowest THD. There are a number of amplifiers with balance pots to include but not limited to: Fisher 500C series, McIntosh MA230, Fisher 70-A, Dynaco clones and others.
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Big fan of your videos, thank you!
It's interesting that adjusting for symmetrical clipping was so inaccurate. A fourth way to do it would be to adjust for minimum 2HD on a spectrum analyser, which is what the process is really all about, rather than symmetrical clipping, which makes all sorts of other assumptions. I would expect this to agree completely with the AA501 THD method.
I used to travel to HAFB in the 70's and 80's to service equipment at their Army Comm Center. Drought in El Paso? What drought? That's the way it is here. Thanks for your comments.
Thanks for showing those techniques. Obviously using the distortion analyzer is the best. I saw another technique suggested for adjusting AC Balance when the output tubes have separate cathode resistors. This technique suggests using a 100 Hz input signal with about 70% power output, and adjusting the balance so that the DC voltage difference between the two cathodes is minimized. If you have an opportunity I'd like to see how that approach works compared to the other techniques you showed.
In many amps it helps to disconnect the Feed Back loop for AC balance adjust.... The reason is because the feedback loop is trying to balance the AC signal and fighting against your balance methods...until you push the drivers out of headroom ..then they are not as effective..
I am in awe of your shop equipment.All i have is a Fluke 77 simpson 260 and an old Tectronix RM 561A also an HP 200 audio oscillator. Never found anything tube or S.S. i couldnt get running again but definitely takes more time n I couldn't give customers the distortion specs cause I never had the bucks equipment as you do.I would sit down with schematic (hand drawn or a Sams if i was lucky) & try to figure out what the design was supposed to do and what was it actually doing based on my measurements.Toughest so far? Early Crown DC-300 not the A theyre pretty easy.And lately a 1949 Midwest 916 radio that is so microphonic on FM you cant turn the volume up too loud! and it breaks into some kind of RF oscillation on the low end of the FM band.I'm thinkin i missed a bypass .01 somewhere.I'll find it. But my hat is off to you for being so well equipped. N2VUW 73's
hi i look at, and look forward to seeing your videos where have you been lately?
I was told by my local repair shop that he could not fix the fm tuner. The rest of the unit works great. I do not work on them myself.
I would check distortion also with the >400Hz filter, just to rule out 120Hz power supply hum that comes up when the output is unbalanced. Some of the distortion you are reading may be hum, if this is the case.
Great video! I have a question: you injected 2000Hz signal; what about its PPVoltage?
Mephysto65 Please give me more of what you mean by PPVoltage and I will see if I can help.
ElPaso TubeAmps Maybe I'm a little bit confused. When you test amp for thd you inject a signal (1khz, 2khz or whatever) in its input. This signal has a voltage: do you start from a value and then increase it until distortion occurs or set a fix value and then increase amp pots until distortion? Thanks in advance
Mephysto65 I understand your question now. In power amplifiers like the Dynaco, Mcintosh, etc. it takes about 1 volt (or more) to drive the amplifier to full power. In a guitar amplifier, at the 1/4 inch plug where you plug the guitar in, it takes maybe only 0.050 volts (50 mv) to drive the amplifier to full power. So, depending on what you are testing, it is a good practice to start out with the driving voltage really low. Assume your generator is set at 0 dB output (which would be about 0.77 volts across 600 ohms) , you might set your attenuator at -50 dB so that you do not overdrive the first state (often times a very high gain 12AX7 state). If you overdrive the first state then everything after that state will be over driven and distorted. So, while watching the output on an oscilloscope, you would slowly raise the drive voltage from a few millivolts upwards until the DUT (device under test) is over driven or starts to clip or THD rises suddenly, etc. . A little experimenting and I think this will make sense. Hope this helps.
ElPaso TubeAmps Great! I'm coming out of the fog. Talking about 600ohms termination: is it a rule for every generator involved in audio test or there's something else to know in order to determine if I have to insert a 600ohms termination or not? By the way: I have 50ohms termination and if I put one of them from generator to oscilloscope I noticed that voltage drops 1/2 value (ie. 4Vrms to 2Vrms)
Mephysto65 If it drops to 1/2 value you have the correct impedance terminator.
watched twice but still wondering where you placed the leads of the scope. across the primary of the output xformer?
The scope is on the secondary side of the output transformer where the 8 ohm dummy load resistor is connected in place of the speaker.
Thanks
hello I just had a fisher 500c restored here locally in Houston. They could not get the fm/ radio to work. Can you help?
+felix serna Hi Felix - I don't take in any equipment for repair anymore but I have been successful in helping others over email in getting their equipment repaired, assuming you are able to work on the instrument yourself and you have basic test equipment.
can i do this without oscilloscope ?
Yes, it think it should turn out nice if you put your volt meter from grid to grid of the output tubes and adjust for zero voltage while driving it at a reasonably "loud" level.