Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: acheld on March 29, 2020, 04:10:59 pm
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Greetings!
I built a standard Hoffman AB763 awhile back -- seems to work well. However, the head cabinet seemed to be a little warmer than usual (it smelled hot, but not burnt) -- and with time on my hands and the need for some precision in the world, I went back and remeasured voltages.
A = 442V
B = 431V
C = 341V
D = 277V
Pin 3 of V7 and V8 = 433V
The voltage drop across the 1 ohm cathode resistors was 40 and 35 mV, higher than expected. I reset the bias to draw 25 and 22 mA. Before doing so, noted that V7's plate had a slight glow in the dark. I think this is why it smelled hot.
While reviewing the build, I was unable to find expected voltages within Doug's project notes. When compared to Rob Robinette's annotated schematic for the AB763 DR, all the voltages I measured are higher than his -- admittedly well within the Fender spec of +/- 20%.
Are these voltages in the ball park what others are seeing for this amp?
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I would not be concerned with your voltages. Here are my voltages using 6V6s...
http://sluckeyamps.com/tdr/tdr.pdf
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Trem-O-Nator candidate
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When undertaking such voltage surveys, I think it highly beneficial to record contemporaneous wall and heater Vac. They provide essential context and reference points, without them half the picture is missing and comparison to other amps (or the same amp across time) loses validity.
It would be nice to have an idea why the ‘as found’ power tube idle current was so high - any thoughts?
Concerns being that the tubes are getting gassy, or something is increasingly loading the bias supply as time passes, eg leaky reservoir cap.
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Always try to use a Variac, Autoformer and set your wall voltage to what the amp is designed to use. Consider AC voltages raise 10%. How much do you think your DC will increase?
If within spec of 20% is leniant for current times. This tool allows you to record your amp voltages and if something changes, but the variac is still giving 117VAC or whatever, you then know something has changed.
Shines lights on mushrooms so I can find them.
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So, these answers are why this forum is so good, so consistently. Much appreciated!
I thought about posting the wall voltage and bias setting -- much later after I had sent the first post, and will go back and do so in the next day or two.
As to why this happened, I am still baffled. I suspect I made an error when I biased the tubes 5 months ago after completing the project. I subsequently used the amp sparingly for a month. The good news is that it smelled hot at some point, and I put the amp aside in my man-lab -- but only got into it this week. Previous biasing method was to use direct measurement of plate current via a Euro-lab clone. This time I measured voltage drop across the 1 ohm resistors.
More to follow, and thanks for helping.
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It's very common for amps to get hot. That doesn't mean there is a problem. If you want it to be cooler, put a little fan on it.