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Post by mauro on Dec 3, 2018 1:30:04 GMT
Usually, after rebuilding an amplifier I use the listen test and it has served me well but once in a while the amp just doesn't sound like its loud enough or it lacks bass.
I have a ton of testing equipment and am wondering if there is a way to quantify the performance through measurement?
I know this is not Seeburg specific but I have 3 HFMA1-L6 amplifiers that are completely rebuilt and sound like crap!
Either not loud enough or lacks bass, I have gone through them countless times and can't find an error or bad component. I'm wondering if I can inject a signal and put a dummy load on the speaker plug, then measure the output and know that it is operating as it should. I'm sure you can but I've been reading countless books and none explain what I want to do.
Perhaps one of the amp gurus can satisfy my curiosity.
I'm pretty sure I have enough equipment to do this but just don't know how.
I really appreciate the time it takes to explain this and hope someone can steer me in the right direction.
Thank you,
Mauro
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Post by robnyc on Dec 3, 2018 11:03:37 GMT
Mauro,
For what are usually final tests on amps, I start with the power stage.
Tools are:
-A.F. Signal generator.
-Dummy load at least 50% greater in wattage than the total expected output from the amp. I use 100w variable type and set them for the intended speaker impedance (8 ohms here). The reason for over-wattage is to be reasonably sure the dummy’s resistance doesn’t change much when fully driven.
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-Accurate (preferably analog) AC meter w/db scale.
I use a Simpson 360. The problem with a DVM is that they become less accurate as frequency increases. At frequencies not much greater than 50-60 Hz they are OK
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-A set of known-good tubes. Use same tubes for all of the same model amp.
For these Seeburg amps I first test the power stage. Go in at Pin 7 (remove the RVC plug) make sure the AGC switch is set to ON so that the full generator signal is coupled.
I’ll use a basic frequency to start such as 1KHz and ramp up to where clipping first occurs. With strong American tubes (RCA-G.E. etc) you see about 12-13 VAC giving 21-22 watts into 8 ohms. My experience with Sovtek 6l6 had them clip at barely 17w. The difference isn’t meaningful here though.
Make sure the 12AX7 driver-splitter is strong, This is not the best tube here, a 12AU7 would have been better in dealing with the final’s load when they start to draw grid current. They wanted the gain of an AX7 I guess.
I also test at 50Hz to see what the power bandwidth is. Amps often show more distortion at the frequency extremes. Since my large dummy loads are not non-inductive I can’t get accurate full output results in the highest frequencies. I do have 20W non-inductive types and limit those tests.
For the preamp stages in those tube amps a simple voltage divider of 100K-1K will bring the generator down to a manageable level for the preamp. Remove the 6SK7 and be sure the AGC switch is set to “ON”. Scope and measure will take place at where it is presented to the volume control at pin-1 RVC plug. Other than frequency response, which will be close to the RIAA curve on these, I don’t recall the max level before distortion at pin-1. In service these amps put out around 2 volts AC to the volume control –that is with AGC functioning at-near fully driven and music.
AGC tests are a separate matter we can go into if interested.
RobNYC
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Post by mauro on Dec 5, 2018 22:27:50 GMT
Rob,
Thanks for explanation, some of it I dont quite understand but when I get set up to test, as you outlined, I'm sure I'll have some very specific questions.
Hope you can guide me on this, it's like anything I else, the first time is going to be a long process but then once I've done it I think I'll be able to repeat it easily.
Thank again,
Mauro
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Post by robnyc on Dec 5, 2018 22:47:48 GMT
Mauro, for what you are looking for in frequency response you will need a good meter. However, for simple power test on location I have a small oval speaker with 70v transformer attached, the tran is tapped for about 5000 ohms (1/10th watt). I connect this across the 8 ohm output along with the dummy load. When I inject the sine wave I can clearly hear the harmonic when the amp clips without being deafened. Take the reading and you're done. The scopes I have; two Tech's and a Heathkit are not very portable -sort of like the old "portable" TV's.
Good luck,
Rob
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