... I mean, so quiet, I had to play it again just to make sure it was working. Here's the story:
If any of this sounds familiar, it's because when I searched for information about this model, there are a few specific problems that owners and technicians experience which seem to come up over and over again relating to "noise". In my example, there were three sources for the noise problems, so I'm probably going to have to break it up into a few different posts.
First: On the ones I've seen, the idle hum is the biggest point of concern for most customers. Often, they request a "cap job' to replace the original electrolytics that were stock to the amp. This isn't unreasonable, and is often the first thing a tech might look at with an amp of this vintage. However, in these models, you can often replace all of the electrolytic caps outright, and even increase the capacitance thinking it's a lack of filtering only to have no effect on the audible hum level. This is how it was with the both of 2 models that recently came in for service (with the second coming in about 2 weeks after the first).
We noticed fairly quickly when testing the amp, that the hum start the moment the switch was turned "on", even before the tubes heated up. So, we pulled all the tubes, powered it on, and the hum was still present! This left the most likely source being the leakage flux coupling through to the output transformer, which was exactly the case. The only way to fix this is to remount the output transformer. Realistically, there are two ways to do this; First, mounting it on the bottom/outside of the chassis. We weren't sure if it would fit using the existing mounting holes with the speaker frame. The other way, which we did, was to rotate the OT 90˙. This can be done without having to do a bunch of rewiring and drilling, if one is very careful. We were able to use one of the existing mounting holes and an L-bracket to position the OT and completely null the radiated PT flux.
It's easier to show a couple of illustrations of how this worked. The first is an overview of the stock manufacturers layout, and the second is the position in which we were able to achieve the lowest noise:
If you decide to remount it internally, I would advise that you follow these steps to make the job very easy. I happened to have some brass L-brackets which fit nicely, but you may have to do a little searching or small amount of fabricating.
This solves the first problem.
I encountered 2 more which I'll write about, and how to fix them shortly...
If any of this sounds familiar, it's because when I searched for information about this model, there are a few specific problems that owners and technicians experience which seem to come up over and over again relating to "noise". In my example, there were three sources for the noise problems, so I'm probably going to have to break it up into a few different posts.
First: On the ones I've seen, the idle hum is the biggest point of concern for most customers. Often, they request a "cap job' to replace the original electrolytics that were stock to the amp. This isn't unreasonable, and is often the first thing a tech might look at with an amp of this vintage. However, in these models, you can often replace all of the electrolytic caps outright, and even increase the capacitance thinking it's a lack of filtering only to have no effect on the audible hum level. This is how it was with the both of 2 models that recently came in for service (with the second coming in about 2 weeks after the first).
We noticed fairly quickly when testing the amp, that the hum start the moment the switch was turned "on", even before the tubes heated up. So, we pulled all the tubes, powered it on, and the hum was still present! This left the most likely source being the leakage flux coupling through to the output transformer, which was exactly the case. The only way to fix this is to remount the output transformer. Realistically, there are two ways to do this; First, mounting it on the bottom/outside of the chassis. We weren't sure if it would fit using the existing mounting holes with the speaker frame. The other way, which we did, was to rotate the OT 90˙. This can be done without having to do a bunch of rewiring and drilling, if one is very careful. We were able to use one of the existing mounting holes and an L-bracket to position the OT and completely null the radiated PT flux.
It's easier to show a couple of illustrations of how this worked. The first is an overview of the stock manufacturers layout, and the second is the position in which we were able to achieve the lowest noise:
If you decide to remount it internally, I would advise that you follow these steps to make the job very easy. I happened to have some brass L-brackets which fit nicely, but you may have to do a little searching or small amount of fabricating.
This solves the first problem.
I encountered 2 more which I'll write about, and how to fix them shortly...
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