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Originally Posted by Ares /img/forum/go_quote.gif
I first would like to thank the forum for the wealth of information that allowed me to build this beautiful piece of hardware. However, I have a few questions.
While my wiring is messy and my parts are budget, (carbon resistors, 610s instead of 510s, generic 180uF capacitors, 24AWG copper wire) the thing seems to function.
1) While the amp normally works well, and glows normally, sometimes after i mess around with the MOSFETs, one tube will glow like a 15W lightbulb and the other will stop glowing. This has never happened with headphones in and I always unplug it before it warms up to the full glow. What is going on here? I think i fixed it (rewired the MOSFETS to try to minimize shorts, but i am still curious).
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No offense, but what is going on is not good. Having a tube flare up like that is caused by over-voltage applied to the heaters/cathode. It's a good way to burn the tube up. So, somehow messing with the MOSFETs is shorting the circuit to one of the tubes so that they're exposed to the full voltage (the tube heaters are in series, so in a correctly wired amp, they'll split the voltage between them).
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| 2) With headphones pluged in and nothing playing, when i touch the side of the aluminum case that I used, a pinging micro-phonic is transmitted through the headphones. Originally, I had cold solder joints on the ground plate, i believe i fixed it (as the torrent of sound that came through the earpiece when i touched the case has stopped but the ping is still omnipresent). Is the ping still a grounding issue? |
This has nothing to do with the case or cold solder joints. The pinging would not transmit unless the tubes picked it up and amplified it. That means one or both of the tubes is microphonic. Unfortunately, this is not something that can be tested for with a tube tester - only by putting the tubes in an amp can you tell that this condition exists. Any good tube vendor will replace a microphonic tube upon request, however.
Just to be sure, swap the tubes and see if the pinging travels with one tube more than the other.
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| 3) I have installed 2 switches into the case. Both times, I would plug in amp after the switch was installed and the amp would make a click and the switch would no longer work. Did the case (which is wired to ground) short with the 48V? What is going on? Why is my amp frying switches? |
The click you hear is in the Cisco power supply. It has an internal circuit breaker that will trip and then automatically reset itself when you unplug it from the shorted device. I doubt seriously that the switch has anything to do with it - most likely your first switch was OK. Reading above about your trouble with the MOSFETs would explain a lot - especially if this is an intermittent issue. Or, you may have a short somewhere else, too.
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| 4) I need a new audio pot (the channels are off by about 15%); what is the cheapest pot that is within about 5% range. I would ideally like to spend less than $5. Many people recommend the ALPS RK27, but i cannot afford that. |
I've not had any trouble with the Alpha pots once you apply the input resistors, but I know people have complained about them. Unfortunately, there's nothing much available under the RK27. The RK097 is a good option, but I've heard just as many complaints about channel mismatching with that one, too.
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| 5) I don't hear an audible click when the amp powers up. Therefore, is it safe to leave my headphones plugged in while turning the amp on? |
Not necessarily. It's possible that the stuttering/slow turn on is enough of a delay by itself to prevent any offset from occurring at the headphone jack.
However, the best thing for you to do is to measure the voltage relative to ground at the headphone jack connections when you power it up. That'll tell you very quickly if there are voltage levels high enough to damage your headphones.
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| Sorry if any of these were already answered, I tried my best to find answers but the post is so long and has so much information. |
No problem - hope this helped!
