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#1
Posted to rec.audio.tubes
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Silvertone 1484 Power Tube Issue
Unbeknownst to me until after all this occurred, my buddy loaded in my
Silvertone 1484 upside down as we travelled 45 minutes to a show. It powered up fine and even when I switched from Standby to operate it stayed on. However, I got no sound out of it, not even the much dispised 60 cycle hum I am so accustomed to hearing. I thought this was strange and I turned volume up on both channels to see if maybe my ears were still messed from an extra loud rehearsal the night before. No sound. I eyeballed the tubes and they were all operating properly-lit up as they should be. When I unplugged the cable from my guitar and touched the tip with my hand I heard a very faint high pitched squeal. Shortly after that I started to smell something burning and the pilot light went off. I checked the fuse and it had popped. I replaced the fuse and hit the power switch-immediately popped that fuse. At that point I realized my problem was deeper than a fuse and I left it alone until after the show. When it had cooled off, I pulled the 1st power tube (6L6) (as the smell seemed to be strongest near it) and saw that the bottom tip of it was black. When I got it home and pulled the chassis I could see that the socket had been burned at pins 2 & 3 and that the wires actually fused together. I'm not sure what caused these wires to heat up to melting point but I'm thinking that the tube may have wiggled itself a bit loose during our drive and therefore been making weak contact. Everything else in the amp seems to be ok, it's just that one socket that is affected as far as I can tell. I suspect that the wires making contact with each other caused the short as pin 3 is the plate and pin 2 is the filament---Is this right? I think the OT is fine as the job of the fuse is to protect anything major from being damaged---Is this right? I plan to add a 3 prong power cable and I may replace the tube sockets while I'm in there. All the output caps are original and look great, no leakage and the amp sounded amazing before this. I've never had a problem with it prior to this. Should I replace the output caps anyways? Also, do I need to replace the tubes? They're Svetlanas; about 2 years old. I'm going to try to get some pix so those that decide to respond can see what I can see. Thanks for reading! |
#2
Posted to rec.audio.tubes
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Silvertone 1484 Power Tube Issue
Barry:
A couple of things could have happened, including what you suggested. But: Something inside the tube could have worked loose and shorted a tube internally. Yes, it could have arced from a poor connection and caused the damage you saw. It is unlikely you were getting signal to the OPT such that it would have been damaged... or if it did, you would have smelled it pretty quickly. From the symptoms, it "sounds like" a local short of some nature. BUT: Before you apply power, check the tubes (all of them) for shorts. Clean up any possible carbon tracks generated by the fault (it could even be 100% unrelated to the inverted transport), and check all the wiring and circuit boards (if any) carefully for bridges or breaks. Good luck and report back, please. I will spare you my rant on fuses.... Phil Allison might rise from his crypt. Peter Wieck Wyncote, PA |
#3
Posted to rec.audio.tubes
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Silvertone 1484 Power Tube Issue
I also forgot to ask...does anyone know how can I build a device to
slowly charge an amp without ruining the caps? I know of the technique of putting a lightbulb in series with the amp, but how is this specifically done? I'd prefer not to buy a variac and I've heard they're not the best option anyways. Thanks for the info Peter, I'll post a follow-up hen I get into the amp and I'll do my best to let sleeping fuse rants lie. |
#4
Posted to rec.audio.tubes
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Silvertone 1484 Power Tube Issue
barry casebeer wrote: I also forgot to ask...does anyone know how can I build a device to slowly charge an amp without ruining the caps? Barry: There are multiple answers to that question based on what you wish to achieve, as follows: a) "Forming" capacitors in amps that have been sitting for many years, are more than (arbitrary cut-off here) 20 years old, or are of unkown antecedents: Don't. Unless it is for short-term testing only based on the full, honorable and complete intention to replace all electrolytic caps anyway (at least). More to follow below. b) You are looking for a soft turn-on, and have a solid-state rectifier: There is a device called a Thermistor that you may install in series with your power-cord that will soften the turn-on to the amp. It should be purchased to the correct size for the load, installation is pretty simple but DO NOT DO IT unless you know exactly what you are doing. If you have a tube rectifier, theoretically, it starts as slowly as the rest of the tubes, and so softens the blow when B+ hits a nearly cold tube. The "more later" part: Forming HV capacitors takes time and attention. Theoretically it should be done with an external DC supply, a meter, time and close attention, and according to a simple but persnickety process. And a good percentage of the time the it either does not work or the effects are temporary. If your unit has a SOLID STATE rectifier, you can somewhat do it yourself by using a Variac and time and attention. You cannot use a Variac (despite rumors to the contrary) with a tube rectifier as the it will not even pass current until somewhere between 60% & 75% of voltage anyway. So the caps get slammed with 60% - 75% of full B+ anyway, hardly a slow-forming process. Mini-Rant: Variacs *must* be equipped with proper metering (voltage and current) or they are dangerously worse than useless. Whether this metering is on board or outboard, it must be there. On the other hand, if you have _ANY_ reason to suspect _ANY_ electrolytic cap inside your unit, replace it RIGHT AWAY. The results could be spectacular and unhappy otherwise. CAUTION: Lethal voltages exist in most tube equipment. Do not mess around inside unless you know what you are doing *and* are familiar with working around such voltages. Peter Wieck Wyncote, PA |
#5
Posted to rec.audio.tubes
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Silvertone 1484 Power Tube Issue
Thanks Peter,
I've been using the amp for a couple of years now and I discharged it last night before poking around in it so it most definitely has not been sitting for long periods of time. I installed a 1/4 " speaker jack prior to learning that the power should be brought up slowly and while doing that I discharged the caps and just turned it back on when I finished. That was about a year ago and didn't seem to have had a negative effect on the amp. The reason I am asking now is that I would like to bring them up properly because I may have been lucky with that one. The rectifier is solid-state in this one, btw. |
#6
Posted to rec.audio.tubes
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Silvertone 1484 Power Tube Issue
barry casebeer wrote: The reason I am asking now is that I would like to bring them up properly because I may have been lucky with that one. The rectifier is solid-state in this one, btw. As long as the discharge did not go through you... that would have made you sit up and take notice! A thermistor might be your best solution then. But caps discharge within a very few hours (or much less depending on design) anyway, so near-as-makes-no-difference, you would be getting only a minimal benefit from that treatment. But a benefit none the less. Peter Wieck Wyncote, PA |
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