| Author | Message | ||
     Eliot Curtis (roboliot) Username: roboliot Registered: 12-2011 |
First off, I want to say thank you for this board. From trolling I have learned a lot about my wonderful Music Man amp, and feel as if I am closer to resolving its demons. The amp at hand is a 410HD One-Thirty combo, with the 12A7 rectifier. About 2 years ago, when the amp first went out of commission, I was playing guitar through it when, I noticed smoke coming out of the amp around the power tubes. I looked back there and all 4 6CA7's were glowing hot. I immediately powered the amp off, and waited an hour or 2 before attempting to power it back on. When I powered it back on, it worked, but I would say the output was at about a quarter of what it should be. After playing through it for another 20 minutes, the mains fuse blew. My first thought was to replace the power tubes and fuse, as the tubes had actually burnt, which I did. After doing so the amp would power up, on stand-by, and I could see the heaters come on in the tubes. I then thought all was well, until I switched the stand-by switch up. I then heard a loud AC hum come out of the speakers, and then the fuse popped. It did this in both the High and Low switch positions. At which point I came to the oracle which is the internet, and that is when I discovered forum. After reading many of the threads on this forum, I decided that I would recap the HV electrolytics and go from there. When I read one of the big 100mf puppies, it appeared to be open. I then ordered a set of 4 spraugue atom axials from Mouser and installed them properly. I am surprised that the longer caps fit in the case so nicely, but they did. I powered the amp back up, and replaced the fuse, with it in stand-by, and I was hopeful, as I saw the heaters power on, and I thought it was going to work. I then flipped the stand-by, and bam, the fuse went again. Back to the forum I troll... I then went in with a DMM and checked all of the diodes to see if any were open, and the all checked out. I checked all of the wire and switches, and everything seems solid. I also swapped the rectifier with another tube that I knew was good, and it had the same effect. I did notice that right when I power the amp on the rectifier tube glows very, very bright for a second. Is that normal? At this point I am at a loss as to what the issue. I really, really miss this amp, because I love its amazingly clean tone. I must say that I am a novice when it comes to tube amps, but do have a background in electronics. I am hoping that there is something really stupid that I am overlooking here. Thanks for your help. roboliot | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Hi Roboliot, welcome to the forum! There are six major components that could be causing your fuse to blow when you flip the standby switch: 1. Output tubes 2. Output transformer 3. Tube sockets 4. B+ rectifiers (diodes) 5. Filter capacitors 6. HV clamp diode/cap on output tube sockets You already checked or changed 5. and 4. - 1. may or may not be a problem even if you changed them. The first thing to check is to remove all tubes (including the phase splitter 12AX7 - and yes, it would be ok for it to light up really brightly for a second). With all the tubes removed, does the main fuse blow when you flip the standby switch? If it does, you need to re-examine the HV protection diodes and caps on the output tube sockets again (D7, D8, C39, C40 in the schematic). If either of these components are shorted, the fuse will blow. If these components are ok you can try disconnecting the OT center tap at point 'H' on the schematic. If the fuse still blows, your problem is in the rectifier/filter/voltage doubler circuit. If you are ok here, re-connect point 'H' and disconnect the OT primary wires from the output tube sockets (tape them up and fixate them really well!). If the fuse blows again, you have a shorted OT. If you are ok, your problem may be in a shorted tube socket (these may or may not show themselves with just a DMM - if the socket material is somehow damaged (burnt), the shorting starts happening at a higher voltage). Keep the amp on the low power setting at all times. Ideally you should have a variable power transformer (Variac) as a troubleshooting tool, but I realise not everyone has one. I hope this gives you something to go on. Cheers, Lars Verholt |