| Author | Message | ||
     Jason Smith |
I have a MM HD130 w/ El34's and a 12AX7. I bought the head used about 6 months ago. Since then I've noticed that it appears to be modified! I've found that the audio transformer is not original and I also noticed that there is no filter choke. What is a filter choke for anyway, and do all MM amps have/and/or need them. I have pictures that I can send if anyone would like to see them to help me better understand what mods have been done to my amp in the past. | ||
| Steve Kennedy |
The filter choke appears to be a part of the DC power supply noise filter for the output tube grids. The grid supply is 1/2 the plate voltage and contains additional filtering because any additional power supply noise injected into the grid will show up amplified at the plate (and on to your speakers). It sounds like this amp lost its output transformer at some time and the resulting failure smoked the output tubes and took the choke coil with it. It was probably easier to jump across the old coil than replace it for whoever fixed it. Without the filter choke, your amp may be a bit noisier but I doubt there is any other drawback to having it missing. The audio transformer was probably either rewound or replaced with an off-the-shelf replacement that was close enough to operate. I cannot comment on how desirable this may or may not be. I would definitely keep an eye on the output tubes (no red-glowing plates) and check the temperature of the output transformer occasionally. If all seems well, then it is probably OK. If you don't notice any problems, then you can probably trust it. My main concern would be if someone threw a transformer in there that was not designed for the 700+ volts that it will see when the amp is set to high power. If this transformer is not heavy duty enough, it could arc-over and short. This would hopefully blow a fuse and not smoke the rest of the output stage! I don't want to alarm you, just provide some feedback. If you are using the amp and it has been working fine the repair was probably adequate for this model. |