| Author | Message | ||
     Thomas Dabrowski (snaedelarj) Username: snaedelarj Registered: 11-2009 |
First things first: Many thanks to the people posting in and maintaining this forum. It gave me good insight about Music Man amp's technical interior. I know a little bit about electronics, largely self-educated, but have close to no experience in repairing amps. I hope this success story could help people having the same symptoms. My second hand HD-130 worked like a charm for 6 years and was the most reliable tube amp head I owned until it stopped with a buzzing, fart-like tone. Exchanging tubes didn't help and the only thing that worked was a loud and short break-through while cranking up all pots and strumming the guitar really hard. First thing I noticed was a significant voltage drop on measuring point F which ought to be 340V. It was a somehow shifting voltage between 95-140V. The output tube screen voltage (G) was fine at 357V. Also the bias was at 0V. I replaced all the electrolytic caps on the filter and rectifier board and exchanged the filter board resistor (R65, 10K). No change. I measured all resistors and re-soldered all connections on the driver board. Still no change...but...after like 3-5 minutes being on and connected to a speaker, there was a sound I would describe as blowing on a blade of grass and the amp was working again and sounded fine. Yeah! Voltages were fine and bias was ok. The happiness only lasted for a few minutes until I switched the amp off and on again. No sound, no nothing. But after approximately 4 mins it turned on by itself again and I saw a blue flash in one of the output tubes. I tried another hint given by some people on this forum and measured the thick output tube resistors (R56+R61, 10 Ohms, 1W), freed one leg to do so and they seemed to be good. Carefully hitting them while the amp was switched on, working or not, had no effect, even hitting them a bit harder. I nevertheless replaced them with new metal film resistors and voila! - that did the trick. Maybe someone here could explain to me the correlation between the output tube resistors and the circuit (F, 340V) that supplies plate voltage for the 12AX7, if I haven't got it wrong completely. Besides having been able to repair the amp, I don't technically understand how it worked. My HD-130 is working again since a month and I was so happy to have it back again after half a year being out of order - the substitution, a Fender Bassman 135 head, couldn't reach the sound quality at all - at least with my setup. You guys here saved me a lot of money. Thanks again. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Congratulations on the beat and fix! Sometimes, they just need a good thrashing! Honestly, I have to assume by what you tell me here is that the cathode resistors(the ones that you replaced) were flaky and allowing the plate voltage to go low at times, thus increasing the plate current. As current goes up, the voltage drops. This is a meatball explanation but it's really all I've got at this point. If you were to measure the voltage on the plates and then change the cathode resistors drastically or change the bias supply voltage, the voltage changes on the plates. I'm not saying that this IS the explanation but it does explain some things. Mike. |