| Author | Message | ||
     UK Jim |
Hello, I have a slight problem with my Music Man amp that I was hoping someone might be able to help out with.... Every once in a while when I'm playing through my amp the volume suddenly jumps up slighty. The master volume control then becomes ineffective below about two or three leaving a 'preset' minimum volume. The fault occurs on both the high and low settings and is usually cured by switching into standby and back or cycling the power off and on. Usually a hissing/rumbling sound like distant thunder accompanies switching into and out of standby when the fault is present. Thanks to this excellent website I have replaced the electrolytics and wicked the bias up a smidge which seems to have resulted in a far tighter more focused sound, but hasn't cured the problem... I have some new valves in the post from Eurotubes as I'm unsure of the current set's condition (two of the EL34s are Mullards the rest have no markings). The amp is an HD130 212 with the 12AX7 phase inverter. Many thanks UK Jim | ||
| michael kaus |
It sounds like you will solve your problem with the tubes but I would re-ternsion the sockets a little while you have them out. You did order the 12ax7 also didn't you? I wouldn't be surprised if that wasn't the trouble. The hissing/rumbling does throw me a little though. You might check the condition of the big cathode resistors on pin 8 and see if they are hot looking or cracked.They take a lot of heat. Let us know. Mike. | ||
| UK Jim |
Hi Mike, many thanks for the reply and advice. The tubes arrived today (12ax7 in there too) so I'll be putting them in sometime in the next few days... I'm not quite sure what you mean by re-tension the sockets though.... Cheers UK Jim | ||
| Michael Kaus |
THe little pins on the tubes go into little metal grippers in the sockets. They have a certain amount of tension on the pins so they don't arc and get corroded. It's a good habit on older amps to get a little ice pick type tool and bend the little tabbs so that there is plenty of tension on the tube pins. Also, cleaning them with a little in and out motion with some tuner/contact cleaner is advisable also. Mike. | ||
| UK Jim |
Hi Mike, I tensioned up the tube sockets, put in the new tubes and rebiased, plugged in my guitar and was rewarded almost imediately with the jump in volume! I'm going to try replacing the cathode resistors with some big old wirewound chaps as you suggested. I had wondered if the hissing/rumbling sound could be capacitor related? Many thanks Jim | ||
| michael kaus |
The resistors on the tubes DO act as a second kind of <fuse> of sorts so I really wouldn't go to wirewounds. I think your amp had 1 watt resistors there so that should be plenty. I put 5 watt ceramics in mine simply because I had them and I overdo everything! If you want to beef it up, 3 watt would be plenty. Mike. |