| Author | Message | ||
     Gilmour Jope (gilmour_jope) Username: gilmour_jope Registered: 02-2012 |
Hello All, I’m having a problem with my 1978 MM 65 Reverb Head (non-12AX7, chassis #2275-65) and I’d be very grateful for any suggestions you might have. About a year ago, I got this amp in non-functioning condition; it had an old (probably original) 6CA7 tube and a Groove Tubes EL34 in it, along with a very dodgy power cord. After searching around a bit, I ordered some JJ EL34L tubes for it and a correct replacement power cord and I brought the amp to a local and reputable (though non-MM specialized) tech, who replaced the two large filter caps , the power cord, and installed and biased the new tubes. It’s been working great since then and I replaced the reverb tank myself with the correct MOD rank from AES without any problems. About a week ago, however, the power has been cutting out after about half an hour after turning it on and this happens in both channels and when using both the ‘HI’ and ‘LO’ power settings on the front switch. On my amp, there is a main power switch on the back marked ‘ON/OFF’ and the front switch is marked ‘HI/STD. BY/LO’. When the power cuts out, the tubes and the light on the front switch remain lit and I can get the power back by switching between the ‘HI’ and ‘LO’ positions on the front switch, but it cuts our again with increasing frequency soon afterwards. I have also noticed that I get an audible, 15-second ‘gurgling’ sound in the ‘STD. BY’ position before the amp eventually goes into silent standby mode. Any suggestions you might have as to possible causes of the problem or sources for replacement parts would be much appreciated! Thanks, Gilmour | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Check the screen grid supply resistor (R63 1.5k 10W). If this resistor is defective and gets intermittent, the output tubes will stop conducting and you get no sound. Another obvious place to look is the standby switch. What points to a bad connection rather than a diode/cap/tube short is that your fuse does not blow. Either way, it would be a relatively easy thing for your tech to troubleshoot with the proper tools. The gurgling sound when switching to standby is not uncommon in the MM amps with the solid state driver. It doesn't hurt the amp but can sound a bit funny at times. Hope this helps, -Lars Verholt www.captain-foldback.com | ||
| Gilmour Jope (gilmour_jope) Username: gilmour_jope Registered: 02-2012 |
Thanks, Lars! Gilmour |