| Author | Message | ||
| Mr.Shiney |
Just got this 65 head, sounded really bad so I put new output tubes in it, well not really new they were spares I had, and that helped quite a bit but still doesnt hold a candle to my boogie. It just doesnt have that sparkle, the louder you turn it up the "harsher" it gets. its gotta sound better then this, doesnt it? what should I have checked by my local tech? thanx for any help | ||
| Michael Kaus |
First off, does your head have a 12ax7 Phase Inverter tube(a little one next to the power tubes?). If it does, it could be biased too cold and giving crossover distortion or too hot and breaking up too early. If no little tube, you have one of the SS driver versions that are set with a separate type of procedure. You have to measure the voltage on the cathode resistors and balance them. If you have the later amp with the 6l6's instead of the 6ca7's, it's pretty much plug in and go. Then you have to start looking somewhere in the signal path for more "sparkle". In other words, pedals. These amp are clean with very little coloring and that's what I like about them. You doctor them to the way you want them. Mike. | ||
| Steve Kennedy |
Another place where "punch" and "sparkle" are lost is in old capacitors. Aged or dried-out power supply filter caps can cause an amp to sound sterile and lifeless (no dynamics or "punch"), even if it doesn't hum. Leaky tone-control caps, Bright/Deep caps or interstage coupling caps can have a dramatic influence on tone (no sparkle, early distortion, low gain, etc.). I recently got around to evaluating an HD-130 head I bought years ago on eBay. I had physically restored it (new back panel, new grill board, cleaned and patched up, etc.) but I had not touched the electronics yet and hadn't used it since. The amp sounds lifeless and distorts early (on the "good" channel). I found that the high-voltage is only about 650V at the high-power position and the +/- 40Vdc supply is only about 28Vdc. This is an oldie... 12AX7 phase splitter and a serial number just over 2000. From all symptoms and measurements, I think this one needs new caps at a minimum (and perhaps new high-voltage rectifiers) to get its groove back. I get tired just listening to it! Oh, and the gain of the first two op-amp stages had been "artificially" increased by multiplying their feedback resistors by a factor of 3 on one channel! Talk about distortion! You couldn't get a clean on that channel! The unit also oscillated at high Volume control settings as well so it was REALLY honked-up... just a bad attempt at making the circuit sound like a Marshall, I guess! When I can get some time, I intend to electronically restore this one, but I need to get a lot of caps as a start! Also keep in mind that units originally built with the 12AX7 typically did their Hi/Lo power selection at the AC input winding of the transformer... a lot like taking a variac and turning it down. All the secondary power supplies will reduce causing earlier distortion than at the Hi power setting. This was changed to a High Voltage-only switch on the transformer secondary in later models. Steve | ||
| michael kaus |
Steve. You mention the feedback resistors on the first two stages of the preamp. I have been wondering how I could tame down the preamp of my MM and that never occured to me that I could doctor that. I think that might be the place to start. Might tame down the ratio a little. Amazing what you think of reading someone else's suggestions. Thanks. Mike. | ||
| Steve Kennedy |
Of course, if you are shoving too much signal at the input, changing the feedback (gain) resistors may not have the desired effect. In a case like that, changing the input voltage divider might work better. |
Tue, 03/25/2014 - 09:57
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