jamiewoody Posted November 21, 2008 Share Posted November 21, 2008 i have this old univox u-45b tube amp. it has been a work in progress, but i LOVE the sound of this beautiful little amp! i unlocked a secret, and i was wondering if any electronic gurus here could give me some insight. there are 3 inputs, "guitar", "mic", "acc". (is acc accordian?!!LOL!). if i take a pedal patch cable, and plug 2 of the inputs, the one left sounds HOT! the amp then becomes crunchy, and sounds like the volume is turned up 2x louder than it is! i guess i need to do some metering, but i am guessing the impedience of the input is changed when the other 2 are shorted out? also, is this sort of thing detrimental? there was this guy who had an old marshall with 4 inputs who did the same thing a long time ago in a band i was in. i never understood why back then. insight please? Link to comment Share on other sites More sharing options...
cGil Posted November 21, 2008 Share Posted November 21, 2008 Your amp is actually a one channel amp. It simply has all three inputs feeding the same input tube stage. It really doesn't matter what the labels on the jacks say they're for. As you can see, they're all exactly the same! http://www.schematicheaven.com/bargainbin/univox_u45b.pdf Without the patch cables you have 23.5k to ground following a 47k input resistor. As is, that thing is dumping about 2/3 of your signal to ground before it reaches the first stage! By plugging in the patch cord, you eliminate that signal dump entirely, so yes, it's gonna sound a lot louder 'cuz you've got a heck of a lot more signal hitting the grid like it oughta. In fact, you really don't need the patch cable; fresh, unwired phone plugs (dummy plugs) would accomplish the same trick for your amp. Oh yeah, and it won't hurt the amp at all. On the 4-input Marshall, something completely different's going on. It's got two separate input channels, with 2 inputs each. Your friend was merely jumping the separate input channels to hit both at the same time. This trick would require the patch cord to split the guitar signal, and dummy plugs alone wouldn't work here. Gil... Link to comment Share on other sites More sharing options...
jamiewoody Posted November 22, 2008 Author Share Posted November 22, 2008 WOW! thanks gil! it is really cool when you unlock precious secrets and sweet spots in your gear. it is even more cool when you learn what is behind it. i really need to take a basic electronics course. what about the tremolo? the amp has relatively new tubes in it (NOS). but, the trem has not worked since before i owned it. any suggestions? Link to comment Share on other sites More sharing options...
cGil Posted November 22, 2008 Share Posted November 22, 2008 My 18watt's trem channel is very sensitive to the tube's gain. and won't work at all with some 12ax7's. JJ's are just too hot and make the oscillator go into uncontrollable oscillation runaway and the depth is nothing less than square wave thumpy. Sovtek 12ax7LPS's are also a little too gainy as well, and won't let the trem slow down below "fast", and won't let the depth get subtle. The tubes that do work for my 18watt are the Chinese 12ax7's and Sovtek 12ax7WA's. Even with a Sovtek 12ax7WA, the trem just won't do the slow shallow crawl that I love to get from the cheep Chinese tubes. But 5751's and 12at7's are both too far down on the gain scale and won't get the trem rolling at all. For my 18watt, it's all in the tube, and I'm pretty much stuck with using Chinese tubes in that channel till I find something better. So, basically, your trem circuit may also be sensitive to the actual gain factor of the tube, like mine is. However, if trying different tubes doesn't show any promise of helping, it may simply need a cap job. Gil... Link to comment Share on other sites More sharing options...
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