zatoichi Posted December 6, 2008 Share Posted December 6, 2008 I bought this new for around $150, over a year ago. In short order, I put new tubes in, & replaced the stock 10" speaker w/ a Jensen C10-Q. Since then, I've been studying the threads here & @ the wattkins family of sites. At this point, I'm looking at this in stages: first the OT, then the DSP, then tweak individual caps & resistors; that way, I can track the changes I want to hear, rather than just "making it sound different" & trying to decide if I'm satisfied w/ that. Pretty much ready-to-go here: just a few unanswered questions. One big question I have is about the variety of ways of bypassing DSP, many of which seem to introduce problems; so naturally, I'm looking for an effective way w/ the fewest side-effects. I got this from a post by hucky on sewatt: he says the thing to do is connect the pre-amp directly to the power amp, and to do this by lifting the right leg of R17 and the left leg of C6 and connect them with a wire. I imagine this will disconnect the DSP, but is there any virtue to doing it this way? I ask because hucky's post was well-received, but virtually uncommented on. IS there a "best way" of bypassing the DSP? Is there advantage to "connecting pre-amp to power-amp"? What is that advantage? Speaker Jack assembly the jacks I picked up aren't what I need to replace the PCB-mounted jacks, so I'm going to punt on this one - keep the PCB, wire it for 8 ohms (as original), skip the power to the foot-switch jack, & remove extraneous wires. One of these wires seems to be a ground lead from the PT: should I leave it, cut-&-tape it, or attach it to the chassis? My Hammond sez the yellow-wire ohmage is 16/8/4 - I should replace the stock OT's red lead w/ this? Gil has already verified the primary leads from the 125DSE: BRN = T8, BLU = T9 Amp was unplugged while on, left so for a month. chassis has been out of the cab for 2 weeks. What do I need to discharge before I reach in & start cutting & soldering? I've read warnings about these tab connectors - even one that recommended soldering the primary leads directly to them: would alligator clips be a reasonable compromise? Thanks as always for the help, guys! Link to comment Share on other sites More sharing options...
cGil Posted December 6, 2008 Share Posted December 6, 2008 For the Hammond, you want a 5k impedance reflected back to the tube, http://www.hammondmfg.com/125SEsch.gif So the yellow wire will be your 8ohm load. The black wire is the OT ground, and it needs to be connected to the ground connection of the output jacks, AND the chassis. As to the DSP bypass... listen to the guy's who've done the mod. I'm not that guy, cuz I just bypassed the entire board (and DSP, too) by gutting the whole amp. There are no ready made turret boards for these, so I'll have to resurrect it with a totally homebrew point to point layout on turret strips. Gil... Link to comment Share on other sites More sharing options...
zatoichi Posted December 9, 2008 Author Share Posted December 9, 2008 The black wire is the OT ground' date=' and it needs to be connected to the ground connection of the output jacks, AND the chassis.[/quote']AND the chassis? The stock OT was not connected to the chassis...? How important is this? Have got the new OT hooked up ('cept for the chassis) & back in the cabinet: it looks like I'm going to have some woodwork to do in order to get things to line up for the screws... Link to comment Share on other sites More sharing options...
zatoichi Posted December 10, 2008 Author Share Posted December 10, 2008 Well, nothing blew up when I hooked it up & turned it on, no magic smoke escaped, no funny smells. Amp sounds a lot bigger now - more like a twin-twelve than a single ten - lots of "character", but not much definition, it seems - still, whatever it is or isn't, there just seems like a lot more there than was before. Understand, I didn't fiddle w/ the TMB at all, just gain & MV to 12. Do notice a hum now - may have been there before, but I don't remember. there's a gain-knob hum, that I can hear when the gain is at 12, and there's an MV hum, which I can hear when the MV is at 12. Not a really bad hum, but not really silent, either. I'm going to have to move a pice of wood inside the amp: the new OT is wedged up against this shelf-strip the chassis is supported by - and so can't move into place for the screws. simple enough thing to do - just odd no-one else has had to do it (maybe they just never thought to mention it in their threads?) Link to comment Share on other sites More sharing options...
cGil Posted December 10, 2008 Share Posted December 10, 2008 That happened to me, too, except I had a new PT too close to the wooden cleat in the cab. I carved it a bit with a Dremel tool and it fits fine, now. As for the hum, OT primaries can cause a squeal not a hum, yet you don't have that gremlin, so it's definitely not your OT wiring causing any problems. It may be due to just a slight movement of the PT's wires during your modding. Try using a pair of chopsticks to move 'em around and see if you can find a spot for 'em that reduces the hum. The heater supply may only be 6.3v, but it's still AC, and will induce a hum into practically anything it gets close to. Twisting the common pairs of PT wires always helps to self-cancel some (but not all) of the generated field strength around 'em. And if they must cross other wires, make sure they do it at 90 degrees. This applies to the pair of 6.3v heater supply wires, the pair of 12v solid state opamp and DSP supply wires, the pair of 270v high voltage supply wires feeding the rectifier diodes, as well as the 120vAC wires coming in from the power cord jack to the power switch, and from there to the PT. Twist those, too, and keep 'em secured away from everything as much as possible. You can find the color codes for the PT supply pairs listed on a label on the top of the PT. And yeah, I'm sad to say that 10" speaker sucks as bad as the 12" in my VStandard. I don't care 'em at all, and didn't really care for the new Eminence either. Wound up selling off my VJr extension cab. I'll probably wind up with a Jensen P10r my VSpecial cuz I have one I can use and I love the tone. I suppose my VStandard will get a 12" Mesa Celestion 90 that I also don't like, but only slightly less than the VStd's stock speaker. Till I decide what it really wants, that is. Can't afford speakers right now. Gil... Link to comment Share on other sites More sharing options...
zatoichi Posted May 18, 2009 Author Share Posted May 18, 2009 Just for the sake of following up: The hum is practically inaudible, so I haven't messed with it. Some quality time w/ a hammer & chisel, and hte chassis now fits into the cab snug-as-you-please. Still anxious about cutting the DSP loop - it depends entirely on the schematic in your box, and the methods are NOT 'one size fits all'. never any word from the guy who replaced his op-amp chips, so who knows if that's ta worthwhile mod or not? Happy so far! Link to comment Share on other sites More sharing options...
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