pablogt wrote:I tried 315ma fuses for HT (closest I could find to 350) and they held out with the amp cranked. You were right, Gaz.
I will probably be building another one soon, this time with Koa resistors and Jupiter caps. I won't know which differences are explained by the caps and with by the resistors, but it should give me an idea of how good the AMRGs really are.
I'll try to have some clips recorded this weekend.
Pablo
Hey, that's cool, and you it's nice to know your fuses will blow more uickly if they ever need to. I'm very paranoid about the issue because I once built an amp Trainwreck style with no HT fuse and 5W screen resistors. Well, one of the screens failed, and screen resistor would not open because of the high power rating. Unfortunately the mains fuse did not go, so the PT went up in smoke instead.
I really feel like this is the major design flaw in TWs. Fenders, with no HT fuse, have smaller wattage screen resistors that will open up during failure. Not the best design earlier, but at least protects against a common failure mode.
I had some clips recorded yesterday by my friend Marco Ciargo, who is a better guitar player than I. They come all straight from a Zoom H2 recorder, using the on-board microphones and have not been edited in any way. The mikes are pretty bad, so the clips really don't do the amp justice, but they'll give you an idea.
Guitar is my Tom Anderson HSS strat, stock neck pickup at half volume for clean parts, dimed bridge humbucker (Bare Knuckles Crawler) for dirt. The cab is a Hiwatt 4x12 with two Eminence GB speakers. The other two ports are closed. No effects were used except a booster in some parts of the "Real life" clip.
The clean clip is recorded with every control at noon except Master and Power which are dimed. The drive clip is recorded with every control dimed, except Master and Power which are at noon. These two clips present the extremes of the amp settings. The third clip, "Real life", presents a more realistic setting with every control at noon except both gains at 3pm and a booster (Xotic EP booster) used at some points.
All clips were recorded at "Modern" voicing. In fact, I will probably change the Modern/Vintage circuitry because it is not useful as it is (Vintage mode is too boomy).
Pablo
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I can hear the amp through the rough recording. That second clip has great dry bite that I love. It sounds like success to me!
Too much fat in the vintage setting? I prefer to build too much of everything into my stuff, and then back off on the settings a bit. I bet you would like my variable tone slope setup from 33-100k. I end up between 33 and 70 or so at medium volumes. It helps clean up the bass response vs gain.
Good deal, great amp!
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
To me it sounds like the best amp I've ever played (the recordings are that bad). I might be biased, though
I might do some radical voicing to the Vintage mode before removing it. The good thing is that all the circuitry related to that mode is mounted on the switch itself, so I can change it completely without leaving any trace.
I thought of putting alternative resistors to the CF's cathode after testing different values with a trimmer. The effect is so evident and there are so many usable values that in a future build I might change the Presence pot to a fixed resistor and put a "Compression" or "Mids boost" control instead. It could also be a nice "Pull boost" option for the Mids control.
Pablo
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Another really nice amp... I love the distortion tones you are getting. I am impressed with your ability to get all that in there and keep everything straight... good work!
thanks for your kind words, rj... Working the layout in detail in Visio has been a great step forward for me. You still find some "surprises" when you build the real thing, like turrets too close for soldering, need for inset screws or tapped holes, etc, but not many. For instance, in my layout I did not draw the chassis lips on the side (the ones that hold the nutserts). That proved to be a big problem when trying to work on stuff that is underneath them. From now on, I'll draw the chassis lips in Visio too!
Right now, I'm going nuts trying to make GMX work without alien noises. If you abstract yourself from those noises the effect both sound and feels great, but I'm about to give up on it after three very frustrating days.
Kevin O'Connor believes it's a lead dress issue, probably related to the output stage elements being too spread out (which is an inherent characteristic of Trainwreck style layout).
Ok, I've run out of ideas regarding GMX. I attached a short clip to show what the problem is. I play the guitar clean with GMX control down, reach back to dime the GMX control and play the same part again.
I've had several long chopstick sessions, changed the GMX's wiring lead dress according Kevin's advice, checked the GMX board, reflowed every solder joint, changed the quad opamp IC for a new one, and changed many aspects of the lead dress of the amp (many of the changes were advised by Kevin). I have attached a photo to show the differences.
Kevin still thinks that it's a lead dress problem but I just can't think of more things to change that might have the slightest relation to the problem.
The only positive aspect, so far, that I've incorporated some lead dress changes that will be part of my "standard" from now on: shielded twisted pair wire for PA grids, PA's ground line twisted with OT primaries, ground and B+ line twisted when possible (no more ground bus), etc.
I'm ready to try any suggestions before removing GMX for good
Pablo
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The GMX board has one, that's the only one that could cause it because the amp works perfectly well without GMX. Even with every control dimed and a 15db boost in front.
As you know I am following this very closely on both sites. I really don't have any other ideas than to possibly try it from a different power node - Vs - which I highly doubt will help. Truly I like the added and more muscular sound of the GMX in your clips, please don't give up solving this problem. There has to be something simple - wrong value on the GMX board or something. I am rather surprised that there has not been more help for your problem - something that makes me a little nervous in building the GMX into my amp....
Yeah, I'm sorry that the end result was not better. Several of the guys over at power scaling have implemented GMX with good success. Thinking here... the only problematic mechanical component would be the potentiometer and then how is the circuit changing as it is dialed up to cause distortion? Certainly one would expect the output transformer to be OK.... Have you tried different tubes also?
Maybe Monday Kevin will offer some other suggestions???
After hearing the sound clip I played it for a coworker who builds amps too. He said in his latest build (without GMX) he had a similar noise which was an oscillation in the output stage.
Would it be possible for you to try to place a few caps from the output tubes grid to ground just for trial. I am afraid without a scope this may be hard to solve.
