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Post by tubefreaked on Sept 11, 2015 13:50:33 GMT -5
Hi. I'm new on this forum and also at reading schematics except for the basics and have no formal training but I wanted to ask if anyone could take the time and review the schematic for an old amp of mine that uses diode clipping in the preamp to see if anyone could suggest alterations to get some of the harshness out from it. the amp is a crate gt50head with four 6v6's and is loud as heck and I love it's voicing with the exception of the harshness. It should be a little smoother and the amp was designed by Lee Jackson who still designs innovative products even after the 80's hair metal thing ended with which he played a decent enough part of. The amp is built and laid out well with Wima caps and metal resistors throughout. With the exception of the transformers (which would be replaced if I can get a little more smoothness and less 'solidish' sounding tone from it) from what I can tell compared to other low cost amps. I think it's a steal if I can tweak it just a little. But anyway, I thought you may like to see the schematic anyway cause it has a hybrid type of thing going that was quite advanced in the late eighties. Thanks. glad to be a member! 11869d1291422517-gt-50h-_50_schematics.pdf (839.22 KB) |
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Post by JohnH on Sept 11, 2015 14:55:00 GMT -5
Hi tubefreaked, welcome to GN2. That sure is an interesting hybrid of an amp schematic! we don't look at a lot of amp designs here but sumgai or ashcatlt will hopefully call in soon and offer a view.
My first guess would be that the diode distortion comes from around IC2a, and if you increased capacitor C18 (currently 100pF) it might add more negative feedback at high frequencies to control things up there a bit more.
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Post by ashcatlt on Sept 12, 2015 12:08:59 GMT -5
Hi tubefreaked, welcome to GN2. That sure is an interesting hybrid of an amp schematic! we don't look at a lot of amp designs here but sumgai or ashcatlt will hopefully call in soon and offer a view. My first guess would be that the diode distortion comes from around IC2a, and if you increased capacitor C18 (currently 100pF) it might add more negative feedback at high frequencies to control things up there a bit more. I think that would be a good place to start. That cap will shelf off more high frequencies as the gain knob is increased. Since caps in parallel add, you can experiment with aligator clips to come up with an acceptable value without having to make any permanent mods. OTOH - it might be as easy as changing speakers.  |
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Post by tubefreaked on Sept 14, 2015 16:50:02 GMT -5
Yeah. I already changed to a Celestion g12H for more low end but I am thinking the vintage thirty may attenuate the highs more. I like the drive and gain of the amp but it seems like a tubescreamer has been turned constantly on when you hit the gain channel with all the diodes and what looks like mosfet stuff. I don't know a lot about that kind of stuff. It definitely is piercing in the high mids and high end, almost like an old solid state treble booster. I see guys changing diodes to 'soften' the sound but I haven't dealt with them before in an amp. I just want to calm the higher freq.'s down a tad because I end up maxing the bass to get any low end. It seems to drop off on the low E string. I just want the distortion to be smoother and more applicable. The schematic is nothing like the tube books I've tried to learn from and mosfets, diodes and the like are totally knew to me though I am not any more against them than anyone slapping a solid state pedal in front of their amp. The amp has a pedal emplaced within it's circuitry is how it appears to me... instead of the front end. It's a good learning experience for me anyway. If you think of anything else that may help let me know. I am gonna try to fiddle with the cap 18 and see what happens. Thanks for your advice!
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Post by JohnH on Sept 14, 2015 17:04:09 GMT -5
Good luck. I have Vintage 30s in my amp and cab. I like them very much but they are famous for a high mid peak. Not sure that wold give yoy what you want relative to a G12H. WGS do a Veteran30 that is supposed to be a smoother sounding (and cheaper) equivalent.
Im also curious about r33. Its 100k. What happens if you put another 100k across it? I will change the tone and is worth a try. That whole opamp stage is interesting and I might try modelling it in 5spice
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Post by tubefreaked on Sept 14, 2015 17:44:59 GMT -5
Yeah. This guy, Lee Jackson, was way ahead of everybody years before an acceptance of diodes and so-called 'artificial clipping' to tube purists, like myself, was accepted in the amp world...except by a lot of true hard rock guitarists who were looking for that sound and didn't want to keep buying the next best distortion pedal. You can take any marshall jmp or fender 50 watt amp from the late 60's or early seventies and slap a ts or similar clipped pedal in front and you wouldn't know the difference. His design is all about producing the "souped up' sounds that were the rock scene at the time (the Reagan 80's) and he did it better than most which is probably why he worked with Ozzy's guitarist and many others. This amp just needs a few little tweaks to get it just right for that kind of sound which I've liked since hearing Black Sabbath many years ago. I just wish I was an electrical engineer and had more knowledge but we are never too old to learn. This amp is worth over half the price it cost brand new and is growing in value as people are starting to accept the sound for itself which is all I'm about as a guitarist. The amp I use has to pull it off for me to play better this I know. It's symbiotic.
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