Name: Ed Goforth
Email: goforthsound@yahoo.com
Subject: Amp noise/hiss
Thread: 278
Time: Mon, 26 Nov 2001 10:11:23 UTC
I am not sure, but perhaps if there is 1458 IC's in that model maybe you could swap out those to LF353 IC's. The LF353 is a very low noise hi performance replacement for the 1458 chip, though I do not know if it would be unstable with a higher perfoming chip, should work though. I once had a Yamaha G100-112, that came with JRC4558 IC's, (4558 is a step or so up from the 1458) which are the same IC's that are desired in the older Tube Screamers, but at that time I took them out (I did not know any better then, haha!) and replaced them with LF353's. The noise was pretty much gone other than when the gain was up high, but not anywhere as much as with the JRC4558's. The tone was tighter in the low end, crispy highs, but just a bit sterile compared to the JRC4558's which sounded more natural in the stock amp, which had NO tubes what so ever! In the MusicMan amps, I believe some of the RD50's used TL072 or LF353 chips which are of very high quality and low noise. I think those MM amps have sockets for the IC's, so swapping them out should be no problem, BUT be sure to turn off the amp before swapping them out! Or have a qualified tech do it for you. Another cause of noise might be resistors or caps. Some amps have those old carbon comp resistors, which sound warm and sweet, but after time and heat, go out of tolerance and become somewhat noisy, . I would check to see if there is any carbon comp resistors that may have high voltages routed through them. I have replaced many CC resistors that have gone out of tolerance or became noisy. I usually change noisy CC's with Carbon Film to 1 watt or at least 1 watt CC's depending on where its location is. If the CC resistor is in a low signal area like in line of the sigal, and in tolerance, not at a high voltage source, you may want to keep them, since they sound warmer to most people. CF resistors keep the warmth, but offer low noise and more stable tolerance. If you want very transparent sound quality, some folks like Metal film or Metal Oxide resistors. I like the later for power supply areas and bias circuits, which help keep noise to a minimum, and keeps the voltages more stable, from my experience. But in tonal areas, either carbon film or carbon comp. Its a subjective subject, haha! :^)
Another trick that some folks do not buy into is changing the rectifier diodes to FRED's. These are Fast Recovery Epitiaxial? Diodes. The bottom line is they rectify faster than than standard diodes, much like a GZ-34 tube rectifier. The one thing I did notice too, is they up your voltages. My old Fender Bandmonster was putting out 440 vdc on the 6L6 plates stock, when I tried the FRED's, it went up to 465, which is fine since I modded that amp to Dumble specs. Another amp I built, based on the Dumble, using a Newsensor power transformer, was putting out close to 500vdc on the 6L6 plates! So I had to put a 250 ohm/50 watt resistor in series to get the voltages in spec! The difference to my ears was much warmer and musically dynamic! Also adding .01uf/1kvdc snubber caps in parallel with the rectifier diodes help cut hiss. Using good quality coupling caps help too! Sprague/SBE orange drops are popular, Mallory 150 series, and others preferably Polypropylene caps tend to be pretty stable and quieter, where if you have ceramic coupling caps, you might get more hiss and microphonics, and maybe even bad tone. Ceramic caps work nicely in overdrive circuits, they tend to add to the breakup in a way that is not as harsh, but smooth and warm. Silver mica's work great in cleaner circuits like the clean channel treble cap. This is just mine and some others preferance though. As far as the FRED thing, I built an amp for this guy a couple years ago, and it had a socket for a GZ-34 tube type rectifier. I had installed a solid state (1n4007) plug in unit built into an old tube (male pin socket). I built another unit like that, but this time used FRED's and snubber caps, took it over and said tell me what you think... He tried it first with his original setup (1n4007) and then we switched them out to the new one (FRED's), he said "Thats just a Fred?" He felt I had snuck a GZ-34 in there! It was warmer, quieter, and more dynamic he said, and he really likes Fred. Anyway thats my story and I'm stickin toit! :^)
Ed