Tonefiend Forum

Didn't want to give the impression that I was dissing BJR mods. The vendors you mentioned have good iron at affordable prices, as do Allen and Magnetic Components in their ClassicTone line. An affordable transformer upgrade, component tuning, decent tubes -- I get all that. It's the people who go all in on upgrades spending more than if they got the amp they really wanted that I don't get. Mercury Magnetics reminds me a lot of that old joke about Bose: "better sound through advertising." The industry of expensive upgrade components for guitars and amps is basically predatory of people who believe in magic rather than electronics.

MDF is great for hifi. But most MDF in cheaper amps (including amps I own and love), well I haven't seen it hold up well to road abuse. That gives me more pause than tonal concerns.

Oh, and I had some much worse amps in mind than the BJR.

Quote from joe on October 2, 2012, 10:23
I'm going to nominate a FAIL, not because it was the worst thing ever, but because it should have been incredible, and was SUCH a disappointment: The low-wattage amp heads Lexicon made in the ’90s, such as the 284. It was a great idea: a well-built, well-engineered 3-watt head designed for getting cranked amp sounds at low volumes in the studio. My hopes were high, because the Lexicon team at that point was quite brilliant. But damn, it was the most flat, sterile, un-sexy tone you can imagine — the polar opposite of a cool tube amp in extremis. I had a sad.

I've seem a repaired a few of these and I agree with you they are very sterile. IMHO these were amps designed by "electronic engineers" that had all kinds of high end test equipment and who were great at all the other high tech Lexicon stuff but didn't really understand what made guitar amps sound good. Most good sounding guitar amps are sort of assbackward low tech designs mostly taken from vacuum tube data books by folks that were not electronic engineers.Rather the people that created good guitar amps by trial and error actually playing their designs with real guitars and speakers and used their ears as a piece of test equipment. They also worked closely with musicians to hone things to meet the actual needs of the marketplace.

A good example was the silverface Fender amps that CBS put out to replace the blackface Fender amps. The silverface Fender amps mostly sounded awful even though the designs were close to Leo Fender's original designs. What happened was CBS's "electronic engineers" began using test equipment to "improve" the amps designs. They saw parasitic oscillation on the output waveform when they hooked the guitar amps output to the test equipment. The parasitic oscillation occurred because they were testing the amps using non-inductive dummy loads (big ass 8 ohm resistors)rather than speakers which have inductance as part of the load due to the voice coil being a COIL. You never see that parasitic oscillation on an amp driving a speaker because the inductance damps it. They added small value caps to the output stage to remove the parasitic oscillation so their test equipment looked better not realizing this would kill off that beautiful high end sparkle that Fenders are known for (in fact a small 8 ohm resistor and a .01uf cap in series across the dummy load simulates the spkr inductance and kills the parasitic oscillation, every amp test bench should have a couple of these to use on amp testing they are dirt cheap). They also put snubbing diodes from both sides of the output xfmr primary winding to ground to avoid back emf damaging the tubes and output xfrmr if the amp was run with no load. However those diodes were there to fix a non-existent problem since the Fender amps used 1/4" spkr jacks that shorted the output to ground if no speaker was plugged in. So those CBS engineers were following what they had been taught in school that it is proper to put diodes on to suppress back emf but in the original design it could never really occur except in the case of a open spkr or bad wiring. However those diodes also reduce the Fender sparkle giving the amps a dull sound and if the diodes shorted out from heat and use (which they often did) they would make the amp blow fuses and short the HV supply (decreasing reliability). This shorting the output may seem like a bad thing and it would be for a transistorized amp (without current limiting). However when an output tube sees the reflected impedance across the output xfrmr primary reduced because the secondary of the output xfrmr is shorted the reflected load impedance on the tube is below it's ideal load impedance and the output tube efficiency drops and the tube current drops to nil rather than the tube being killed. Most output tube amplifiers are fine with shorting the output xfrmr secondary but leaving it without a load is a big problem and can cause a lot of damage to the tube, socket and even the xfrmr itself hence the shorting jacks.

There were many folk at the time that traded in their blackface Fenders for the new silverfaces that were really bummed and if they were lucky they went back and got their old blackface Fenders back. This is one of the reasons that anything pre-CBS was highly desirable in Fender equipment and CBS really tarnished the good name Leo Fender had built. Some amps like the Fender Deluxe Reverb and Princeton escaped this at least for a while. CBS engineers also did other nasty things in the name of good electronic practice. But if you get a silverface Fender and "blackface it" they sound very good (you are basically removing all the things those CBS engineers put in). The problem was the CBS engineers used test equipment rather than guitars and their ears and did not give credence to the complaints of the musicians that used the equipment. This is why I always cringe when the "electronic engineers" get involved with some types music equipment. So those CBS "electronic engineers" were also a FAIL!!!

Quote from s.huck on October 7, 2012, 20:03

Quote from mwseniff on September 9, 2012, 06:17
JCM900's can sound a lot better if you replace the output xfrmr with one from a JCM800 or plexi. Also almost every JCM900 I've ever had on my bench was biased in correctly and if they used anything but 5881's the feedback was on the wrong xfrmr tap which makes a huge difference. That being said the preamp was very different from other Marshalls it even had LEDs as clippers in the distortion channel, but I have heard some folks make JCM900s sound great.

I've always wondered why some guys get great sounds from JCM900's when the ones I have played through sounded so thin. Does replacing the xfrmr with one from a JCM800 "open up" the amp, and give it more depth? If so I may have to take another stab at the JCM900 series. Also should I order from somewhere like Mercury magnetics? Or try to find a used one? Sorry I don't know much about the internal workings of amps.

Replacing the OT improves bass response and seems to reduce some of the murkiness in the mids and highs. Opening up the sound is a reasonable description. It is also important to have good properly biased tubes which is not a given (most amps are underbiased and that makes for vey poor sound). I have never worked on a tube amp that I did not also rebias. As for replacements Mercury Magnetics stuff can be great but they have several lines some are no better than other mfrs in the same price range. One thing is that in terms of resale the Mercury Magnetics name means a bit more pricewise. The big thing is that the xfrmr in a jCM800 is different beast, much heavier laminations and windings. I believe the manufacturing process is different in the two xfrmrs. I have used the "generic replacements" for customers on a budget with good results. The JCM800 xfrmrs are physically larger and size matters. As to some players sounding great thru JCM900s, I agree but most of those amps are stock units with original xfrmrs. There are differences between players that transcend gear IMHO. I know players that can make stuff sound amazing but if I played it I would sound awful. The main reason these mods were good was that the JCM900 were dead cheap used in the 1990s & 2000s and worth doing a bit to upgrade, I have been away from the market in the last few years so I don't know the economics today. But if you can find a JCM900 with a bad OT cheap it is a great way to go.