Reader Ask Rack Noise and Jumping Blondes

I’ve got some problems with my current rackmount setup. Here are the specs: Marshall JMP-1 preamp (from Britain, I use a converter so it will work in the states), MosValve MV-962 Power Amp (solid state), Digitech 256XL Effects Processor (soon to be replaced with TC Electronic G-Major), Juice Goose Power Conditioner (1800 watts), Korg Pro Tuner and a Dunlop Crybaby. I run my Les Paul to wah to preamp – the rack is controlled by a Behringer MIDI foot controller. It runs through a Marshall 1960 lead 300-watt 4x12 cab

This amp sounds unbelievable except for the extreme noise problems. When I add gain in the preamp, I have to switch to a clean channel whenever there is a break in the song to keep the noise from being audible. The causes may be using a volt converter for the British JMP-1 – which also requires the use of a three to two prong adapter – or the MosValve Power Amp, which only has a two-prong power cord (nonremovable). I think the combination of sacrificing the ground wire on the preamp and the power amp may be the cause, but it’s all that can work.

I have heard the G-Major effects unit has a noise gate that will at least partially solve this problem, but for the money I’ve invested in my setup I would like a total solution so I can enjoy it to the fullest. Any ideas?

- Andy

While too many ground connections can cause what’s known as “ground loops,” there is always the necessity for one good ground connection in a system. That said, it looks like you’ve already called out what I believe to be the source of your problem: the lack of a good common ground connection. This is a big mistake that cannot only be the source of your noise, but also a major safety concern.

Without a ground connection for the JMP- 1, your guitar cable shield is not at ground potential, which also means your guitar is not at ground potential, which means there is no noise reduction possible from the internal guitar shielding. But the problem continues in the other direction as well. Because of the lack of ground connection through the AC line cord, the chassis of the JMP-1 is not shielded, which can lead not only to the unit being susceptible to picking up external noise and hum, but emitting hum and digital clock noise as well. There are all sorts of digital noise suppression circuits inside the JMP-1 that are completely useless without an earth connection (ground).

You also run the risk, should the AC hot leg inside any piece of gear come in contact with its case, of placing yourself at a 120V potential. Grab a grounded microphone, or worse yet, another other guitar player that happens to be holding his guitar strings, and you’ll both “see the light!” In reality, it’s no laughing matter and could get you killed.

Luckily, I believe there is a solution. According to the schematic of the JMP-1, it uses a dual primary transformer, and there are internal jumpers on the PC board that can be configured for 120V or 240V. Take your unit to a qualified tech and have him convert it to 120V operation and install the correct fuse. Once that’s done you can use the standard IEC cable, complete with ground connection, and your rig should only make the sounds you expect it to.

Now you can do your part to help stamp out noise pollution.

I recently acquired a ‘64 Blonde Fender Bandmaster. My question is, can the Bandmaster be “jumpered” like the old Plexi Marshalls? If so, does the patch cable simply go from one of the dual inputs to the other channel (i.e. guitar to Input 1 on channel one, run a jumper cable from the second input of channel one to first input of channel two)? Thanks for your time!

- Daniel

Hi Daniel,
Nice amp. The Blonde Fender amps are some of my favorites. By the way, the vibrato in the Bandmaster with six preamp tubes is as good as it gets in a Fender. Now on to your question. Generally any Fender amp that does not have reverb is channel-jumping friendly, with one exception: the brown amps with vibrato. Unfortunately, the vibrato channel signal is processed through an additional gain stage, which flips the signal out of phase with the Normal channel. The result is that at certain volume settings, one channel will tend to cancel the other. This is also the case with Fender Reverb amps. The only solution I know of is a pedal manufactured by Barber Electronics called the Launch Pad ( htm). It has a normal as well as an inverted output to alleviate this problem – plus up to 20db of gain for a good deal of added crunch if necessary.

Now you’ll truly have a Blonde with big tones!

Jeff Bober
Co-Founder and Senior Design Engineer – Budda Amplification
©2007 Jeff Bober

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