Diaries of a Guitar Tech: 'Who Thought That Would be a Good Idea?'

GB Guitar tech + cheap 60s amplifier = improved tone      03/08/13

Maybe it was worth it. As well as giving more gain, removing the feedback also changes the sound of the amplifier, both at low levels and when pushed into clipping, and customers may have preferred it like that. Clearly adding another valve would have been too difficult. 

There was still the reverb to sort out, and here the amp showed another unique point: the spring was driven not by tiny electromagnets as used in every other reverb tray in the world but by record player cartridges. I kid you not; the ends of the spring sat where the styli should have been. Reviews and memoirs suggest that the reverb on these amps was nasty sounding and prone to feedback if turned up high. Electrically it does make sense; the cartridges are high impedance (Hi-Z) devices that are a good match to the mostly high impedance valve circuitry. But amplifiers from the U.S. that had reverb all used electromagnets. Were these reverb trays simply too expensive to import? Did Bird think they were being clever or radical by using something cheap and home grown? Did they think guitarists would put up with the feedback and tinny sound of the reverb? Did they really think it was a good idea? 

Time had taken its toll and reduced the crystal elements to dust, with a few sticky patches. Identical devices are no longer available, but slightly more recent ceramic cartridges were still manufactured for low end record players until quite recently, and are electrically quite similar and still available. Fitting a pair of these (one to send, one to receive) restored the reverberation. 

There were still problems; while the gain of the amp was low, the gain of the reverb was much too high. It was set by a control on the back of the amp, which had to be at almost zero or horrible feedback would ensue. Changing some component values cured the second problem while removing much of the residual hum as well, and I came up with a radical solution to the first which also cured the amp’s lack of brightness. 

(Photo: The dead bits)

Bird left the guitar amp business fairly quickly and went back to making electric organs. This amp is rare because it didn’t sell well, and it is easy to see why: there were many failures of thinking. Having got familiar with it, it seems a shame; except for the reverb, the major components are of good quality (making it more of a mystery why they cut corners on the small, cheap ones) and a few minor changes improved the amp a great deal. Even my fix for the low gain didn’t involve major surgery. I could have gone further; removing Ultra Linear Mode would be easy, and squeezing a little more output power even easier, although that might melt the speakers. 

I hope I have done a job that will mean that the amp’s current owner will get more pleasure out of it than its previous owners did. That is where my job satisfaction really lies.

If you want to find out more about Nev's work, or get in touch with him concerning a job, you can contact him on the ZenWorks Facebook page

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