A great replacement diode for guitar amps: the Vishay SF1600–and perhaps some digressive philosophical background discussion…

When it comes to guitar amp electronics, there are two thought paths. On the one hand, the 1960s and 70s are considered to be the golden age of tube guitar amps. The ones that I am aware of that are considered by many to have seemingly magical powers and are collectible are Marshall ‘plexi’s, Hiwatts, and Fenders. The Marshall amp had some nice distortion and was used by almost all of the classic hard rock guitarists–Jimi Hendrix, Jimmy Page (Zeppelin), Ritchie Blackmore (Deep Purple), and Clapton. The Highwatt amps had better build quality but were designed for a powerful clean tone. They would not distort until driven to the highest level, and then you got the sound of Pete Townsend (The Who).

Fender amps had a generally clean tone, and were ubiquitous among American guitarists. Leo Fender sold his company to CBS in 1965, and CBS was considered to have implemented cost saving measures on its products, leading to the term ‘pre-CBS’ and creating a golden age and collectability for the older amps. Hiwatt went out of business shortly after its founder, Dave Reeves, died in 1981. Marshall was not thought to have lowered its quality, but the circuits and parts were changed, and when the faceplate was changed from poly carbonate to aluminum in 1969, the so-called ‘plexis’ (plexiglass) became collectible. I personally liked the sound of the JCM 800 series from the 1980s, but nonetheless the perceived magic was in the plexis. The aura of magical powers surrounding these amps were in no small part due to the great music of that era, but whatever the reasons, there is a golden age associated with many guitar amps produced in the 1960s and to a lesser extent the 70s.

So the two thought paths are that the original parts must be used if at all possible, the other being that some improvements can be made with newer parts that are better performing. I share my opinion here, knowing that others may vociferously object to my thinking–that upgrading parts is good in many circumstances–unless the amp is considered 100% collectible and its operation as a ‘player’ is not important.

Getting back to the diode replacement business… I have a Highwatt 50 watt custom that was built shortly after Dave Reeves died, branded as a ‘Biacrown’ model. I note it was still wired by Harry Joyce, which is a wonderful thing. It did not have the legendary Partridge brand transformers, and the Biacrown era amps are not considered very collectible. So, I set about to upgrade some of its parts. Replacing the electrolytic caps was a no brainer, but the older diode technology has always concerned me.

Of relevance to this discussion–solid state technology has improved a lot since the 1960s, certainly in the realm of diodes. The devices available then did not have the capacity for the 600 volt levels driving the power tubes, so they were built with two or three diodes in series. I like to replace these with newer diodes. The diode in the older tube amp circuits needs to withstand a peak reverse voltage twice the ac peak value from the transformer. So, if the transformer puts out 500 volts rms, the peak voltage is 500 times the square root of two (roughly 1.414) which is 707 volts–doubling to 1,414 volts!

I discovered that finding a suitable replacement diode was not as easy as I thought it might be. There are not a lot of diode models for this high voltage. I found one model but got hung up in a discontinued product/back-order situation. I settled on an excellent device, the Vishay SF1600. It has a peak reverse voltage of 1,600 volts, a peak forward surge current of 30 amps, and a 1 amp average forward current rating. Sufficient for my Hiwatt with plenty of over-specification to help reliability. The diode itself is tiny at 4 Millimeters long, and costs 63 cents apiece if you buy 10 of them from digi key.

It also has a 75 nanosecond reverse recovery time. Reverse recovery time measures a little chirp of conduction the diode makes as it switches from conduction to non-conduction during the 60 cycle ac input. The 1N4007 technology reverse recovery time is 1-20 microseconds. To the extent that this produces an audible difference in a guitar (or even hi fi) amp is debatable, but it’s another attribute that has been improved up by the SF1600.

The very popular 1N4007 runs maybe 17 cents, but for the small amount of work I do, the price difference is trivial and there is no reason to stock two different diodes for tube amp rectification, so the SF1600 is my go-to high voltage diode. I use it in my Hiwatt as well as my capacitor multipliers.

An additional note. When ordering another batch of high voltage diodes, I ordered 30 BY448CT-ND diodes at 58 cents each. The differences I could find from the SF1600 are this doide handles two amps as opposed to one for the SF1600. It also has a slightly lower reverse recovery time, but still in the nanoseconds, so these do the job as well.

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