17 Aug Fender Hot Rod Deluxe Rising Bias Problem
Aah, the good old Fender Hot Rod Deluxe, one of my favorite guitar amplifiers of all time, and the one that started it all for me. You see, back in the day when I was still a young chap, the Hot Rod Deluxe was my first experience with working on tube guitar amps. Internet was still a new thing, but luckily a friendly guy with the name of Justin Holton put up a website – called the Unofficial Hot Rod Deluxe owners guide – that documented his experiences with modding his Hot Rod Deluxe. The site contained everything that a starting tinkerer of guitar tube amps could wish for: tube amp maintenance, how to bias the amp, how to solder, mods, and of course safety.
Always looking to learn something new I opened up my amp, and I never looked back. That was the starting point of years of working on guitar tube amps. And here we are again: looking into the open back of this all too familiar amp. Feels like coming home.
Justin Holton’s site disappeared for some time, I guess he had other interests to follow, but a friendly pastor dug it back up from the dungeons of the world wide web and hosts it now. Some info is outdated, but generally it is still a good repository of knowledge about this tube amp that has been used by rock Gods like The Strokes, Arctic Monkeys and Franz Ferdinand.
Anyhow, the particular example that I have on my bench now is the first incarnation of the Hot Rod Deluxe, from the time these amps were still made in de USA, somewhere around 1995. Later on Fender moved production of these guitar amps to Mexico, and I think we are now at version 4 of this iconic amp.
The owner had a clear description of the problem: it didn’t work.
Upon inspection it soon became clear why. Can you guess?
Time for a retube. Anyhow, these are the original Groove Tubes the amp came with. And since USA Hot Rod Deluxes were made somewhere in the 90’s, I guess you agree it’s about time to have some fresh new tubes installed. So I ordered some new 6L6GC powertubes, and some JJ ecc83, plus nex filter caps, and we’re good to go. Installing tubes, biasing, and measuring the amp on all important test point. Eazy Peazy.
Or so I thought.
It soon became apparent why one of the previous tube blew, and the other had scorch marks.
I installed the tubes, biased the amp, and let it burn in for some time. But then I noticed some red plating was developing, checked the bias: way to high and rising. Quickly turned the amp off.
Time for a head scratch.
Anyhow, the problem was somewhere in the bias circuit.
But before that I replaced these Illinois filter caps, as they get a bad rep. And also the two load resistors responsible for providing +/-16V to the opamp switching cirquit.
Anyhow, the problem of the rising bias was still present after these mods (which I expected). Anyhow, surfing a bit around I discovered that I’m not the only one having this problem. Apparently Fender put a blob of paint on the bias pot, probably from preventing to shift during transport. However, this paint can enter the bias pot, and cause all kinds of problems. So replaced the pot, and voila, we have a stable bias on an amp that runs like a sewing machine.