Fender Valve Guitar Amp Repair
I got this beautiful Fender Hotrod deluxe guitar amplifier for repair.
It looked to be in pretty good shape, and is used mainly in a local church. I could see that it still had the original factory installed Fender Groove Tubes. The set consists of 2x 6L6GC power tubes and 3x 12AX7 preamp tubes.
Contrary to popular believe valve amplifiers are still highly sought after in the hi-fi and pro musician communities, especially amongst guitarists. There are however only 3 factories left in the world that produces quality valves. Most of them have been going since the days of WW1 & WW2.
Anyways, powering this amp up revealed some audible hum and some noisy scratching sounds from the speaker.
I opened her up, and could clearly see that the electrolytic capacitors were failing. This can be seen on the positive side of the e-caps. The electrolyte had started oozing out from the positive lead entry point. These caps clearly had reached the end of their life and had to go!
I could also see some coloration on the pcb around the two power resistors.
This is due to heat, and leads to cold solder joints.
Time to remove the board.
NOW A WORD OF CAUTION WHEN WORKING WITH VALVE AMPS!
THESE AMPS CONTAIN LETHAL VOLTAGES, AND THE CAPACITORS CAN STORE A LETHAL CHARGE. DISCHARGE ALL CAPACITORS AND VERIFY THAT THEY HAVE BEEN DISCHARGED WITH A VOLTMETER. ONLY THEN IS IT SAFE TO COMMENCE WORK ON THESE AMPLIFIERS.
First remove the knobs from the potentiometers. Then remove the nuts and washers, including those from the jack sockets. Then remove the pcb screws. Carefully push down on the PCB and the pull it outwards so that you can gain access to the bottom solder side. I marked the bottom of the board with a sharpie so as not to confuse myself when installing the new capacitors.
There are a total of 4 filter capacitors, 1x 47uF 500V, and 3x 22uF 500V. They are glued down onto the pcb. To make removal easier I take an exacto knife and cut the plastic labeling from the caps. Then remove the actual capacitors, and then peel the labeling from the board.
The glue or silicone is easily scraped off the board.
The new capacitors are ready to install. To my dismay I didn’t have the needed 47uf 500V in stock. So I decided to use 2x 100uF, 250V, 105 degrees C, Nichicon capacitors connected in series. This will give me 50Uf, 500V. Close enough!
When connecting capacitors in series, especially in power supply filtering applications, you have to add a resistor across each capacitor to force equal voltage sharing. I usually use between 100K and 500K for tube amplifiers.
Luckily this amp had some extra pads and links on the pcb that I could use to make the two capacitors lie flat on the board.
I then soldered the two resistors to the bottom of the board.
After all the capacitors were installed, I quickly scanned the board for cold solder joints. I reflowed the solder at places, and finally cleaned the bottom of the board with an old toothbrush and some IPA.
I reinstalled the pcb to the chassis. When doing this remember to install and tighten the nuts from the potentiometers and jack sockets first before installing the pcb screws!
I then added some silicone to the caps to keep them in place.
When installing new filter caps in a valve amp such as this one, it is very important to “reform” the capacitors. This ensures that the dielectric is charged up to its full capacity. This is especially important with high voltage capacitors, and if they have been sitting on a shelf in storage for a few years.
To do this, I connect the amp to my variac without any of the tubes installed. I then switch the amp on and bring the variac up slowly in increments of 50V. I let the amp sit for a couple of seconds at each 50V increment, until I reach 230V in my case (230V in my country).
The capacitors are now “reformed”.
Next up, I replaced the tubes with a fresh set. The 2x 6L6GC tubes need to be a matched pair. I prefer matches of 5%. You can buy them matched.
With the new tubes installed, its time to bias the amp.
A lot has been written about biasing, but basically we are again ensuring optimum operation of the output stage. This is a class AB amplifier, and we want the tubes operating in a region were there is no crossover distortion happening.
Luckily on the Fender Hotrod amps, there is a bias test point indicated on the board.
Set your DMM to mV and probe between chassis earth and this point. Adjust the trimmer for a reading of 68mV.
Let it sit there for an hour or so and recheck. Make adjustments if necessary. Here you can see the output waveform with a 1khz input signal with the crossover notch “just” visible.
Lastly I cleaned all controls and jack sockets with some contact cleaner. This concludes the repair of the amp. Tube amps are exciting, and have a sound second to none!
Lets keep the culture alive and remember to “keep ‘em burning” To find out more about tubes and tube amps, check out Uncle Doug’s YouTube channel.
This article was prepared for you by Riaan Diedericks. He runs his own electronics repair shop in Pretoria, South Africa. He specializes in Pro Audio repairs.
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You can also check his previous repair article below: