Summary of this amp
A lovely Fender Blackface Deluxe Reverb 1966 came to us. It was extremely noisy and the reverb was crackling and popping. It was a USA 110 volt AC model and so used a step down transformer to enable it to run in the UK on 230 volts
The amp was all original apart from the power supply capacitors and the choke having been changed.
It is very unusual ( at least to me) to see the choke changed. The main part of the noise was a failed disc signal capacitor. The amp now quietens down . However the amp is still noisy. I have a Silverface version with the same AB763 circuit and this is my comparison.
So now comes the challenge. Folks like to keep an amp all original but old components wear out? You have the choice of having an all original ornament/museum piece or a fixed amp that sounds like it did when it was new. A fixed amp can be reliably used every day and does not let you down at gigs.
The Amp’s Problems and Solutions
Many of the preamp anode resistors had gone noisy, so these needed replaced. The decoupling capacitors on the preamp valves were original and one or two had failed. They are just open circuit passengers. Time to replace them all !
So now the amp is nice and quiet. Sounds great. Let’s try some reverb and hear that lush Fender reverb. Oh No ! Cracking and popping is back again. The reverb intensity control is just a noise and crackle volume control!
The bottom line is that the reverb drive transformer is the culprit so we order a new one and we fit it in the amp. Wow! lovely quiet reverb is back. During this exercise, I removed the reverb tank to check it out. The fine wire connections in the reverb tank can have issues as can the phono plugs which oxidise over time and make poor connection. Especially if the amp resides near the salty air of the seaside.
When I removed the reverb unit, I we see that there are multiple holes in the case where the reverb tank had been removed and replaced. This amp obviously has had reverb issues for a long time and previous folks were checking the reverb tank while the issue was with the reverb drive transformer.
So what are the common faults with Fender reverb circuits?
The most common faults are oxidised RCA/phono sockets on the reverb tank and the reverb send/return sockets on the back of the amp. The outer parts of the plug fatigue too and these need a gentle squeeze with pliers to tighten them up.
The reverb drive circuit is where most of the failures happen. There is a 12AT7 valve with both triodes strapped together to make a little audio amp of around a couple of watts. This drives the reverb transformer. It is connected to the 410 volts B= supply. This poor little 12AT7 is being made to work at or beyond its maximum ratings. It is not surprising that this is the valve in the amp that wears out first. You get crackling and popping or really high noise. When the noise goes through the reverb it sounds like the distant rumbling of thunder. When I get reverb issues in a Blackface or Silverface amp first thing is to change that pesky 12AT7. Most of the time that will fix the problem.
It is unusual to have a reverb transformer go noisy but it does happen. The reason is that the insulation on the windings slowly fails. I heard the story that this is caused right back when the transformer was made. Back in the 60′ and 70’s the folks winding the transformers wore cotton gloves to prevent the acidic sweat on their hands getting onto the windings. Sometimes folk would eat citrus fruit at lunchtime and it would be on their hands. As we are all human and they touched the windings of the transformer acidic material gets into the windings. It takes many years or even decades but the impurities, combined with heat, break down the insulation.
I have fixed a lot of Fender amps and I have yet to see a faulty reverb pan. I am sure that day will come. I have seen broken solder connections on the reverb pan itself and these are always easily fixed.
The vibrato channel was dead and the first valve had a gas leak and gone all white inside.
Can the valves make this amp sound good?
The valves were all worn and needed replaced.Onis completely dead. Most folks put standard ECC83/12AX7’s in this amp. The amp was designed with 7025’s – low noise low Gm valves.
Check out the original valve sticker in the amp and you will see not a standard 12AX7 is used
The only 12AX7 in the amp is a medium gain 12AX7A for the vibrato oscillator. Fender chose the 7025 to keep the cleanness in the amp. The result is that lovely vintage Fender tone with gentle bluesy /jazzy tones when driven hard.
Our Custom Valve kit
We developed a valve kit specifically for the Deluxe Reverb with the exact valve types. Check it out the two versions . One uses the 5U4 rectifier for the Silverface version and the other uses the GZ34 for the Blackface version