Guitar Amp Repairs, Custom Amp Builds, Speakers – JPF Amplification

This a photo of the filter caps from a Blackface Fender Twin Reverb that was recently brought in for repair. Note the early ’64 date code on the reverb transformer. The main filter caps have been replaced with Sprague Atoms. Can anyone see the error the previous repairer had made?

Hope you all had a good break. I certainly did.

Having not seen a Rivera-era Fender for several months we had two Deluxe Reverb II amps in for repair within a couple of weeks! Amazingly they both had faults with there speakers, which we replaced.

As I’m sure you know, Paul Rivera was employed by CBS/Fender in the early 80s to revamp their amp line. As well as bringing in a range of new amps with features such as channel switching and high gain channels, Fender also upped their game with respect to construction, and, in my opinion, this era of Fender amps are the most sturdily built.

Whilst the lead channel isn’t to everyone’s taste, these amps can still be picked up at a reasonable price, and are good value for money.

Anyhow, here’s a chassis shot of one of the amps. Typical Fender eyelet board construction.

The grid wire to one of the output valves had not been soldered on at the factory. Amazingly this wasn’t the problem with the amp, although I did solder this.

One area of weakness in the DRII (and the Princeton Reverb II) is the 47 Ω resistor that feeds the bias supply. This is only a 0.5 W rated part so is prone to failure; and as bias supply failure is not good, I always change this resistor to a higher rated component.

The “burnt” resistor in the photo below is not the original resistor, and so must have been installed during a previous repair. Needless to say, the replacement resistor seems to be as “shandy” as the original, so I replaced this with a 5W cement box.

I will be using our blog to show any cool and interesting valve amps we have in for repair service. This very early, rare Matamp GT-120 was brought in recently. It  had expired in the late 70s and hadn’t been turned on since!

Opening the chassis revealed the likely problem; the white powdery coating on the inside of one of the valves shows air has got into it. Although the glass envelope can be damaged by receiving a knock, valve envelopes are often cracked by overheating. The other valves also showed signs of overheating too.

Expired power valve on left. Sadly a Mullard.

If an amp hasn’t been energized for several years there is a good chance that the electrolytic caps in the amp with have deteriorated to a point where they can’t be reformed, so it is always wise with an amp that has lain idle for a considerable time to bring the voltage up slowly with a variac and a series current limiter.

Having done this, most of the caps seemed to be in good order. However, I did change the caps in the bias supply (the blue 10 µF/160 V Erie caps shown in the photo of the board) as these were were leaking electrolyte. It is always worth changing the caps in the bias supply before they fail. If these caps leak too much current, or fail short circuit (a common mode of electrolytic cap failure), there will be insufficient negative bias voltage on the power valves to prevent excessive current draw. If you are lucky the HT fuse will blow (assuming there is one!); if you are unlucky the collateral damage can be very expensive. Think mains transformer. I’ve seen many amps where this has happened.

Light blue bias supply caps