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

I’m taking a week’s holiday in the US, during which I will be attending the Dallas guitar show 2010. As a result of this we won’t be doing any repair work until I return on the 21st April.

I will have a digital camera with me (sadly slide film is getting harder to find) and so will — hopefully — manage to take some photos of cool amps and guitars at the show.

Check out the JPF amps blog on my return for a report of proceedings.

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?

Although most of our work is repairs, we also do mods on amps. Here are a couple of recent examples of our mod work.

The Fender Blues Junior is a very popular model in the Fender valve amp range. However, like many amps that use EL84, they do run the output valves v-v-v-very hot. In this case the very knackered looking output valves were dissipating nearly 20W each at idle, which is slightly more than the data sheet maximum of 12W!

Although the Blue Junior is fixed bias, there is no means of adjusting the grid bias voltage. Here I installed a trim pot on the PCB that allows adjustment of the idle current to a less incendiary level.

The second mod was in a Musicman RP-65 amp. These amps date from the late 70′s/ early 80′s when Leo Fender was involved with the Music Man company. Like most amps Fender was involved with, these are well built and reliable amps. Although Musicmans have a solid state pre-amp and run the power valves almost in class B, they do sound good, are well built, and can be picked up for not a lot of cash.

This series of Musicman amps has gain and volume controls, employing a circuit similar to that used in a TS9 Tubescreamer to generate distortion. The owner of the amp like the distorted sound and the clean sound, but wanted to be able to switch between the two sounds.

I added an extra pair of controls for gain and volume, conveniently where the (unused) line out jack sockets were located, and only needed to drill one hole in the back of the amp for a footswitch. As the pre-amp is solid state running off +/- 15V rails, it made sense to use JFET switching. Below is a picture of the switching board. Don’t worry it wasn’t left flapping in the wind, but was bolted to a conveniently placed piece of cardboard underneath it. (The card is there to stop anything being poked through the line out jacks touching the fuses).

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.

Just to let you know that we will be shutting for business on Monday 21st December and will be re-opening in the New Year on January 4th.

In the New Year we will be adding a Tech Info section, which we hope will present some information not found elsewhere.

We will of course continue to post any interesting amps/ repairs that we have in.

Have a great break and all the best for 2010.

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

I’ve recently purchased a copy of Merlin Blencowe’s book “Designing Tube Preamps for Guitar and Bass”.

This is the best books I’ve read on guitar amp pre-amps, which will come as no surprise to anyone who has visited Merlin’s excellent website. Although the approach is quite academic, even if you aren’t into the maths (sad to say I am), there are a ton of useful ideas and practical tips.

Thoroughly recommended.

The only disappointing feature of the book is that the author chose to you “tube” in the title rather than “valve”!

OK, with our new blog facility (!?) we aim to keep the cyberworld up to date with JPF Amplification activities.

Once we’ve worked out how to add picture to this we will also bring you details of any interesting amps we have through the shop.

Keep it country!