Wed 3 Feb 2010
Do you think this valve has overheated?
Posted by Frank under Guitar Amps
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Fri 8 Jan 2010
Fender Deluxe Reverb II repair.
Posted by Frank under Guitar Amps
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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.
Fri 18 Dec 2009
Christmas Holiday
Posted by Frank under Bass Amps, Guitar Amps, Uncategorized
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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.
Mon 7 Dec 2009
Cool Matamp GT120 for service.
Posted by Frank under Guitar Amps
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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
Mon 23 Nov 2009
Great new valve guitar amp book.
Posted by Frank under Guitar Amps
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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”!
Mon 2 Nov 2009
JPF Amps Blog
Posted by Frank under Bass Amps, Guitar Amps
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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!