Category Archives: Amp repair

Investigation Of Noise In Fender Pro Junior Guitar Amplifier

By Frank | Published: July 7, 2021

The Fender Pro Junior amplifier is popular amplifier. It sounds good, is portable and loud enough for smallish gigs (the sort I play for example!) if you don’t want ultra-clean. Unfortunatley it is prone to being noisy – often to the point of unusability – so we did some investigation into the sources of noise […]

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Gallien Krueger MB 150 Mains Transformer Replacement

By Frank | Published: June 30, 2021

The Gallien Krueger MB 150 is a popular bass amp, especially with double bass players. We recently had an MB150 in for repair with a blown mains transformer, GK part No. TTO-10879-02 (equivalent to TTO-10879-01). Unfortunatley, sourcing a replacement was tiresome. The UK distributor quoted 6 months lead time! I couldn’t find an off-the-shelf mains […]

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MESA M-Pulse 360 Bass Amp Repair

By Frank | Published: May 31, 2021

We recently had a MESA Pulse 360 bass amp in our new workshop for repair, with a fault description that it was powering up but not passing any signal. I’d repaired an M-Pulse 360 last year with a identical fault condition, so we wondered whether the fault in both amps was being caused by the […]

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Upgrade PCB for Fender Blues Junior

By Frank | Published: June 3, 2019

The Fender Blues Junior (BJ) is probably Fender’s best-selling valve amplifier. However, it does have a number of poor design features; in particular the layout of the PCB on which the EL84 power valves are mounted. The EL84 is a well-designed valve. The high voltages on the anode (pin 7) and screen grid (pin 9) are located […]

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WEM / Watkins Copicat tape echo repair extravaganza

By Frank | Published: October 22, 2016

We had a bit of a WEM / Watkins Copicat repair extravaganza at JPF amps. Here’s a nice photo of three of the 8 (yes eight!!!) we had in for repair.   Great sound that can only really be done by a genuine tape echo.

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Leslie 122 Relay Repair

By Frank | Published: October 3, 2015

Here’s an emergency repair I did for a Leslie 122 cabinet. The fast / slow relay had died, and I needed to find a solution with parts from Maplins! The fast / slow replay relay switching is quite interesting in the 122. The cabinet is connected to the organ by a 6 pin connector, however […]

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Fender Tonemaster (Tone-Master) Guitar Amp Repair

By Frank | Published: August 27, 2015

Although much credence is given to the notion that hand-wired amps are inherently superior to other methods of construction, this is certainly not always the case. Here’s the inside of a Fender Tonemaster amp, which as you can see is a total rat’s nest of wires. I dread to think what Harry Joyce would have thought […]

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MESA Carbine M9 Bass Amp Repair

By Frank | Published: June 15, 2015

Here’s some photos from an emergency repair I did for the band Swans. I got phone call from their tech saying that one of their amps. a MESA Carbine M9, had died with smoke emanating from the innards. Swans needed the amp turned around quickly as they were on tour. I said that this didn’t look […]

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Hughes & Kettner TubeMeister 18 Repair

By Frank | Published: June 4, 2015

I’ve had a few Hughes & Kettner TubeMeister 18 amps in for repair. This is an 18W fixed bias amp that uses 2 x EL84s in the output stage. These amps have a quite sophisticated biasing system that automatically adjusts the output valve grid voltage for optimum bias, and also shuts the amp down in the […]

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70’s Vintage Marshall JMP 100W Guitar Amplifier Repair

By Frank | Published: April 30, 2015

We often get 70’s Marshall JMP amps in for repair; these are excellent rock amps and are still very good value for money. Ahe major issue with these amps are the voltage and impedance selectors which are prone to falling out. Someone had done a really good job of wiring the mains selector in this […]

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