Upgrade PCB for Fender Blues Junior

The Fender Blues Junior (BJ) is probably Fender’s best-selling valve amplifier.

However, it does have a number of poor design features, not least the layout of the PCB on which the EL84 power valves are mounted.

The EL84 is a well-designed valve as the high voltages on the anode (pin 7) and screen grid (pin 9) are located on pins away from the low voltage control grid (pin 2) and the cathode (pin 3).

EL84 pin out

Fender negate this sensible layout on the left-hand side EL84 looking into the back of the amp by running a PCB track from the grounded cathode to the ribbon cable from the bottom PCB to the main PCB via the gap between pins 6 and 7, i.e. right next to the high voltage anode connection on pin 7, which the EL84 was designed to avoid.

Stock Blues Junior PCB. Note PCB trace from Cathode (pin 3) running next to the anode (pin 7)

This is a mistake, and we have seen many Blue Junior PCBs damaged due to a short from the anode to ground via this trace.

Fender BJ PCB damaged by short from pin 7.

Fender have addressed this problem in the current Mk IV version of the BJ by slashing the PCB trace from pin 3 and hard wiring pin 3 directly to the ribbon cable. We now perform this mod on all BJs we get in.

Modification, as used by Fender, for BJ PCBs to stop arcing.

Regardless, given that there are plenty of BJs out there with the older style PCB, so we designed a replacement PCB that avoids this problem.

Our PCB is made from 1.6 mm FR4 (this is a better material than the synthetic resin bonded paper used in the BJ), 2 oz copper traces (1 oz is standard), double-sided, and plated through holes.

JPF Blues Junior replacement PCB showing valve sockets

The PCB is designed to accommodate Belton noval sockets, which are used in some versions of the Blues Junior, and are very good quality sockets. You can recycle the sockets from your old PCB.  Sometimes if a valve short occurs one of the power valve sockets will be damaged, and we can supply replacements.

We have added pads so the filament wiring to the pre-amp valves is hard wired off the PCB with tightly twisted wire, which should help to reduce noise. Again we can supply the PCB with this wire in place.

Filament wiring on JPF Blues Junior PCB.

Additionally, there are pads for 2 snubber capacitors (C1 and C2). Fitting these will eliminate the tendency for the BJ to oscillate at around 50 kHz. This of course not audible, but stresses the valves and the filter caps. We recommend 220 pF, and can supply these. Alternatively if you are removing the main PCB, you can change the 47 pF capacitor across the phase inverter to 220 pF; this is designated C33 or C14 depending on which version of the BJ you have.

JPF PCB installed into a Fender Blues Junior.
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Electro Harmonix Big Muff Pi Repairs.

A significant proportion of our work is pedal repairs.

You’ll often not see any of one type of pedal for months, but then, just like buses, a load of the same type will come along.

Here’s a selection of EH Big Muffs (and indeed Little Muffs) that we recently repaired.

BigMuffs

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Ibanez TS-808 Tube Screamer Reissue Switch Repair.

The Ibanez TS-808 Tube Screamer is, rightly, regarded as a classic pedal, and has been reissued by Ibanez; this is lucky as an original TS-808s will set you back several hundred pounds.

 

Ibanez TS-808 Reissue

 

Unfortunately the switches on the reissue (and indeed the originals), are not that robust.

This pedal came in with a broken switch.

In the past I have simply replaced the whole switch assembly; however I had run out of TS-808 switches and the only replacement I could find for sale was in the US and $20, which would have made the repair expensive.

I thus investigated whether I could repair the switch, so I removed this from the pedal and disassembled it.

 

 

TS-808 momentary switch

 

On the left we have the defective switch, which was stuck on. Amazingly I was able to source a replacement, which is an inexpensive surface mount momentary connect switch, and it was a simple job to swap out.

Here’s the final switch assemble ready for reinstallation in the pedal. Total cost of the repair £23, which is almost certainly less than the cost of ordering the switch from the US!

 

TS-808 switch assembly.

 

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Regent Sounds Website up and running.

Regent Sounds, who are Central London’s longest running Fender & Gretsch specialist, and where we are now based, have (finally) got there website up and running so you can browse their stock and even make a purchase!

As well as Fender and Gretsch, Regent Sounds also stock Musicman, and Danelectro guitars, Blackstar and VOX amps, and a wide range of effects pedals, including Fulltone, Chase Bliss and Exotic Effects.

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WEM / Watkins Copicat tape echo repair extravaganza

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.

copicats

 

Great sound that can only really be done by a genuine tape echo.

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Leslie 122 Relay Repair

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 as the audio signal to the cabinet is balanced, there is no spare pin for switching the relay.

The way around this is that a DC voltage is superimposed on both terminals of the audio signal. As the input is balanced this is a common mode signal and thus not amplified by the 122  (see schematic below).

122

This DC level is typically 60-100 VDC. This is applied to the grid of a 12AU7 that energizes the relay. The relay had died (due to a Leslie motor shorting out), and needed replacing.

I couldn’t source a direct replacement, so used a DPDT 6V relay which could switch 240VAC at 5 A. As I only needed a SPDT relay I paralleled the two switches for extra current handling.

I derived the power for the relay from the filament supply to the valves in the 122 amplifier; the relay only draw 83 mA which will not have any effect on the filament winding. To switch the relay I used a TIP31 NPN transistor which will turn on when a positive voltage is applied to the base. I limited this voltage with a 47k resistor and a 12V zener.

Here’s the final circuit.

122relay

 

 

 

 

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Fender Tonemaster (Tone-Master) Guitar Amp Repair

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 of this!

 

 

 

Fender Tonemaster1

 

 

 

 

Fender Tonemaster 2

 

The amp needed new valves, but was also oscillating at higher volumes.

The problem was the length of wire from the phase inverter output to the power valves.

Normally grid stopper resistors are attached to the valve control grid to prevent oscillation, however although there were grid stopper resistors in the amp they were attach to the far end of the the grid wires, and NOT directly to the valve sockets.

Adding 1k5 grid stops to power valves (under the heat shrink in the bottom photo) cured the problem.

 

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Gallien Krueger 2001rb Bass Amp Repair

Gallien Krueger make amps that are both well engineered and good sounding.

I recently had a GK 2001rb bass amp in for repair.

This is a complex machine (for a bass amp: see photo of innards); it has two 500W class G amps to provide do the heavy lifting in the low end (one each under the heat sink fan assembly in the photo below), and 2 x 50W integrated chip amps for the “tweeter”. (Can’t say I’ve ever seen the point of a tweeter in a bass rig, but there you go).

GKRB2001.2

The 2001rb has a very sophisticated processor controlled protection circuit and the amp was going straight to fault mode.

Class G is an ingenious method of increasing amplifier efficiency, and hence reducing waste heat. To achieve this class G amps employ two (or more) pairs of power rails, in this case +/- 75 V and +/- 38 V. Power is only drawn from the higher power rails when the signal greater than 38V, and thus power is saved. A good discussion on class G amps can be found at Rod Elliot’s website.

The pre-amp of 2001rb runs from +/- 15V rails -a very standard arrangement – which are derived from the +/- 38 V inner rails for the power supply using LM317 / LM337 regulators.

After some inspection, it became clear that the LM337 regulator for the -15 V rail had gone short circuit. The LM337 was located under the heatsink for one of the 500W power amps.

Here’s the amp with the heatsink removed. The LM337 has been removed, and was at the top above the left side row of exposed transistors.

GKRB2001.1

The consequence of the LM337 failing was that -38 volts was imposed on the circuitry down stream, including the pre-amp, and as a result there was substantial collateral damage. For example the TL072 op-amps in the pre-amp didn’t like it up ’em.

GKRB2001.3

Most of these needed replacing, as did several of the DG419 CMOS switching ICs.

Often repairs like this are iterative; ie once you have repaired the main point of failure you then need to find what else has been damaged.

 

 

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MESA Carbine M9 Bass Amp Repair

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 promising but I would have a look at it; smoke is usually a bad sign, and if say the output transistors had died it’s unlikely that I would have the correct parts in to affect a repair.

Anyhow, when I got the amp the source of smoke was fairly obvious:

MesaCarbine2

 

This an insulation displacement connector (IDC) which connects the power transformer secondary windings to a PCB. IDC connectors rely on blades in the connector to pierce the insulation in the connecting wire.

IDC connectors are generally used to aid assembly/ disassembly. However you can experience problems with IDC when passing high currents eg in a 600W amp! Other manufactures have had problems with IDC connectors for the filament supply in valve amps, eg Bugera in their 333XL.

Anyhow, the I removed the connector and soldered the wires directly to the PCB and we were up and running again.

MesaCarbine1

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Hughes & Kettner TubeMeister 18 Repair

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 event of valve failure to prevent further damage. They also have a nice plexi panel backlit with blue LEDs.

18HK

 

Output valve operational status is indicated by 2 LEDs on the back panel (one for each valve), and this amp came in with one LED flashing which should have meant that the corresponding valve had failed.

When I’ve had these amps in with this fault before it’s simply been a case of replacing the output valves and the amp is up and running, however in this case changing the valves had no effect on the fault; the flashing LED still indicated that the valve was faulty.

Looking at the schematic for this amp, the power valves are turned on and off by MOSFETs in the cathode circuit. After much head scratching and probing I found that one of the MOSFETs had a short from gate to source, and this was causing the fault condition: ie the valve protection circuit had failed!

Here’s the PCB with the offending MOSFET removed:

 

18HK2

 

I replaced the MOSFET and this  cured the problem.

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