Fender Pro Junior Hum Investigation

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. It does have an Achilles heal which is that it prone to be noisy, often to the point that players find their amps unusable, so we have investigated the sources of noise in the Pro Junior.

There are two sources of noise from the heater supply. Firstly there is the usual 50 Hz hum from the 6.3 VAC heater supply itself. There is also a more insideous noise source which manifests itself as buzz. The source of this noise is quite cryptic, but actually originates from rectifier switching spikes on the HT (aka B+) supply. These spikes contain a large amount of high frequencies, and couple via stray capacitance from the transformer secondary HT winding into the heater windings, and thence into the pre-amp valves via stray capacitance between the cathode and heaters.

It is hard to see this using an oscilloscope on the LT and these switching spikes are obscured by the relative large 50 Hz heater voltage (Fig 1).

Figure 1. Oscillograph of filament supply showing switching spikes induced from the HT (B+) rectifiers switching.

In order to see the spikes we have employed a technique used by Rod Elliot. A high pass filter is used to remove the 50 Hz heater voltage, leaving the switching spikes induced on the heater winding. Fig. 2 shows the switching spikes induced onto the heater winding with the stock 10 nF snubbing caps across the bridge rectifier diodes removed. What you can see is a relatively large initial spike (peak just under 2V, 500 mV / division verticle scale), followed by a damped train of oscillation, a phenomenon referred to as ringing. The very steep rise time indicates a significant high frequency content, which of course does not need much stray capacitance to couple into other windings etc (impedance of a capacitor goes down with frequency).

Figure 2. HT spike from Figure 1 visualized by removing the 50 Hz filament supply using a highpass filter.

Fig. 3 shows the stock circuit which has 10 nF caps in parallel with the rectifier diode (C19, C20, C21 and C23 in schematic I have). These caps are included to reduce coupling of rectifiers switching spikes into the heaters by not only the magnitude of initial spike, but also the frequency of the subsequent ringing. By lower frequencies will induce less noise into the heater winding and should be less insidious. The vertical scale is 100 mV / division (cf 500 mV / division with the unsnubbed rectifier), the first peak is about 380 mV and is not as steep as the unsnubbed ringing.

Figure 3. Lower amplitiude and ringing due to addition of 10 nF snubber caps in parallel with each rectifier diode (stock cricuit)

The extent of the ring is actually not surprising as we have added no resistance to the circuit to damp the resonance, we have simply altered the frequency of the resonance.

Another way to snub a rectifier is to add a Zobel network/s either across each rectifier diode, or across the HT secondary winding. A Zobel network is simply a capacitor and resistor in series, the resistor provide damping by converting the electrical energy in the circuit into heat (this is analagous to mechanical damping). For best results the capacitor and resistor values need to adjusted to the circuit they are being used in, and we tried a variety of values.

Fig 4 and 5 show results with 100nF / 10 ohm and 100 nF / 100 ohm networks. As we can see these are a massive improvement even on the stock snubber network with 100 nF / 100. Both networks show a lower initial peak. Note 100 mV / divison verticle scale.

Figure 4. Ringing due to addition of 100 nF/10 ohm Zobel network across the HT winding.
Figure 5. Effects of 100 nF / 100 ohm Zobel network. Higher valve resitors ehances damping, at the expense of a sharper initial spike.

Another option is to try some soft recovery diodes in the rectifier. We installed MUR1100 diodes instead of the stock 1N4007s. Fig 6 shows the MUR1100 diodes with no snubbing caps. This is a dramatic improvement on the stock diodes. This is on the same verticle scale as Fig 2.

Figure 6. MUR1100 diodes instead of the stock 1N4007s. Same scale as Figure 2 shows reduced peak in ringing and lower frequency.

We have added the snubbing caps back in, and get a similar result to the stock diodes; not surprising as the snubber capacitance swamps the diode capacitance (Fig. 7).

Figure 7. Stock 10 nF snubber caps with MUR1100 diodes.

The 100 nF / 100 ohm diode Zobel network again gives pretty much the same results as with the stock diodes (not shown).

I have also experiments with some other capacitor and resistor values, and have managed to reduce the ringing at the expense of increasing the amplitude of the initial spike, so I am not convinced that the 100 nF / 100 ohm option is optimal.

Two other areas that seem to improve noise are reducing the current draw of the power valves (these amps can be biased very hot), and using a hum balance pot on the filament supply.

We have now designed a replacement power valves PCB that incorporates a trim pot that can be adjusted to minimise hum.

Noise can still be a problem, and I guess part of this is due to the layout with unshielded PCB traces running to and from the first valve stages.

To be continued……..

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Gallien Krueger MB 150 Transformer Replacement

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 trying to source a replacement was tiresome. The UK distributor quoted 6 months lead time, and the only other supplier I could find was in France; they didn’t have the transformer in stock either.

Unlike some other amps (eg Ampeg SVT 350) I couldn’t find an off-the-shelf mains transformer that would do the job, so got in contact with Tiger Toroids who have done several jobs for us over the years to get a custom unit made. Amazingly Stephen at Tiger Toroids had been sent a blown MB150 transformer a few weeks before us, and already had the specs to make a replacement.

Custom replacement mains transformer for the GK150 made by Tiger Toroids with correct colour coded connectors.

Due to construction contraints, the replacement transformer (see above) is slightly taller than the original (a good thing electrically), but space in the MB150 chassis is tight, so this transformer could not be mounted safely using the original mounting plate; what you must NEVER do with toroidal mains transformers is allow the mounting bolt through the centre to touch the top and bottom of the chassis as this will make a shorted turn and the transformer will be destroyed in short order.

Mounting transformer with original mounting plate

Alternatively we can partially fill the centre hole in the transformer with potting compound, and drill a mounting through this. The transformer can now be bolted into the chassis without danger of the bolt touching the other side of the chassis. If I need to order another MB150 transformer from Tiger Toroids I’ll get them to do the potting.

Centre potted mains transformer now mounts safely in the chassis.

I added a plastic sheet on top of the transformer for extra insulation. The amp is now working correctly

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MESA M-Pulse 360 Bass Amp Repair

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 same component failure.

The M-Pulse 360 used tantalum capacitors in several positions in the pre-amp (see figure below). Whilst tantalum capacitiors are considered a “premium” part over standard aluminium electrolytics, they also have a reputation for unreliability. Rod Elliot of Elliot Sound Products has this to say about tantalum capacitors: “while many sing their praises, I do not recommend their use for anything, other than tossing in the (rubbish) bin.”

Arrow pointing to one of the tantalum capacitors in the pre-amp

So why use tantalum capacitors? When compared to aluminimum electrolytics tantalum capacitors can be made smaller for the same capacitance, very useful if PCB space is tight, and have lower equivalent series resistance (ESR) which is useful for decoupling power rails in some applications. However, in this amp PCB space is not at a premium, nor is low ESR a major consideration, and if it was there is space to add a smaller value poly cap in parallel. Some people nay also use them because they are more expensive so must be better………

Anyhow, we openned the amp up and low and behold the same tantalaum cap was shorted as in the other M-Pulse I’d seen! To test the caps I simply unsolder one end and measured the resitance with the multimeter. The cap that has shorted was decoupling the +15 V rail supplying the op-amps in the pre-amp, dragging this rail to ground; without the +15V rail no signal was being passed. We replaced the cap with an aluminium electrolytic and the amp was back up and running.

And here is the amp being tested on the bench.

Fixed amp being tested on the bench using “ammo box” dummy load, and analog ‘scope.
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JPF Amplification now at 5 Denmark Street.

We have relocated our work shop to number 5 Denmark Street, WC2H 8LP.

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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.


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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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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.



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).


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.






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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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