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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Hammond Organ and Leslie Cabinet repairs

I’m a massive fan of the Hammond organ: well who isn’t?

Anyhow I’ve recently been helping a friend of mine, and Hammond organist extraordinaire, Joe Glossop, get his Hammonds and Leslie cabinets into working order.

Joe recently acquired a couple of 60’s M series organs and a Leslie 145 cabinet (to go with his C3 and other Leslie 145!), however the method of connection of the Leslie cabinet to the M102 organ (that’s the white Hammond in the photo below) it came with was eccentric to say the least. Our aim was get the Leslie and M102 wired in the standard fashion so that all the Hammonds would work with either Leslie cabinet.

M102-1M102-2

The white Hammond is a factory split M102, which it appears was made only for the UK market in the 60s.

Stevie Winwood had one of these; there is great Youtube footage of Stevie using an M102 from 1967 with the Spencer Davis group on Finnish TV.

It’s also the model occasionally played by Monty Python’s Terry Jones, so could also be useful for a Monty Python tribute act!

Part of the problem with the Leslie cabinet was that it had an amp from a 147 cabinet that ran on US voltage, so required a step down transformer; however the relay in the amp for switching between fast and slow speeds was a 120V relay, so could not be switched in the usual fashion from the organ, necessitating a non-standard wiring in the Leslie cabinet.

Leslie145.1

We replaced the relay with a 240V model and reverted the wiring to standard.

Ultimately though we will install a UK mains transformer to make the cabinet more roadworthy. Tube Amp Doctor in Germany sell a suitable replacement.

We also needed to rewire the Leslie connector from the M102, and reinstall a speaker for the reverb, which he got from another M100 (in Joe’s basement!!) that’s a more long term restoration project.

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Vintage Marshall JMP 100W Amplifier Repair

We often get 70’s Marshall JMP amps in for repair; these are excellent rock amps and are still very good value for money.

One the major issues 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 amp!

JMPmains

 

Not only is this a total hazard, the amp is actually wired to the wrong voltage! Suffice to say I hard wired the amp to 240 V, and disconnected the unused voltage taps from the voltage selector.

The input jacks also had the wrong value resistor soldered to them; the resistor in the photo is 4k7, but should be 470k. It looks very much like a mistake at the factory.

JMPres

 

Additionally one piece preventative maintenance I always do with this era of Marshall amps is to change the caps in the bias supply. If these go leaky then the amp can go into thermal runaway.

 

JMPbias

 

 

 

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Ibanez Tube Screamer TS9 DX Repair

Here’s an Ibanez TS9DX Turbo Tube Screamer we’ve just repaired.

 

TS9DX2

 

The pedal came in with a common fault: the switching was intermittent.

This is usually due to the foot switch wearing out, however the foot switch in this pedal was fine.

After some prodding with a high tech implement (a chopstick!) I found that moving one of the transistors in the switching circuit turned the overdrive off.

Further inspection of the PCB, showed that the solder joint on one of the legs of said transistor was broken. I’ve had to explode the photo quite a bit, but the duff joint is in the middle.

 

TS9DX1

 

As is often the case with pedal and amp repairs, it’s finding the fault rather than effecting the repair that takes majority of the time.

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Better Call Saul theme by Little Barrie using JPF Regent 25 amp.

New American TV drama Better Call Saul, which premiered on 8th Feb, is a prequel to Breaking Bad.

The theme tune for this exciting new series was composed and performed by Little Barrie and features a JPF Regent 25 amp that was used both for the guitar and bass guitar.

Click here for a sound clip.

I have to admit I didn’t see any of Breaking Bad, but I heard it was very good!

You can also see Barrie demoing this amp on our media page.

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Ashdown MAG300 Bass Amp Repair

I’ve had quite a few Ashdown bass amps in the workshop for repair.

Here’s the inside of a MAG300 I had in a recently.

Ashdownmag300.4

The amp was not powering up, and on inspection the mains fuse has blown.

On energizing the unit employing a current limiter (a 60W light bulb in series with the amp), it was obviously that something had shorted out in the amp.

The most likely candidates in a transistor amp are the output devices; however these were fine, as was the main bridge rectifier (which I’ve seen fail in these amps before), so the finger of suspicion now pointed at the mains transformer.

AshdownMAG300.3

 

Unloading the transformer and applying mains to the primary showed that the fault was indeed with the transformer and a replacement was ordered.

Interestingly the replacement mains transformer (on the right in the picture below) is larger than the one originally fitted, suggesting that the original may be on the marginal side for power rating (a larger transformer has a greater power rating).

AshdownMag300

 

As expected fitting the new transformer fixed the amp.

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Ampeg SVT Repair.

Here’s a rare example of a 70s Ampeg SVT that I had in the workshop recently.

The SVT has, rightly, gained the reputation of being one of the greatest bass amp ever made. Even today the SVT is the industry standard bass amp for live work.

Working on these amps is not for the faint-hearted, with an HT in region of 700 VDC.

Unusually this was a UK voltage version; most of the 70s SVTs I’ve seen have been US voltage, necessitating a (very large) mains step down transformer.

 

SVT

 

The massive transformers don’t help with the weight.

SVT1

 

This amp needed a couple of the 6550 output valves changing. Fortunately I was able to match a couple of valves with the existing valves in the amp, avoiding the expense of a complete new set of 6.

SVT3

 

The amp achieved well in excess of 300W on the bench.

 

 

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Fender Blackface Deluxe Reverb repaired at JPF Amps

We’ve repaired hundreds of vintage Fender amps, and it’s always a pleasure to get these classic amps back in tip top working order.

It’s a testimony to Leo Fender that so many Fender amps from the 1960’s are still being used today.

Here’s a picture of the inside of a vintage 1960’s Fender Blackface Deluxe Reverb that we repaired recently.

 

 

FenderBFDchassis1

 

The amp was working, but at higher volumes would intermittently cut in and out.

On further inspection, we found that one of the ground wires from the eyelet board to the brass ground bus had become unsoldered.

FenderBFDeluxeChassis2

 

Not the best photo!

Anyhow, the mechanical vibration when the amp was being played disturbed the ground wire and hence the intermittent signal.

I’ve seen this problem on several Balckface and Silverface Fenders, so I now routinely check the soldering to the ground bus whenever I’m looking inside a Fender chassis.

Resoldering the wire solved the problem.

 

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