A Very Quiet Fender Twin Reverb

I hadn’t seen John for some years, but he turned up out of the blue with the Fender described above.

Even if I looked at the amp from a distance of a lot of miles I would have known it was a ‘Reissue’. I’ve just looked up ‘Reissue’ in ‘Ballooning for Aquatics’ and it said ”Consult ’Skiing for Quadrupeds’.” Which said ‘Consult “Brain Surgery with Lump Hammers”.’ So I finished up guessing. It must mean, I thought, something old brought out again, much later. Wrong.

What it really means is ‘Nick a label off a really nice amp and make a new one that’s nothing like it.’

The overriding problem with current reissues of ‘classic’ amps, is that it’s impossible to do unless you have a raft of prospective customers with bottomless back accounts. I’ve got one of those, but only because it has a big hole in the bottom. A reissue amp has to use pcb’s. If it used solder wells, tagstrips, turret board; as the original Vox, Hiwatt, Marshalls(old ones), Fender (old ones), Selmer, Supro, Carlsbro (old ones) etc….etc…. it would be monumentally expensive (there are still a few of this breed around) because people would have to do much of the building, as opposed to computers.

This gets me (eventually) to John’s Fender Twin reissue. The almost insuperable problem of valves and pcb’s not being mutually on speaking terms rears its head. In this case the output valve bases (4x 6L6) are soldered directly through the pcb, and…… as the valves hang from the bases the heat rises and…..they get hot!!!! And if we heat up a solder joint a lot of times (it doesn’t even need to be all that hot; the melting point of tin is 232degrees c and lead is 327 c so anything over 232c burns off the tin and leaves the lead.) So it drives off the tin content of the joint and you finish up with lead. And usually cracked lead. The alloy of solder is usually 65%tin to 35% lead, and the resistivity of tin is 11,5 ohm.m whereas lead’s resistivity is 21,3 ohm.m. This means that the solder joint has twice the resistance when the tin has gone.

Another property of lead is that its flexibility is poor, and the expansion and contraction of the metal with heating produces cracks and poor contact with the joint. At its most extreme the joint can become an insulator, and your nice guitar sound has become nothing at all at the other side of this joint (which is known as a ‘dry joint’ or a ‘cracked joint’).

It’s about here when I wish I had a bit more interest in photos and things, because it would be useful. However….the fault with John’s amp was minimal, but as with a lot of pcb-based amps, not that cheap. To correctly replace a component in a pcb, you have to be able to get to the to the UNDERSIDE of the board. Pheeew! That can mean taking off wiring, marking where it came from, taking the pcb out, all to get to the track side of the pcb. If you don’t do that (it’s possible to cheat by cutting off the component and soldering to the resultant wire sticking out) you run the risk of dry joints, shorts to chassis and missing any burned tracks that might be under there. In the original version, the solder wells are visible, and the pcb dismantling zero, because there aren’t any. So the job takes five minutes, and the rest of the time can be usefully spent cleaning up and servicing.

Also, on this amp there are two wirewound dropper resistors (270 ohm if memory serves) that run hot. That’s ok, they drop the voltage from around +- 50 volts down to +- 17 volts to supply the chips and relays that the amp uses for channel switching. These two resistors are flat to the board (not ok), and so heat up the tracks underneath, and are also close to two 1000uF 35 volt capacitors which don’t like the heat. For an amp of this age (1994-ish and on) these components are ready for replacement and you need to have a good look and resolder all the joints under there.

Just as an aside, it’s interesting to note that any mortgage you might have taken out to buy a Mega-Hugely-Marvellous-Platinum Plated- Radar Controlled…..erm….guitar lead….would be a totally daft investment if you had a dry joint on the input socket.

To put this problem right would cost you 0.00000001% of your investment with Wonderful Guitar Leads Inc.

Which is but a small increase on my macaroon outlay for the decade. Tea.


Universal Audio S-610 preamp

This is brief set of impressions regarding a nice American made valve preamp. This one blew fuses, so not quite as nice as it should have been.

On the back of the unit there is a lot of blurb about which fuse to use for which mains voltage. What it is less than clear about is that you don’t just swap the fuse and everything is ok for the different voltage. What it says is that a 250mA fuse is for 110 volt operation and a 120mA fuse is for 240 volt operation. What is not obvious is that there has to be changes made inside the unit to effect the voltage change. You might notice that if you trawl through the user manual, but not necessarily.

There two four pin connectors mounted on the internal pcb near to the IEC input, and the free plug that fits these (looking from the back of the unit) needs to be on the left hand one for 240 volt operation or the right hand one for 110 volt operation.

Unfortunately, if you’ve bought one of these units, and it was internally set for 110volt operation, and have plugged into 240 volt mains, it will have blown the fuse in the IEC socket. You might be lucky and after changing the voltage setting to 240 it might be fine. But the electrolytics in there will have been subjected to a high over-voltage and it would be sensible to check these.



Ampeg (i.e. Loud Technologies) BA300 115

If you happened to be one of those few misguided folks who followed this blog, you may have noticed that….there hasn’t been any for quite a while. My excuse is that I have to get enough steam up to vent my spite on unsuspecting pieces of electronic equipment, and, more often than not, their manufacturers.

The Ampeg BA300, whether in the 1x 15″ or the 2x 10″ versions, seems, from the various forums I’ve read, to be leaving a trail of very disgruntled musicians in its wake.  Many of them having had these amps switch themselves off, mid-phrase. The amp then switches itself back on again after a leisurely 10 seconds or thereabouts, and then, if the volume settings remain the same, does it all again. Although the amp itself seems not to suffer any embarrassment, the same cannot be said of the bass player.

I’d like to be able to say I’d successfully repaired one of these, but the honest statement is that I’ve done no such thing. But I have spent many hours inside one, and can let you in on a few conclusions.

The first mystery is why any builder of amps can think that it’s a great idea for the amp to switch itself off if too much power is demanded of it. The traditional answer to that problem is that the amp goes into clipping distortion, the player hears that, (not pleasant) and turns it down. If the music demands a heavy distortion, the amp just doesn’t go any louder if turned beyond it’s rated power output.

This amp is a ‘clever’ amp. This means that it’s complicated, not necessarily for any good reason except for, maybe, profit boosting purposes. It is a ‘class D’ amp. The ‘D’ stands for ‘dismal’ in my book. But it actually means that it uses a modulated carrier signal (of about 450 kHz if memory serves). There are different versions of the modulated carrier technique; this one uses ‘pulse width modulation’.

They also use a switchmode power supply. It’s a lighter alternative to a traditional isolating transformer, but it has its own problems.

The power amp section is unusual, and not a bad idea. It uses a bridgemode technique, which means that the speaker is driven from both sides, instead of the usual hot drive to positive and ground to negative. The power rail voltages can be much lower than in the traditional configuration, and this results in better efficiency in  the power amp section.

There were two faults showed up on this amp. The first was easy and fairly obvious. It didn’t work. With certain faults in mosfet devices, (these are !RF640 used in a push-pull configuration) an output signal can be produced which disappears when there is no load (speaker) connected. This amp has four !RF640 mosfets. Each pair produces a push-pull output, and the two pairs are out of phase; and applied to the speaker at either side. This was the bridgemode arrangement described earlier.

These devices were replaced and…It worked!

Un…fortunately I found out that a transient (a ‘pop’ or a slap on bass) switched it off. It returned after a delay and would do the same thing again. Etc…etc…

The ‘protection’ (if that’s what it is; one could argue that it’s musical assassination) switches off the carrier signal mentioned earlier.

Clever? Yes, but not very sensible.

Time for tea.

Accutronics spring reverb

The Accutronics spring reverb has been around for a long time. They haven’t changed much, except that there are more options for the transducer specs. The idea of this little missive is to get to find out when they might be repairable. The answer to that is ‘Quite often’, and for no more money than a blob of solder. But let’s clear up what a ‘transducer’ is, for a start.

The yellow thing is a transducer.

A transducer can be many things, but basically, it’s any electronic wozzit that converts one form of energy into another. In this case, this wozzit converts movement into electrical voltage/current. Or the opposite way round, i.e. electrical current/voltage into movement.

This means that the speaker in your amp is a transducer, as is any form of microphone.

The Accutronics has two transducers, one at one end of the springs we can see in the top pic, and another (looks the same but isn’t) at the other. One is the input transducer, the amp puts out a voltage /current to this, which creates a magnetic field in a dooflicky called ‘the armature’ and this acts on two very small magnets connected to the springs. This effectively creates movement in the springline which is an analogue of the electrical impulses applied to the transducer coil. That’s basically what the transducer is; a coil. The coil at the other end (the output transducer) of the spring picks up the movement of the spring, converts it to electrical signals, and the amp’s circuitry amplifies these small signals and mixes it with the straight sound through the amp.

So the amp circuitry for the input side of the reverb is completely different to that of the output. Depending on how old (or expensive) the amp is, the drive to the input might be a chip (op-amp, and cheap), or a transistor (still cheap,) or a complementary pair of transistors (not so cheap) or a valve driving a transformer (definitely not cheap. Most early Fenders had this arrangement and the later ‘reissues etc.’ not.) It makes a big difference to the quality of the sound through the Accutronics device.

From the above we can see that, if you plug the unit into your amp the wrong way round, you are up for a disappointment, because it won’t work. The two transducers have completely different specs; the input being quite low impedance and the output very much higher. If you’re going to swap a unit in an amp, you should look on the Accutronics/Belton website, because there are a lot of variations of these things these days, whereas the early models were a spring line unit that worked, and nobody bothered too much about the numbers. Anyway, there is a code printed on these units , and you need to compare this to the various specs on their website. Not difficult, just irritating. Or maybe that’s me. Damn! I was going to do this blog without any niggly quips etc.

So what might you be able to repair in one of these?


The first thing to check with a multimeter set on a low ohms scale, is the readings on the phono connectors. The input should read a few tens of ohms and the output, a couple of hundred or so. If these read ok, you’re looking for a fault in the amp reverb circuitry. A check on this is easy. Turn the reverb up and put your finger on the plug of the output lead. It should buzz. If not, problems. Not easy to test whether you have a drive to the input. Grounding on the unit comes in a variety of forms. The old ones used to be grounded through the frame of the unit via the rivets that held the phono sockets in. Corrosion often messed the ground integrity up. A ground wire between the two phonos will usually sort that out. Have a good look at the wire from the phonos on the unit to the transducers. They often break because of the constant vibration of the spring unit. A break at the phono end is repairable (easy); a break at the transducer end is usually not.

Much of this can be sorted out with the unit out of the amp case.

A cup of tea. And a macaroon!


And Now…..KAM !!!! Another fully paid-up member of ARS

“So!” I hear you ejaculate (?); “What is this ‘ARS’ that you insist on my sensibilities?”

ARS is a fine example of the group of phrases known as ’acronyms’. It is also, by the by, a fair description of the companies who deal in ‘Anonymously Ridiculous Subterfuge’. These people’s productions are soooo….clever, that they are quite convinced that there is nobody else can repair them. This not all that surprising when you discover that the schematics are encoded in Sanskrit and buried under the Sphinx.

I have to admit that most current and recent gear blends into a sort of mess of colourless nonentity when I have to recall what they are. Or what they’re called. Or even what they do, sometimes. So I can’t really be expected to remember this KAM mixer because it also blended into the undergrowth of standardised, erm, undergrowth.

The last DJ-esque thing I looked at was a Neumark Dimension something or other power amp, and the thing I remember most about it is that it had been imported from planet Zarg; and the schematics, so far as I could work out after various expeditions up the Unpopo, were still residents of Zarg. I was bloody minded enough (on that occasion, and usually, anyway) to draw the thing out with my trusty pencil on the back of a fag packet, which enabled me to prevent the owner paying out a few hundred quid to Neumark. So it was worth the effort just for the opportunity to deal a hefty fiscal boot to the groin of another member of ARS.

Back to KAM. It’s a while ago now but the most memorable bit of it was that it looked like everything else. From my point of view that means ‘made of tin and sharp edges and most of B&Q’s stock of self tappers’. And it didn’t work. I often forget that bit because when I get something it nearly always doesn’t.

In this case the left hand output was down. After week or two’s trawling the internet, I had to admit that KAM was definitely a member of ARS. In that situation one is reduced either to:-

a) giving up; b) eating the workshop carpet; or c) having to think.

This one wasn’t that bad. If you look at the mixer from the front, on the right hand side are the two output faders. After removing a tinful of screws, the pcb is revealed, as are five SIL (single in line) chips. Four of them are situated in a square formation between the faders. The really interesting one is at the top of that pcb. This is the final amplifier before the output, and this was the one that had faulted. Get your favourite integrated circuit data book out and find the pinout arrangement for that chip. All you need then (and this test works for most op amp chips) is your multimeter, set on a volt range of say 0-25 volts or so. This is a dual chip so it has two outputs. At those two output pins you should see a zero volt reading, or at most a few millivolts. A faulty chip will often give out rail volts (that’s plus or minus 12 or 15 volts-ish). That was the case for this KAM mixer. And, once again, the customer did not spend a few hundred quid with KAM for a new mixer; but about £40 with me. I doubt, however, if he was going to heartily recommend KAM to anybody except for a good kicking.

All’s Well that Ends Well. Shakespeare said that. “And now for a cuppa and a macaroon.” I don’t think he said that.

Fings Ain’t What They Used to be….An unreliable history lesson

This is a digression. It won’t help you to fix anything….except maybe make a start on the World….but then, that is too far gone to warrant the effort of throwing a spanner in it.

This little dissertation comes about from my various involvements with Carlsbro, both the real one of yesteryear and the imaginary one from China. Or wherever.

The photo is of the TC60 (actually the one after it but it looks about the same). This was Stuart Mercer’s first production (not unlike the AC30 from a distance but utterly different from every other point of view). That gentleman started the business and designed the first amps. The school group (‘band’ as it would be called now, but they were ‘groups’ then) that I accidentally started in 1959 bought a complete set of this gear from a stable block in Jenford Street, Kirkby-in-Ashfield, where he made them, in 1963. That was a couple of guitar combos, a bass top and cabinet, and a 60 watt PA system with a pair of 2×12 speaker cabinets. A total of 240 watts rms of valve power, and it was bloody LOUD. The whole lot was also £600+ and in 1963, for a group of school kids, that was a lot of dough. On the never-never (hire purchase) of course.

This was the Carlsbro bass stack. Amp missing but that looked a lot like all the others.

As with most gear of the time, schematics were not considered a black art and were easily available. In fact they were often supplied in the bottom of the cabs. Or in the case of Leak, Wharfedale and the like, they were actually stuck into the bottom chassis plate. Very sensible.

To find a current schematic for most things out of China, Indonesia, Korea, India, Botswanaland, Mars etc, you would need to get up an expeditionary force to the upper reaches of the Unpopo because that’s where they’ll turn out to be. (Under the box marked ‘Man eating crocodiles do not disturb’.)

Why is that? Too complicated? Too clever? Too self-interested? Narrrr…..none of that. They just don’t have ‘CLASS’. You can’t buy it, you can’t put a price on it, but you know when it’s there. Don’t ask me why, but a falling-to-bits AC30 or a Binson Echorec absolutely reeks CLASS. Whereas your state-of-the-art Mackie desk, or Blackstar whatsit just doesn’t.

I mean just look at this thing. TOOO COOOOL. And they sound….pheeewwww.




Back to the ‘history lesson’. It must be pretty obvious that I am terminally biased (?) against modernity. The price of being an old git. But I don’t care. Anybody who hates touch screens and apps and phones that take photographs and double as life rafts and do everything, but badly, can’t be all bad. You can argue with that if you like, but as I don’t care what you think, you would need to find somebody else to argue with.

Stuart Mercer started Carlsbro, as we said, in stables in Jenford Street. Presumably after the cows had been put out. He then did a lot of sweating, building prototypes. Much of this sort of grafting work is no longer done in modern design. In fact, a bloke with a degree in computer software would be better suited to designing a modern amp (even though he knew more about elephants than electronics) because this is where the ‘models’ come from. Software programs.

But Mr. Mercer had a problem. He was a qualified TV engineer, and TV engineers know little about the design of big valve power amps, but plenty about valve preamps. So he could design the tone circuits and such, but what to do about a power amp? This is where a stroke of brilliance, much representative of those days, struck him.

He bought in a Leak 50 watt rms power amp, and SCREWED IT IN THE BOTTOM OF THE CABINET!!!! From there he had to build a connecting lead from the preamp to the Leak amp and Walla !!!! A perfect solution. Doubly so, because keeping the power and preamp sections physically away from each other is a great piece of design strategy.

There might be more of this dismal rubbish; I’ve really enjoyed this tirade of insults to current thinking. I don’t really deserve a cup of tea and a macaroon, though. But what the hell….

Righty-ho clever dick….YOU design an amp……

There is the possible beginnings of a story to this one. So, as we are all ‘friends’ (in wonderful internet-speak that is. And bearing in mind you don’t know me from Adam. Or Eve for that matter) I let you in on an interesting prospect.

A gentleman I knew from a long way back called me out of the blue to ask if I would care to figure out why the amp project he had built does nothing but make nasty noises. This is regardless of what kind of nasty noises are being shoved into the front, you understand. The answer to that enquiry, although the problem I find very interesting, would usually be something like ‘GET THE HELL OUT OF HERE!!!!’ There are cans of worms and bucket loads of them, and sorting out somebody else’s design is definitely a bucket load. But Will is no idiot, a genuine bloke, and a good businessman. I don’t know about the first two but I definitely do not fall into the last category. The idea of somebody else doing business-thingies while leaving me to sort out technical issues might work out alright, thinks I.

The problem he has is with parasitic oscillation. Basically the amp is unstable (and unusable.) Not to give away too much, part of his idea was involving a class A output stage. A push-pull (class AB of some description) output stage is a naturally stable design. The reason that power amps become unstable is often that the power rail (this is a valve amp) carries some of the signal of the output back to the preamp stages and produces a positive feedback loop, not unlike acoustic feedback, which is also a positive loop. In a push -pull stage, one valve conducts through the primary winding of the centre-tapped output transformer, and the opposite valve conducts through the other half of the winding.

This means that the power rail that supplies the centre tap, has no signal content because the two halves of the transformer cancel each other out. It’s not quite like that, but close enough. Class A, for all that is trumpeted about its wonders has three serious failings. Firstly, It has to take a high dc current from the supply. It has to be biased so that the valve(s) will conduct the entire signal. Secondly, there is no cancelling effect from the dc supply rail. So the smoothing has to be very substantial, in order to kill off the inherent signal in the dc rail that is pulled by the output valves. Thirdly, the push-pull arrangement also cancels out smoothing ripple at high current drain. Again, class A relies entirely on major smoothing capacitors, iron-cored chokes etc to eliminate the 100hz ripple generated by high dc current drain.

So, although I’ve not seen the amp design yet, these are the things I’ll be looking for. Class A amp building is not an easy assignment. I’ll keep you posted on the hair loss and macaroon situation.



QSC CX 404….Don’t try this at home

I had a worrying thought. It might have been yesterday. Having not very many of them (thoughts, that is) I ought to be able to remember one. So that was a bit worrying as well. It was regarding the prime minister. I realised that I didn’t know who it was/is; then I was even more worried by the fact that I didn’t care who it was/is.

He has paid me a hundred quid towards my heating bill, and I thought it would be nice to send him/her a note. Obviously to 10, Downing Street.  But you can’t address it to ‘Somebody, 10, Downing Street. The cat might get it. Anyway, lead onwards and outwards, Macbeth!

Ah….yes! The CX 404. The amp with more things to go wrong in it than a politician. It has a complex switch mode power supply, with a load of things to go wrong in that. It has four separate amps in it to go wrong. It has a bagful of protection circuits to protect it from going wrong, all of which can go wrong. The only way you should take on a job like this is with a bazooka levelled at your head (or somewhere) and Mills grenade taped to both hands. That’s the most useful information I might come up with. Suffice it to say, I recently repaired one of these. This I put down to stupidity of a high order.

The power supplies had blown. These are IGBT (insulated gate bipolar transistors) that do the power switching, and they’re not cheap. There are two chips that generate the switching pulses and provide the drive to the IGBT’s. If the IGBTs are cleaned out it’s pretty likely that so are these two chips. But if there is a fault in any of the power amps, the amp will cycle on\off etc. and the power supplies won’t operate even if they’re ok, because of the dc sense circuits. Or, of course, it could be because the sensing circuits are sensing something that isn’t there at all.

The power transistors you need to take out to check if they are the ones in the schematics. The one I had, had two pairs of completely different ones in it, to the ones in the schematic. The drivers were the same MJE15032 and 15031 I think.

There is another issue with these. The heatsinks clamp down onto the top of the power transistors (there are four separate ones, one for each amp. That’s fine when they’re first built, but I discovered that different manufacturer’s devices can vary a bit in the physical thickness (back to politics) which means that your heat sink might not make such a good contact with the device. Not good news.

I got fair number of useful lessons out of this amp. The main one certainly to be very wary in taking them on.

Being deserving of at least several cups of tea and a pallet load of macaroons, I shall attend to those matters.


Korg SDD3000 (The old one).

I like rack mounted effects units. I know the hot fashion for little boxes that sit on the floor and you stamp on them. But they really don’t do your stage persona much good when you’re grubbing around to find a three millimetre diameter knob amongst a wash of Carling Black Label and fag ends. Still, each to his own, live and let live, and all that.


This is the inside of the rack mount Korg SDD3000. Unfortunately I was too late to do a ‘before and after’ thing on it. I’d done the job before I caught up with the brain cell that deals with ‘before and after’.

This is the battery that was in there before the thing I replaced it with. Sorry ‘with which it was replaced’. I could do it in Italian, but I don’t know any. This original one was a NiCad, rated at 250 mAh @ 3.6 volts. The mAh (miliampere-hour) rating tells you that it will put out 250mA for an hour. Or 500mA for half an hour. You get the idea; it’s the capacity of the battery to store charge. It’s not that important electrically, especially in this context, where the current draw is minimal. In other words they last a long time. This particular battery had drained to about 2.6 volts. With an old unit like this, checking the backup battery is first job.

But the voltage is important. And in this case, if the battery voltage drops too far, not only do you lose your custom settings, it also doesn’t work. At all. Dave’s unit came in to be checked over and serviced. Servicing is generally straight forward. Separate all internal connectors and spray them out with something like Servisol 10 (NOT WD40). Same with pots and switching and jacks.

Here’s also a timely warning. There are various static sensitive (Cmos) devices in here. They don’t like fingers much. Strictly speaking, these blogs are not a sort of instant-do-it-yourself-manual. Sometimes they might save you a few quid, but often they are just interested in informing.

The only battery I could find that was 3.6 volts and would fit in (with a shoe horn) was an AA size lithium battery. It did fit in with the correct clip, the only other mod being moving the 100 ohm current limit resistor. The custom presets had to disappear because the original battery voltage was too low to hang in a temporary d.c. supply, but Dave was ok with that.

There was another problem waiting to happen on this one.There are two separate bridge rectifier networks in this. The four diodes in the pic have replaced the originals. There was nothing wrong with the originals, but they had obviously been running hot for a long time, and joints underneath them had dried/cracked and the pcb was discoloured. I replaced these because with the longer leads they can be spaced off the surface of the pcb and dissipate heat, so that future dry joints won’t happen.

Time for tea. And maybe a macaroon, to celebrate something.





More Members of the Secret Police

Don’t get too excited about this. There won’t be too much information in it. But even if there were none at all it would be (usually) a 100% more than the average amp/instrument manufacturer wants you to know about.

This last insult to the musician’s intelligence was from Roland. Here’s a little test for you. Have a go at finding a schematic for a DB500 bass combo. They’ve probably been around for twenty years (a guess). It would be a simpler proposition to find a flowering begonia shrub on Pluto. After an hour or two of brain-numbing frustration, I came across a random piece of information. ‘Random’ is the key issue here.

The Secret Police of selected Amp Companies would never be party to helping you get your amp fixed. Unless you wanted to take it to Japan, or Korea (or Pluto?).

Here’s the sacred phrase. The DB500 is, apart from minimal output device modification, EXACTLY THE SAME AS THE DB700.

This one you can find schematics for, if you are sufficiently bloody-minded. Which I definitely am.

So I was able to sort out the DB500, which turned out to be a blown resistor/zener diode power supply fault. The bits were about 20 pence.

Instead of being led by the nose by Roland to buy their latest offering, for several hundred £, the gentleman paid me a few quid to get back his amp, which he didn’t want to change anyway.

The next time you happen to be looking round at a prospective new purchase, at the bottom of all the bullshit, insert a little phrase of your own: “but don’t expect to be able to get it fixed out of the year’s warranty”.

Tea time.