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.

 

 

Extra terrestrial messages from an endangered species

Well, alright, it’s me really. But I am.

“What is that daft old git talking about?” You would be well within your rights to make that comment, even if it is fairly disrespectful. “So, explain yourself!” you ejaculate (?).

Alright, I will. On your own head be it. Whatever ‘It’ is.

There are few weeks go by that I don’t look inside an amp and wish for x-ray vision. Or at least an electron microscope. You can at this point visualise me (not a pretty sight) squinting at a piece of circuit board with 574 components on it. That will easily fit in my tea cup. It’s about at this point that I go to the many sites on the Wonderful Web that stack up schematics. An hour after that I’ve pulled all my hair out and am just starting on my toenails. Shortly following that, the innocent-looking pcb is in the fire bucket and I’m stomping off kettle-wards.

I started my apprenticeship in 1963. It lasted five years but I had expired well before that. (“In your dreams, mate”). It was intended to be an electrical engineering apprenticeship at the outset, but fate stepped in and after two shakes of a dead lamb’s tail it became an industrial electronics apprenticeship. This was in spite of the fact that industrial electronics and electrical engineering are as mutually compatible as bulldogs and arses.

The reason for this metamorphosis was pig ignorance by those leading the way for the electrical department, coupled with my own brand of pig ignorance. Not forgetting that electronics gear was flooding through the doors like a blizzard and there was nobody in the factory knew any more about it than how to fit a plug on the end. So I finished up at Chesterfield Technical College for seven interminable years.

So how does that equate to anything relevant to the topic? In 1963 valves (which had been around for, say, forty years plus) were being gradually supplanted by semiconductors. At the time transistors were mostly germanium and blew if placed near a stiff breeze. Whereas, valves had evolved about as far as they were going to. (The duodecatron valve was about the most complex and the mercury-arc rectifier the most powerful. In a darkened room the mercury-arc looked like something Frankenstein might have relaxed in.)

The point being (There’s a point?) that the folks who had the misfortune to be involved in electronics at the time, and also throughout the following sixty years or so, found themselves on a technological ice flow whose shape and constitution was changing almost week to week. There was no way anybody could usefully design a course to accommodate electronics servicing, because for the next decades it was changing in ways that few people could guess the direction of.

And the motivation for these technological changes was largely the pursuit of excellence. Most pursuits currently in vogue are running after greater and greater….errm…profit margins. And compared to those decades just mentioned, the changes that happen these days are painfully slow and usually restricted to being able to put more pins on a chip you can scarcely see. The ideas don’t change much at all.

I have designed a spectacularly useful electronics service course though, for current electronic repairs. I’ve called it ‘How to Plug In a New PCB”. You can do an endorsement to this which I’ve called “How to throw it Away and Buy Another One.” To keep abreast of current new ideas, we have the course entitled ‘How to Paint it a Different Colour’.

It’s very obvious that the situation will never again exist that involves living and learning through the changes that had so many ideas developing at such an extraordinary rate. The digital age has solidified like grease in a chip pan and its developments (personal opinion) are shams of convenience. But we can always expect something faster; and paint it a different colour.

There must be a kettle round here somewhere. And….a macaroon?

 

 

 

 

 

Put the ‘Cuffs on, Mate, I did it….!

This is a layman’s guide to how to get away with murder. Or close to it, anyway.

Let’s (just for a minute) consider the terrible repercussions of my stealing a box of matches. Not a robbery, you understand; just a slight misappropriation of funds. Call it creative accountancy, if you like. I did not (yer honour) waft a firearm about with abandon; neither did I threaten bodily harm, nor even the slightest suggestion of a frown did pass my forehead. I just nicked the box of matches. Displaying a certain skill, I might modestly add.

Now let’s consider the Countess of Canterbury’s diamond necklace. Using similar levels of skill and identical nonviolent techniques, I nicked it. Yer Honour. Is this a more serious felony than my nicking (in exactly the same circumstances) Cyril Crabtree’s box of matches? If the long plonker of the Law comes down harder on the diamond necklace felony than on the box of matches felony, surely the Law is more concerned with the irrecoverable nature of the knocked off goods than the crime. Will I get six months in Dartmoor for the theft of a fiver and ten years for the theft of a hundred quid? If not, why not? Which gets us to my theory that most folks on the internet should be doing time, in a big way.

“Why” quoth the affronted internet population “should’st thou level that one at us, o varlet?”

It goes like this. The real reason that the diamond thief might have got strung up in days of yore, and got a mere ducking in the village pond for half-inching Cyril’s box of (damp) matches, is relative to the ease that the contraband would be replaceable.

The most irreplaceable possession any of us has? TIME. TIME. TIME. You just cannot get it back if somebody wastes it for you.

So the next time you read some brain-dead Tweet, twitter, twatter, facebook, bong, don’t just suffer it. Send ‘em a bill. Or better still, a summons of court.

If dealing with official bodies? Send ‘em a bill. Multiply it by a few thousand.

Better still, accumulate a load of their pamphlets, circulars, polls, customer reviews, opinion polls, and post them to a deserving cause. The council, say. It doesn’t matter which one, just steal their TIME. They can’t touch you for it; sadly.

I need that cup of tea. Preferably cold , to dowse the steam blasting out of my ears.

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.

The Unfortunate Case of the Roving Vox AC30TB

There is a short but entertaining story to this, which deals with a range of matters from lack of respect for quality to wanting to kill people from a distance to destroying irreplaceable artefacts to dealing with insurance companies to fixing the unfixable to boldly going where……you get the idea.

This little story, although sort of entertaining, was definitely not so for Marc; who shall otherwise remain anonymous. He sold his lovely old (mid sixties) Vox AC30TB to a bloke in Spain who desperately wanted to buy it. My own involvement up to that time was that I had rebuilt the output stage for him a couple of years earlier. Although you could have bought one of these new in 1965-ish for a hundred and twenty quid or so, you certainly can’t now. So, the deal was done and the lovely Vox sailed away to Spain having been packed and cased by Marc, who is the soul of conscientiousness. So it would have been very securely shipped.

After a week or so, the Spanish bloke decided that he wasn’t actually as keen on buying as he had been, and took this up with Paypal. He had a full refund, and sent the amp back. All this, although a sad indictment on honourable dealings seems (to the modern mind, at least) alright. But it didn’t stop there. The lovely Vox AC30TB came back in the in same box without any packing. I did mention ‘lack of respect for quality’ did I? It was, unsurprisingly, very much the worse for its journey, with the front baffle and casework badly damaged. It also appeared to have had less than expert hands inside it, than it deserved. It’s about here where I get involved again. Marc needed an estimate for repair for Paypal as he was making a claim for damage in transit.

The big problem with an estimate in this situation (i.e. major accidental damage) is that you need to do all the repairs to find out what it will cost. There is so much in an amp that might look (and test) alright, that actually isn’t. Not reliably so, anyway. The valves would all need to go, for a start, whether or not they tested ok.

The result of this was that the amp was a write-off, mainly because the speakers  (original Vox Blues) were damaged, and therefore it would not have been possible to bring the amp up to original spec.

What a shame. These amps should be treated with kid gloves, not hobnail boots.

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.

A Travelogue….dedicated to Mark, the third reader of this blog

Yes; it came about after I had relieved Mark of £110 in aid of resurrecting his lovely Fender bass amp. It had blown a fair chunk of the power supplies, and fried three output valves. He admitted that he regularly read this blog. I had to believe he read it because there was no way he could hang his laptop on the nail in the bog for more useful pursuits.

So here’s one for you, Mark. Hope you and your lovely Fender are keeping well.

                       A Travelogue:-

                         How to go Somewhere Foreign.

           If you’re English, it doesn’t take you very long to catch on that there is something seriously wrong with you.

Your first problem after being born, which was finding out how to breathe, (in England, we have a Government Pamphlet for that. I know that because we have a Government Pamphlet for everything) you are then given another pamphlet that tells you who is Foreign. It’s quite a short pamphlet. It just says ‘Everybody’.

So ‘Going Somewhere Foreign’ is a really easy book to write, because if you live at ‘27, Bargery Street, Dipstick, Doobyshire,’ for instance, you know for a fact that ‘29, Bargery Street, Dipstick, Doobyshire,’ is Foreign. As are all the other numbers in Bargery Street and Everywhere Else.

So my Travelogue could be about, say; ‘32, Bargery Street’. I would then go on to discuss useful matters for the tourist of 32, Bargery Street.

The climate for instance; ‘Overbearing’ about covers it. ‘How to get around in 32, Bargery Street’; which of course contains bus timetables, when to hitch a lift on their dog, and avoiding the bedroom at certain times.

But on this occasion, just this once, you understand, I’m going for the adventurous approach. I’m going to do ‘the Solar System’.

Think big, just this once.

‘The Solar System is Very Big’. There we are, done that.

Onwards and upwards to…….A cup of tea and…..A macaroooooon!!!!!!

 

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.