Samson XP308i Expedition. Lost in the upper reaches of the Unpopo. I wish.

This a nice picture of a Samson XP308i.

 

Shortly after I’d dropped it off a cliff. Or maybe just before. In this case it wouldn’t have made much difference because it didn’t work.

 

 

I know this might sound Neanderthal, but I don’t like switchmode power supplies. Uggy-wuggy-grunt. There you go, I am Neanderthal. I’m Stone Age enough not to run my business on bullshit and subterfuge, which currently shrinks my playing field down to the size of a postage stamp.

So what is wrong with switch mode power supplies? More to the point, what is a switchmode power supply? And also why is it? And what is the alternative?

All these riveting issues you’ve fallen asleep over, will be rivetingly detailed in our next week’s riveting issue, called ‘How to sell something you haven’t got.’ No, not really…… onwards and inwards.

 

This is a picture of the main bit of a switchmode power supply. The finned thing is the heatsink, upon which is screwed a switching regulator (in this case a Fairchild  KA1M0880B power switch which is the nearest device you see screwed to it. The next one along is a high current diode and the one furthest away is a mosfet transistor which beefs up the output from the switching regulator to supply the amp circuitry. There are few different ways to design these things but the idea is that the mains input is directly rectified to produce something like 330 volts dc. This an impractical voltage as it stands. It is too high to do anything with for a semiconductor amp, and also, d.c. is not readily manipulated to produce the various different voltages that the amp needs; in this case probably +15 volts, -15 volts, 5 volt digital supply, and then maybe +30 volts or so, and -30 volts ish.

So we’re starting with a voltage from the supply that is way too high, and we can’t easily modify it.

An orthodox power supply has an isolating mains transformer. The primary side is connected to the mains supply, and there is no direct connection to the electronic gubbins in the amplifier. The secondary side of this transformer has several windings (or coils of wire, if you like) wound on the same lump of metal as the primary winding. By having various sized windings, many different voltages can be available, and the whole idea is relatively simple.

The switchmode idea has no isolation from the mains, it goes straight into a bridge rectifier and comes out of that as high voltage d.c. as we’ve already said. So where do the voltages come from? Well, the switching regulator we mentioned ‘chops’ the d.c. and produces a high frequency (in the Samson case 75 kHz) and this can be transformed to give the various voltages by much smaller transformers, because of the high frequencies involved.

So what was wrong with the traditional power supply, that we need to do away with it?

One of these things you might have already guessed. Wait for it……..a switchmode……

IS CHEAP!!!!! Yayyyy! Even including the little transformers a switchmode like this is unlikely to cost the manufacturer more than a few quid.  On the other hand the ‘old’ transformer arrangement is probably ten times the price to build.

Another big plus for the switching supply, and especially in the case of toy p.a. stuff like this Samson is, is that it’s lightweight. A traditional power supply is a lot of things but it is definitely not that.

So why don’t I like them? It’s fair question. Might it be that I don’t like anything less than fifty years old? Well………….alright, you got me there. But I do like ideas that save the good old planet’s resources, and it does save a lot of copper and steel by not building a big transformer. But……………………

Switchmode has many problems that I don’t like. First (whatever they say) I see a lot that are blown to hell. They have many protection devices in there (overvoltage, overcurrent, overtemperature, overblown claims of longevity…… well maybe not that) but they still seem to hit the rocks very regularly. Second, there is no isolation between you and the mains supply. I Don’t like that. Third, it doesn’t have the same inherent filtering of mains noise of an isolating transformer. Fourth, as with many unnecessarily ‘complicated’ things there is a lot more to go wrong.  Fifth, (again, whatever they say) the switching noise generated inside the unit will leak into the mains supply. We don’t need more crap in the mains that we already have. Just one of these things, well, ok; but a hundred thousand, say? It just seems to be a step in the wrong direction, to me.

I’ve actually led you astray, here. (So what’s new?) Because this Samson Expedition thing I haven’t fixed. Yet.

 

This is where I get into another ‘Pet hates’ tirade. The pic shown is of the sheaf of paper I have so far assembled in the interests repairing this amplifier. Why did I need to do this? It’s because of the increasingly secretive nature of the so-called ‘free information society’. After spending several hours on the web searching for schematics for the XP308i, I was forced to draw the conclusion that there isn’t any.

I’m sure there are a bagful of bullshit reasons for this, but the obvious one, you rarely find mentioned. Samson don’t want these things repairing. Just a guess, you understand.

Being a very bloody-minded person, that, I find is a great encouragement to figuring out what it’s all about, how it works, and fixing the damned thing. I’ll bring news of this job in due course, but just as a hopefully useful aside, to anybody unfortunate enough to get stuck inside one of these:- Beware of applying too much heat to the tracks, they’re very flimsy. Check that you have 330 volts dc out of the mains bridge rectifier. Check, that from pin one on the KA1M0880B(that’s the nearest pin to the edge of the heat sink) to the negative pin on the main rectifier, you have a significant ac signal. Don’t use a grounded scope (or anything else grounded) on this switching part of the circuit, as it is all referred to virtual ground. If none of this makes any sense, probably best not to go there.

We will be back on this amp, we will fix it, and I will let you know how it was done. And hopefully Samson, and all those of a similarly secretive fraternity will get it as a sound kick up the arse.

Time for tea.

 

Fender Deluxe reverb

Marc is a nice bloke, and an intelligent one, until it comes to the issue of GAS. When this rears its bankrupt head, it’s true that most of us of a musical persuasion lose any grip we might ever have had on the excuse for reality that we have forced on us.

For the uninitiated, GAS is the acronym for ‘Gear Acquisition Syndrome’, although you would have lived forever at the bottom of a well, not to have come across it.

I have a Stetson hat and welding goggles for my GAS attacks. “How does that work?” you might wonder in an idle moment under the duvet. It goes like this. The bloke who walks into the music shop with welding goggles and a Stetson with the brim pulled down to his feet, is me. I stare at the floor, ask for my bag of strings and get the hell out before anything hanging on the wall commands me to buy it.

What has all this to do with Marc and his Fender Deluxe reverb? Well, he does seem to buy a lot of things that end up in my hands being fixed. The latest being his nice Deluxe that didn’t sound right when he cranked it up. Bearing in mind that the amp is only (but worthily) rated at 22 watts rms, might he just be asking for it to do more than it reasonably is capable of?

It was a strange fault, and this is why I’m jotting about it. Strange, but in essence, simple; when you know what it is. The same could be said about Blackpool rock, Dark Matter, quantum physics, and the Dyson vacuum cleaner. All simple after it’s been done; by somebody else.  

If somebody complains about something not sounding right, crossover distortion and intermodulation distortion come high on my list of things to look at, because they both have symptoms not easy to describe. If your amp sounds like a bear’s arse being filed down with a hatchet it’s not a subtle thing. That depends on your degree of sensitivity and whether or not you happen to be the bear. But the subtle faults are notoriously hard to describe.

Crossover distortion often makes an amp sound ‘weak’. If biasing is overdone on the output valve control grids, the positive and negative -going signals cross over badly and you get a weak sound and early clipping, so lower power output.

Intermodulation distortion is when your nice guitar sound gets mixed in with some other noise like, say 100hz buzz (poor smoothing, check the reservoir/smoothing caps) or 50 hz hum (check the hum balance if you’ve got one).

This was neither of those things, and in the first place, didn’t exist at all. One of the things you figure out early on in your engineering training is the effect a bench has on all things broken. It often makes them work. Briefly. So the MesaBoogie just brought in by a suicidal musician who watched it disappear under the wheels of the 8.24 from Paddington, is shovelled onto the bench where it rests for a second or two, before producing the most glorious music. So he says “It never did that for me, after the train ran over it”.  

Extreme case, I grant you, but it happens all the time. Back to Marc’s Fender Deluxe. It was perfect. These things just look right on a ‘scope if you’ve looked at scopes for a long, long time. So he had it back. No fix, no charge. I’d be very wealthy if I charged for the things I didn’t fix. You know, like the microwave in the Aspidistra Hotel in Llandudno. I’ve never been there but it’s the same principle. Just send a bill somewhere if I get a bit short of the readies. I’d probably do well in the tax office. Anyway, he got it back home and it was just the same. I was getting the ‘Black Hole ’ feeling about it.

To cut an already tedious story to the slightly less boring. 

It turned out that many(!) if the power valvebase contacts had been poorly manufactured (it had to be a manufacturing fault, because the thing looked nearly new even though it wasn’t) a bit of a jiggle around in the car, and the screen grid contact on one of the 6V6′s open circuited, and resulted in that particular valve putting out a fraction of its share of the power. I slotted a watchmaker’s screwdiver down each side of all the valve base contacts to tighten them up, and everything was, and still is, hunky dory.

Often the repair is far easier than the figuring out of the what it is you should be repairing.

Tea time.

How to Make a Lot of Money Very Quick

A notion, at which you might shudder as it slithers out from under the slimy rock next to your settee, is fairly pre-puberty in its tower block subtlety. It has a mathematical probability approaching certainty that, considering the glut of advisors on this very subject, nobody is telling you anything that might be remotely useful to your acquiring your first million. Or first fiver, for that matter. Otherwise we’d all be rolling in it. Money, that is. You can procure, with no more than garcon-esque twitch of the little finger, enough paper to start Guy Fawkes off. Just so long as none of it looks like a ten pound note.

So, just to clarify.

First: anybody who is willing to advise you free of charge, is going to send you a bill in the next post. Except that you can take money off folks a lot quicker these days with a PayPal account. The free gift offered will not, strange to say, be a brush to paint the red numbers in your current account black. More likely it is a wobbly plastic dinosaur that makes your cornflakes taste of urine. Maybe that’s what they taste like without the wobbly plastic dinosaur, but I’ve never had a box without one so it’s hard to tell.

A simple litmus test is all we need to prove the aforementioned pecuniary point. First of all you need to locate a barrister, solicitor or some other member of that august body. You can tell an august body member by its shape; they’re the ones that drop their leaves (and anything else that may give a career leg up) at the merest mention of an annuity. You may need to nip back to the rock next to your settee to pinpoint one of these members geographically, however.

“How does this test go, then?” you might wonder. It is very, very simple. The whole profession is very, very simple so that should come as no surprise. If you really want to join it you need no more than extraordinary memory for things that didn’t matter four hundred years ago. And a completely vacuous brain for anything that might, even from a vast distance, seem like it might make sense. Back to the test.

Our barrister, we will conjecture, is progressing along the street towards us. They don’t walk. This one has just emerged from a litigation meeting and is swinging from the bells of Notre Dame but without the bells. This is because they are paid by the second in gold bullion and insist on it in cash. Or if it looks like a scorpion poised to stick something nasty in you, this one is very, very important and just can’t quite get its nose far enough away from its arse. Back to the test. You ask it the time. The answer will be

“£746.73″

This will be announced without reference to any form of timepiece, but will certainly be accompanied by vigorous scratching’s on a triplicate pad. The following morning, a buff-coloured watermarked manila envelope will fall on your doormat. This will contain a bill for £746.73 and a statement of the time “3.24 pm.”

Now that’s How to Make a Lot of Money Very Quick.

 

Numark Dimension4

This Numark power amp was a real challenge to my bald spot, which threatened to broaden to an area which could comfortably accommodate a five-aside football match. And the crowd.

Don’t ask me why I walk into these death traps; I put it down to hereditary traits of insanity. This was one of those amps that, so far as the customer is concerned, was worth far more dead than alive. The repair bill would go along way to buying a new one. So, I acquire these things. Until, that is, the pile becomes so great that the sphinx would have a hard time seeing over it. Then, I buckle down and get my ‘bloody minded’ hat on, which means I will stare these things down into oblivion, until they get so bored that they throw up their hands and say “Alright! Leave me in peace! I’ll work”. On this amp, I stared at the same three inch square piece of pcb for a week. It should have been embarrassed, but it wasn’t.

So, how does this situation arise? You might well ask. After forty years, I still find things that I never suspected could exist on a planet governed by the laws of logic. Or even this one. I look at it like this. Or maybe the other way round on a bad day. It is something I could file under ‘I do not understand this.’ But as that covers 99% plus, of everything, we haven’t narrowed it down, much.

So let’s take the specific case in point. The Numark Dimension4. People keep secrets, which is fine as I don’t want to know anything about anybody else anyway. But not good if the people who built the God-forsaken machine want you to think that it’s some kind of miraculous invention that it’s dangerous for people to know about. So they keep everything associated with their bloody silly machines in a filing cabinet surrounded by a minefield and razor wire, and mark it ‘radioactive’. Now you’re going to say ”But what about ‘Intellectual Property’ and all that?”

And I’d say “To be a possessor of intellectual property, you first must be the possessor of an intellect.” Sorry, that’s just bitchy. But let’s take the opportunity to straighten out whose property this intellectual stuff is.

Sticking to the power amp (although a similar reasoning exists behind much ‘modern’ electronics); the basic topology of the orthodox semiconductor power amplifier has changed very little over probably sixty years. It’s had bits added and new devices do this or that differently, but the idea has evolved hardly at all. And even that idea evolved from way before that. That means then, that everybody steals the intellectual property belonging to the bloke who first developed the idea. Doing the odd modification here or there is like claiming the invention of grass is yours because you happen to cut it.

So people like Numark, are effectually protecting the stuff they nicked in the first place.

The fact that they change this or that resistor does not amount to a new idea. It has all been done before. How much of this sounds familiar?

Back to my worst fault in the world. In order to get hold of a Numark schematic there are many hoops to be negotiated. I don’t like that. So I drew out the relevant section of the circuit (with an archaic pencil and paper, would you believe?). It took a long time. But anybody who can stare down inanimate objects can hack the ‘long time’ bit.

Not a great reproduction, sadly.

Anyway, I figured out what the problem was, fitted various transistors, and……it didn’t work. The worst fault ever, is the one you think you’ve fixed, and even worse than that is the one where a replacement component you fitted is faulty. But I’ve found another, even worse than that, and it took the Numark Dimension4 amplifier to demonstrate it in all its nastiness.

The component I fitted (a 2SA1364) had the WRONG FUCKING NUMBER ON IT!!!!!

Apologies for the graphic language. Time for tea.

 

Cheap as Chips.

After a fallow season of things to moan on about, again the ‘pro’ world comes to my rescue with its usual clutch of the overbearing, attempting to befuddle and disinform the unwary and trusting. This time it’s Laney.

It’s a tough job to work out who owns what, what owns whom and where you might find somebody who would own up when it goes tits up.

Take the word ‘Mini’ for instance. I used to be able to fix my  mini in the middle of a desert with a screwdriver. So long as I have my handy diagnostics computer and a main dealer in tow, it’s not impossible that I still might, with today’s incarnation. But very improbable.

So a current ‘Mini’ is as ‘mini’ as a Winnebago is ‘tent’. Why not call it something else?

Tricky question, but it’s definitely related to the ‘celebrity’ thing. Let me tack together a few thoughts. Always assuming I can find a few to do that with.

Back to my little mini of the ’60′s. Falling to pieces. But, if it had ‘Bently Turbo’ stuck on the front somewhere, and I did a bodywork relocation of the whole Bently Turbo in total, it would be a very different animal so far as KUDOS goes. The fact that would have a hard time ripping away from zimmerframe dragsters at the lights is beside the point. It looks like something it isn’t and that (currently, anyway) is all that matters.

Back to Laney. Laney bought Vox. The name anyway. This Vox AD50VT was made in Korea, under the instruction of the r+d dept of Laney UK. Which means that it was made somewhere else, by somebody else, and maybe overseen by somebody who had never seen it.

Laney is not Vox. If they survive another fifty years they might be. They might not.

But this is not a ‘kick Laney’ exercise. Although you may not have guessed. It’s a ‘Why not tell folks what they’re buying, honestly’ exercise. Laney designed a lovely little amp called an LC 30; the subterfuge is pretty obvious even to a trusting old git like me. ‘LC 30 approximately equals an AC 30′. No it doesn’t and never will. If we go no further into it than to appreciate that the Vox AC 30 was hand built from the ground up, and no pcb ever got any nearer to it than the moon, that statement that would flatten almost every amp made in the 30 years or so, not just Laney.

I think that this particular ploy has cost Laney dearly in Kudos. The little LC30 is/was a great little amp in its own way. The company could have trumpeted that fact because it was worth the effort. But they chose to hide behind the fact that it has some circuit parallels with that legend, the ‘Vox AC30′. So it became an also-ran behind the legend, when it should have been sparring at its own weight. And it was very good at that.

Things are different and worse, now. Nondescript manufacturers based possibly in the orient (or somewhere) and staffed largely by robots, buy names; or ‘logos’; call it what you like. The stuff you find inside these ‘Carlsbro’s’ and ‘Vox’s’ and ’Wharfdale’s' etc. etc. bears no resemblance to those companies’ productions as they once were.

The plus side to this is that lower end gear tends to be at least affordable. At a time when a mini cost a bit over £300, the Vox AC 30 was something over £120. And a Fender Strat was a bit over £160. To get hold of either of those cost you a few years on the ‘never-never’ (hire-purchase) so it was a big commitment. At that time you could buy cheaper gear (Bird, Elpico, Watkins,) but you did know what you buying. It had their names on it and they were proud of it.

 

 

 

 

The Little Red Clip light comes on/ the Protect light comes on.

I read a forum the other day where a lot of fellers were scratching their heads about little red lights, on a beefy power amp.

Does the clip led come on when you wind up the volume, without any other indication? (Like green signal present; or orange, signal level ok.) Do get no sound out of it?

It’s only mosfet amp will do this.

Most power amps won’t bother if you run signal through them without a load (speakers etc.). Remove the speaker. Bring up the signal through the amp. The led’s function as normal?

You have open gate mosfets, probably all of them. This means that the control gates in the output devices (mosfets) have blown open and aren’t controlling anything. This sort of fault can happen progressively. Because mosfets will power-share, if one goes open, the others will take over and share out its load between them. This means that they are all working a bit harder, getting a bit hotter, so the weakest will blow open. This is obviously a progressive thing, which is why you get to the completely open output stage, eventually. Anyway…….

Answer? Replace the mosfets. Probably all of them. This is not really a DIY job.

This fault will not happen in a bipolar amp. They short and take out power supplies etc.

If the protect light comes on at switch on, it often means that one half of the output stage has fried. It’s a rebuild, often.

Tea time.

Fender Bassman 135

This was a silverface Fender Bassman 135. It was bound to be really, because they were only made in silverface design, the 50 and 100 being earlier versions that made the transition from blackface to silverface.

This was a ‘come back’ job. They don’t come along very often, but just once in a while……..

Well, the whole story goes like this. It came in, in the first place having lost power and the output being distorted. The output valves were from way back, and on the scope, the signal was very bottom-heavy. One half of the output stage wasn’t pulling it’s weight, one output valve having lost the internal contact to its screen grid. In which case that one did very little.

A new set of output valves went part of the way, but it turned out that the phase splitter was giving an unbalanced output. (They sometimes call it a ‘driver’ these days, but it isn’t. It splits (inverts) the phase to the output valves so that they work in opposing pairs.) There are explanations on this site which go into more detail (that’s in between the silly asides). The phase splitter has to supply the same rms value signal to one half of the output stage as it does to the other. Otherwise, any biasing-balancing you might do will be a waste of time. So there was a repair to made on the phase splitter.

All this was sorted out and the biasing set up, and the amp ran on test for an hour or two without problem. It went out.

It came back. The guys had got into the studio and…….it didn’t work. At all. Bugger. The lights came on. the heaters came on, but no sound.

Now we get to the point, and it actually applies to quite a few different Fender amps, from many eras. The output jack to the speaker is right next to the extension speaker socket. The extension speaker socket is switched, and the signal to the main output is sent through the switching in the extension socket. Bad/dirty contacts there and you get nothing out to the main speaker jack. That was what it turned out to be. Answer? Turn the amp off. Spray contact cleaner into the ext jack, whack a jack plug in and out a lot. Remove jack plug. Walla! Done. Job’s a good ‘un.

Any more problems might indicate a jack socket change, but this was a happy little Fender Bassman 135. Intermittent faults can be a real dog, but getting excited doesn’t usually solve much.

The tea pot calleth.

 

Bugera BC30-212

This the first blog on this site for I think about a year. By not writing any blogs I suspect I postponed a sackload of hair loss. Unfortunately I don’t have the sackful I lost over the previous year as a comparison.

Just so that you might not think that it’s all your fault, the reason that this literary triumph has been in suspended animation for this long year (or short year if you happen to be one of those folks misguided enough to read it) it is because of the attentions of various internet factions.

Far be it from me to point the finger of an irate god at those internet vandals who have wonderful stretches of time to waste on buggering up other folk’s miniscule creative efforts. (Clearly it’s not ‘far be it’ because I already have.) Rather than waste similar amounts of my own time in putting it right, I decided to let it go to the dogs (‘Gods’ for those of a theistic persuasion). Time is one of those strange things that you can waste in truckloads if you have plenty. I suppose that goes for just about anything.

Now I’ve got that steam pressure blown out of my ears (or wherever), onwards and upwards to the Bugera BC30-212 combo. Maybe not ‘upwards’.

Make sure, if you do insist on poking around, to UNPLUG, LEAVE IT A WHILE, AND THEN ASK NICELY WHETHER OR NOT IT’S GOING TO ELECTROCUTE YOU. These things have never told me, before running a few hundred volts up my arm, but you might catch it on a good day.

Just to get a handle on where we are, this is looking into the chassis of the amp from the back, towards the rear of the front panel.  The orange cable goes to a spade connector which carries around 350 volts. You can’t see it too well, but the pcb is badly burned where the connector is soldered to the front pcb.

 

The the fault description was that it fizzed and smelled. The customer switched it off (wise chap), but then he tried it again, and although everything seemed to light up, and there were no further fizzes etc, it just didn’t work.

Here’s another confusing picture. The mirror-like plate facing, is a bit of screening foil stuck to the top of the cabinet. The amp fits into the slot below, and the circular burn mark in the lower left hand corner, was directly over the burned contact we looked at in the last pic. So it had obviously produced some smoke and heat.

 

I have to admit, I don’t like Behringer things. And this a Behringer product, I understand. Behringer is German, this is probably designed in Africa, built in China from parts sourced on Mars. But how would I know? I do know there is plenty of ‘Behringer knocking’ goes on, some deserved and some not quite, but their gear does fill a useful place in the market, as they make something that looks good, and functions often pretty well, and makes available these things to the lesser financially affluent. Such as me. It’s just that I don’t like to move them about much, in case bits fall off. Sorry, that’s just bitchy.

My real problem with Behringer, and a few others of their ilk is that they are just far too precious (for something that is really paste). Try getting hold of some information in the form of a schematic. After a lot of looking over a long time, I’ve never found one. When it comes down to it, there are not many original designs around. The stuff that Leo Fender was designing in 1945, is little different to the same sort of thing that Marshall (for instance) is doing this week. Except in 1945 they were built with a different ethos. They wanted the amps to keep working, and, 70 years later, many still are. Bugera in 2080? Can’t see it, myself.

Back to the plot, if there was one and if I can remember what it was.

Valves and pcb’s don’t generally go very well together. There are many reasons for this, but one in particular is relevant here. A cheap pcb often will not have a very high breakdown voltage. This means that high voltage can arc or track through the insulation; and this means that the insulator ionises and becomes a conductor, shorting across the copper tracks with high voltage differences. This is much more likely to happen between copper tracks and pads etc., that are close together.

The burned contact in the pic was partly down to this arcing/tracking, and partly that the joint to the pcb had been poorly manufactured, had heated the pcb and helped to get it to arc across.

It’s simple enough to correct the burned connector. Scrape the laquer from the track, run solder along it, and solder a link to the spade connector. Unfortunately, that won’t cure a tracking fault. Once the pcb has ionised, there’s not much you can do about it.

But here’s a couple of possibilities, neither of which would will work in this situation, as the tracking is extensive in a completely different part of the circuit.

One possibility is to scrape away at the burned section of the pcb (sometimes you need to scrape a hole through the pcb). Sometimes this will work, but often not. Usually worth a shot, though. The other way will work if you have room to do it. Cut the tracks so that you isolate the complete section of burned pcb. In other words, the copper track remains but is cut either side of the burned section of pcb. Then you scrape away the laquer down to the copper on the copper tracks either side, flow solder onto it, and solder an insulated link (piece of wire) to those, thereby isolating the shorted bit of pcb.

Some years ago C Audio had some desperate problems with their RA3000 amps. Great, high quality, rackmount, high power (3000 watts rms) amps, that had a thing about fog machines. After some time, the fog that was taken in by the amp’s fan rotted the laquer of the pcb and caused tracking all over the place. The ‘standard fix’ was to cut a load of tracks and link them. Those links went from the front to back of the amp and were about a foot long!

The traditional way to build a valve amp is with either point-to-point wiring, tagstrip or turretboard. The times those things track across or burn are almost never.

Tea time!

 

 

 

 

So you think that’s Smooth-er

This promises to plod along the same tedious route as the last (So you think…..). But WILL IT? You might well ask. Or maybe not, if you have some fascinating alternative pending. Say, sorting out your sock drawer, for instance.

I had a sudden, and alarming brainstorm. A very bad thing to happen if you’re in carpet slippers; your sock drawer can become a thing of the past. No bad thing in my case.

I’ve been drawing symbols for things like resisistors and capacitors, diodes and the like, and we don’t actually know what one looks like. This sort scatterbrained thinking can be VERY DANGEROUS. Frank Zappa said so in ‘the Dangerous Kitchen’. For instance; let’s say you’ve been taking notes about a symbol for a dog. You go to pat said dog in a friendly manner, and it takes your arm off because it turns out to be an alligator. Come to think of it, it could also have been my grandma. In the interests of a full complement of limbs, here’s a picture.

The Problem with Cathode Bias….(RSC valve amp)

Here’s a picture of something you don’t want to see in your amp.

The grey tube in the centre of the pic is (was) an electrolytic capacitor. The silver foil stuff used to be inside the tube.

This is an example of a cap that has exploded. It’s part of early ’60′s RSC amp

 

 

 

 

 

 

Needless to say, it doesn’t work anymore. So what did it do, what is it there for, and why doesn’t it do it anymore?

Here’s another picture.

 

 

 

 

The destroyed capacitor in the pic is the bypass capacitor in the schematic diagram above.

The purpose of bias voltage in any valve circuit (and also in semiconductor circuits) is to control the current through the valve (or transistor). The grid 1, which is also called the control grid, is held at a negative voltage to the cathode voltage. So if the cathode was at zero volts (ground) the grid would be held at around - (that’s ‘minus’) 36 to 40 volts.

The effect of this bias voltage, which is d.c. and is set to a predetermined value, is to ‘pinch’ the current through the valve, to say, a few tens of milliamps, though this varies considerably and sets the ‘class’ of the valve operation. Class A has least bias volts, Class AB, AB1, AB2, have different levels of bias voltage. But elsewhere on this blog there are more details about biasing and the class of operation of amps.

Associated with this capacitor is the cathode bias resistor.

There are two different ways of applying the control grid bias voltage. One is to apply the negative voltage from a source straight to the grid. This kind of bias is adjustable and often has a preset pot to vary the bias voltage.

The other way of doing it is to put a resistor (the cathode bias resistor) between the cathode and ground. This does effectively the same thing, that is produces a grid voltage that is negative to the cathode, but it does it differently. The current through the valve develops a voltage across the cathode resistor which means that, by raising the cathode voltage, and leaving the grid at zero, the grid is effectively at a lower voltage than the cathode. The bigger the cathode resistor, the more the bias voltage. The capacitor bypasses the signal (sound component) of the current through the resistor.

This is known as ‘fixed bias’ or ‘automatic bias’.

And now we get to the reason for the blown cap. If for some reason the valve goes soft (also known as red plate or short) the current through the valve multiplies. This often because the internal elements (often grid/cathode) distort and short within the valve.  The cathode bias resistor is in the main current path of the valve and the voltage across it increases beyond the working voltage of the bypass cap. The cap doesn’t like this at all, and does what it did.

Another cause of this symptom is the bias resistor itself which can open circuit (as this one did). But the result (and the reason for it) is the same.

The best know amp to use this kind of bias is the Vox AC 30 (and also AC 15) , but there is a difference. In the Vox’s all the output valve cathodes are connected together and have a common bias resistor/bypass capacitor arrangement. In this RSC amp, each output valve had its own seperate circuit.

 And this is the replacement bias network.

After a a blowout like this, it’s a good idea to check for tracking and burning on the valve base.

 

Tea time.