Valve rectifier circuit and how it works.

This looks horribly intelligent.

Which means (by inference) that my coconut macaroon is fading into the sunset.

On the plus side, it’s not impossible I might get a laugh out of this. Not likely, just not impossible.

Onwards and upwards, bull by the horns, goldfish by the goolies……maybe not that…..

The first thing we need to supply our amp is something to plug into the socket on the wall. On the end of this cable, we go into an IEC (kettle) connector, and this has a line voltage, a neutral, and an earth. The line voltage and neutral connect to the mains transformer ‘primary winding’. This ‘primary winding’ is quite sensibly named. In a world of electronics bullshit, you should hang onto anything that might be sensible. Because there’s not much of it about.

This primary winding is the first thing that the mains voltage sees (so it is primary, or first) and it’s a winding, because it’s a long length of copper wire, wound around some metal former. In the drawing, that primary winding is represented by the block marked 240 volt. Also wound onto this same former is another (sometimes many) more lengths of wire. This is the secondary side of the transformer, and in this case we have two HT (stands for ‘high tension’ or ‘high voltage’) windings which supply the two anodes of the rectifier valve. American for ‘anode’ is ‘plate’. But this doesn’t make all that much sense because the cathode is still ‘the cathode’ in American. Don’t ask me??? We also have a heater winding, which is much lower voltage (5volts or 6.3 volts usually, depending on the valve. This heater winding supplies the rectifier valve heater (and possibly other valve heaters.) The property of ‘induction’ enables the transformer to function. The ac voltage applied to the primary winding has some fraction of that voltage ‘induced’ in the secondary winding, which is why a voltage appears at the secondary output without there being any direct connection between the windings.

The heater of the valve often (not always though) is completely separate electrically from the operation of the valve. All it does is heat the cathode.

Here we get to actually how the valve rectifier (and nearly all valves actually) works. By heating the cathode (usually made of a tube of copper with the heater inside it) we excite the free negative electrons on the cathode. I knew we would get to something exciting eventually. The copper cathode is often coated with something like boron, which gives off a lot more free electrons than copper, so making the valve more efficient.

This does next to nothing at all, other than to create a ‘space cloud’ which is a cloud of electrons that surrounds the cathode. However, if then we switch on mains voltage, the secondary of the transformer supplies a high voltage to the anodes, which is positive in one direction. Positive attracts negative, which means that the electrons from the space cloud are drawn towards the anode when it is positive. This has now begun an electron flow from cathode to anode (but only when the anode is on its positive cycle).  Conventional current flow is actually the opposite (positive to negative), because electron flow is negatively charged.

So what we have now is positive voltage appearing at the cathode (marked HT dc out).

On the end of this terminal, we have to provide smoothing for the HT output. That voltage is not usable as it is. So we will look at the choices of smoothing circuits next.

Tea. And not many laughs. Sadly.

 

Valve rectifier?…..solid state rectifier?????

Further to the amp design series. This is part three. If I can get to three without suffering brain fade there must be a slim chance I might know something useful. Eh?

a) So what’s the difference? b) Is there a difference? c) Will I notice a difference?

a) One uses a valve. Duh.

b) Not if you’re deaf.

c) See above…..and…..some people wouldn’t notice a difference if a herd of wildebeest stampeded through their kitchen.

A rectifier of any sort has one purpose, and that is to convert (rectify) an ac (alternating) current/voltage to a dc (direct) current/ voltage. But the valve rectifier has certain properties that the solid state rectifier doesn’t. From a point of view of how it affects the sound, the valve rectifier has the property of ‘sag’. I know how it feels. The valve itself has an internal impedance, and what that means is that the current path from anode to cathode (the conducting direction) isn’t a short circuit, it has a certain resistance. (Impedance, strictly speaking.)

This is a very complex characteristic and varies from valve to valve in imponderable ways. This will not endear the rectifier valve to the digital fraternity. They like to put these numbers into this black box and it does this. Every time. No valve will do that, because the valve is a physical device and as such every one is different (within certain tolerances). Not only does it vary from valve to valve, but also depending on the current it is drawing, how hot it is, what is the smoothing arrangement on the dc side. And what day of the week it is, for all I know?

The characteristic of ‘sag’ in the rectifier, means that as the valve takes more current, the output voltage tends to drop, which tends to limit the output of the amp it is supplying, so providing compression. This is not a simple compression. It is affected by frequency, amplitude, phase angle, power amp loading, even speaker impedances. So far as I know, there is no compression circuit that can simulate this form of compression, it is not very predictable and varies (as we’ve already noted) from valve to valve.

A solid state rectifier, on the other hand, has very little impedance in the forward direction. It will run itself without varying the output voltage, to destruction. So it does not compress, and has no sag factor. This is a big difference, as any guitarist will tell you who has wound up a valve amp with a valve rectifier.

But there is a downside to the valve rectifier. It’s expensive and complicated. It needs, for a start, an extra valve base. It also needs a transformer that will give a heater supply voltage for the valve, which is often a different voltage to rest of the amp heaters. In the case of a GZ32 or GZ34 or 5Y4….etc….5 volts. This means that the mains transformer has to have an extra winding to supply this voltage.

You don’t need any of those things for a solid state rectifier circuit. Just two (or four) diodes, or a bridge rectifier. Nevertheless, our amp (should it ever get to that state) will have a valve rectifier. If we keep it small, say 15 watts  rms, we can do the valve rectification with an EZ80 or EZ81. ….So what?…..ah well….these valves have 6.3 volt heaters, so the mains transformer is much simpler; we don’t need a separate 5 volt supply.

So we’ve actually made a decision. Which is quite something in a life of doing my best to avoid them. We use an EZ rectifier valve and design the output to be around 15 watts rms. And that solves another decision. If the amp is going to be some variation on class AB (we’ll look into this a bit further into the proceedings), it is likely to be using a pair of EL84′s. Not unlike the Vox AC 15, or the Watkins Dominator.

The thick plottens………..

Tea! Macaroon! Oh yessssss!

Puctec ZD-987 solder/ desolder station

This is a bit off the beaten track, but might be useful. There’s a first time for everything……

I’ve used a desoldering hand pump device forever, and they work ok. For about a week. You really can’t expect too much from something that costs a couple of quid, can you? And also, if you have a lot of desoldering to do you can get a serious case of ‘pumper’s thumb (?)’ We will not go into that; but it hurts.

The big plus for this soldering/ desoldering station was that it was cheap. I may as well be brutally honest. Most of the other soldering stations (Weller, Antex, etc.) would need me to sell my grandma to drum up the cash. As I don’t have a grandma any more, my style is somewhat cramped in that investment area. It was actually surprisingly good (I’m off grandma’s now) but for one piece of missing information. It was like building a flat pack bookcase from Korea (or somewhere else foreign). On following the instructions to the letter (usually an interpretation from the early Sanskrit) it looks remarkably like a food mixer. A food mixer with a lot of screws missing.

There is a glass tube with a spring in it that should fit behind the sucky-pipe-thing. I found that a jack for a Transit van was helpful in installing this. You really have to have powerful thumbs to put this in. It is probably worth an investment in a thumb-building-up course as you have to take the tube out on a regular basis to clean out the solder waste. Except there isn’t any waste if the temperature isn’t effectively set, because it won’t desolder.

Here is the first real crunch. In the instruction sheet, it says that leaded solder melts at 180 degrees C, and leadless solder 220 degrees C. Which it does. However after some experimentation, we find that after setting the temperature to around that recommended level, we pull off the tracks quite well, but don’t actually desolder anything.

You need to desolder at around 335 degrees C. At that level it does work very well.

However, another problem (which is described in the instructions) is that at high heat levels (see workable operating temperatures) the solder tends to clog the intake pipe with a lump of lead. This lump of lead (also according to the operating instructions) you can’t get out. That’s because all the tin content of the solder alloy has been burnt off, which leaves you with a lump of lead that you can’t shift with the poking tool. Fair do’s though; Puctec do inform you of this. And also that your nice desoldering machine is unusable. Decent of them.

There is a way round this. You might have to do this a lot if you are soldering valvebase joints where there is a lot of solder to get rid of. You bring the temperature up above 400 degrees C and thump the cleaning stick in and out repeatedly. As the temperature gets up to around 380+ the solder will shift and you will have a clean tube. Sounds a bit brutal, but it works and is a lot better than chucking your nice new machine away.

Cuppa tea! And I certainly deserve a macaroon for this one.

The Second One in the amp design series.

The general idea in designing/ inventing anything at all, is to get together some wonderful ideas, build them, and then find out why it don’t work. Unless you use a Computer Aided Design piece of soft brain (sorry, software), in which case you can get it to virtually design it, virtually build it, virtually test it, and come up with something you can then virtually poke around with until you get something new. Except it won’t be, because the intuitive human input  has been relegated to accidental poking about and the imagination relegated to accountancy and marketing.

I am not, then, very likely to be a CAD person. In which case I shall have to think a bit. Oh dear.

We’ll start with the power supply. “Why would you do that?” you might enquire. Because if we get that wrong I am not going to get a macaroon. If that part of it is less than it should be, the rest of the amp is unlikely to be worth plugging in. So what do we want from a power supply? At its most basic, some device that takes the ac mains (240-250 volts in UK) and converts it to a useable dc voltage for the electronics. There is little difference in that requirement whether you are supplying a semiconductor (solid state, transistor; call it what you like) or valve circuit. The difference is that semiconductors tend to be higher current, lower voltage, and valves tend to want the opposite. So a power valve will be likely to have several hundred volts at the anode, whereas a semiconductor amp will need voltages of less than hundred volts. The actual voltages depend on the power output of the amp. The power transistors will put out amps, and the power valves, tens (or maybe a few hundred) milliamps.

Clearly an over simplification, and the actual figures will be dependent on the sort of power rating of the amp. But it at least illustrates the general principle.

There are two different possibilities for the power supply, and the main difference between those alternatives is in the way the incoming ac voltage is rectified.

 

 

This is a basic arrangement for a full wave rectifier circuit using solid state diodes. It could also be a bridge rectifier arrangement, but the result is the same, so we will stick with this one.

A diode will only conduct in one direction, and the transformer’s job is change the 240 volt input to a useful voltage for the amp to work with. The primary side is the 240 volt input and the secondary is centre tapped, which enables the ends of that winding to act in opposition. That means that when the voltage increases at one end, (with respect to the grounded centre tap) the other end decreases. This enables the diodes to conduct alternately in their forward direction, so producing a dc voltage from the ac input.

All this sounds fine and dandy, but the output is like a rowboat on a ski jump. Not smooth in other words. And therefore not useable for the amp, as it stands, because what you would hear most is a 100hz buzz. Most tunes really need a bit more than that.

So far there is no difference between that which the valve amp requires from a power supply and that of a transistor amp. The difference comes in with voltage/current requirement, and that brings in a big possibility with the valve amp. It can use valve rectification, and the solid state amp does not have that option.

In the next blog we shall consider the why’s of that, and what the differences are.

And just in case you might think  I’ve turned sensible, I shall, especially for you, dear reader, come up with some really, really daft ones.

Have a good macaroon…..I mean…..day.

 

 

This might be a bit of a shock

…..and surely will be if you were silly enough to trudge through the last, extremely daft, post.

I have mentioned (probably many times) my ambition to design an amp. After a year or so I’ve got a bit further with this ambition. It is called ‘A Guitar Amplifier’. Slick, eh? At that rate of progress the second great flood will have swept the planet before I get down to soldering something. At least you will know who it is paddling the ‘Guitar Amplifier’ across the frozen wastes  of Biggleswade.

My second idea (on a roll, here) is to entertain you, oh erudite blog readers, with a sequence of blogs that will take you from the very first stages of amp design, though to the…erm….very second stages, etc, etc.

Alright, let’s make start. Questions first, answers second.

What level of marketing bullshit are we going for? Zero, because I don’t care if I sell any, because it’s for me, and because it’s just a fascinating thing to do, to build something that skirts around all the stuff that doesn’t work (or doesn’t work very well) and takes the stuff that does work to another level. Hopefully.

Another question. What colour is it? Don’t care. Next question?

Valve or semiconductor or hybrid of some sort? This sounds a bit serious, and I can see my macaroon dwindling. To come up with an answer to this one, we have to consider what a Guitar Amplifier actually does. The text book answer for this is that an amplifier makes whatever it is going in, bigger when it goes out. It has the property of gain, in other words. But in my Guitar Amplifier, this is only part of the story. A semiconductor of more or less any sort will produce (all things being equal) an output very close to the original input, but bigger, because its characteristics are close to being linear. A valve will rarely do this, it will add things; harmonics for instance, because its characteristics are rarely linear; and also certain sustain capabilities, because it will tend to ring in some degree. A guitar player might find this very attractive, or obnoxious. Two guitar players might have both opinions, it is a subjective thing. As this is for me, I can poke around with it until it sounds as I would like it. Bearing that in mind, I am very likely to produce, at the end of this process, something based on valves.

And CERTAINLY NOT a ‘digital modelling amp’. This is something that can sound like everything but not much like anything.

There we are, a start. Zen saying “A thousand mile journey starts with one step”. But so does a journey to your front door. Doesn’t it?

Tea. Macaroon. But only because it’s nearly New Year. I preferred the ‘Charter Flight’ blog. I got a laugh (and a legitimate macaroon) out of that.

 

Fender Hot Rod Deluxe. You really don’t want this kind of fault.

Of any fault that can beset an amplifier, oscillation is the most damaging and also the most obscure to fault find, unless your happen to have an oscilloscope about your person. Not everybody has those. Even the symptoms are not easy to interpret.

So what is it, and why is it? The ‘what’ is easy. The amp puts out its full power, but you can’t hear it. That’s because the oscillation frequency (often) is way above any Earthlings’ hearing. I have heard that folks on Calisto whistle at frequencies in excess of a MHz, but you shouldn’t believe that because I just made it up.

The ‘why’ of this can be tricky. Without the benefit of a ‘scope you would be able to hear a fairly heavy buzz (this is the power supply putting out everything it’s got), and distortion on anything you plug into the input. That is the signal fighting with the huge oscillation signal.

But the heavy buzz could also be reservoir/ smoothing capacitor problems (which could also cause the oscillation), or possibly an output bias fault, (check bias volts, around minus 55 or so is a safe one to go for 6L6 valves), or possibly a heater/ cathode short on any valve (interaction with the volume/ tone controls will narrow it down to the first stages) before the phase-splitter.

The causes are often reservoir/ smoothing caps drying out, especially on older reissue amps. There are three 22uF and a 47uF in the Deluxe/ Deville amps, and the best plan is to change them all. Another cause is shorted windings on the output transformer. The centre tap becomes not centre because the lacquer sealing the windings has melted. You can check for that by measuring the impedance (ohms) of either output valve anode to the positive on the 47uF. They should be the same but anything up to 10% or so discrepancy should be workable.

A high voltage (1kv working) poly capacitor (say, around 0.0022 uf) across the anodes of the of the 6L6′s can work wonders in stabilising an oscillating output stage. Not a cure for all ills, though.

There we are; an almost sensible blog. I’ll need a cold shower before my macaroon.

So when do you revalve….????

As soon as somebody (usually somebody trying to sell me something) sets off with “This ECC83 (JJ…EH…Groove Tubes…Mesa…TAD….etc…etc…) has a great, really tight bottom (?), a very cool detailed mid range, a glistening array of shimmering highs”; I go into ‘dark cupboard mode’, and check the bike lock on my wallet.

Were I a NORMAL, say, twenty-year-old, (are there any of them? Were there ever any of them?), my top end range of hearing would fall just a bit short of a dog whistle. Around 20kHz. As I’m not any longer a twenty-year-old (being rather closer to a hundred than twenty) my hearing falls off at around 14kHz in the right ear and nearer 11kHz in the left. So, obviously, there is little point in sales pitching me a valve with ‘shimmering highs’ if I can’t hear them.

Extrapolating this logic, how does anybody who sells valves in Russia, or Watford, or wherever, expect to tell me (or anybody else) how we hear this, that, or the other? Bullshit and bollocks? It starts to smell like it, don’t you think?

So when might we need to swap valves? And this is not a simple question, because there is no valid answer like, say, every year or every two years. I have a 1963 Fender Tremolux amp that is completely original. Which means that the valves, and everything else in it, are 50+ years old. It still sounds great. And here is the downside of revalving an amp. The valves in your amp acquire a character (as does the rest of it; speakers, capacitors, even resistors and transformers) which becomes an irreplaceable unit. It becomes a unique piece of electronics even after a year or two, and certainly after fifty years.

The tell-tale signs that valves need replacing are not difficult to pinpoint, generally.

A loud buzz that isn’t affected by the controls……look at the power valves. The grey anodes (the metal box inside the valve) shouldn’t glow. Glow either end is ok, that’s the heater. Switch off. A valve change (all the power valves as a matched set), but don’t fit them until you’ve had the amp’s bias circuits checked. If that has a problem, your nice new valves will be destroyed, more or less straight away.

Loud hums that might (or might not) be affected by the controls, are often caused by a preamp (ECC83, say) valve fault called ‘a heater-cathode short’. It means the heater has distorted and shorted to the cathode. Change the valve. You can isolate this by removing the valves one at a time.

Whistling, ringing noises. If you tap the side of a preamp valve with pencil, a microphonic valve will ring. This can also happen with power valves. Change it (them).

Just gently moving the valve around in the valve base can often clear poor contacts on a valve base. Worth a try.

I’ve just been tapping this mug of tea with a pencil and it sounds great. The macaroon didn’t. Must be faulty.

The Kustom ’36 Coupe….In the Interests of Saving you a Lot of Money….

I can remember when Kustom first came out. I saw Buddy Rich’s bass player with one. That would have been mid-seventies, but they had been around since the early sixties. They were unusual (they looked like something out of a spaceship) and doubly so, because they were all solid state. Very few transistor amps were around at that time, nearly everything was valves, and for good reason. The transistors of the time were very dubious beasts, and we (the folks who had to fix ‘em) knew not very much about them. Whereas valve technology had been evolving for probably fifty years.

The OC and AC range of transistors were about all you could get, and they didn’t like high voltages, high currents, or heat, very much. I never did find out what they did like.

But to get back to the plot. Whatever it was.

The gentleman who owned this last one I saw (I’ve seen a few in the last year or two), had revalved it, and I think had a replacement mains transformer fitted. All because it didn’t work. The actual fault was a 5 watt dropper resistor (3.9k) that had gone open circuit. A couple of quid or so, as opposed to £100-ish for a revalve and 50+ for a transformer. And all the labour.

If you are handy in amps (you can always tell a reasonably proficient electronics feller; he has two working arms and hair that isn’t black and smoking) the easiest way to check the state of dropper resistors, is to switch on the standby and put a meter on pins one and six (the anodes) of any ECC 83 you might find.

All this is all far easier if you have a drawing. A schematic is essential if you are concerned about wasting time and money.

Un….fortunately, schematics for the ’36 Coupe are kept in a cupboard guarded by Tyrannosaurus Rex, somewhere on Mars. This is how to keep your customers happy? I don’t think so.

Tea and macaroons are now necessary. Just after I’ve jumped up and down on a picture of the MD of Kustom Amps.

The ‘Fender’ 5f1 ……a kind of Champ kit.

This was interesting, a lovely little ‘Champ-esque’ amp in a nice tweed case. Unfortunately, it’s chief claim to excellence was its chainsaw impression. There was no point in playing anything through it if it didn’t sound like a chainsaw. If I’d been a bit quicker in the brain department I could have come out with the ultimate pedal. The Chainsaw….forget the notes…chop down a tree.

On looking into the design a bit further, I had problems figuring out how even the original could have been used to record stuff like ‘Layla’ and ‘Rocky Mountain Way’. The Monty Python ’Lumberjack Song’?…..maybe. This brings us into the realms of phase cancellation. No, really.

Most valve amp heater circuits have a 6.3 volt ac supply. Which means, on the face of it, that you introduce a big 50Hz signal into the amp. And 6.3 volts rms is indeed a big signal compared with the few hundred millivolts of the input signal. As this arrangement has been working well for a long time, there must be more to it?

The problem with the 5f1 was that the heater hadn’t been grounded. In the schematic, one side of the 6.3 volt supply was grounded. Sorting that out made a big difference to the chainsaw ripping through the speaker. But I also realised that this was not going to be the ultimate in low noise amps even with the ground fitted. I explain.

If we solder a preset pot of around 100ohms, with the two ends of the track (that’s the right and left tags) to the heater terminals of, say, the preamp valve, we produce a hum balance pot. Nearly. The ground has to be lifted from the heaters and replaced onto the wiper contact of the preset. You can then trim out the heater hum by adjusting the wiper position.

How/ why does it work? Before you do this mod, you need to make sure that there is no internal ground connection to the heater winding in the mains transformer. Or your trim pot will short the heater supply to ground. Important, then.

The heater wiring is twisted together for good reason. As one side of the heater goes to a higher voltage, the other goes lower. They are opposite in phase in other words. By adjusting the wiper of our preset pot, we have effectively produced a grounded centre tap which we can adjust so that the positive phase exactly balances the negative, producing a very low resultant signal to upset the amp. Most ac heater supplies have some arrangement of this sort, often with an internal centre tap that is not adjustable. In that case you can’t fit a trim pot.

Although this made the amp useable, and reasonably quiet, it still wasn’t as quiet as it could have been. Which gets us to smoothing capacitors.

There is no doubt that a valve rectifier makes a significant, and positive contribution to the sound of an amp. But it has limitations. The main one being the surge it is able to tolerate. Within the characteristics of the valve a maximum reservoir capacitor value will be stated. In the case of the GZ34, say, it’s 50uF. It’s this capacitor that makes a big difference to the 100z component, which generates the noise in the output stage. More so in a class A amp than a push-pull design (class AB, Ab1 etc) because the class AB amp has inherent noise cancellation properties.

So this little Fender amp could have been quieter with a bigger reservoir capacitor. The original was 16uF and then a load of decoupling stuff after it for the preamps. If that was doubled, the output stage noise would be much less. But you can’t just hang an infinite capacitor on the end of your 5Y4 rectifier, because it will blow the hell out of it.

So 32uF is probably about it. And now…..away with the sensible….!!!! Ha!!!!

I only have one issue with ‘sensible’. There are millions of sensible folks and yet the world is still a miserable place. Which is why I’m very happy to write a loada rubbish. Just so long as it makes me laugh.

I am presently devising a macaroon that will stir tea. I’ve tried it on Costa Copper and McDoodle’s Doughnut Dugout, but they weren’t keen on taking the doors off to get it in.

The Hermit-o-phone is still doing well though. I didn’t get a phone call from the tax office or the gas company. Or anybody else……….

Tea and a macaroon call-eth.

 

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.