A thank you to Gareth for his Donation to my Macaroon stash!

I didn’t charge Gareth for my perambulations (didn’t think I knew that one, eh?) through his Cornell Romany Pro amp. If I charged folks for not fixing things I would be very well off. Anyway, it turned out that his problems with the amp were not faults, it just didn’t do what he wanted it to, so I didn’t actually fix anything, but spent a lot of time working out how it worked.

Make no mistake, this is a beautifully made amplifier; but I got the feeling in my water that there was some faulty thinking gone into the design of the amp.

Gareth turned up for his amp with a couple of packets of Macaroons! Chocolate and plain. Bliss!!!

So as a small “thank you” I offer this little piece of history of the origination of the macaroon thing.

A long time ago, when I was up to my neck in an industrial apprenticeship, I worked for some time with a hilarious cockney gentleman named Cyril Haffenden. If a problem reared its head, or if he just felt a bit stressed by it all (or any other reason, for that matter) he would pronounce in a loud cockney accent “THAT’S IT! BAGGER IT! A CAPPA CORFFEE AN’ A MACAROON!!!!

That’s where it came from, tempered by poetic license of my own.

Thank you, Gareth.

 

Cornell Romany Pro……had me going for a minute there.

This amp knocked a year or two off my life expectancy, and foot or two off my hairline.

A schematic is the most valuable attribute to repairing an amp; with a working brain cell running a close second. I’m saying nothing about the latter, but the Romany Pro does not subscribe to the former. I found an owner’s manual (for the Romany) which told me where the knobs and switches were, but not much else. Back to the Secret Society again, then.

Nearly all valve amps have a working topography of:-  preamp stage(s) (usually ECC83 based valves) into a phase splitter (if it’s a push-pull class AB amp) and into a push-pull output stage. The phase splitter arranges the signal to the output valves so that one half goes positive while the other goes negative. This results in the push-pull of the output stage.

This Romany Pro was different, and it took a while for me to figure out how it could work. Everything I tested, from the phase splitter on, was in phase. Without going into too much detail, that can’t work because the output transformer tries to sum two identical in-phase signals the resultant of which is zero. A zero watt amp is never going to catch on.

Then I noticed that the phase splitter, isn’t. It doesn’t (as the usual arrangement does) split the phase, it just puts identical signals to the output valves. The only way this can work is by reversing one of the secondary windings of the output transformer. I’ve no idea why people like Mullard spent years developing the phase splitter design. Unless it’s a lot better, of course.

I think a pallet-load of macaroons might be appropriate in this case, to go with my tea and oversized toupee.

 

What to do After You’ve Written a Few Sensible blogs.

In the interests of staying insane, or at least becoming normal, one has to get away from the ‘sensible blog’ syndrome.

So don’t expect this to help you to fix anything that a couple of aspirins won’t sort out.

So here it is…………….

                                 The Allen Invasion of the A5 Services

                                                                                          

            Wailingpolice sirens belly-ached into the night as half the motorway cop cars in the county picked out the A5 Services outside Bromfield in their strobing beams. This was all highly irregular as the belly-aches were usually inside the cafe.           

                          #

            “It still looks like a burger in a bun to me” said the prone cop sighting at it down the barrel of the .45 calibre automatic pistol. Slamming his stomach down on the tiles after a bagful of chicken nuggets had not been a wise move, the sergeant quickly realised. The officer sprawled next to him raised his Heckler and Koch and dug around in his shirt pocket distractedly.

            “The bloke on the phone said they all look like burgers; ‘there’s hundreds of ‘em’, is what he said.”

            “There would be; this is a burger bar,” said the sarge; he didn’t take his eyes off the burger, though; “but it’s dead in my sights if it tries anything funny.”

            “If that bloke was right, we’re outnumbered, sarge” said Algy, with a nervous twitch. There were, indeed, hundreds of them dotted around the floor like acne.

            “They don’t look armed, Algy.” A silence descended on the deserted restaurant, broken inconsiderately by a waitress diving for cover behind the cutlery trolley. “But would I know what sort of killer weapon a burger in a bun might carry in its, er, pocket?”

            “Don’t that one look bigger than all the rest? What do y’ think, sarge?”  

            “Of course it’s bigger. It’s a Double-Whopper-Angus-Burger. What would you think if you ordered a Double-Whopper-Angus-Burger and a sausage roll turned up?” A cleaner lady with a power floor washer walked in without looking up and plugged it into the wall.

            “Are you mad?” said the sarge.

            “I’ve been doing this job for twenty years, what do you think?” She pushed things in and pulled things out of the machine with the efficiency of a robot. Algy’s face turned puce and he looked up her rumpled stockings from the floor.

            “Look, madam, we’re on a..a…stakeout for this… er…just a minute.” Algy pulled out a crumpled reporter’s note pad triumphantly and flicked through its tacky pages; “Allen Invasion!”

            “A what?” said sarge, his eyebrows standing well clear of his forehead.

            “That’s what it says here,” said Algy.

            “You’re telling me we’ve surrounded the Blastoff Burger Bar with sixty men on the whim of a bloke who can’t spell ‘alien’?”

Just then a disembodied voice shouted something that sounded like

            “’Ten…Shun!” and the burgers that had been scattered randomly about the floor of The Blastoff Burger Bar shuffled around willy-nilly before clicking into several ranks behind the Double-Whopper-Angus-Burger. A floor-washer started up behind them and a thoroughly spooked Algy redesigned it into a cheese grater with his Heckler and Koch while surfing the soap bubble waves cascading across the tiles.

            “Help” said Algy, careering towards the ranks of the Allen Invasion Force as they opened fire with ketchup and Worcester sauce.

            “It’s ok Algy, I’ve got you covered,” shouted sarge.

            “There’s no need sarge, I’m covered already,” said Algy.

            “And you think I’m mad!” said the cleaner, formerly of the rumpled stockings and now of the floppy gumboots, as several of the Allens blasted off into the dizzy ceilings of the Blastoff Burger Bar. The sergeant battled to get a grip on his walky-talky as it danced out of his hands in the monsoon of ketchup from above and the tide of ‘Washy-Floory’ suds surging below.

            “The situation is getting messy, sir” said the sarge into the phone that was grotesquely distorting into something more Allen than the Allens.

            “Bring your leader to us!” boomed the Double-Whopper-Angus-Burger, in radical break with tradition.

            “No,” shouted Algy to the ceiling, “that’s all wrong. Your line is ‘Take me to your leader’. I saw it in ‘The Allen Goes to Hollywood’.” Just then the glimmering of dawn was obliterated by an unimaginably vast Super-Dooper-Triple-Quadruplicately-Huge-Whale-Burger that cruised, silent and deadly, over the Blastoff Burger Bar and hovered, its ketchup cannons primed like a million bristling cocktail sticks.

            “Bring us the President of the World!” said Double-Whopper from his ceiling tile.

            The sarge at last dropped his eyebrows as there was nowhere else for them to go.

            “That could be tricky” called up the sarge.

            “What! You refuse! Puny services-ling?” Double-Whopper bristled. “In that case we shall eliminate your silly civilisation.”

            “Psst. Sarge.” Algy nudged the sarge’s elbow.

            “Not now, Algy. I’ve got to think this out,” bubbled the sarge through a mouthwash of Washy-Floory suds.

            “She’ll do.” Algy pointed at the cleaner with the floppy gumboots. The sarge, aghast, looked at Algy as if he was an Allen.

            “They’ll never go for that. Just look at her.” The sarge had another look just to make sure. He shrugged. “Here she is, then;” he called up; “the President of the World.” The President of the World looked up from her ceremonial mop and bucket.

            “No!” A panicky voice rang out from the Ceremonial Ceiling Tile. “You must not invoke the gods. It will be the end of everything as we know it.”  The President of the World stubbed out a distressed fag-end and smoothed down her floral piny.

            “What you got against dogs, then?” enquired the President of the World, rolling up her sleeves and bridling her bosom.

            “PSSST!” pist Algy at the President of the World. “It said ‘gods’: not ‘dogs’.”

            “Don’t you ‘pssst’ me mate. I know my rights. That’s oppression of a Pet Owner’s Society member. I’ll set my god on you.”

            “Aaaiiiieeee! The President of the World invokes the dogs yet again.” The Double-Whopper-Angus-Burger wailed pathetically. While outside, the Super-Dooper-Quadruplicately-Huge-Whale-Burger pulsated as it sent down a majestic silver stairway through the ceiling of the Blastoff Burger Bar; along which a crowd of Allens stepped equally majestically, each Allen carrying a majestic mop and tin bucket and primrose yellow rubber gloves. They raised their majestic plungers in salute.

            Things had become very confused by this time; so they might have praised their joss-stick slungers in palute. It was hard to say.

            “No; leave all that bowing and scraping stuff out” said the President of the World, “it’s embarrassing.” The plunger-gesticulating crowd of Allens looked shamefaced, their plungers drooping to an impotent half-mast.

            “What is it that you want?” said the sarge having strung together a couple of brain cells.

            “We are collecting for ‘Uranus in Need’ and doing a bit of market research on the side” said the Double-Whopper-Angus-Burger, grandiosely.

            “It seems like a good cause, sarge” said Algy. “Nice to think somebody is collecting for mine; even though it’s not in need, so far as I know.”

            “On what terms do you do the market research, Mr., erm Double-Whopper-Angus-Burger?” enquired the sarge.

            “Oh, the usual; you get to fill out a questionnaire that takes a mere twelve hours a day for six months and get a fluffy badge for it.”

            “It’ll never catch on, here on Earth.” The sarge looked dubious.

            “According to our market research it’s what everybody does for a hobby when they’re not shouting at a computer.” The Double-Whopper-Angus Burger looked as smug as one might imagine a burger in a bun could be.

            “So,” said the sarge, tentatively exploring the limits of lunacy, “if we have a whip round and donate a few quid, you go away. Is that about it?” The burger-boss clicked its crust impatiently.

            “Don’t forget the market research!”

            “And the market research; yes.”

            “Then we must move on. One of the little moons of Mars next, I think.” If it could have smiled it probably would have. “Nice spot, Phobos.”

                            #

            To cut a short story to about the same length; i.e. tedious; in the wake of the exit of the Allen invasion force of the A5 services, (it having collected £1.52 and given out several hundred thousand market research leaflets), a month or two later followed a queue of brave politicians and journalists, both of whom were writing a blow-by-blow, fly-on-the-wall account of the event; in the same way that a duck-billed platypus did of the invasion of Greenland by aquatic camels. These intrepid seekers of truth descended on said cafe, having first taken the precaution of booking into the Dorchester for lunch.

                         #

            The leaflets from one section of the archetypal activists read:-

 

              Politician Does Something    

The Conservative party candidate for Crackbracknel, the Right Honourable Mouthmore N. Getplaces single-handedly sent people to quell an Allen invasion at the A5 services near Bromfield. He says.

 

He also says that his bravery in sending people to quash this intergalactic attack far outweighed any previous bravery as reported by any of the other parties. Whichever they were.

Sporting an open mouth and a megaphone he modestly described his battle as ‘Heroic’. Some spectators erroneously interpreted this as ‘diuretic’, which was about when it hit the fan.

The report from the police department was somewhat different.

                      #

            ‘We didn’t see hide or hair of Mr. Mouthmore N. Getplaces. We did get some fluffy badges though.’

 

                   The End (is in sight).

 

That was definitely worth tea AND a macaroon. I feel so much better. I mean, there’s nothing actually wrong with normal………..?

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.

Laney VC30. The ‘VC’ stands for ‘Very Cwiet’.

Well, the light came on. Always a good sign. But the speaker did not speak to us.

A quick look through the rear grill told all. It was dark as the Munsters’ lounge. Or Trump’s thought processes. Not a working heater was to be found.

Inside the amp a toroidal mains transformer is mounted on the chassis, and from that, eight cables connect from the secondary of that transformer to an eight way, dual-in-line connector. The female end of this is flying and the male counterpart is fitted into the chassis. There is an interesting piece of design thinking here. Two of the cables carry the heater voltage/ current. The voltage is 6.3 volts rms. But the heater current through this connector is the total of all the heaters in the amp which is around three amps.

The connector, however, has a rating of one amp, and therefore has to carry a current that is three times its rating. Sooner or later, this is going to burn up, and it did.

The practical method of sorting this problem out is to cut all the cables on the transformer side, and solder long insulated link cables to them. The male part of the connector we prize out of the chassis. You need to make notes of what cables went to which when the connector was fitted. Then all the cables are cut to the connector. This makes it easier to desolder the cables from the pcb. Then your links from the transformer are soldered to the pcb in the order that you (hopefully) noted when you cut the cables.

This has effectively got rid of the burned connector and hard-wired the transformer wiring to the pcb.

I’ve had similar problems (for the same reasons and with similar repairs) on some Fender reissue amps. It’s a time-consuming fix, but at least the fault will not occur again.

Unless you are particularly slick with a soldering iron, this would be best left to your local tech gentleman.

Tea calleth.

 

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.

 

DBX 160xt …….under pressure….as Freddy would have had it.

The DBX 160 compressors are definitely a cut above average. They use a chip called a VCA chip. These have been used in analogue synthesisers, more or less from their inception.

You might be wondering at this point why I might want to use the word ‘inception’. Apart from it being a wondrous bullshit word, it reminds me of the Jenson Interceptor, a car with more carburettors than sense. I always thought of it as a sort family saloon AC Cobra. A great way to get rid of your ears, as it would rip them off at the first dab of the gas.

The VCA chip stands for ‘Voltage Controlled Amplifier. There were also, of the same ilk, the VCF and VCO. They were/ are respectively ;Voltage Controlled Filter’ and Voltage Controlled Oscillator’. They all worked in basically the same way. A dc voltage (the control voltage) is applied to the control pin of the chip (Before the advent of chips there were….fish. No, no! Come on, get a grip.) Let’s try that again. Before the advent of chips these devices were built up from discrete transistors. Before that, they were built from valves.

I remember a valve analogue computer at Chesterfield tech college. It had six operational amps and they shifted it from room to room on a fork truck.

The output of a VCA is proportional (I don’t know a car of that name…..wait a minute…..a pro Porche…nal. That’s relief, I thought I was turning sensible.) to the input dc control voltage. In other words, its output amplitude goes up as the dc control voltage increases. On a VCF the filter effect frequency rises or falls according to the voltage, and with the VCO the oscillator frequency rises or falls similarly.

What has any of this to do with the DBX 160xt? Well, this compressor has a VCA chip in it. The control dc voltage is sensed from the input level, and also from the setting of the compression level.

This probably won’t help to fix it, but might help a bit to understanding it.

Here’s a practical fix it bit. The DBX160 xt compressor uses a six pole push switch to bypass the compressor. If this goes bad, it won’t work, and you can’t buy one. I have heard there are plentiful stocks on Calisto, but DHL don’t ship from Mars. But you can get four pole push switches (alps switches). You can use these if you know how to a) solder and b) not panic when tracks peel off.

Take the original, bust, switch out. Tricky, but not impossible. A good desoldering device is advisable. Put the new switch in so that it occupies the 12 holes furthest from the front. Solder it in. Get a piece of heat shrink sleeving, and cut it so that the front bypass switch will push the switch operator forward. I also used a bit of the pipe from an aerosol can inside the heat shrink to make it more solid to the touch.

This will work and also switch in/ out the bypass function. What it doesn’t do is switch the led on the front which remains on. If you can live with that, you’ve fixed it.

I’m now going to phone somebody on this macaroon. It is, of course, my hermit-o-phone for the week. Last week it was a Tesco radish.

Tea…..ahhhh!

 

 

A useful Laney LC 30 mk2 blog.

I promise this will be useful. Not to Mesa, Fender, Marshall, Valco, Hiwatt, Matchless, users/ owners of course. Nor will it be much good for those needing information on house training a rhinoceros. But there will be a smattering (you didn’t know I knew that one eh?) of almost interesting information.

This (the Laney LC 30 mk2) is a noisy amp to my mind. One reason for this is that the centre tap of the heater supplies is grounded (via a 2amp fuse) to chassis. Another reason is that the pcb is grounded via the pot screws to the front panel metal work.

One thing you really don’t want in an amp is ac currents through the chassis. It introduces noise at every stage. So the best way to ground the heater CT is at a star point at the incoming earth. (At the IEC mains input in other words.) Make only one connection to each section of the chassis and lead these wires back to the same ground star point. The ground to the pots is best hard wired, again to the star point.

Sorting that out will produce a significant improvement. You can’t do much with the heater wiring. AC heater wiring is traditionally twisted together; for good reason. The twisting produces a cancelling effect of the heater current-generated hum. In this amp the wiring is not twisted anywhere in it. This amp, it seems, had an optional pcb, that converts the heater supplies to dc. In that case (if it is done well) the heaters don’t generate hum. But if it’s not a good supply design it will generate 100Hz buzz. Oh well.

The third problem that you may be able to rectify, is the fact that the design omits two control grid leak resistors on the first and third preamp valves. If you connect a 220K resistor from the centre grounded pillar of the first valve to pin 7, and repeat the procedure on the third valve, the noise levels will be attenuated by a lot of db.

I thought this was a sort of responsible blog. But I don’t get a macaroon because I didn’t get a laugh out of it. And there are few things more significant than that.