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.




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.

Supro Thunderbolt Reissue (well, the box is pretty close anyway.)

When is a ‘reissue’ patently not a reissue?

Taking an extreme view, anything in a Vox AC30 box, would be unlikely to be called a ‘Fender Twin reissue’. I would be, I would guess, in serious trouble if I was selling a goldfish on ebay that was actually a rhinoceros. You can’t do that sort of thing because it must contravene a trades description thing. Mustn’t it? You surely see my point, even if it does emanate from a cynical old git. That’s me, in case you were in any doubt.

And so, on to the Supro Thunderbolt ‘Reissue’. Apart from the fact that in the original had the power amp in a completely different place (bottom of the case), a quite different preamp arrangement, totally different biasing circuit…..well, the reissue is just the same.

But this was a repair job, so I should be at least a bit more useful than that. It (the reissue) has three power settings on a four pole three way rotary switch. This switch, in the one I had in, had fallen to bits. It effects the power levels in two different ways, and there are two different biasing arrangements associated with this switch. The first position, 60 watt setting switches in a 270ohm 15 watt resistor into the cathode circuit of the 6L6 output valves, so this is cathode (or automatic) bias, and much like the original circuit. In the other two positions (5watt and 1watt if I remember right) the cathode resistor is switched out and the control grids of the 6L6′s have a negative bias voltage applied, which are the other two functions of the power switch.

I think if I owned this amp, I would be wary of switching the power settings without first switching off standby. Just a thought. And now another thought. Tea and a macaroon. Nice thought.


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.


Messages from the Virtual Idiot Dept

My partner has a phone. It looks like a spaceship. I have a phone that looks like a loofah. The difference between these ostensibly similar devices is that my phone does not work well in the bath either as a backscrubber or an underwater communication device; whereas I can’t make my partner’s do anything at all even standing next to EE’s gargantuan mast.

That’s because I’ve never worked out how to switch it on. By the same token I’ve never figured out how to switch it off either. She says it will make movies, take pictures, recite Shakespeare, jump up and down to the rhythm of the latest 007 Theme, act as a life raft in extreme circumstances. But I can’t ring anybody on it. Meanwhile, back at my loofah-phone, I can’t ring anybody on that either. I am told by nauseating folks at Vodaphone that it’s because I don’t put money in it. I bought it for ten quid twenty years ago; what more do they want?

I drool after those phones in the black and white ‘B’ films that people whir around with a middle finger, talk into a black stick-looking trumpet thing, and say something like “Give me Whitehall 1212, please operator”. Now, I have to key in a 54 digit, digitally secured code (that turns out not to be secure because some flea bitten company like Norton or Avast or somebody, wants to charge me a mortgage to ensure that I am secure) and after doing that I find I’ve straightened out the National Debt while waiting a day and a half in a queue for somebody from the phone company to come back off holiday to tell me how much I owe them.

So I now employ a completely revolutionary device called a ‘Hermit-o-phone’. By using this I don’t have to ring anybody at all. The first Hermit-o-phone I had I came across accidentally. I picked a banana up in Tesco and, lo and behold….it didn’t  ring! This is the phone for me, thinks I. There was also, I soon found, a bonus. I couldn’t ring anybody else, either. Another big advantage is that nobody has sent me a bill. For anything. I’ve tried this with various ‘apps’. A stick, a dog biscuit, a JCB tyre; they don’t ring you on them either.

So I’m stretched out on my blow-up mattress watching the rain outside my shed, banana on the one hand, tea and macaroon on the other.

I might have a peaceful conversation by macaroon, shortly. You’re never alone with a macaroon……..

In Praise of People we Pay Good Money to Break Things For Us

The latest in a long line of parcels that arrived as a shovelful of bits in a box that was once square-ish and had metamorphosed into some distant cousin of the arthropod species,  convinced me that sending anything to climates more distant than next door is seriously questionable. And if you got through that sentence without retching, you are made of stern stuff, and I doff my hat.

I would have to say, in the interests of fair appraisal, that my curiosity in any place more distant than arm’s length, on the Richter Scale of zero to ten, is about minus five hundred and seventy three. And you can multiply that by a lot if it’s raining.

Even taking this into consideration, I have toyed with the idea of stowing away in one of my parcels booked in for shipping (pick your own company; in my experience their records of losing/ breaking/ delivering to people I’ve never heard of, are remarkably similar), with my trusty machete and Heckler and Koch automatic. Unfortunately, I would have to be built of sheet stainless steel to survive the journey. So I shelved that plan.

So what is the most effective packing for a piece of valve equipment? Bearing in mind that there are few materials (in this Universe at any rate) that will stand being run over by a fork truck, this gets to be a problem that Einstein might balk at. So I phoned him.

“Is your name Einstein?” I asked.

“Yes” said Einstein.

“Would that be Albert Einstein?” I asked.

“It’s actually Einstein Trimblestrop; actually” said Einstein.

“I’m sure you’ll do” I said hopefully. “How would you pack a parcel to be delivered by courier to Spain? Or anywhere, really.” I asked.

“I wouldn’t send it at all” said Einstein. He thought a bit. “I did send a Victorian cast iron commode to Venezuela once. They said it looked like a toilet when it got there. I suppose that was near enough.”

“How did you pack it, Mr. Einstein?”

“I dropped it on the courier’s foot and he packed it in his plaster of Paris splint.”

So that’s solved that one. I’m now looking for my brand new packet of Macaroons shipped from Venezuela.  Easily mistaken for a box of nails when shaken.

Reconstituting smashed Venezuelan macaroons is an extended project, I can tell you. Almost as bad as repairing a new Studiomaster Powerhouse.

Now, where’s my brand new packet of tea from Venezuela………?


Universal Audio S-610 preamp

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

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

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

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



Extra terrestrial messages from an endangered species

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

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

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

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

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

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

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

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

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

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

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

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






Put the ‘Cuffs on, Mate, I did it….!

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

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

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

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

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

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

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

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

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

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