Beware…..the Vox Concert 501 !

I do carp on a bit sometimes. You noticed? Ah, Shakespear had to write a ton of stuff to get noticed, I just have write a few sentences of drivel to get your attention.

This particular carping is regarding the afore-mentioned 1980′s Vox Concert 501. I can only think of one thing worse than not having a schematic for a piece of gear. And that is when you think you have a schematic, but, in fact, you don’t. This drawing had ‘Vox Concert 501′ written on the bottom, I had a combo in the workshop with ‘Vox Concert 501′ plastered all over it; but it may as well have been a ‘Mongoose Training for Beginners’ manual for all the use it was. Unfortunately, being a trusting sort, I thought it was the real McCoy. Until I discovered there were chips in it.

None in the schematic though. “Oh-oh” I belatedly thought. That was when I found that the third valve stage wasn’t there. At all.

Yes, I admit, I watched snails speeding past me as I wrestled with the fact that the drawing just didn’t fit what I had on the bench. From this demoralising situation, I managed to gain a positive thought. “Burn the schematic” was my best shot.

Even worse than all that, there was a switching transistor missing. This dealt with reverb footswitch operation. I hadn’t the faintest idea what it was, so I concocted a hard wired resistive network. I was convinced that this would introduce switching noise. But it didn’t!

So what the transistor (whatever it was) was for, I’ve no idea.

I most definitely get a tea-laden macaroon for that one, even if only as an anti-frustration measure.

Sound City Concord….it was quite a buzzzzz.

There can’t be that many folks about who remember Sound City. And even less who want to. I hold up my hand. I actually used to sell them, when I worked in the venerable establishment of C.E.Hudson and Sons of Chesterfield (that’s Derbyshire, England.) That would have been in the ’70′s.

Some of the best, and certainly most useful, times of my life were spent during my six year sojourn there. At that time Dallas Arbiter were a big name in musical wholesale, and it was that company who was responsible for the introduction of the Sound City name to the market. The amps were designed by Dave Reeves (later of Hiwatt fame), and some big names used them; Pete Townshend being amongst them. But this all sounds too much like a boring old git carping on. Let’s just make the point that these amps were (when the design settled down) very well made and well thought of.

The one that turned up at the workshop was a Concord, which was a fifty watt 2x 12 combo, and now not far off fifty years old. It had a quite distinctive look to it, having faders for the channel volume, treble and bass controls. This one did buzzsaw impressions at a level that would have completely drowned a buzzsaw next to it. The main reservoir capacitors had dried out and that seemed to be the extent of it. After replacing the two 100uF capacitors……it stopped buzzing….and started humming. This was not quite at the level of the buzz (which had disappeared) but it wasn’t messing about, and certainly wasn’t usable.

This was a mystery. It sounded (and looked on the scope) like heater wiring hum. If the heater wiring has been moved around it can pick up. So you get a pair of longnosed pliers and (very cautiously) move the heater wiring around. The effect of that can be extreme. In this case it didn’t make any difference at all. Sometimes the heater winding on the transformer can develop shorts, so that the cancelling effect of the ac heater wiring is upset, and you get hum. One thing you can do to get round that (but not if the heater winding has an internal centre tap) is to fit a hum cancelling preset pot, which can trim the out the hum. That was not possible on this amp because it had a grounded internal centre tap on the heater winding.

It didn’t make any sense to me at all, so I shelved it, and sat in a corner for a month sucking my thumb. Then I realised that there was a distortion if a signal was put through it. Difficult to make anything of, when you have a monumental hum going on. But it was not far off half wave on the scope, which meant that half of the output stage wasn’t doing anything. THAT WAS IT!!!!!!! Eureka. Half of the phase splitter wasn’t putting a signal to one of the EL34′s.

On most phase splitters (not Ampeg) an ECC83 or ECC82 valve is arranged so that each anode (pin1 and pin6) puts out an identical (but out of phase) signal to the grids of the output valves. So that explained the distortion. But what about the hum?

There was an open circuit anode resistor, which was responsible for the lack of half the signal. But this meant that there was an open circuit at the anode of the ECC83 and that was connected via a coupling capacitor to the EL34 control grid………and picked up the hum from the heater wiring!!!!!!!   Yes!!!!!!!!

Although there were few laughs in this job, I award myself a congratulatory macaroon. And tea.

 

 

 

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.

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.

 

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 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.

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.

 

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.