QSC CX 404….Don’t try this at home

I had a worrying thought. It might have been yesterday. Having not very many of them (thoughts, that is) I ought to be able to remember one. So that was a bit worrying as well. It was regarding the prime minister. I realised that I didn’t know who it was/is; then I was even more worried by the fact that I didn’t care who it was/is.

He has paid me a hundred quid towards my heating bill, and I thought it would be nice to send him/her a note. Obviously to 10, Downing Street.  But you can’t address it to ‘Somebody, 10, Downing Street. The cat might get it. Anyway, lead onwards and outwards, Macbeth!

Ah….yes! The CX 404. The amp with more things to go wrong in it than a politician. It has a complex switch mode power supply, with a load of things to go wrong in that. It has four separate amps in it to go wrong. It has a bagful of protection circuits to protect it from going wrong, all of which can go wrong. The only way you should take on a job like this is with a bazooka levelled at your head (or somewhere) and Mills grenade taped to both hands. That’s the most useful information I might come up with. Suffice it to say, I recently repaired one of these. This I put down to stupidity of a high order.

The power supplies had blown. These are IGBT (insulated gate bipolar transistors) that do the power switching, and they’re not cheap. There are two chips that generate the switching pulses and provide the drive to the IGBT’s. If the IGBTs are cleaned out it’s pretty likely that so are these two chips. But if there is a fault in any of the power amps, the amp will cycle on\off etc. and the power supplies won’t operate even if they’re ok, because of the dc sense circuits. Or, of course, it could be because the sensing circuits are sensing something that isn’t there at all.

The power transistors you need to take out to check if they are the ones in the schematics. The one I had, had two pairs of completely different ones in it, to the ones in the schematic. The drivers were the same MJE15032 and 15031 I think.

There is another issue with these. The heatsinks clamp down onto the top of the power transistors (there are four separate ones, one for each amp. That’s fine when they’re first built, but I discovered that different manufacturer’s devices can vary a bit in the physical thickness (back to politics) which means that your heat sink might not make such a good contact with the device. Not good news.

I got fair number of useful lessons out of this amp. The main one certainly to be very wary in taking them on.

Being deserving of at least several cups of tea and a pallet load of macaroons, I shall attend to those matters.


Peavey VK 100……no, seriously.

The VK 100 head is the same amp as the VK 212 combo. I’ve had two of these in the last couple of weeks; the first was fairly mundane….but the second! Wo-ho! There have been a few reports on various forums etc. about anode resistors blowing in these things; the reason for this is interesting. But, to start at beginning…..

Anode resistors do blow. Older amps are more susceptible than newer ones. No they’re not, necessarily. Resistors tend to go high as they age, and that can make them heat up more and that can make them go higher, and that can make them……you get the idea. The anode resistors in an old amp are often half watt rated, sometimes more. On a new-ish amp they are often quarter watt and sometimes less.

There is a term in engineering design known as ‘over-engineering’. This means that the design in some way uses higher rated components than necessary. This makes for better reliability and (sometimes) performance. Peavey’s designs and constructions when built in the States, were plentifully over-engineered. Which is presumably why there are still plenty around from way back. Same could be said of Fender, Ampeg, Matchless, Gibson and a load of others. Open a new-ish offering of those things, and it becomes pretty obvious that it’s the profit margins that are over engineered (in China?) and they cut their fingers on everything else (personal opinion). This is where we are with the Valve King 100.

The two red marked sections are heater diagrams, and they reveal something very  peculiar about this amp.

V4, V5, V6 and V7 are the heaters of the output valves. They are in series, so that’s a bit strange; the heater supply is dc; so that’s also a bit strange; the three preamp valve heaters are in parallel with each other, and the whole lot are in series with the output valve heaters. So what?

In the first place, there is about 32 volts to the heaters. Each valve has a 6.3 volt heater and the voltage drops through the chain of heaters so that each heater gets it’s 6.3 volts. So what’s wrong with that? Nothing, until a fault develops. Here we need to remember that a hot heater has a radically different resistance to a cold one. When first switched on, there is a surge, because all the heaters are a fraction of the resistive value that they are a few seconds later when warming up.

In this amp, if you take out a power valve, none of the heaters will work, because of the series connection. However, if you take out a preamp valve, this will upset the heater balance and put, say 9volts to the other two. Take another out and that becomes, say 12 volts. Heaters don’t last long in that situation. But there’s worse to come.

If, having blown the hell out of a couple of preamp valves, you put in a cold valve, this will upset the heater supply to all the valves, because of the low resistance of the cold valve. But there’s worse to come.

A fault that can happen (and had on the VK100 I had) is that an over voltage on the heater can distort it so badly, that it shorts its length against the cathode tube it is housed in. In this situation the dc heater voltage can be applied to the cathode. This upsets the bias of the valve no end and it draws a lot of current which can blow the anode resistor. Maybe then you might do a bit of swapping around with this dud valve and take out more anode resistors. R103,R104 and R101 were open on this one, but R144 and R145 on the phase splitter could easily have gone the same way.


Yes, alright, the capitals are a bit over the top. But so is your overdraft if the matched quad of mesa valves you got a second mortgage on just made a lot of smoke.

A nice cup of tea……..and maybe…….a macaroon!




Elm Tree Soup

You might have guessed from the title that this is not going to be helpful. You may even be able to buy Elm Tree Soup off the shelf (so to speak) at your local garden centre. Any shop that has the gall to sell recycled plastic reindeer droppings with little bells in June might just do the same for Elm Tree Soup. At that last resort, I would have to say that anything home made will taste like soot. If I made it.

There is a serious philosophical dissertation coming here, so dust off your MA in soup-making. This is how it goes.

If I could find something so utterly gormless to build my thesis around, I wouldn’t have to agree with anybody, and still get a ‘First’. Or in the realms of Elm Tree Soup-making, a ‘Thirst’. The problem with getting all these ‘levels’ (you know ‘A’ or ‘O’ or ‘Spirit’ or ‘Mezzanine’) is that you have to AGREE WITH PEOPLE. It’s no good on the exam paper saying, “I don’t like the way you fart, so my answer is much better than yours.”

That’s a non-starter. What you have to do is memorize by heart every last hiccup of the examining board, disengage any suspiciously subversive brain cell activity, and write like hell. Anything. Unfortunately, I have a lot of trouble agreeing with anybody, which is why I repair electronics. If I disagree with it sufficiently, I can stamp my climbing boot on it.

This option, I hasten to add, is not usually one you might come across in an exam room. Neither do they ply you with tea and macaroons. Which is something else I can’t agree with.