On the subject of luck in fault finding….

I need to ramble on a bit. A long time ago, oooh….about mid-’70’s, I was working in the service department of Hudsons Music in Chesterfield. There were three of us; Dave, Danny and me. And the very excellent guitar teacher, Fred Baker, used to wander through the workshop from time to time.

You might think that luck has nothing to do with electronics, but you would be badly misguided in that opinion. Of the three of us, Dave had the most ridiculous degree of good luck. He could fall into a barrowful of horse shit and jump out smelling of Chanel. Danny would fall in and come out smelling of horse shit, and I would fall in and never get out again.

What, you might wonder, has this got to do with anything? Well, it eventually might (with a bit of luck) get to a Marshall preamp/ recording/ headphone drive/ pedal. But to resume. On this occasion, Dave had checked through an early ’60’s Selmer Treble and Bass amp head, and it had gravitated to my bench with the advice that,

“It just wants a couple of preamp valves”. Dave had switched it on earlier, and apart from not making any noise at all, had sat on his bench, quietly, for the best part of an hour.

I took the back off, and the amp, performing to an audience of three (Dave Danny and Fred) blew the main reservior capacitor to bits, decorating my jumper with its insides, and filling the room with smoke. This was thought to be a hilariously funny Christmas tree impression. My point is that, had I been the one to donate the amp to Dave’s bench, it would have behaved with perfectly good manners.

Onwards and upwards, to the Marshall preamp pedal. Belonging to Paul, he freely admitted plugging in a dc supply of the wrong polarity. Inside most pedals, there is a diode which protects the internal chips. The idea is that if you apply the wrong polarity supply, the forward-biased diode shorts the dc input and blows the fuse in the power supply. Unless you are unlucky. In which case, the diode blows open, the protection doesn’t protect and the internal electronics are fried.

In other words your nice pedal is converted to a pile of junk. Why they don’t use a series diode to protect the protection diode I don’t know.

Suffice it to say that my decorated jumper was well worth the three cups of tea and a macaroon.       

Marshall TSL 122 (drifting along with the breeze)

This an excerpt from an email I recently sent to Rob, a beleaguered TSL 122 owner.
  This was after some considerable head-cracking problems.
I pur it down verbatim, as time is trickling away towards the yard-arm.
Hello Rob,
                   there’s a lot of puzzled folks on the internet regarding the bias voltage drift in the TSL122.
I rarely take much notice of the forum stuff, preferring to draw my own conclusions. Anyway………
This might not make a lot of sense, but I’ll give it a go.
There are two root causes of the bias fault. That’s not counting the problem we had with the damaged valve base contact, which made a mess of at least two valves.
The pcb material is very flimsy and of poor quality. The insulation properties become compromised over time, (and especially if exposed to damp/ condensation etc) and tracks and pads that are close together and have large voltage differences tend to leak over from the high voltage rails to low voltage (particularly bias voltages).
Also the bias voltage supply maximum is around -46 volts. Originally this was up to around -55 volts which means that the bias voltage range has been downrated by 25% or so.
Marshall came up with the answer to this (as you reported); replace the power amp pcb. A new pcb would eliminate the track-leak problems. However, it would also set up the same problems for later down the line, because the pcb material is still crap. Just newer crap. And also the bias range would still be limited as in the later versions.
I came up with three answers:-
Replace the bias supply caps. They are on the power amp pcb and work at a voltage close to the working voltage of the caps. So I uprated them to 100 volts instead of 68 volts.
Modify the bias supply resistor. The latest value they use is 10k, it was 3.3k, so I replaced that for the original value. It gives a much greater range of bias and the impedance is less, so the drift factor is also less.
I drilled through the lacquer on the pcb, in various places between the anode pads and the contol grid (bias) pad. That got rid of any leak through the surface lacquer from anodes to grid.
So far, the bias is stable, but I can only use a couple of your matched valves, the others being in not good shape, and the other new one you brought being a very poor match.
It’s been on for an hour or two, and seems ok, but I’ll give it another whirl tomorrow, and let you know how it’s shaping up.
The real fix is to replace the power amp pcb. As Marshall suggested. Not sure how long the new one might last……… as I remarked………cynically.
Onwards and upwards, not much to be macaroon-ed in this one.

Across the vast desert of the internet……

……there is the odd water-hole of commonsense. I flatter myself that I am indeed odd enough and common enough. The jury is still out (and on it’s 54th tea break) on the  sense.

The reader of this blog (that’s the Redoubtable Dave, should you not have been following) must surely have noted that there are expanses of the internet desert that seem sporadic, so far as the ReVamp blog is concerned. My footprints in the drifting sands disappear without so much as a by-your-leave.

There is a reason for this. If I can only remember what it was. Ah, yes! There’s nothing happened in the decade since the last blog. Were I of a political persuasion, that would not shut me up. Indeed they thrive on the dearth of information. It’s when you might want to know something useful they tend to clam up.

This is about something. The Dr. Z Maz Junior NR 18 watt.

This is a ’boutique’ amp. Not knowing much about the ‘coolness’ of what things are called this could just as easily be a J.R. Tolkien amp. I wouldn’t care and neither should you if it’s on your bench to be fixed. Boutique amps have a common thread . So a Dumble is going to be like a Bumble, like a Vox HW, like a Dr. Z ….etc. By virtue of the facts that they cost several limbs to purchase one, and there is very little inside them. I could probably make the most expensive amp in the World if I put nothing in it at all.

The one thing I do like about the extravagantly priced ranges of amps that have little in them, is that there are no two exactly the same. Because they are hand made, by hands. People in other words, and people have this propensity for getting a bit fed up on Monday afternoon, a bit giggly around half past three on Wednesday, maybe a bit pissed after Friday lunch break. So serial number 36 is not going to behave exactly as does serial number 37 (who we shall call Brian, for ease of remembering).

To current digital thinking that is terminally bad. Serial number 256,437 (who we shall call 256,437 for ease of remembering) has to be exactly the same as serial number 1. Or any other number you might care to come up with. This is of course fine if you happen to be a digital musician, because you will be doing exactly the same stuff as every other preprogramed machine. Why this lack of humanity? Because robots stick ’em together and robots don’t get pissed on Friday. So far as I know.

The fault with this Dr. Z amplifier was that after a few minutes it made noises more suitable to a rifle range than a musical performance.  The setting of the gain controls made no difference which would indicate that the fault was in the last stages. Un…fortunately there were two faults. The amp has two EL84’s in the output stage. A light tap or two with some suitably non-conductive tool soon sorted out that the fault was in the output valves. New pair fitted, bias checked. This is an automatic biased amp. (Which means that there is a wirewound resistor in the cathode (s) of the output valves. The bias is generated by the volt drop across that resistor which lifts the cathode above the control grid of the valve, thereby creating the negative voltage required for the bias. In this case 13.5 volts.

The other fault, which showed up after sorting the output valve fault, was poor contacts to the first preamp valve. The fix? Slide a soldering spike down the side of all the contacts in the valve base, so tightening them up.

Job’s a good ‘un!

One or two macaroon-worthy giggles in that.

Vt100 schematic. Here’s working link.


This is probably amongst the most brilliant sources for schematics on the net. Electrotania have helped me out of more holes than I could dig for myself in a lifetime.

You don’t have to register or pay, or any of that rubbish.

So let’s hear it for ELECTROTANIA, the most awesome site on the planet. !!!!!!!!

This is definitely serious macaroon time.!!!!!!!

Beware…….the Vox VT100

I know, I know. It has as much similarity with a proper Vox as the Hoover tumble drier that broke this morning. More or less the same shape, and makes about the same noise. None at all in either case.

There are various forums that will tell you that the preamp section of the VT100 is ‘about the same’ as the VT50; it’s just that they are different power outputs. No they are not the same, and excepting the first and last analogue chip on the preamp, completely different.

Here’s a rough appreciation of how this preamp section works. It is rough because to get a valid schematic you might have to shoot somebody. But it works more or less like this. I think.

The jack input is applied to the first chip (which is secreted under the rotary switch that (theoretically) gives every amp simulation since the megaphone was invented.) This is a triple op amp the output of which goes to what I think is an analogue to digital converter. This chip also has inputs from the tone filters and these (I think) are VCF’s (voltage controlled filters). This is in some way (don’t know about this because there is a DAC missing from the pcb on the 100 watt version) routed to the master volume and last chip which then arrives at the power amp pcb.

This is more or less how functions. Un…fortunately, I don’t have an electron microscope for eyes so soldering a 3mm square chip with 16 pins is no joke. Well, it is, because I don’t do it. Some surface mount stuff is worth a shot, but DSP processors and the like? No, forget it.

What you have is an amp that is fine if it works. But not repairable if it don’t. Not by me, anyway.

I’ve not seen the redoubtable Dave recently (I’m sure he’s pleased about that), so the macaroon stash has taken a serious dive.

Marshall DSL15c…and Marshall’s Wonderful Parts Department

Before I start (and before you switch channels) I have to say that, whatever else they’ve got wrong over the last sixty years (or thereabouts) Marshall’s parts department can hold up a flag that says ‘this is how we do it, and this how you should do it’, and I for one would sign it as many times as they want.

I’m not necessarlily the greatest fan of their gear, but you have to admit that if the ‘Marshall Sound’ is what you want, nothing else is going to do it for you and you’ve got to buy one. And even though the little DSL15c has a completely none-traditional (for Marshall) power valve complement, it still sounds like a Marshall. That is quite something in my book, even though I don’t really know why.

The valves they use to produce their 15 watts rms in the DSL15c are unlike anything that has gone before, to the best of my knowledge, being a pair of 6V6’s. EL84’s would be historically their choice for this sort of power output (very British) and not 6V6’s (very American). However, this is not a tub-thumping Marshall fanzine (is that what they call those things?) but a bit of my own history on repairing one of these.

About five months ago I repaired one for Pete, and it had last week shorted an output valve. I don’t usually chase about trying to get a warranty to stick on valves. I once had an experience with Chelmer Valve Company with an expensive Russian valve that turned up with a crack in it. After several fruitless conversations and teethmarks in my keyboard I concluded that the time I had wasted was irreplaceable and warranties on valves was a joke. In that case, at my expense. So valve faults I book down to experience and just pay up and grimace. Pete was happy enough to pay half towards the replacement valves and I checked the amp over and sorted the biasing for nowt.

Fair play all round, everybody’s happy. Except it sounded very crap. It turned out that the shorted valve had shorted a lot of windings on the output transformer.

This is a good reason not to run an amp if has buzzes, or seems hotter than usual, because if you have a red-plating (soft) output valve, the output transformer can go down before the anti surge fuse blows.

Output transformers are fairly tolerant of shorted windings. What can happen with the primary side (the anode load of the output valves) is that some windings on one side of the centre tap can short due to possibly a valve fault (or a poor bias setup). The output stage can often not be too worried by this, maybe if the imbalance in the windings is no more than say around 10%. But this was nearer 90%, and that is a seriously knackered transformer.

Swapping the transformer on this amp is two screws and a spade connector wiring job, so pretty simple, and neither is the transformer very expensive. But be cautious about the bias when you’ve swapped the transformer. The best plan is to check the bias voltage on the centre pin of the preset bias pots, BEFORE you fit the 6V6’s!!!! It will be different when you fit the valves, but by checking the bias values you at least know the bias network is functioning correctly. If you set them around -26 volts your valves will be happy enough. A scope will tell you about any cross-over distortion and you can adjust the voltage to suit.

After doing all this, it left me with another problem. If you wind everything up (particularly treble and presence controls) the amp became high-frequency unstable.

No idea why as yet. But I might commiserate with myself and a macaroon. There’s always a plus…….




The political wing of the ReVamp blog

I have long nurtured the idea of being a political commentator. But I do have significant problems with this, before I even get out of bed. The first one being that I don’t know any. I know that there should be a prime minister (somewhere) but I don’t  know who they are. No, really. The nearest I get to knowing one, is the bloke with the platinum blonde yardbrush on his head. And I only know him because of a ’70’s Who song (‘Boris the Spider’).  But you can spot him from quite a distance and take evasive action.

You can spot any of them from quite a distance because they talk rubbish through a megaphone. None of which explains why I don’t know any. That’s even though I pay them, (which is very galling because I don’t know what for).

The reason I don’t know any is because I fall asleep just looking at them. Excepting the really, really stupid ones (there is one called after an anal explosion I think), they are all incredibly boring. And worse than that, for anybody in electronics. They make no sense.

The thing about electronics is that you just can’t bullshit your way out of a being an idiot. It’s no good saying to this non-working piece of printed circuit board ‘It’s the Conservative party’s fault because…..blah….blah….blah’. Unless you can reason your way through a problem that seems to make no sense, it is never going to work, you are not going to eat for the week, and people will be knocking at your door with knuckledusters.

Whereas, if you spout total cobblers for long enough, people fall asleep, and you don’t have to make sense. I rest my case.

What is even more of a problem is that it’s not really very funny (so I don’t get a macaroon). That is because these folks ( that I don’t know) have, can, and will in the future I have no doubt, get us shot at. By the million. By telephone (or email etc.). Without even rearranging their hair appointment.

So, by staring fixedly at a piece of circuit board that doesn’t work, I can blame somebody else.

Bad day. I’ll find something useful for the next one. That’ll be a first.


Things that are out to make me look stupid.

The Marshall TSL 601. This one definitely didn’t like me.

Having said that, I’ve never been very keen on the amp, and although I wax lyrical about Marshall’s parts department and their wonderful Imagebank, I don’t like some of the more recent design ideas. ‘Recent’ in my time scale (Old Gits’ calendar) stems from around the Norman conquest….of….erm….Norman(?).

The major problem (unless you take out a second mortgage) with amps built in the last, say, 25 years is that they have PCB’s AND VALVES. The valve base and soldered contacts associated are quite often situated on the PCB (I won’t be doing this on our ‘Most Wonderful amp………blah, blah). You will have noticed that a valve gets hot…..(Hmmm….the clever stuff eh?). What happens over a period of a few years is that the soldered contacts on the valve bases melt off the tin content of the solder, and it’s that stuff that is the main conductor of the solder alloy. There are different alloys these days but the effects are very similar. The tin content also provides a certain flexibility, and when that is gone, the solder joints crack due to vibration.

Anyway, to get back to the point, which was me being bitchy about a TSL601.

An interesting symptom came up, which I should have latched onto quicker (hence the ‘things that are out to make me look stupid’ bit.) In this case it was because I was being stupid. Most ECC83 valves designed as a preamp have an anode voltage which might be anywhere between, say, 100+ volts and 200+ volts. All the preamp valves in this amp had 400+ volts on the anodes. What this means is that the ground to the cathodes of all the preamp valves has a fault. No it didn’t. So all these valves were not working; they were taking zero current. A quick check across the cathode resistor (usually around say 820 ohms to 1K+) should show a voltage of around a volt or thereabouts. They didn’t. They were as silent as my dog, Rex, who has been dead thirty-five years.

This called for serious action. I have this four by two plank in the workshop that I batter my head with. The philosophy behind this is that the plank of wood will think it out faster than me. It worked. The plank said, ‘Are the preamp valves hot?’

I said “Duh!”.

I then looked at a schematic, which is a good idea to do BEFORE you’ve wasted an hour guessing.

The preamp heater supply in the TSL601 is rectified dc, and the bridge rectifier had blown.

I cannot rightfully claim a macaroon on this one…..BUT….the Redoubtable Dave (in the absence of a macaroon stash in his local supermart) brought me a box of very nice shortbread biscuits. So that was alright.

Thank you for saving my life yet again Dave.

To sum up. Assuming I can add up

I did mention, oh….a year or two ago….. that I would like to design and build the Best Amplifier In the World. I didn’t? Well, I meant to. It’s your fault if you can’t follow simple smoke signals and temporal-time-warp-speke.

We have actually got some little way into our Best Amplifier….blah…blah…design.

Mr. Dumble based his designs on somebody else’s (Leo, no less), and a stratospheric pricing philosophy. There’s no doubt if you’re going to copy somebody (allegedly) you would be sensible to make sure that the ‘somebody’ knew what they were doing. There would be no point codging your neighbour in the next desk’s answers to your French exam if he happened to be doing the geography of Beijing.

The overiding piece of understanding we need (stratospheric pricing philosophies notwithstanding) is that we have so far loosely put together a number of parts.

Mains input transformer primary…goes to secondary…..goes to rectifier….goes to smoothing. IT IS MODULAR !!!!! And so is the rest of our Best Amp in the….blah, blah.

The power amp section of a solid state amp is less simple to section off into a modular form, because there are so many negative feedback loops (ac and dc), but it is still modular in essence.

So we have got as far as the smoothing section in our Best….blah, blah. What next?

Well, staying within our so far unmathematical approach, we get to the decoupling stages of the various dc rails, which supply the different preamps and power amp.

Onwards and upwards…………

A post for the Redoubtable Dave, of Macaroon fame.

Yes it’s true! My favourite person in the whole Solar System has yet again, in his infinite generosity, awarded me another packet of macaroons!!!!!!!!

So this is to digitally set my gratitude in stone. This is, un…..fortunately, rather like setting it in wax over a hot grill pan. Such is the immense value of the free information society. The stuff that is really valuble is written on rice paper and kept in an alligator’s anal tract.

That is not to detract from Dave’s wonderful generosity. Thank you Dave, you are indeed a gentleman. Un….fortunately I’ve eaten them all.

So it only remains for me to be miserable until Dave’s gear next breaks down.

Tea…..oh, oh…….