DIY Valve Guitar Amp Head

[Marcel]

I think at least one of my DIY guitars uses a .1uF PIO cap as the tone cap, they are pretty good sounding, to my ears the common garden-variety 100V Greencaps seem to have a tone that has a harsh high-end to it, or maybe I'm just imagining it....lol.

Many years ago I ran a workshop in the Valley in Brisbane servicing all sorts of amps and gear. I very quickly learnt that happiness of the customer was paramount, and a substantial part of that happiness was catering to their beliefs about gear. Which gear sounds best, which gear is crap, is it worth it to do this or that modification, and so on. Quite often I'd end up replacing a perfectly serviceable part with another because the other had some reputation that the muso wanted in his rig. I'd charge the muso accordingly , they'd walk away happy, and recommend me to their friends... so it was all good. Nine times out of ten I couldn't tell any difference, but they could, so I lived with it, it was their money that they were spending. In a sense I felt they were buying happiness, which made them play better, but I couldn't tell.... These days I have the time to spend looking at and repeating for myself those changes I did back then, and despite my own issues (Tinitinitus or whatever) and in my own way I'm starting to see and hear what those muso's back then and now were on about.... go figure...

My ESR meter works fine, as does my Z meter... I'll post some pix soon to illustrate what you said there Doc... I have a few new and old dud caps floating around here somewhere...

[DrNomis_44]

Many years ago I ran a workshop in the Valley in Brisbane servicing all sorts of amps and gear. I very quickly learnt that happiness of the customer was paramount, and a substantial part of that happiness was catering to their beliefs about gear. Which gear sounds best, which gear is crap, is it worth it to do this or that modification, and so on. Quite often I'd end up replacing a perfectly serviceable part with another because the other had some reputation that the muso wanted in his rig. I'd charge the muso accordingly , they'd walk away happy, and recommend me to their friends... so it was all good. Nine times out of ten I couldn't tell any difference, but they could, so I lived with it, it was their money that they were spending. In a sense I felt they were buying happiness, which made them play better, but I couldn't tell.... These days I have the time to spend looking at and repeating for myself those changes I did back then, and despite my own issues (Tinitinitus or whatever) and in my own way I'm starting to see and hear what those muso's back then and now were on about.... go figure...

My ESR meter works fine, as does my Z meter... I'll post some pix soon to illustrate what you said there Doc... I have a few new and old dud caps floating around here somewhere...

Cheers mate, that would actually be good so that other forum members can see what it's all about, it'll be interesting in any case.

[DrNomis_44]

And on no.1, the working voltage should ideally be half that or less of the caps rated value. Above that, the capacitance value drops off (so you might need to use a bigger value capacitor than normal if that's the case). And when working with AC circuits, if the cap only has a DC voltage value on it, then remember that the RMS voltage of an AC circuit is less than the peak voltage achieved by the AC signal. For a sine wave this is 41% greater than the RMS value.

Good point there Simon, I've actually got first hand experience of what happens to an Electrolytic cap if it's working voltage is exceeded, I've got a variable DC power supply that I built from a kit of parts that I bought from Jaycar Electronics, one day I powered it up so I could use it to power a circuit I had breadboarded on one of my solderless breadboards, about a minute after I turned the power supply on I heard a loud bang from inside the power supply, I quickly powered it down and disconnected it from the mains power, after I had recovered my nerves I took the top plastic cover off the power supply so I could have a look inside to see what caused the loud bang, it turned out to be very obvious just what caused the loud bang, one of the main power supply filter caps just after the bridge rectifier had exploded and blew it's aluminium casing off so that the casing was now lying on the bottom of the power supply's plastic casing, as luck would have it, I still have the pics I took of the exploded cap so I'll post them here.


Here we go, this is to illustrate exactly what can, and will happen when a cap exceeds it's working voltage:








As you can see in the pics, the cap in question was a 2200uF/25V Electrolytic cap, my guess is that the voltage across it was a bit too high and the result is what you see in the pics, the other 2200uF/25V cap is in parallel with it so maybe the one that exploded was inherently faulty in some way when it was manufactured, I ended up replacing both of them with a couple of new 2200uF/35V caps if my memory serves me right, and the power supply has never given me any more trouble since.


A very classic example indeed.

[Marcel]

A Dud cap tested.

It has the correct uF reading on the capacitance meter and reads high ohms when tested with an ohm meter, but it also has high ESR and is leaky when a high DC voltage of 250VDC is applied. This failed cap will allow hum to pass and will sag badly when the amp is pushed causing loss of dynamics and loss of headroom.

[Marcel]

A almost good (and very old) cap tested

This cap has lower capacitance than the marked value, close to 20% less. The insulation resistance is good at near 2Mohm but the ESR is marginal. So given it is quite old and was borderline performance on 2 out of 4 tests it was swapped out for a new HT filter cap. Truth be told this cap could easily have seen a few more years faithful service and nobody would have complained, particularly the wallet of its previous owner.

I've included a precautionary photo of the insulation tester and a volt meter. Note that the Insulation tester I use sends out negative 260VDC. This is crucially important thing to note when you testing large filter caps as they very rarely respond well to incorrect voltages being applied to them as illustrated in Doc's earlier post... Check the polarity of the insulation tester that you are using before doing filter cap tests like this...

[DrNomis_44]

Cheers for that Marcel, that ESR meter looks exactly like the one I built from a kit of parts I bought from my local Jaycar Electronics store, I'll see if I can find it and do some restoration work on it if it needs it, it's a very useful piece of test gear to have cause it will easily identify caps that have gone bad but otherwise look perfectly okay.

[Marcel]

In recent days I've been trawling through circuit diagrams of amps looking for a half decent 2xEL84 output design with two or three 12AX7 driver tubes to build and I think I have found something I like. It is essentially a Marshall circuit with 18W output which has nil negative feedback circuit and even includes a 2nd channel with tremelo. For something to do at night I might try and rearrange things into a single channel design so it can have switchable clean/crunch/OD and/or tremelo on the one input jack...

[Dedman]

I've always thought of electrical wiring the same as plumbing....except with acid.

[Marcel]

I've taken the plunge.... Bought all the electrickery bits I need but don't yet have to make me a 15W EL84 amp. A couple of transformers, a few HV caps and a few tube sockets only set me back about AU$400.... Just need a pretty box to house it all in and a pair of EL84's.

[DrNomis_44]

I've taken the plunge.... Bought all the electrickery bits I need but don't yet have to make me a 15W EL84 amp. A couple of transformers, a few HV caps and a few tube sockets only set me back about AU$400.... Just need a pretty box to house it all in and a pair of EL84's.

You could try doing what I did and make a chassis/box from 2mm Aluminium sheeting, I found that 2mm Aluminium sheeting is not very hard to cut with a jigsaw, although you do need some method of folding the sides and tabs up, like a panbrake metal-bending machine, if I could get a decent one I'd be doing more amp builds.