The start or end of a journey?

Rocker

Who knows but it is a journey that has come upon me and, in the scale of things, is small.  But necessary.  Please bear with me.

Around twenty years ago I built a combo guitar amplifier.  A single ended valve amp from a kit supplied by Dennis Cornell and sold through one of the guitar magazines at that time.  Supplied with a 6L6 output valve and an ECC83 preamp valve and a Jensen 10" speaker, it worked first time of asking and has been my mainstay amp ever since.  It went by the name Stinger and it features the unusual option of switchable tone controls, referred to as Cool [tone controls in circuit] or Cookin [tone controls out of circuit].  It worked well, sounded more Country than Rock but at least I could honestly say that actually I built it myself!

Lately I felt that it was sounding a bit off.  A spiky sound like you sometimes get from an MP3 of someone playing a Telecaster on YouTube.  Not in the least musical.  The tone controls did little to tame the spikiness though, oddly enough, swapping cables between my guitar and the amp sometimes improved the sound.  It was a guitar cable supplied by Russ Andrews many years ago that tamed some of the spikiness. 

Something was not right in the electronics so I wrote a very garbled message to our own @ICBM with an attachment of the circuit and the circuit board layout and awaited his response.  Unsurprisingly he came up with a few suggestions and he generously sent a few resistors and a capacitor with detailed instructions on where the components were to go.  And an explanation as to why these changes would achieve what I hoped to get from my amplifier.

That was a few weeks before Christmas.  Then I had a recurrence of Sciatica.  The pain simply refused to go away.  The amplifier upgrade would have to wait, as indeed a few other projects had to wait, until the pain receded.  Which happened over the weekend, the pain is not totally gone but it is bearable so fingers crossed that it will get better and not worse in the coming days and weeks.

Despite reading and re-reading the assembly instructions recently, I still had no recollection of what I would find when I removed the back cover.  Which I did today.  The interior is very cramped so the work is going to take a little time.  But over the next few days I hope to get the new components soldered in position.  I will probably avail of the opportunity to replace the input jack socket when the circuit board is out.  Watch this space for further progress........

SteveRobinson

I've still got the Stinger I made. Mine is one of the first with the tweed cabinet.

Rocker

Thanks for your comment @SteveRobinson, mine is tweed covered too.

Yesterday I set about the component upgrades.  Firstly I replaced the input jack socket as the original always 'felt' a bit insecure.  This enabled me to compare the supplied 2 Meg Ohm input resistor with a replacement 1 Meg Ohm one.  I kept a guitar at hand to test each upgrade as it happened.  I was surprised to discover that the amp sounded a lot more 'normal' with the 1 Meg Ohm in use.  Amazing that one simple change could make such a difference.

I fitted the 68K Ohm grid stopper resistor, none was supplied with my kit, to prevent parasitic oscillation between the input capacitance of the valve and the inductance of the source.  There was no audible change as a result of this addition but none was expected.

I replaced the tone stack treble capacitor for a Polystyrene capacitor.   This had the effect of 'flattening' out the response a tiny but.  But audible and the treble control operates more in the way you expect it to.  Big improvement.

The above listed upgrades managed to keep me busy for several hours.  Testing before, during and after each component change.  Space in the chassis is limited and the main circuit board had to be unbolted, the treble pot and the 'Cool'/'Cookin' switch removed to facilitate the capacitor change.  All the while ensuring that the PSU caps were discharged before putting a finger near the electronics.  Glad to be able to report that the amplifier worked perfectly when it was finally re-assembled.  During the work I used the original valves but after the final re-assembly, I replace the valves with a pair of new tested valves.  The result was a revelation.  I don't go for a lot of distortion from my guitar sound, much more Country than Rock, with the old valves in place the notes sounded clean but with authority.  Not spiky as before but a bit more rounded in texture.  With the new valves in place, the sound took on a different level of clarity.  This was unexpected but very welcome.

My biggest problem yesterday was my eyes.   I have great difficulty is seeing details like resistor colour codes, pins on valve bases etc.  Some time ago I bought an illuminated magnifying glass from Aldi and this did great service yesterday.  But I came to the conclusion, somewhat reluctantly, that this would be my sign off on amplifier building and upgrading.  At my age, I will turn sixty five tomorrow 16th, it is time to simply enjoy playing and leave the building, repair and upgrading of amps to those whose career it is and who know what they are doing and why.  I am glad I did the upgrade, satisfied at the reasonably decent build I managed and happy at the sound from the Stinger.

My heartfelt thanks to @ICBM for all his help, advice and the components he so generously sent to me for this work.  One oft mentioned 'advantage' of self builds is that you can tailor the sound to suit yourself.  That is true but you have to know what to change to alter the sound without risking blowing up the amplifier!  I am comfortable dismantling, re-assembly and soldering but I know nothing of the effects a component has on the workings of the amp.  John explained what he thought needed changing, why and what the expected effect that change would make.  All technical explanations are taken from John's email.  I had a printout of his email in front of me as I worked and everything he wrote happened before my eyes.  So I take my hat off to you John, you are a wonder and I will be forever in your debt.  Genius and a gentleman is the minimum I can say about you.

The Stinger has been working for over twenty years and I hope to get a few more years from this little box. My wife has often wondered why I persist in using this little amp and why I don't buy a new one.  Fair question but as the Stinger simply keeps on working, and now better than ever, I will persist with it.  If and when if finally stops performing, I will check out what is available in the shops. But until then.........