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I just posted this on DTB's audio forum, thought maybe a few here may be interested, I've just copied the thread verbatim:
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In 1983 I had been made redundant from my then job as an Industrial Radiographer and looked elsewhere for a different profession. For some reason and to this day I don’t know why I decided I’d like to fix televisions for a living.
I ended up doing a year long course in Electronics at what was then the Deptford Skilcentre and is now Lewisham College. Together with a year’s Day release course some three years later I gained certification in City & Guilds part III Electronics, in Colour TV Reception and Digital techniques.
Whilst learning of things I never knew existed I bought the November copy of Elektor magazine and there was this Valve Amp Project. ‘I fancy that’ I thought to myself. At the back of the Skilcentre classroom was a stockroom which I rummaged through and to my surprise found a stockpile of mains transformers and output transformers that perfectly suited the Valve Amp project.
The tutor approved of me having a couple of each and I figured they were the biggest expense of the project so I decided to go ahead and build. I ordered the PCB’s from Elektor magazine (these were to be my most expensive purchase for the project) and decided to mount each monoblock in a pair of home-made aluminium chassis’ that we had used earlier in the course to build a power supply in.
After I’d finished making them, they worked perfectly and I measured a steady 11 watts each into an 8 ohm dummy load after feeding a sinewave into them directly at a level just below clipping point. There was no hum present and they sounded sweet. I was actually using a Citronic disco mixer as a preamp to play music through them. Not ideal, but it gave me an idea of the sound quality they could produce.
That was a little over 24 years ago and the sad thing is I never did incorporate them into any home setup. I tried them several years after construction with the preamp section of a NAD amplifier, a 3020 I think. This arrangement made me realise just how sweet these amps could sound.
To quote from the magazine article:
“An EF86 pentode acts as a preamplifier, a double-triode ECC83 forms a differential amplifier and finally two EL84 pentodes make up a push pull stage that drives the loudspeaker via an output amplifier”
The heaters are powered by AC.
As you can see from the photographs here, my construction technique leaves much to be desired, they ain’t pretty. But, it was the first electronic project I’d ever built and they did work straight off.
I have included scans of photocopies within this thread which give a much more detailed description of the build and the amplifier’s operation. These scans have been reduced from around 4000 pixels wide to 800 pixels wide to make them comfortable to view within the forum.
If any body would like a copy of the original full size scans let me know and I will host them for download. They total 144Mb in size.
Along the way I decided to mount both monoblocks into a 19” rack unit. When I next tried the amps they had developed a hum, one monoblock noticeably worse than the other.
They had been stored in a lockup for years and I suppose must have been subject to a little damp, though the lockup is dry.
Two valves failed, which I replaced and I also cleaned all the valve pins. Still that hum, which is actually not noticeable when music is playing but it’s annoying the hell out of me. The only preamp I have at the moment is an old Quad 33 but if I can get the valve amps working satisfactorily I will invest in something better.
The amp(s) is now residing in my home which should give it a nice dry warm atmosphere, that may help. I suspect rewiring in some areas may help, although as I said earlier, the amps did work fault free initially.
Dave Da Boiss, him of the six stringed Apeggio, has agreed to have a look at this wonderful construction of mine and see if he can tweak the hum away, so to speak.
So I dare say I will point my wheels eastward towards Dartford and plonk it on his good lady’s dining table quite soon.
Here’s some pix I took a couple of hours ago, I note that the flash shows how grubby or marked the case is, I feel some fine wire wool may be needed here.
And here’s the scans of the original article:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In 1983 I had been made redundant from my then job as an Industrial Radiographer and looked elsewhere for a different profession. For some reason and to this day I don’t know why I decided I’d like to fix televisions for a living.
I ended up doing a year long course in Electronics at what was then the Deptford Skilcentre and is now Lewisham College. Together with a year’s Day release course some three years later I gained certification in City & Guilds part III Electronics, in Colour TV Reception and Digital techniques.
Whilst learning of things I never knew existed I bought the November copy of Elektor magazine and there was this Valve Amp Project. ‘I fancy that’ I thought to myself. At the back of the Skilcentre classroom was a stockroom which I rummaged through and to my surprise found a stockpile of mains transformers and output transformers that perfectly suited the Valve Amp project.
The tutor approved of me having a couple of each and I figured they were the biggest expense of the project so I decided to go ahead and build. I ordered the PCB’s from Elektor magazine (these were to be my most expensive purchase for the project) and decided to mount each monoblock in a pair of home-made aluminium chassis’ that we had used earlier in the course to build a power supply in.
After I’d finished making them, they worked perfectly and I measured a steady 11 watts each into an 8 ohm dummy load after feeding a sinewave into them directly at a level just below clipping point. There was no hum present and they sounded sweet. I was actually using a Citronic disco mixer as a preamp to play music through them. Not ideal, but it gave me an idea of the sound quality they could produce.
That was a little over 24 years ago and the sad thing is I never did incorporate them into any home setup. I tried them several years after construction with the preamp section of a NAD amplifier, a 3020 I think. This arrangement made me realise just how sweet these amps could sound.
To quote from the magazine article:
“An EF86 pentode acts as a preamplifier, a double-triode ECC83 forms a differential amplifier and finally two EL84 pentodes make up a push pull stage that drives the loudspeaker via an output amplifier”
The heaters are powered by AC.
As you can see from the photographs here, my construction technique leaves much to be desired, they ain’t pretty. But, it was the first electronic project I’d ever built and they did work straight off.
I have included scans of photocopies within this thread which give a much more detailed description of the build and the amplifier’s operation. These scans have been reduced from around 4000 pixels wide to 800 pixels wide to make them comfortable to view within the forum.
If any body would like a copy of the original full size scans let me know and I will host them for download. They total 144Mb in size.
Along the way I decided to mount both monoblocks into a 19” rack unit. When I next tried the amps they had developed a hum, one monoblock noticeably worse than the other.
They had been stored in a lockup for years and I suppose must have been subject to a little damp, though the lockup is dry.
Two valves failed, which I replaced and I also cleaned all the valve pins. Still that hum, which is actually not noticeable when music is playing but it’s annoying the hell out of me. The only preamp I have at the moment is an old Quad 33 but if I can get the valve amps working satisfactorily I will invest in something better.
The amp(s) is now residing in my home which should give it a nice dry warm atmosphere, that may help. I suspect rewiring in some areas may help, although as I said earlier, the amps did work fault free initially.
Dave Da Boiss, him of the six stringed Apeggio, has agreed to have a look at this wonderful construction of mine and see if he can tweak the hum away, so to speak.
So I dare say I will point my wheels eastward towards Dartford and plonk it on his good lady’s dining table quite soon.
Here’s some pix I took a couple of hours ago, I note that the flash shows how grubby or marked the case is, I feel some fine wire wool may be needed here.
And here’s the scans of the original article:
