CHAMP ELECTRONICS -" THE VINTAGE VALVE
AMP HOSPITAL"
NOTTINGHAM, ENGLAND
THE CHAMP CBA-200/200 (8 x KT88) STEREO AMPLIFIER
Hi everybody. A good few months ago back in 2010 I received an ‘out-of-the-blue’ phone call
(which is quite often the case regarding enquiries like this) from a really nice gentleman named Andrew. Andrew was
looking for someone to build him a stereo tube (valve) power amplifier of 200 watts per channel, and specifically asked if KT88’s could be used for
the output….eight of them….four per each channel! The reason for these specific requests were that back in the seventies, Andrew had an
amp like this but hadn’t owned it for many years and wanted to replace it. So, now after a good chat and agreement on the
plan-of-attack, I took the task on and….. here we go!
THE CHAMP CBA-200/200 STEREO AMP:
The finished amp looking cool and groovy I must say! Since doing this project I have had more enquiries to do
others, which includes a 300 watt per channel stereo version to go out to France!
Both lovely output transformers finished but not varnish-dipped yet….and there is a reason for this. When I design and make transformers and particularly output
transformers, I always test them before dipping. As the transformers have many windings which when finished have to be series/paralleled in lots of various ways…it is very easy if one gets called away for a phone call or other distraction etc
that, on returning to finish this said wiring, a mistake can easily be made in the sense of remembering which is the start
and finish of each of these windings and then can easily be wired the wrong way around! This has happened to me three times in my life of making transformers, so I won’t be caught out again.
Doing the dipping after testing is now the rule-of-thumb!
The 2mm mild-steel chassis with underneath ventilation.
Everything standing roughly in place
in order to get all the correct measurements. Note the power transformer in the middle has already been tested and varnished.
Now the
laborious bit….doing all the marking out. This is not one of my favourite
tasks in amp designing/building, but it has to be done….and it has to be done accurately.
All the larger holes done. Although I do have all the various size chassis cutters for doing
this kind of thing, they are good for up-to 1.5mm mild steel and anything above this
is a struggle. As the chassis are 2mm mild steel, I gave them to my good friend/metalwork man Steve to cut all the holes with a pillow-drill which he uses at his workplace. I must also at this point give Steve a big mention and thank-you again for making/supplying these chassis…thank you Steve. You will also notice that there are two chassis here in this photo. This is quite simply
because it is easier to do two at the same time; the second one is for the gentleman in France (Edward). His will have 8 x KT100 run on higher voltages to get the 600 watts (300 per channel).
Next, I
moved on to making the two output tubes’ driving boards. I have used my own design on this board numerous times, and they are perfect for driving any four output tubes (in fixed-bias)…two above the board & two below. With four individual lots of drive components on each
board, all that is required is simple linking to the associated tube bases!
All bolt holes now drilled and everything bolted into place (apart from the one missing
Speakon because I was out of stock at the
time!). Just to be certain that everything is OK before the chassis is powder coated.
As above but the front view. To some people, bolting everything into place and having to remove it all again for spraying may seem a waste of time,
but for me, at least everything is checked and verified before the spraying by doing this, plus…having now sorted all the various size nuts, bolts washers etc.,
I simply put them all in a bag and they are ready to use for the real thing when back from the powder coating.
I don’t have to sort them all out again!
After drilling all the smaller bolt holes etc., which were of course no problem for me to do in the 2mm chassis, and
with both chassis now at the sprayers for the powder coating, (by-the-way, both Andrew here in the UK and Edward in France chose the old grey Hammerite look….good old-school stuff!), I now moved on to the pair of pre-amp drive & splitter boards. These boards are ‘handed’ as they are left & right for stereo. The two 20-turn pots are for the drive balance….my own idea that always works wonders. Also, though not shown in the
photo, the right-hand board ended up with another 20-turn pot, this time in the feedback loop. This is a perfect way for balancing the two stereo channels of a tube amp as the tubes and corresponding components are never exactly the same,
no matter how good the tube matching! Transistor amps are usually perfect for equal channel gain, but it’s just not the same with tube amps. By putting this trim-pot in the feedback of just one channel, you can tweak this said channel to equal the fixed channel.
It works perfectly and can be re-adjusted when any of the tubes have to be changed!
Back from the powder coating….looking bloody mint!
Here is where another
of my friends gets a big, big mention and thank you. At school I was actually top-of-the-class for many years at technical-drawing, so I am quite capable of doing this myself but….I asked my dear friend Howard (from Matamp) if he fancied doing it as he does this kind of thing on a regular basis for the guys up at Matamp. He jumped at the chance and drove down to Nottingham, from Leeds (about 90 miles) the following Sunday to do this for me. Lovely neat job of the A2 actual-size scale drawings for the front & rear panels….big thanks Howard. Also
many thanks to Geoff and Derek at Matamp for doing the computer controlled cutting of the said panels too.
A tentative start made on the building here.
The two buss-bar runs, here covered with brown & clear
silicone sleeving, are for the KT88 heaters. There is no point in having hand-wiring
and twisting for the heaters of output tubes as the gain on them is so little. Here the heaters are done in this buss fashion with 1.5mm silver-coated copper wire,
then the 6.3 volt supply from the power transformer is applied to the centre of the
buss resulting in equal current pulled from each side and no voltage drop
whatsoever.
Also, all the very simple wire links from the two output boards to the relevant tube bases
have been completed here.
Now the centrally mounted main fuse
board
which also carries the bias supply is finished here, as well as the independent separate HT supply for the pre-amp boards.
On big tube amps like this, I always use a separate HT supply for the pre-amp
and splitter/drive circuits. The reason being that as the output tubes start to draw current, the main HT for them obviously drops. However, by using the separate supply for the drive sections and not running them from the same HT as the output
tubes, the grid-to-grid drive voltage remains virtually constant....which is exactly what we need! Other than full voltage stabilisation
everywhere, this is about the next best way to keep an un-clipped, good, clean, drive voltage! The
amp, on full sine-wave drive with both channels driven simultaneously, is doing a perfect 212 watts RMS per channel!
After testing and confirmed to be fine, the two output transformers are dipped and drying in the English sunshine.
A rare occurrence……and I do mean the sunshine!.
All finished and on a full test as always. Pretty neat do you not
think? Something worth mentioning here is that the last time I made a stereo amp was back in 1967 at the age of 17! Over the many years, all my customers have always wanted mono-blocks, but hell….its no big deal….you just do everything twice but have to be very careful of ground/earth-loops when both inputs are ground-linked,
as they would be when connected to the preceding pre-amp!
With the quite tall feet fitted to allow for airflow from underneath.
Completely finished rear view.
Completely finished front view.
And as I always say, ......... peace and good-health to you all. I hope you enjoyed the project and article.
What better way to end this than with Andrew’s (the owner’s) testimonial.
Cheers, John.
From Andrew - Champ CBA 200/200 Stereo 8 x KT88 Amp - August 2011
"I wanted to replace a 400 watt valve amp that I had custom-built back in the early seventies but hadn't owned for many years. Searching on the internet, I could find what I can only call gimmick designer amps and second hand guitar heads; not really what I was looking for. Just as I was about part with a lot of money for a little amp that seemed to be the only choice I could find, I stumbled across a picture of a 1000 watt valve amp and followed the links to John's website.....Wow what a result!
I phoned John to see if he could help me. He knew exactly what I wanted as he had made many amplifiers like this over many years. When I asked him meekly if he would build one for me, he said "yes....no problem" and the "CBA 200/200" was realized!
During the build, John emailed me with regular photo and textual information on the progress, and with his ideas. Also with regular phone conversations taking place, John involved me at every stage. We live about 150 miles apart but I could have been sitting next to him in his workshop...extraordinary inclusion and a real privilege/added value. I met John for the first time when the amp was completed and ready for collection and, yes the rumors are true......he does make a lovely cup of tea!
What a great amp he has built! The thing that leaps out is the quality of workmanship, with the tidiest wiring I have ever seen. It's robust, with no hums or hisses even when turned up to max...and that's even with an open circuit input! The care taken at every stage is extraordinary; John is a conscientious craftsmen who takes great pride in his work and he is a lovely fella, which is not something you get with most things these days. At the moment the amp is hooked up to 2 x 18" drivers / 4 x 15" drivers / 2 x horns and will easily handle double that. With plenty of power, it covers all the desired ranges Hi & Low. My vinyl has not sounded this good for a long time!
I am extremely pleased with the amp that John has built for me, and what a great guy whom I now consider as a good friend. If you need anything related to valve technology be it hardware or knowledge, John is the man you need to speak to (no time wasters please), and be nice!
Andrew Laidler."
....... and a further email from Andrew after he noticed this article about his
amp on the website:
"Hi John,
Saturday morning (well.......midday really!) 10/09/2011 and I've just read the article about my amp on your
website. Fantastic and well done!
The amp's doing great, I'm a very proud owner of a Champ Amp! I smile every time I see it, I'm a very happy man. Thanks for all you've done for me. Hope
you and mum are well and not working too hard? Once again John, thanks for everything.
All the Best, Andrew."