"After the research and tremendous success of all the experimenting that I have undertaken with the EL36/6CM5/EL360 family of tubes (please see THIS ARTICLE), late one night I just got it into my head to design and build 'one hell of a mother' 500 watt bass guitar amplifier using this non-sort-after, ten-a-penny, virtually forgotten but highly underestimated family of tubes!

After achieving 185 watts from four quite easily, I calculated that twelve would give me around 555 watts (obviously!), but in reality would likely be a bit over 500 watts as the massive amount of current drawn would cause at least some extra HT voltage drop.

I first found an old Carlsbro 200/7 PA amp chassis. This was large enough to take the monster. Next came the design and winding of the power and output transformers. (See picture below)



The power transformer worked out to about 1kva and is a 3 inch stack of type 638 laminations, in English this translates to “big and heavy!” There are four secondary windings 1 x 330 volts @ 2.5 amps….yes 2.5 amps!! HT-1 for the plates (anodes) and pre-amp circuits. 1 x 163 volts @ 250 ma HT-2 for the separate screen grids (grid 2) supply. 1 x 40 volts @ 500ma for the bias, slow start and protection relays, and lastly 6.3 volts (CT) @ 20 amps for the twelve output tube heaters!! Wow.

After cutting all the extra holes in the old Carlsbro chassis for the eight extra output tubes (it had four KT88’s originally), I started to bolt everything in place. Then came the power supply and output tube wiring. The heaters of the twelve output tubes were wired in a buss fashion using 16 gauge silver coated copper wire to eliminate “voltage drop” The 6.3 volt winding from the power transformer was then divided into three lots of cable feeding both ends of the buss and the centre too in a kind of triple-ring circuit. This ensures perfect symmetry of the 6.3 volt supply all around the circuit.



Next came the two HT supplies. The main HT-1 is about 435 volts (on load) and the main, single, smoothing capacitor is a 2000uf @ 500 volts. The HT-2 supply is about 200 volts (also on load) and the capacitor for this is a straight 1000uf @ 400 volts.

The negative bias rail is around 52 volts. This is also perfect for the four pre-amp tubes' heaters by putting them in series (50.4 volts), and is also beneficial as they are now run on DC.

I had to design and install a “slow start” circuit (see picture below), as during power-up there is 3000uf to charge-up, and this proved a bit too much for a 5 amp power fuse! After installing the slow start circuit, she powers up with no thump or problem whatsoever.



The final part of the bias supply is a 48 volt relay across the said supply with its contacts in-line with both HT’s 1 and 2. In the event of a “bias failure” this would then instantly remove all HT from the output bottles, protecting them from a total catastrophe!!

I will also point out that the bias rail comprises of twelve individual, 20 turn pre-set potentiometers (one per each output tube). This is essential for an amplifier running multiple output tubes and also eliminates the need for accurately matched tubes!

When it came to designing the output transformer I was beginning to run out of room! This being the case, I decided to do two smaller output transformers instead. This doesn’t change anything other than allowing a bit more “juggling around” on the chassis. The two transformers are simply paralleled to give the same desired results. They are both impregnated and interleave wound of course, also using the highest grade M6 type 248 laminations.


One of the Output Transformers under Construction.


The Completed Two Output Transformers.


Having now gotten this far, and on power-up setting the twelve output tubes to 30 ma (combined plate and grid two) cathode currant (each), I then proceeded to design and make-up the final pre-amp and drive boards.

Using an ECC83/12AX7 as the first pre-amp, gain and tone circuits, plus a further ECC83/12AX87, a 1 x ECC81/12AT7, and 1 x ECC82/12AU7 as the second stage gain, phase splitter and cathode follower/push-pull driver, the whole thing burst into “real life”. Man, this thing is doing 512 watts true RMS with no effort at all; just as I had expected it would! It sounds absolutely incredible/amazing on bass guitar (and in-fact on anything else for that matter!)

You may ask why use these old black and white TV line/sweep-output tubes, particularly as these tubes are now not manufactured anymore (and most probably never will be again.). My response to this question would be as follows........ Firstly the EL36/6CM5/EL360 family of tubes are out there in abundance and very cheap (about £2.50 each tube on average!). Secondly if the supply of them ever did diminish/run-out (though that would be a very long time), any amplifier that actually uses them could quite easily be converted, in time, to use something more readily available.

Another couple of other factors important here are:

(a) this family of tubes are extremely cheap and are very unlikely to ever command unprecedented prices. And ....

(b) using twelve output tubes in an amplifier of this magnitude is advantageous in that if one tube went down, other than re-setting the trip and (or re-placing the fuse), the show would go on with only one twelfth of the power lost; virtually un-noticeable!! 

This family of tubes are so cheap you could not even buy one single KT88 (even the later production types) for the price of these 12 x EL36/6CM5/EL360 “good ‘ole” family of TV tubes! They have much more to offer than is recognized.

This prototype has now been bench-tested nearly everyday now for the past six weeks (as at 12/05/06) and has more than proved itself. I build these (or any other amplifier for that matter) to order. However as this particular project has turned out to be so successful I will, over the next few months, be constructing the first production model. This will be properly cased and on a new chassis; not full of unnecessary holes like this prototype. It will be properly sprayed accordingly with powder coating. The power input and speaker outputs will be both “power-con and speak-on” as, whilst the output jacks are fine for testing purposes, they are far inferior in reality for such a beast as this!

By the way, I realize that the two smoothing capacitors are lethally mounted like this!! Nevertheless it goes without saying that on the first production model they would be mounted the other way up; through the chassis correctly. Also all the main wiring would be immensely neater, of course. This is after all just the prototype. There will also be three cooling fans fitted in the case. It is amazing how, by keeping tube temperature to a minimum, this expands the life of tubes!

I will in time be presenting this first production model on the website, along with all the goodies photos. So............. as they say, “Watch this Space”. In the meantime, I have now included the Schematic Wiring Diagram for the CBA-500 on the website for your interest.

I can not impress enough on you all how your interest in my work and knowledge of the “tube world” is appreciated.

Many thanks, John."


Input, tone circuit, and pre-amp board.








The three cooling fans located in the top of the amp's temporary cabinet.