Hi all you tube/valve high-powered amp enthusiasts out there! I am really sorry for this long-awaited Part 2 of my Champ CBA-20807 project. Since first putting Part 1 on the site (April 2009) things have been so very busy. However, over the festive period of 2009/2010 I took a complete break from everything else and made tremendous head-way on her. I know many of you out there have been looking forward to this and I thank you all for the vast amount of interest that this project has created!

Like any new design and project, sometimes things get changed (usually for the better) along the way. There have been a few small changes during the progression but I have detailed these changes and the reasons for doing them in all the photo texts below.

Have fun digesting this one!

What better place to start than with the making/winding and laminating of the humongous output transformer! I went round to my friend John’s (J. Wood Electronics & Transformers) where we keep our jointly owned winding machine and stock. I arrived there at 9:45pm one evening and got back here at home the following morning at 6:30am! This is how long it took just to wind the coil! Just check-out all those windings…..fourteen all-told, and the DC resistance between each plate to the centre-tap is spot-on at exactly 9.6 ohms…….perfect!

Both monsters side-by-side and finished - just waiting for a coat of lacquer (varnish).

I deliberately pre-wired all twenty of the 807’s heaters prior to any chassis spraying. In the past, when trying to fuse low voltage/high current heaters, finding the right type of fuses has been a problem. However, I have found that the best ones for the job, with very little voltage drop, are vehicle 12 volt blade type fuses. The 807’s are .9 amp each, so we are looking at 18 amps total (obviously!). So….wiring all twenty tube bases on a ring-circuit using silicone-sleeved/covered 16swg tinned-copper wire in a buss fashion then, feeding each end of the ring with 2 x 6.3 volt 10 amp windings (instead of one 20 amp winding), and bringing all four leads from the power transformer through 4 x 25 amp blade fuses did the job perfectly. The 6.3 volt remains absolutely consistent throughout the whole circuit. When striping the chassis for the spraying I didn’t remove all this wiring. I simply removed all the bases in one-go and then replaced them back when the chassis was ready.

All holes drilled and cut. Everything in-place (apart from the transformers) and doing a final inspection before striping and preparing the chassis for the powder coating process. Note how with the size of the chassis it was very convenient to put virtually everything on the front panel…in three banks. The left-hand bank is six Speakon output sockets, 2 for 2 ohm, 2 for 4 ohm and 2 for 8 ohm. The centre bank consists of two inputs (high & low gain), a gain level control, bass & treble (using my own breath-taking circuit design!), a master volume, and finally a slave in/out socket. The right-hand bank is (working from right to left) voltage selection, main power and output tubes’ grid 2 fuses, power-in, power-out, power-on, on-lamp, and standby switch. The back panel (not really shown in any detail in these photos) only contains an on/off switch for the internal illumination, a 21-way rotary switch (positions 1 thru 20 for each output tube bias-check, position 21 = off), and a red/black pair of 2mm Banana sockets for the connection of a meter….for setting the said 20 bias positions.

As above. Everything in place and just checking for anything missed.

Chassis now striped and complete here with its base-plate, ready for powder coating. And here’s hoping I haven’t missed or forgotten anything!

Now back from the powder-coating. These guys that I use are really brilliant….they always do a cracking job and the coating is really thick and tough. With wanting to keep the theme of the amp “blue”, my choice of this colour is perfect. Its product code is called Dark Blue Hammerite. Plus...........I mustn’t forget a big “thank you” to my metalwork man/friend Steve, who kindly made/supplied this 2mm thick mild-steel chassis and base for me, together with the cutting of some of the larger holes, for which I didn’t have the correct size cutters.

A tentative start now made on the wiring of the amp. Apart from the four PL509’s heaters (which are run in series/parallel on the 80 volts DC supply that supplies them with their fixed-bias too), the rest of the heaters are all up and running here. As the 6SN7 is a 600m/a heater, I have done four individual twisted pairs… for each tube. This keeps consistency on these heaters too….saving the need to run a 2.4 amp capable wire for the first run to the four tubes. Likewise are the pair of EL36’s heaters I have done in series on 13.6 volt. With this, the long run to them from the power transformer only needs to carry 1.25 amp as opposed to 2.5 amp if they were paralleled on 6.3 volts. Apart from the 807’s, all the other heater supplies are fused accordingly with standard fuses and are fine. Oh yes, one other small point here. I have run the four 6SN7 heaters on a standard 6.3 volt, centre-tapped AC heater supply. I didn’t fancy the chances of running these from half-way on the fixed bias DC supply…this was getting a bit too much for one of the 24 volt regulators and I wasn’t comfortable with this.

The fans plate now powder coated “chrome”, installed and checked to all be working OK. Also, the four strips of each six “Hi-Bright” blue LED’s.

Close-up of one of the LED strips. These come on a roll and you can cut them in minimum increments of three LED’s. I have cut them for six LED’s on each strip. As they are 12 volts per strip, I have simply put the four strips in series and, along with the fans on a communal fuse of F250m/a, connected them to the 50 volt bias rail. You can also see here how the cooling fans are rubber-mounted, along with the fact that there are five of them in series (60 volts really). At 50 volts (about 70m/a), this slightly slower speed along with the rubber mountings, makes them really quiet. I always mount my cooling fans like this. By using an M4 screw through a snug-fitting grommet and then a ny-lock nut, the nut can be tightened until it just grips without actually compressing the grommet…….perfect. Note the gorgeous chassis paintwork (powder-coating) finish too.

I absolutely love this photo…..please let me explain why. Here, all the power supplies are up and running perfectly. You can see two big 10 watt green resistors paralleled and hanging in mid-air. I am using these as a dummy load for the 300 volt output tubes’ grid 2 supply. Here it is constant at 180m/a (only 220m/a will be needed for the screen grids at full power, which it will quite easily do from the pair of EL36’s) and the lower meter is showing this rail voltage.’s the bit I really like. In order to load 750 volts at quite a high current….to test and check the regulators (the 4 x PL509’s), there is only one sensible way to do this. Although out-of-sight on the photo, (trust me, they are hanging on the wall above the two meters), I have soldered in series three 240 volt, 200 watt incandescent lamps (near enough for 750 volts) and the same again with three 150 watt lamps. The two sets of lamps are now paralleled and across the 750 volts as a dummy load. It doesn’t take much working out to find the current here but, just in case you don’t know how to do it…..the answer is: 1050 watts (all six bulbs) / 750 volts = 1.4 amps (although she will go much more but I ran out of large light bulbs)! This is constant without any let-up, in other words just like sine-wave drive. The four PL509’s sat here for hours like this…..absolutely no problem for them at all! 1000 volts on their anodes (plates) and 750 volts coming out of their asses (cathodes). The upper meter is showing the 750 volts with no movement whatsoever between no current and full current (all six light-bulbs switched-in simultaneously)…..bloody great! I certainly didn’t need any heating on in the workshop whilst she was under this test….the heat from the light bulbs alone was more than enough…..phew! The two meters are showing both the main 750 and 300 volt HT supplies under pretty-much sine-wave loaded conditions.

Also, in the left of the photo notice the individual cathode fuses for the 807’s. In the event of a shorting tube (which inevitably will happen at some point in time), these 125m/a ultra-fast-blow fuses will protect the 1 ohm cathode resistor, the 100 ohm grid 2 resistor, or even both.

The tedious making of the 807 hammocks. (Well…that’s what I call them anyway…..That’s what they remind me of). All the wires and sleeving I cut to equal lengths first and checked with a ruler, so it makes all the wiring neat and consistent. I have also run another pair of wires the full length of the whole loom, putting both sets of push-pull plates in two ring circuits. This yet again makes for perfect symmetry of the whole anode loading and performance.

OK, here on this photo I can offer you a lot of information. On the introductory text I mentioned the fact that some changes had been made during the construction, and most of these changes are covered here. Starting with the main fuse board on the right of the photo, initially I had made this 24 tag pairs board full-length with nothing but full of fuse holders. I made another full-length 24 tag pair board full with 1200 volt / 1 amp bridge rectifiers and a smaller board (9 tag pairs) specifically for the 50 volts negative bias. At the time of making these three boards, I had stupidly forgotten the fact that the two regulator PCB boards had all their own on-board fuses and bridge rectifiers (8 fuses & 6 rectifiers in total). This being the case, a re-think and re-design brought all three of these boards down to just one board….the one in this photo. It comprises of all remaining necessary fuses for the amp and includes the two regulated power supplies for the total 50 volts negative bias too.

In the middle of the photo you can see the block of five vehicle blade fuses…with a cover over them. The four at 25 amp are on the main 807 heaters as explained earlier. The fifth fuse in this block is simply a 5 amp, and is on the heaters' winding centre-tap to ground. This allows the use of a lesser gauge wire for the centre-tap which, without the fuse, would have legally needed to be capable of 20 amps.

Below this fuse block is the bias protect/soft-start board. Ordinarily I would use only one 48 volt DPDT relay here connected to the bias supply, with one pair of its contacts across the soft-start resistor and the other pair in the negative end of the main HT power supply…to ground. But in the case of this beast, I didn’t fancy putting this negative end through a relay’s contacts with it being 750 volts DC and with a supply as solid as this is……capable of around 3 amps at this voltage. This would have been asking for trouble! Instead, I have used the first relay’s pair of contacts, as normal across the 10 ohm / 100 watt soft-start resistor (this relay’s coil is on a one second delay) but then the second pair of contacts is simply switching the 48 volts coil of the second relay. Now, both remaining sets of contacts on this second relay are series wired into the two heater supplies for the pair of EL36’s and the quad of PL509’s. The scenario that is envisaged here being that if the bias supply should fail (or not even start-up) then neither relay will operate. This then leaves the 10 ohm soft-start resistor permanently in series with the power transformer primary, both main smoothing caps will still charge-up but, as we have no heaters in any of the six regulator tubes……we have no HT anywhere on the amp and hence… blown-up 807’s! And finally, once again you can see on the left some of the 807’s individual cathode fuses. I have mounted these between the on-board 1 ohm cathode resistor tag and an insulated pillar…screwed to one of the securing screws of each base.

And now this is where I am with her as at 25/01/10. All heaters, power supplies and output stage (all 20 of the 807’s) are all up and running…..absolutely fine. All twenty 807’s are set to 16m/a cathode currant (each) meaning a total tick-over current of 320m/a. She has been on (off at night whilst I am in bed) and like this for about ten days on soak-test now, and believe me…the blue plasma glow in the 807’s (which they are quite famous for doing) is getting better and better by the day! My next move is the pre-amp and drive stages. However, as most of the front panel sockets etc mount through the front, I can’t continue with this yet as I am still waiting for the front and back Traffelite panels to be silk-screen printed. I don’t intend to mount and wire all these with a view to having to take them all off again for fitting the panels. Patience is a virtue.

And finally the glow from the twenty 807’s with the twenty-four blue LED’s illuminated and bouncing off them. This is only for show of course and can be turned off via a rear mounted switch. Nevertheless, can you imagine how cool this will look, when she is finally in her blue Tolex covered case and with a completely see-through, 5mm full-sized Perspex top!!

OK everybody, I think that’s about it for Part 2. I will endeavour to get the final part…Part 3 done as soon as possible. When I do, I will be posting the full results of the finished amp, along with the schematic and a list of useful notes too.

This link will take you to the data sheets that I have based this amp, idea, and design on….page 3 (dependent on your screen resolution size). It is the page showing the data as: AB1, 750 volts plate, 300 volts screen [grid 2] -35 volts grid 1, 12,000 ohms plate-to-plate and at this 72 watts RMS for the pair of tubes:

I hope you’ve all enjoyed the photos and article as much as I have in designing and building her…….thus far.

As I always say, many thanks indeed for looking, and peace and good-health to you all.


Click HERE to view Part 1 of the Champ CBA-20807 Project