I REALLY ENJOYED THIS LITTLE CHALLENGE!!
This Champ CBA-225 really started off life as a Carlsbro 200/7 PA Amp using 4 x KT88’s (please see photos below). However the project was originally inspired by yet another simply “out of the blue” phone call; like the very many I receive regarding my web-site!
This initial call came from a nice gentleman down in Wales, UK, called Garth. Garth had rung me with the story of how he had built many tube amplifiers back in his younger days during the sixties, but had not done anything for many years. However, he still retained the knowledge to do so, and he had recently won a Carlsbro 200/7 PA amp on E-bay. He further explained that he had sold the 4 x GEC KT88’s for more money than he had paid for the amp itself (not surprisingly), and was still left with the chassis, all the components and the Partridge transformers. He was wondering if there was anything he could do with these components, but (a) he didn’t want to make another PA amp (a general guitar amp or slave-amp would be fine), and (b) he also fancied using multiples of 807 tubes as he had rather a “soft spot” for this type of tube from the “old days”. He also had plenty of 807s lying around too!
After taking Garth’s phone number and telling him to “leave it with me”, a little light went on in my head...........i.e the fact that this pair of Partridge 200 watt transformers could be suitable for an octet of 807’s After checking the 807 data sheets to refresh my memory, my suspicions were proved to be correct!
On all the Carlsbro 200 watt amps, the four KT88’s were run with what I would class as a “stacked HT supply”. This was done with one HT of around 400 volts which fed the screen grids (grid two’s) of the four KT88s + the pre-amp/driver stages of the amps. (Please see this Carlsbro schematic). Then, stacked on top of this was a further 320 volts to give a total of 720 volts on the output tubes’ plates (anodes). Carlsbro seem to be the only company to run their 200 watt, 4 x KT88 amps like this. All the other brands appear to have used the original data specifications from GEC by using a single HT line of around 550 volts, ultra-linear too, for the 200 watts from a quad.
Nevertheless, with this Carlsbro “stacked-type" HT line it was also possible to reverse this”! i.e, 320 volts for the bottom half of the supply and then stacking on top the 400 volt rail still gives us around 720 volts for the output tube plates! This is almost perfect for 807’s. They are quite happy with around 300 volts on the screens (grid two) and around 600 – 750 on the plates/anodes. Please have a look at these data sheets - Page No 3.
As can be seen from these RCA data sheets, we should be able to get around 72 watts from a single pair of 807’s with around 750 volts on the plates and 300 volts on the screens (grid two’s) but, what is critical here is that these voltages would need to be stabilized at these values. This we wouldn’t be able to achieve with the Partridge transformer windings, so the lesser output of about 56 watts (per pair) would likely be more realistic with around 290 volts on the screens (grid two’s) and around 650 volts on the plates (anode’s) both under “loaded conditions” with this power transformer. I would have expected around the 50+ watts from a pair, basically still 200 watts or a bit more from eight with these voltages. Although not really being any more powerful than the original 4 x KT88s, at least 807s, having been around since the late thirties and still used by the military of today, are out there in abundance and even still manufactured these days. This really makes them a good replacement, and a cheaper choice of readily available bottles!
As the plate-to-plate load for a pair of 807s used on these average voltages of 300 volts screen and 600 volts plate (or up-to 750 volts plate) is between 10,000 – 12,000 ohms respectively. The Carlsbro (Partridge) output transformer (part number TH1499) has a plate-to-plate load of 2,250 ohms. This is just about perfect for eight 807 bottles, being just a little out but not enough to make any substantial difference.
The Partridge power transformer (part number TH1498) used by Carlsbro on their 200 watt amps has an 8 amp heater winding. With 807s only being a 0.9 amp heater (as opposed to the KT88 being 1.6 amps), eight of them along with three or four pre-amp/driver tubes at 0.3 amp each just falls into the “OK bracket” here. Similarly, the main HT windings on this transformer are rated at 650 m/a. Checking data sheets on the KT88 for a refresh/confirmation shows for four tubes (screen and plates combined) that the total currant at 200 watts would be around 630 m/a. Eight 807’s at around 56 watts for a pair would in-fact for the eight (also plates and screens combined) be around 620 m/a. This is slightly less but nevertheless is perfect for the job! The bias winding on these transformers is 300 m/a so as usual, I did a series heaters DC supply for the four pre-amp and driver stage tubes. This is always beneficial!
OK, so having checked and sorted all this out for Garth, I was so encouraged and enthusiastic about this project that, as I also had a few old Carlsbro 200 watt chassis and Partridge transformers laying around that I had stripped down, I decided to do this project in parallel with Garth. I could therefore supply him with all the necessary information and results that I obtained as my amp progressed, and this would help him end up with a “good working project!”.
I was so confident with this project that I decided to build it not as a “prototype” but……..as a first finished “production model" which was likely to sound superb on guitar (especially bass!!).
This being the case, and after cutting all the now necessary new holes, I had the chassis powder-coated in dark grey "Hammerite”. I also persuaded Garth to have his rather scruffy looking chassis to be powder-coated too, which I believe he was later very pleased that I had done so!
There will be some of you out there who will think that dismantling a 200 watt Partridge output transformer is sacrilege. However, I was not uncomfortable with doing this as I already have all the winding details to copy most of the Partridge guitar range of transformers. I did in-fact do this to mine for two reasons. Firstly, the 100 volt-line was not needed so I added a 2 ohm facility here instead. My output transformer now has 2 / 4 / 8 and 16 ohm facilities……much more practical with today’s impedance requirements. Secondly whilst the transformer was stripped down I changed the plate-to-plate load to the correct 2,500 ohms (8 x 807’s @ 300 volts screen and 600 volts plate) rather than the original 2,250 ohms. That was no big deal whilst it was apart anyway. Also, I re-laminated it with today’s highest quality M6 type 248 laminations.
As the power transformer was pretty rusty, this got the “wire brush in the drill treatment” and whilst also managing to save both the “Partridge Transformers’” foil labels and re-uniting them on the pair of now “new looking” transformers, I also re-sprayed them both with clear lacquer.
Very early on in this project I had already decided to use my own-design circuitry. This included my “world beating” phase splitter and push-pull/cathode follower driver stages. However, we had just one problem. As we had now turned the original pair of HT supplies upside down, we only had about 320 volts on the lower half. This is far from enough for the driver stages. I then proceeded to make a separate, small power transformer to give me the 500 volts needed for the pre-amp and drive sections. I wound two of these baby transformers (one was for Garth’s project) and did them at 150 m/a. Yes, I know this is a bit of an “overkill” but, it is always a lot easier to wind transformers with thicker wire and……..there’s no better substitute than “plenty of headroom”! Also, by mounting this little transformer at the output transformer end of the amps chassis, the physical weight balance is now perfect too! This small transformer was wound with a single AC input of 240 volts and is simply wired across the original Partridge transformer's 240 volt taping. Any change now of the incoming input voltage selector would result in the main Partridge power transformer’s primary acting as an “auto transformer” for the added small one.
Once the driver stage was up and running, as always, it was brilliant. In-fact, without the feedback connected (think about it!) I can drive the eight 807’s into almost perfect square-wave, and yet the drive is still perfectly clean!
As all the Carlsbro amps had an aluminium “L-shaped” plate installed between the output tubes and the pre-amp/drive tubes, I simply cut this plate to house 2 x standard 3”, 12 volt, 100 m/a computer fans. Using the 6.3 volt heater supply through a full-wave bridge rectifier and a 4,700uf cap @ 10 volts gives us a perfect 7.5 volts DC (on-load) to run the 12 volt fans, as a gentle air movement through the amp, keeps everything cool! By the way, I had all the cap brackets, transformer “end clamps”, the fans plate, and all other ancillary metalwork powder-coated in “chrome”………. it looks wonderful!!
As the amp took shape, all became superb. The final result is 224 watts true RMS, and it will certainly go much more than this too if a degree of distortion is allowed.
Garth finished his amp around the same time as I finished mine. (Though it's not quite as neat, but he is quite happy at this). Both amps sound great! Any questions, please don’t hesitate to e-mail me.
Many thanks for looking. I do hope you enjoyed the project!
Footnote:- My next move will be to re-cover the old Carlsbro case with nice, modern looking charcoal-grey carpet, and to fit flip-out sprung handles together with professional looking nice front and back panels of course. I will add photos of the exercise to the site as the final touches take place. When I get time to draw the schematic, that will also be added.
The initial idea.
Completed component boards, checking everything for a good fit.
Same again, checking for everything fitting OK.
The re-wound Partridge output transformer, ready to be re-laminated.
Things now starting to take shape.
"Showing off".......all the chromed metalwork!
Finished as far as a slave-amp would be........drive and output.........perfect!!
Close-up of fuse bank #1 and main rectifier board.
Underneath..........completely finished including pre-amp and tone sections.......looks beautiful!
More "chrome work" and pre-amp controls. (Please note that the "universal holes" in the old Carlsbro chassis will be covered by the new front panel....in time.)
The amp now completed. Cabinet work compliments of Bob Kingsbury Joinery.
Rear of the unit. Note the Powercon connector & the 2 ohm facility.
Showing the internal illumination. Looks cool.......but can be switched-off at the rear of the amp for those "stage black-outs!".
Garth decided to stop at the slave-amp section. Hence, no pre-amp stages or controls. Nevertheless he has allowed for the addition of the pre-amp in time, should he wish to do so.
Nice effort Garth, works and sounds as good as mine!! Congratulations........John.