This is the continuation of the original Exodus amplifier. In conjunction with my new AER drivers in the Oris 150 horns I needed to have my Exodus output transformers rewound. The previous PM4A units were 8 ohm speakers, the AER are now 14 ohms. Another reason to have them rewound was the switch in output tube. The UX210 has been moved up from driver to output tube. It delivers a whopping 1.2W of output power at the settings I am using. A bonus is that the VT25, VT25A, 10Y and 801A can be dropped in without modifying anything in the circuit. This will allow me to spare those ancient UX210/CX310 beauties when playing music just for background noise.
The driver is the 71A, a very nice power tube by itself. It has a low gain meaning the amp will have a rather low input sensitivity. Seeing that the 10 is running at around -30V this means I don't really need a lot of swing anyway. The preamp delivers about 10-15V of swing, that leaves me with more than enough input signal. Give or take a volt or two... The miller capacity on the 10 is at it's worst at 30kHz and 25V swing. It requires 1.8mA of drive here. The 71 will have no problems at all with this load, this is one of the reasons I chose the 71A as driver.
I was thinking about using the 12A here as it has more gain (8), but at 6mA bias I don't like this as a driver that much for the 10 in my case. The 10 can go into A2 a little requiring 2-3mA into the grid, so a bit more current is a nice thing to have. The driver is IT coupled to the 10 by a 1:1 ratio, this is the nicest ratio for getting a broad bandwidth (specially in the highs). All iron was made with a low frequency cut-off of 100Hz in mind.
Why change from the 45 people will wonder. I love the old 45 tubes, but the tests with the RS241 and 10Y (not much in coming between these two tubes, granted) left a very nice impression lingering in the back of my mind. The 10 sounds very crisp and delicate in the highs, voices are very focused. The 45 beats the 10 in regards to bass and mid-lows. It is a compromise. Maybe I am just getting older?
The 71 as power tube was a bit muddy in the highs and lacked the extension that the 45 had. But that was as output tube where it had to do some work. As driver the 71 has very little work to do as the 10 will only ask around 2-3mA at a worst case situation swinging 30V into the grid at highs frequencies (30kHz). A piece of cake, a walk in the park. The 10 can even go into A2 a bit, giving around 1.5W of output before running out of current. The 71 is a good tube here, it can swing the needed voltage and also handle the change in grid impedance of the 10.
The preamp will be expanded by a cross-over section to filter the signal going to my bass amps. In most of my projects I have built amps solely for the mid/high horn and used a filter on the output to drive the bass cabinets through a separate power amplifier. This time I will integrate the filter into the preamp section. This means two sections per channel will be added (input and output buffered). Having said this, the actual frequency range of the 26 (preamp), 71A and 10 (poweramp) will be limited to what the horn can handle; from around 80Hz and up. This relieves all three section of the bass reproduction since the signal is tapped from the input and handled by the cross-over now. What this means is that the loads used on the three tubes can be chosen lower (induction) resulting in few windings and possibly smaller cores for the transformers. Less is more is also true with transformers. I choose inductions so that the roll-off should start at around 50Hz.
The 10 will be loaded by a 30K load. "What, 're you nuts?!"... I can already hear people saying this :)) After plotting some load lines for the 10, taking into account the power I actually require and what the 10 can do into positive grid swing, I find that this load will give excellent linearity plus all the swing I could desire. Only prerequisite is that the driver is up to the task of delivering grid current to the 10. Using the 71A with IT loading will allow me to do just that. The curves point out that with the -30V bias I can swing the grid all the way to -80V and +20V before running into "problems". That's at least 50V of grid swing!!! Entering that into some calculations shows I get about 1.8W of output. That is with grid current of course... Normal A1 operations will deliver around 0.7W.
© Jim de Kort