845 is a very attractive tube! It have the clear, air and transperence as 300B, but more linear and natural like 2A3. Just lost a bit detail and smooth but you it is easier to match with your speaker. Have you feel the power of 845? When you love to listen any classic overtune or drum present, you will love 845 (or 211 as mine).
845 is one of the most difficult tube. It not only their high plate voltage. Also it need high voltage swing with low output impedence driver. Many branded product use 300B to drive it. But it rise another problem. Since 300B also hard to drive. It don't need very high swing, but also need low output impedence driver.
To simplify and lower the difficulty, we need to add a step up interstage transformer. That IT must be very high quality. I suggest Tango NC16. 1:2 IT can increase the voltage gain but still very low in the DC resistance. Even it is not cheap, but no-way! (Tell me if you got another idea)
845 cheap? Yes on the new production. RCA, Amperex NOS cost more than HK$3000 (US$380) and very rare! So forgot them. New production of China 845 still sound excellent. Many branded product using them with great result.
To help non-expert DIYer, I draft a simple circuit for the 845 Single Ended and hope you can comments on it.
I think 750V is much easier than 1000V. Also, 5k Output is much cheaper and smaller than 10k. 15W power higher than 300B, the headroom of it is much much higher. So you a 300B cannot get the same power feeling.
I think the circuit is simple enough. But the Power Supply unit? Later on, I will follow up!!
Welcome for any comments