Describe your needs and they might make something custom that's both good and cheap. But it'll probably cost way more than $100. 6SN7 Mu Follower for Comparison This one does well, only 3 db down at 80 KHz driving the 30 feet of COAX. He'll wind something custom that'll be exactly what you need and be utterly magnificent. If that price doesn't scare you, then go see Dave Slagle at Intact Audio. Unfortunately, it costs something over $100. The Lundahl LL2734 is meant for SS outputs, but I've known people to use it with tubes. The good news is that a 4:1 step down is a lot easier to get right than the much larger step down of a typical output transformer. 6CG7 (9 pin) or 6SN7 (octal) and 6N2p are candidates and I can borrow a. Since your output stage is SE you need a transformer that can handle DC idle current. The second valve is a cathode follower of a particular flavour so its gain is. In your case, to drive the 2.5k ohm load at, say 2.5Vrms only requires 1mA, so a 6V6 should have no trouble delivering even if you do jack it up with NFB by using pentode mode.Īs far as a step down transformer goes, finding a good one is the whole problem. The whole circuit falls flat on its face. The common cathode circuit has plenty of voltage gain, some current gain, and high output impedance. How could it possibly deliver 2mA? The answer is that it can't. The cathode follower (officially known as a common anode circuit), like the common cathode, takes its signal at the grid, but the output is taken at the cathode, not the anode/plate. But the poor little 12AX7 is only idling at something like 1mA. The AC signal current through the resistor will be i = v/R = 25/12000 = 0.002 amps = 2mA. But what happens if you try? Let's say you use it to out 24Vrms signal across a 12000 ohm resistor. So, using the 10x rule of thumb, it should have no problem driving a load of 10x1200 = 12000 ohms. As you pointed out earlier, it has an output impedance of something like 1200 ohms. To see when it might be a problem consider a 12AX7 cathode follower. The lowered output impedance is mostly due to negative feedback, so it's only useful as long as you don't ask for more driving current than the tube is capable of, but that shouldn't be a problem in your application The output impedance of a cathode follower is roughly 1/gm, and gm is WAY higher in pentode mode than triode mode so connecting the screen to the cathode through a capacitor (really, the R and C combination) will result in a much lower output impedance. (Look at pentode input stages to see what I'm talking about. The RC time constant needs to be big enough to decouple at low frequencies. The simplest and most common way to do that is to use a larger resistor between B+ and screen and a capacitor between screen and cathode. Im curious if theres any evidence that the grid of the follower seeing full B+ at startup is actually a problem. To make a pentode cathode follower you need to make the screen to cathode voltage constant even in the presence of signal. ![]() In either case, this is a triode connection, which I'm not saying is either good or bad. It will provide a very small amount of negative feedback, but not really enough to think about. I don't see the 100 ohm resistor changing the behavior much.
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