dsavitsk said:
Yes the project has changed substantially. The circuit is as follows:
The input stage is a parafeed E182CC running at 15mA rc coupled to a conventional output stage using Magnaquest "Robinhood" 5K output transformers. The operating points for the output stage are 210v B+ at the plate with a 820R cathode resistor biasing the grid to approx 22.5v with 27mA through the 12B4s. The output stage will be bypassed with a 100uF cap. The coupling caps are 1uF and I will be using 200R grid stoppers on the E182CC and 1K grid stoppers on both of the grid pins on the 12B4s. All of this will be powered with a regulated B+ supply using all film caps. Any suggestions?
G said:
Well ... I hesitate to ask this, but what good is the 12B4 doing? Why not load the E182CC with the transformer, run it at 35mA, and call it a spud. Eliminates lots of extra stuff, allows the B+ to run lower such that you can get away with a $12 120:120 isolation power transformer. Maybe you'll lose too much output power doing that, but if it were me I'd breadboard it to see.
Edit: I'm not sure that that's a particularly linear point on the driver. Used as a driver, I'd use a CCS to back off the voltage a bit and bias it in the 3 to 4V range.
dsavitsk said:
Because I have 12B4As that I want to use and I am wallowing in my own ignorance?
Seriously, I can't give you a good reason other than I'm not comfortable running my "expensive" E182CCs that hot and also I didn't think of it.
I do have a whole slew of 5687s that I can try that with using some Edcor iron in the future. Sounds like a good idea.
See my edit in the last post.
Also, is the preamp built? There's a lot of gain here. Maybe just drive the 12b4 with the 396a and skip the preamp? That 100H choke is about the same load as the 15K transformer, so you should be OK, or at least as OK as you already are. A 6dj8 or a 6N6p (or your 5687's) might be a little better as a driver (lower rp), but also probably not as cheap.
Also, is the preamp built? There's a lot of gain here. Maybe just drive the 12b4 with the 396a and skip the preamp? That 100H choke is about the same load as the 15K transformer, so you should be OK, or at least as OK as you already are. A 6dj8 or a 6N6p (or your 5687's) might be a little better as a driver (lower rp), but also probably not as cheap.
dsavitsk said:
The preamp is not built but if you will notice it uses a 15K:600 output transformer. Shouldn't give more than a gain of 10 or so. I prefer a E182CC to a 5687 as a driver. I like the sound better. They are essentially the same tube though along with a 7044. I have gobs of 5687s but not so many E182CCs and even fewer 7044s. I thought that I designed this well. Evidently I didn't. I think the output stage will clip well before the input stage does and I should be able to get a clean 1.25 watts out of the amp. More than enough in my small listening area.
G said:
I wouldn't listen too much to me. I was just saying what I would do, which is generally different form what most others would do
Other than a lot of gain and a bunch of stages, the only real issue is that while the e182cc won't necessarilly clip, it is not a particularly linear part of the curves. It doesn't get too bad until you put 3 or 4 volts into it, so maybe you'll never notice it. Oh, and I have some nickel core 15K B7's, and they did not do well for me with a highish (6K) rp tube. They are fine with some C3g's. It could have been the rest of the circuit, so your experience may be different.
dsavitsk said:
I wouldn't listen too much to me. I was just saying what I would do, which is generally different form what most others would do
I think you are confusing a E182CC with another tube. The E182CC/7119 has a pretty low rp. Say somewhere in the 1.2K range. I use these in circuits designed for 5687s which is most certainly not a high rp tube.
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