Originally Posted by keabler
It's not that I have no idea what is going on I am just a bit confused as how to start.
So is the VAC from the transformer the peak voltage (which makes VRMS ~ 177V)? Is multiplying the VRMS signal from the transformer by sqrt(2) correct to calculate the DC voltage from the rectifier the correct calculation? I don't think I'm looking at this circuit right because my numbers don't come out.
I do have a more-than-beginners- knowledge of electronics and I am in E&M right now so the theory isn't too heavy either. I don't think I am starting the calculations right.
The documentation is somewhat confusing between figure 12 and figure 13.
figure 12 is using an "original style" transformer arrangement, with one transformer feeding the other. NOTE! The second transformer is marked as a 24:230v unit BUT it is being fed with only 18v from the first transformer! This is important. (230v/24v)*18v=172.5v
Before the rest of this, FrankCooter who I respect greatly once said something like this:
"Once you build the circuit, a well stocked junk-box is worth more than any calculation or sim"
You can plug away at the calculator until you know every voltage and current to a freakishly tight tolerance. Even after that you will sometimes find that your junk just dosnt work the way the simulation said it would... Do your best with a pencil and paper (or whatever software tools you have) but be prepared to tweak things on the fly.
Here is how I would go about "solving" this.
The voltages around the audio part of the circuit are given in figure 4. Using ohms law you can calculate the currents through the output stage, and the input stage. Add them to get the total curent for 1 channel, and double that for a stereo amp. Using this method I get 26mA, VERY close to what stixx said. Good memory, and with real tubes its sooooooo much work to get things within this margin of error.
I would then go through the power supply like you did with 26-27ma, and see what you get.
You are correct in how you got the first 350V, but as mentioned you used 50mA where you should have used 26-27mA. Even accounting for a reasonable DCR for the choke the numbers dont add up! This could be due to a number of reasons: its possible that the mains voltage where the designer lives is always low. Its possible his transformer sagged a LOT under load. Its also possible that he used an (mains):220v or 240v transformer rather than mains:250. Mains:220 or 240v transformers are QUITE common and priced very affordably, where 250v is harder to find except for stuff designed specifically for tubes. the schematic also indicates that the voltage on the first cap is 320V, which is somewhere between what you get with a 220 or 240v secondary... As a last factor the schematic indicates that the B+ and heater transformers are separate units, which improves my confidence in the assumption that he actually used a mains:220 or mains:240v transformer.
If you recalculate starting with a 240v transformer, I think you will get much closer to 220V at the output. a 230v transformer is actually about perfect. If you can find a choke with a DCR of about 1Kohms the numbers line up realllly well. The Triad magnetics C-1x for example: 15H, 20ma, 1Kohm. I'd redesign around a choke with a 30mA current rating, but eventually things make sense.
Now, if you are using a transformer with a "real" 250v secondary you will need to redesign the whole thing :)
Edited by nikongod - 3/20/11 at 5:43pm