|Author:||Henry Ramsey (registered user: 3015 posts )|
|Date:||Thu, Apr 19th, 2012 @ 18:38 ( . )|
On 04/19/2012 @ 15:12, Merlin wrote :RATS! I'm going to have to find a bigger PT if I'm going to continue with the plan of two preamp tubes and the two 12k5s. NOT Good!
On 04/18/2012 @ 19:27, Henry Ramsey wrote :
: Isn't there a rule of thumb to not exceed 80% of a PT ampacity?
: Typical power factor for a capacitor input filter is 0.65, so if a transformer is rated for 2Arms then it will probably reach its limit when you suck
: 2 * 0.65 = 1.3A rectified DC.
That explains why it was humming so loudly then.
At full power last night it was drawing close to 1.7A I think; 900mA for the 2x12k5 heaters, 600mA for two 12a_7 6V heaters through the regulator(s) and up to another 85mA for each SC grid and 8mA for the plates of the 12k5s. I think these are max numbers at idle, but that's up to 186mA more.
The preamp plates were unconnected. Looks like this is not going to work at all w/ this PT. By the power factor I'd need at least a 3A PT.
Maybe I'll pull one 12k5. They're parallel SE so I can pull one and save over 600mA which should be enough for the extra preamp tube. I'm not sure what it'll do to output sound. In my past experience using a single 12k5 all I could ever get was harsh, nasty, ugly distortion.
The tube was biasing near or at cutoff and changing the cathode resistor up or down didn't solve it. It was also drawing huge SC grid current.
Two in parallel seems to have enough equilibrium to not have that issue.
Radio shack used to sell a 25.2VCT@3A PT, but I guess they reduced it to 2A for some reason. :-(
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