|mikepukmel||5/15/2018 8:49 PM|
|JTM 45 build 400v plate voltage, how to calculate OT primary impedance?|
I asked a similar question before, the suggestion was to learn the graphical technique. I have Merlin's book, but not sure what Im looking for (what part of the graph). The Amp should have around 400v plate voltage, with KT66 tubes. The output transformer has 9k, 8k and 6.6K primary taps.
What part of the graph would tell me the target OT primary impedance?
|Helmholtz||5/16/2018 8:59 AM|
|I don't have Merlin's book but I guess it explains graphical loadline construction in PP amps. The primary impedance (Raa, Zaa) corresponds to 4 times the inverse slope of the optimal loadline. |
A quick check shows that the 6.6k tap should be closest. Higher Raa will reduce output power and increase screen losses.
This old OSRAM datasheet gives some examples:
You want to look for the fixed bias configuration.
This said, original JTM45 with Drake OTs used 8K. The earlier RS OTs had 6.6K and 8k taps.
|mikepukmel||5/18/2018 4:37 AM|
|Thanks Helmholtz, yes its the graphical load line technique. 6.6k, thanks! I tried picking through the data sheet but a common problem I have is finding definitions for the symbols. |
Also, thanks for the link as well..
Is the switch to 6.6K and 8K something to do with switching to EL34? I read someplace that the early JTM45's had Radiospheres OT and KT66, then they switched transformers and output tubes as well.
|bob p||5/18/2018 4:52 AM|
|Maybe Steve Bench's tube pages will help. |
Part 3 - Of Loadlines, Power Output, and Distortion
|mikepukmel||5/18/2018 5:10 AM|
|Thanks for the link, Bob. Reading ...|
Another question, probably should have asked before: The graphical techniques draw lines on a graph that already has the tubes characteristic family curves drawn. Where can we get good quality images with these curves? I checked the data sheets and they usually don't have graphs as detailed as the ones in the example, like the "Part 3" web page.
|Helmholtz||5/18/2018 6:46 AM|
Is the switch to 6.6K and 8K something to do with switching to EL34?
I read someplace that the early JTM45's had Radiospheres OT and KT66, then they switched transformers and output tubes as well.