mikepukmel 5/15/2018 8:49 PM
JTM 45 build 400v plate voltage, how to calculate OT primary impedance?
Hi All,
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?
Thanks!
Mike
 
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:
https://drtube.com/datasheets/kt66-osram.pdf
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?
No, all the 50W EL34 versions came after the KT66 versions and used Drake OTs (types 784-128 and 784-139) with Raa of 3.2K, which is just fine and provides max. output power with EL34s.

I read someplace that the early JTM45's had Radiospheres OT and KT66, then they switched transformers and output tubes as well.
The early RadioSpares (RS) was a multi-tap general purpose (DIY) OT. There is some good information in Michael Doyle's book "The History of Marshall". The KT66 datasheet recommends Raa=8K for cathode bias operation only. I have no idea why Marshall used 8K with fixed bias and actually don't know if the Radiospares OTs were wired for 6.6k or 8k. Undistorted (THD=10%) output power of originals with 8k was around 30W or lower depending on actual voltages and KT66 quality.