My journey of building a tube amp (12AX7 and EL84 SE)

[Tubetec]

Looks like close to half the supply voltage is dropped across the output transformer primary ,

 

[zeh]

Looks like close to half the supply voltage is dropped across the output transformer primary , View attachment 120736

@Tubetec you mean for the screen grid? yeah, i have seen this several times already, i asked about it also since my OT has
a tapped primary, but its not marked as such so i dunno what its supposed to be used for if not for the screen grid...

 

[Tubetec]

No,
the anode supply is dropped by half through the transformer , normally you'd only see 10-15 volts drop across the primary .

 

[zeh]

@Tubetec iam sorry but i dont understand you mean here there should be less voltage loss? so there should be about 280V at the anode usually? my OT is dropping too much voltage?

where do you see 'half the supply voltage, or half anode voltage' ??

 

[Tubetec]

Ah ok that makes more sense ,
Id go for a cathode bias resistor of 270-330 ohms , you can also add in an extra stage of RC or CLC smoothing off the rectifier ,

 

[zeh]

Ah ok that makes more sense ,
Id go for a cathode bias resistor of 270-330 ohms , you can also add in an extra stage of RC or CLC smoothing off the rectifier ,

ok cool thanks, yeah i will do this filtering in the future probably, but there is no unwanted noise so i think its fine for now..

 

[zeh]

i decreased the bias resistor to 180 to drive the tube a bit hotter because it was suggested by another member, i will probably bring it down a bit again so the tube will last longer

 

[zeh]

@Tubetec ah, sorry for the confusion, there was a wrong screenshot where i changed values without rerunning the simulation resulting in wrong voltages displayed

 

[zeh]

hi there, could somebody explain to me the purpose of R16 here? is this a kind of feedback? thank you guys!
it connects the plate from the ouput tube with the plate of the second input stage

 

[rock soderstrom]

hi there, could somebody explain to me the purpose of R16 here? is this a kind of feedback?

Yes, this is negative feedback from plate to plate grid. For instance, the mic pre amp Gates SA70 use this technic, too. (here R4+R8 and C4)

 

[Matador]

plate to plate

Plate to grid?

 

[rock soderstrom]

Yes, plate to grid. Misspelling, I correct the original post.

 

[zeh]

cool, thank you a lot for the information! i never saw this thing before, only the 'standart kind' of feedback where the output transformer feeds back into the preamp, also i think i never came across this type of negative feedback in any book i read so far, any suggestions on what book i should read that covers this topic? thanks in advance, i really appreciate your help guys, iam a self tought person who tries to figure out stuff, thanks

 

[rock soderstrom]

suggestions on what book i should read that covers this topic?

RCA Radiotron Designers Handbook

Chapter 7.2

https://www.google.com/url?sa=t&sou...UQFnoECA0QAQ&usg=AOvVaw05jjuUwPe5A1UnQciQ-S90

 

[zeh]

@rock soderstrom yes man, 1 chapter only about negative feedback, thats exactly what i was looking for, thank you!