Capacitor value between preamp "stages"

[martin manning]

Ignoring the bright cap and the Miller capacitance of the second stage, the output impedance of the first stage goes from 39k with the cathode resistor bypassed to 69k when it is unbypassed. The shift is frequency dependent, and is determined by the value of the cathode bypass cap, which is 1u. The mid point of this transition occurs around 125Hz, where it's partially bypassed, and this happens to be pretty close to the point where the response of the first stage is at -3dB from its peak.

There's a really nice spreadsheet posted here that will do all this stuff for you:
https://tubeamparchive.com/viewtopic.ph ... highlight=

But, working through it is a great way to learn.

MPM

[vibratoking]

I can't find any Excel file attachement. There are notes pointing to other posts, but there is never any file when I follow the links.

[surfsup]

martin, thanks for confirming the 69k. I also downloaded your XLS. It is fantastic! Its a bit much for me right now as I am still learning a lot, but I can see what you're doing. My spreadsheet has 7 worksheets to calculate the values after inputting them from load lines (graphically getting the numbers) and going from there - with reference JPGs to remind me what I'm supposed to do on each worksheet and I think I still need those reminders but one day soon I am sure I will be upgrading to your XLS because yours seems to do it all automatically, including the load line graphs, transconductance, gain, etc. Sweet and thanks for this!

A question:

On row 48, you have the Anode Characteristics data all in yellow, but it is also on the worksheet called "device library". If I click on the fields the hard numbers are listed in each cell, but if I click on the tube type button on top right fo sheet, the numbers are all updated to what's on the "device library" sheet. How did you do that?

EDIT, never mind I see you wrote a VB module to do that...gonna check out that code now.

[surfsup]

I can't find any Excel file attachement.

Its there, scroll down the first page to the 4th from last threaded posting. Or try this link...

https://tubeamparchive.com/download/file.php?id=8099

[FunkyE9th]

To see the link to the spreadsheet you have to be logged in.

There are also some nice calculators here... http://ampbooks.com/home/amplifier-calc ... capacitor/ His books are also very good.

[C Moore]

hey hired hand, if you're going to go through the PDF, I found an error already (well, more an inconsistency).

The schematic I posted is the most recent at AX84.com. It shows R18 as Ra=100k

The load line in the PDF Ken posted is using R18=120k which is confirmed in Fig. 15 on page 16, where the schematic is actually DIFFERENT than the one I posted (those sneaky little ...)! Took me a minute to figure that out (but I did figure it out!). Here's what I'm talking about:

[img:496:367]http://chicagocadcam.com/ChrisHahn/stage2.png[/img]

I think it has obviously changed since this PDF was first written. The tone stack resistors and pot values are also different. i hope that helps!

BTW, my math I posted earlier for everything is right (for the first triode), so if you wanna compare notes I doublechecked it through page 14 already. (I have already built a spreadsheet to automate all the calculations based on a few inputs from me , so it only took a few seconds)

Thanks A Lot -
I need to read your posts a bit deeper, I may have a question or two. In fact, I have a question right now.....?
I was just on the AX84 sight (logged in if that matters) and I cannot find the section that has the info Ken posted. Can you tell me where to look for that stuff on the AX84 websight...?
Thanks So Much

[Ken Moon]

Go to AX84.com (not the forum, just the ax84.com website), and look under Classic Amp Projects, then under P1-eXtreme, and you'll find all the P1-eX stuff.

[Dr-Joned]

Surfsup,

If you are looking for a good book on math, MEGA math, check out this book. It will give you a very analytical and concise breakdown of the 5F6A circuit.

http://ampbooks.com/home/books/bassman/

[C Moore]

Go to AX84.com (not the forum, just the ax84.com website), and look under Classic Amp Projects, then under P1-eXtreme, and you'll find all the P1-eX stuff.

Thank You......

[surfsup]

FYI, There's another error on pg.18 at the bottom where the author lists the variables in the equation. The Rp listing should be RL because RL is used in the equation.