Trying to pickup a bit more tube theory...

[Tonegeek]

I could use some help trying to understand the relationship between rk and rp. Specifically why do you usually change both of them rather than just say changing the plate resistor? My guess is it is to keep the frequency response the same for that tube while changing the gain. Guess this would be most important for tubes with cathodes bypassed by a cap. So a practical question would be what would happen in terms of gain and frequency if I change a plate resistor from 200k to say 150k (without changing anything else)?

I have read what Aiken says about tubes, but still have trouble with the mental picture.

thanks

[mhuss]

I'm guessing you're talking about a normal common cathode stage.

In a *normal* amplifying stage, you want maximum clean signal and symmetric clipping. To do this, you need to set the idle plate voltage to a certain point. This depends on both the idle current and the plate resistor.

In the CC case, the cathode resistor primarily sets the bias for the tube. So if you have a certain bias/idle current set, you need to select an appropriate plate resistor to match.

--mark

[gearhead]

Been down part of this path myself. Check out my q, PRRs reply and my reply on this thread:
https://tubeamparchive.com/viewtopic.php?t=3219&start=0

Don't know about frequency response due to plate resistor changes.

There is clipping due to both amplitude distortion and something called saturation (plate). Download a copy of NEETS module 6, and look at pages 1-27 thru 1-29.

NEETS download: http://www.phy.davidson.edu/instrumentation/NEETS.htm

[d2camero]

An excellent article, simple to understand, on exactly your question can be found here:

http://www.audioxpress.com/resource/aud ... /index.htm

Select "Designing Your Own Amplifier Pt. 1: Voltage Amplifier Stages" tells you why and how you select different plate resistors and cathode resistors.

[Tonegeek]

Thanks for the holiday reading list! I briefly glanced at the Audio Express material and it looks like it will be very helpful. I will look at NEETS also.
cheers!

[Structo]

For some down to earth knowledge about tube amps I enjoyed Gerald Weber's books specifically Tube Guitar Amp Essentials.
They are pretty much geared toward Fender circuits but he doesn't go overboard with technical theory. Just kind of practical stuff.
I know many here are way above this kind of thing but you have to start somewhere. If you have basic knowledge of electron and component theory these books can help bridge the gap between that and tubes.
I am nowhere near the design level but I can build an amp with confidence with knowledge gleaned from these books as well as a couple others. Plus with the help of the internet with this forum being a great place to learn.

[sebastian]

Talking of common cathode 12ax7 preamp stage:

I can set Rp and Rk to set the voltage 1/2 of B+ of the stage,to make a simmetric amplifier stage.I can set the resistors also unbalanced,if my voltage swing don't clip each "rail" of the B+

But,I noticed some big differences in TONE of stages polarized cold (example 220k plate resistance with 2.2k cathode resistance) and stages polarized more "hot" (ex. 100k plate and 1k cathode resistors).The bias through the class A - CC amplifier stage of a 12ax7,actually change the sound and feel !!! (without any bypass caps across Rk)

Example,If I take a 1987 marshall amp and change the resistors of the last stage with a 220k Rp and 1,8k Rk ,instead of 100k/820R ,the sound is more midrangey with less bass and more crunch,than the "big and fat" sound of the stock values.This mod change only bias through the stage (and the output impedance but the stage is cc coupled with a cathode follower) ,the gain remain similar.

[tele_player]

Seems to me... that the typical 12ax7 gain stage doesn't clip by hitting the rail. Its clipping behavior is determined by the bias voltage and the swing of the input signal.

[sebastian]

Yes I know,but the "coloration" of the audio signal is also with low signal levels