Cap and resistor in series

[benoit]

I see this all the time in amp circuits. What exactly does it do. I want to know both what purpose it serves sonically and how the components interact with each other and the rest of the circuit electrically.

[Bob-I]

Once you read the ax84 theory of operations document, and study it for awhile, you'll have the answers to many of these questions. Here's another good source of info. http://aikenamps.com/TechInfo_2.htm

The cap that connected to the plate of a preamp tube is used to prevent DC voltage from passing through. A cap will block DC and allow AC through. The resistor that follows may have other purposes, it may be part of a voltage divider, or it may be used to couple with Miller capacitance of the tube to block Radio Frequencies (RF).

[benoit]

arg... I meant parallel.. I typed that at 3 in the morning.. but your answer does help in making sense of it all. Thanks.

[Bob-I]

arg... I meant parallel.. I typed that at 3 in the morning.. but your answer does help in making sense of it all. Thanks.

Again, there's several purposes.

In the case of resistors in series with the signal flow, these can couple with Miller capacitance of the tube, blocking some high frequencies. The small cap in parallel will allow some high frequencies to pass.

In the case of cathode resistors (designated Rk) the cap adds gain and low frequencies.

In the case of the plate load resistor, the resistor from the power supply to the plate of a preamp tube (designated Rp) the small cap in parallel is often called a "snubber" and used to remove extreme high frequencies to prevent oscillation. Dumble placed these caps in parallel with the tube itself by placing the snubber from plate to cathode. He used it to tune out the harsh highs.

[Markusv]

What would typical snubber cap sizes be?
Do they differ if you put them across a plate resistor instead of over the tube from plate to cathode?

Any insights much appreciated

Regards,
Markus V

[jimipage]

What would typical snubber cap sizes be?
Do they differ if you put them across a plate resistor instead of over the tube from plate to cathode?

Any insights much appreciated

Regards,
Markus V

I'm curious to know the answers to these questions, too. Anybody?

[mhuss]

A cap can be used like this also to reduce HF response in the audible range, e.g., to reduce harshness.

A cap in parallel with the plate load resistor works by decreasing the plate load at higher frequencies, which in turn reduces the stage gain only at those higher frequencies.

A cap in parallel with the tube works the same way, but uses "real" ground to shunt the HF (vs. the "AC ground" of the B+ power supply).

Values for these caps are generally in the pF range, something like 50 to 500p.

--mark

[jimipage]

thanks, Mark!

[drz400]

Like they all said but he is some more info

159,200 / RxC(in uf) Gives you the 3db point
You figure out if it a low pass or Hi pass circuit
First order filters are 6db per octave after the 3db point so
In an IDEAL and I stress IDEAL world (which it never) when the driving impedance is very low or at least 10x lower than your RC circuit and the Load is 10x higher, then lets say a high pass 1M with a .001uf across it will be 159hz boost allows the highs to start coming through more than the voltage divider by 3db at that point, then 320hz add 6db so you have 9db and 640hz you have 15db, so basically this will put the focus on the mids and above since 9 and 15db is when you really start to hear it.
Again the impedances around the circuit can change everything and for this you need to do some studying.

On a cathode you can use the same formula but have to account for cathode impedance (which is not covered in LonPower book). But basically across a 2k resistor a 1uf is going to boost upper mids and high end, a 22uf will boost everything as compared to no cap, no cap is sweeter spongy less gain and has more negative feedback.

Best thing you can do for yourself besides using the ears is to get a good signal generator like a Loftech TS1 and a Scope. The Loftech will read your signal and tell you db so you can compare any db at any frequency.
Look at each stage to see what is going on. Aiken has some great info on his site as well. But hands on is the best way to see what happens.

I like snubbers across plates cause you are not making the tube work at those frequencies. a 100pf across a 100K is just going to help reduce noise, a .001 is going to reduce high end for sure. Also scaling is easy
1/2 capacitance and double resistance = the same freq so a .001 across a 100k hits the same freq as a 500pf across a 200K (In an ideal world where there are no outside influences)

[David Root]

Thanks drz400. I have two questions related to your post.
1) While I understand the basics of RC circuits and input/output impedance relationships, I need to deepen my knowledge on overall impedance/RC relationships between stages and other connected sub-circuits, ie the "real world" as you put it. This has particularly made itself obvious to me in my first four-stage preamp.

What sources would you recommend I "do some studying" on?

2) Plate snubbers--I have not yet used these and I have heard some opinions that they adversely affect tone, especially in hi-gain architecture. However, I see them both in exactly such Dumble and Soldano designs. Is their use necessarily a compromise between best tone and best stability with respect to oscillation suppression?

[drz400]

Thanks drz400. I have two questions related to your post.
1) I need to deepen my knowledge on overall impedance/RC relationships between stages and other connected sub-circuits, ie the "real world" as you put it.
2) Plate snubbers--I have not yet used these and I have heard some opinions that they adversely affect tone, especially in hi-gain architecture. However, I see them both in exactly such Dumble and Soldano designs. Is their use necessarily a compromise between best tone and best stability with respect to oscillation suppression?

1) Impedance is something that takes a while to understand, here is something I found http://www.kpsec.freeuk.com/imped.htm and I would strongly recommend picking up the London power books, 99% of it is great information even though you might find different results. Also for more complex reading the Audio Cyclopedia http://cgi.ebay.com/Tremaine-Audio-Cycl ... dZViewItem or Radiotron Designers handbook. http://cgi.ebay.com/Radiotron-Designers ... dZViewItem
2) Nothing wrong with them if they are not abused. I think it is a great way to trim high end and to my ears sounds much better than running a cap to ground. I would never use them to suppress oscillation but only to reduce hiss and tone shape, the amp should be stable with no supression IMO. Stability is in the layout.

[David Root]

Fast response! Thanx! Must be a three day weekend. I have 4 of the 5 TUT books and Radiotron Handbook 4th Edn. I have heard of the audio encyclopedia but not seen it, and I'll check out the other link too.

Snubbers--thanx too, I'll bear that in mind. Makes sense to fix the fundamental issue rather than band-aid it.