non-HRM Findings

[kleinm]

Gil, this is really excellent information. I appreciate you sharing this experiment with all of us. You saved me time and energy! I was considering going to a 100k/1k5 Rp/Rk setup in my non-HRM, but it sounds like the higher Rp setup is just right for me.

Thanks again, Gil!

[talbany]

I generated this up maybe this will help. based on 100k plates



Interesting Exercise -3db points for cathode resistors any bypass caps
With given Rk/Ck values the frequencies will be 3db down. Notice that the frequencies are not cut off. They are only 3db down or cut by half – an audible change.

So, with a typical 1.5 cathode resistor, any bypass cap value above 10uf is a waste. Or is it? Of course, this is pure math based on RC time constants. The real frequencies are dependent upon parts tolerance, tube capacitance, etc., etc. We know that electrolytics and poly, mylar, ceramic, etc caps of the “same” value sound different. Why?

Using the Capacitive Reactance formula alone you can find the band pass or “cut” frequencies.

F = 1/2piRC (pi = 3.1412 rounded)

1/6.2824 * 1500 (Ohms) * .00001 Farads (10uf) = 1/.094236 or 10.6 Hz

All frequencies (within the amp bandwidth) above 10 Hz will be passed and 10 Hz and below will be cut

12AX7 w/100K plate
Rk/Ck .68 uF 1 uF 2.2 uF 5 uF 10 uF 22 uF 33uF 47 uF
470 643 Hz 437 Hz 199 Hz 88 Hz 43 Hz 20 Hz 14 Hz 10 Hz
510 605 Hz 411 Hz 187 Hz 82 Hz 41 Hz 18 Hz 13 Hz 9 Hz
560 564 Hz 384 Hz 174 Hz 77 Hz 38 Hz 17 Hz 12 Hz 8 Hz
680 490 Hz 333 Hz 152 Hz 67 Hz 33 Hz 15 Hz 11 Hz 7 Hz
820 431 Hz 293 Hz 133 Hz 59 Hz 29 Hz 13 Hz 10 Hz 7 Hz
1k 379 Hz 258 Hz 117 Hz 52 Hz 25 Hz 12 Hz 8 Hz 5.5 Hz
1.2k 340 Hz 232 Hz 105 Hz 46 Hz 23 Hz 10 Hz 7 Hz 5 Hz
1.5k 301 Hz 205 Hz 93 Hz 40 Hz 20 Hz 9 Hz 6 Hz 4 Hz
1.8k 275 Hz 187 Hz 85 Hz 37 Hz 18 Hz 8 Hz 5 Hz 4 Hz
2.2k 252 Hz 171 Hz 78 Hz 34 Hz 17 Hz 7 Hz 5 Hz 3 Hz
2.7k 232 Hz 157 Hz 72 Hz 31 Hz 15 Hz 7 Hz 4 Hz 3 Hz
3k 223 Hz 152 Hz 69 Hz 30 Hz 15 Hz 6 Hz 4 Hz 3 Hz
3.3k 216 Hz 147 Hz Hz 67 29 Hz 14 Hz 6 Hz 4 Hz 3 Hz
3.9k 205 Hz 139 Hz 63 Hz 28 Hz 14 HZ 6 Hz 4 Hz 3 Hz
5k 192 Hz 131 Hz 59 Hz 26 Hz 13 Hz 4 Hz 4 Hz 2 Hz

Tony VVT Amps

[talbany]

Sory everybody I'll repost this in a word file with a table easier to read.

[Johnhenry]

Thanks Talbany, that's a very interesting chart, I've got a couple of chart's simular to your's stored on my old P.C
on why a cap sound's the way it does ? mostly due to what it's made out of and what kinda voltage and the circuit it has to deal with.
I've had 100 brand new orange drop's to choose from and i can actually hear a difference in them by switching from 1 to the other on a rotory switch where it can be done quickly, but if you take an orange drop apart very carefully on each end you'll see why just bending the wire improperly could change the way it sound's by loosening the connection of the standoff wire, it's not connected very well to the foil inside, it's like the outside cover was meant to hold everything together, Strange !
I may be F/O/S but that's what i was thinking when looking at the inside's of most cap's i've took apart.
I know they all can be used and made to sound good in the right circuit !
The Metal film resistor is the one that amaze's me, I had never used them except for 1/4 watt's for pedal building, until reading about them here on the Forum.
Johnhenry

[ayan]

I generated this up maybe this will help. based on 100k plates
...

Is your table based on the real formulae, as shown at www.aikenamps.com, for example? The values will be different than with the "first approximation" calculation you showed, 1/2Pi()RC, which would apply if every part of the network was buffered. And then, there is the frequency shaping at the output of the plates, which is in most cases the dominant factor in Dumble's design -- most of his high pass filtering effect takes place at the output. I do have a spreasheet based on the Aiken stuff, if anyone wants it. At any rate, those are just guidelines and as already stated, and sadly for us, not all parts are created equal.

Cheers,

Gil

[talbany]

Gil

Absolutely this is an approx. especially in some of the newer tubes, being manufactured where who knows how these tubes were tested, if tested at all. Gain factors may very, are the 2 halves even closely matched etc. all of which determines where that places you within the operateing point of the triode.
I guess my point here Gill ,is you are dealing with many variables . To give you another example why Fender (and in some Marshall's) used 100 uf bypass caps in the earlier tweed bassmans. judging by the chart that would put the lower sub frequency's way down below an audiable level, yet the amp was never muddy.
Could it be for harmonics?. I learned quickly that these calculations dont always apply to guitar amps. I use them to understand the intention, not the result.
And no I didn't get this from Aiken though I love all the tech info he posts. sharp guy.


Tony VVT amps

[dogears]

Tony,

Your formula is way wrong... For example, you use:

F = 1/2piRC (pi = 3.1412 rounded)

1/6.2824 * 1500 (Ohms) * .00001 Farads (10uf) = 1/.094236 or 10.6 Hz

When in fact it is
=1/6.2824*.00001 Farads (10uf) *Rkout =22Hz! Big difference.

You neglected to take into account Rkout (Cathode output impedence). This is Rk//Rk'
Rk= Cathode resistor
Rk' = (Ra+ra)/(mu+1)

As you can see, just using the cathode resistor is really wrong. The true breakpoint is much higher.

Some examples.

5uf and 1.5K (100k plate) cathode resistor =47hz
5uf and 2.2k (150K plate) cathode resistor = 33hz
1uf and 2.2k (100K plate) cathode resistor = 170k
1uf and 2.2k (150K plate) cathode resistor = 156k (notice how even the plate resistor size effects the knee!)
1uf and 1.5k (100K plate) cathode resistor = 222hz

The knee will always be too low if you don't factor in the parallel Rk' with the cathode resistor. Fwiw, even the impedence of what the stage feeds matters!! For example, the clean 1M master or the 320K impedence pre-od net of the non HRM. With the 100K/1.5K/1uf example, the V1b knee changes from 211Hz to 222Hz just by switching to the OD.

Interestingly though, your chart is basically correct as it says the knee for 100k plate and 1.5k Rk w/10uf bypass is 20hz.... Seems your chart is from a different source??

[talbany]

Hey Scott

Using the Capacitive Reactance formula alone you can find the band pass or “cut” frequencies.
When ac voltage flows through a capacitance an opposing change in the initial voltage occurs, this opposition or impedance to a change in voltage is measured in terms of capacitive reactance.

Capacitive reactance is determined by the formula:

1 / (2 * pi * f * C)

where: 2 * pi = 6.2832; f = frequency in hertz and C = capacitance in Farads

I only did this for 100k plates I know changing the plates changes the knee .This formula just made sense to me since I only wanted the cutoff or more like roll off. though I am sure yours is more accurate. Thanks for correcting me.

[dogears]

I always use my trusty spreadsheet! But, the formula needs the right values. Bottom line is that the cathode resistor has stuff in parallel with it and all that definately changes the results. To the tune of a full cap size.... Otherwise, you may use a 10uf with a 1.5k cathode resistor and think the knee is 10hz when in fact it is 22hz! See? You'd really need a 22uf cap to get your 10hz knee.

This is not my formula, but the correct formula. The only way to determine what you are looking for is to take everything into account.

Btw, your chart is right. I assume that is copied from some text?? Notice that the results are very close to mine... The chart is assuming an infinite output impedence which accounts for the slight discrepency.

You mention that the plate matters, but then in your example there is no accounting for it.

I only did this for 100k plates I know changing the plates changes the knee .This formula just made sense to me since I only wanted the cutoff or more like roll off. though I am sure yours is more accurate. Thanks for correcting me.

[ayan]

Tony,

Your formula is way wrong...

In the real engineering world, any time you are in the same order of magnitude you're doing real well. I would reserve the use of the term way wrong when istead of a 10uF vs a 20uF cap, the alternate approach proposes a 10uF cap vs replacing one of your car's tires.

I'm attaching my spreadsheet for those who want to check it out. 100% credit goes to Randall Aiken for supplying the forumulae.

Gil

[tomrasdf]

Alright, I have to know:
In real world tweaking, how much of your flying is by the numbers? Are you really able to hear an amp and say, "hmm, let's bump that knee up to 40Hz. that'll take out the mud," and then take out the ol' slide rule, make the calculations, and replace the appropriate part?

I could see how that could save me a lot of soldering/resoldering, but I have a hard time believing that works all that well. I use the formulas to understand the relationship between components and what changing values does, but the rest is all up to my ears. I've never used math to tell me what parts I need to swap for tweaking frequency response. Am I missing out?

(love this thread, by the way. Very informative!)

[ayan]

Alright, I have to know:
In real world tweaking, how much of your flying is by the numbers? Are you really able to hear an amp and say, "hmm, let's bump that knee up to 40Hz. that'll take out the mud," and then take out the ol' slide rule, make the calculations, and replace the appropriate part?

I could see how that could save me a lot of soldering/resoldering, but I have a hard time believing that works all that well. I use the formulas to understand the relationship between components and what changing values does, but the rest is all up to my ears. I've never used math to tell me what parts I need to swap for tweaking frequency response. Am I missing out?

(love this thread, by the way. Very informative!)

I don't believe your question is directed at me, but I personally do not use math much for tweaking. It is interesting to correlate what you find out with what theory predicts, but that's the extent of it. Of course, if I will be designing a power supply, or creating some extra filtering, I will want to have a starting place for which a pencil and piece of paper come in handy. But the ears are always the final judge as far as sound goes.

Gil

[talbany]

Hey Guys

I hear it from my wife everyday. Besides I don't know why I posted this here in the first place it really didn't have any relevance to the topic at hand. Sorry Gill!! you are no closer to the truth. As you were




T.

[ayan]

Hey Guys

I hear it from my wife everyday. Besides I don't know why I posted this here in the first place it really didn't have any relevance to the topic at hand. Sorry Gill!! you are no closer to the truth. As you were

T.

I don't understand, Tony. I think it's interesting this came up. I will post results of live playing with the amp after Sunday.

Gil

[dogears]

Well, wrong is wrong..... If you look at he knee of a 1uf on CL2 both ways, it is almost an octave. Very noticeable in the real world, hence way wrong.....

Tony, sorry if I came of strong.... I did not mean to. Just pointing out the fallacy of computing this knee without all the other junk in the formula. It is a very common misconception and I have seen people post this incorrectly many times.

As far as math or ears..... Ears for sure. But then I always check the math to see why I hear what I hear. Or I may use the spreadsheet (thanks Gil!) to make adjustments to values when compensating (to keep knees similar....stuff like that)

Carry on!

Tony,

Your formula is way wrong...

In the real engineering world, any time you are in the same order of magnitude you're doing real well. I would reserve the use of the term way wrong when istead of a 10uF vs a 20uF cap, the alternate approach proposes a 10uF cap vs replacing one of your car's tires.

I'm attaching my spreadsheet for those who want to check it out. 100% credit goes to Randall Aiken for supplying the forumulae.

Gil