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Discussion in 'Shock Brother's DIY Amps' started by King Fan, Oct 21, 2017.
Nice. I think I agree. One of the best amps I’ve tweaked to benefit from switable NFB.
All credit to you, sir, I probably wouldn't-a tried it without your advice. I found getting the cap / resistor balance on the 5f2a was complex, and on the PR the game probably wasn't worth the candle -- is that a thing?
@King Fan , being as 6G2's are all the rage around here these days, I got motivated and went back into my clone to experiment a little.
As you and I have clones, we can mod, care free!
If you recall I had added a 3.3 uF cathode cap in parallel across the cathode resistor, V1 second stage, in an earlier 6G2 thread a few years ago. The stock 6G2 circuit has no cathode cap here. That was on a vintage 6G2 that I used to own. I sold that amp. Today I went into my clone and did similar. I gathered what smaller e-caps that I had to experiment. I only had 2.2 uF and 4.7 uF options. I soldered in both caps + leads at the 1500 ohm resistor. For live testing purposes, then I elevated a micro switch above the circuit. Chassis in cradle on the bench. I took turns trying each cap that I had, grounding the other end. Using the switch to turn each option off an on. Guitar around neck, wailing. Actually different guitars. Single coils and humbuckers. Wow. I love it. I settled on 2.2 uF for my tastes. It offer a little volume and gain boost. My understanding is the different cap values will effect a certain frequency boost. I think I could hear that. The higher value got more mid boost it seemed. The cap value, if I recall causes certain electrodes to accumulate, or boil off the cathode. Someone please correct my theory on that if needed.
With my 6G2 volume on 4 to 5 now, I get some really, really, really nice sparkle with a little hint of dirt. There was less dirt at that point before. The on-set of dirt certainly comes sooner on the dial. The 2.2uf cap gave me some nice balanced drive . On 8 and up its a bloodbath of overdriven sweetness. Crazy good. Like oh my, silly-ness good.
With the NFB permanently disconnected on my 6G2, and now with the second cathode cap, this 6G2 circuit is sooo good. In my opinion. For home use, smaller venues, and folks who like dirt, and don't want to be too loud to get it naturally, this is the creme de la creme. Give it a shot Doc, I recommend!
Yes. The bypass capacitance acts in conjunction with the cathode resistance to produce a frequency shelving filter, below which the frequency gain drops off at a rate of 20dB/decade (which is the same thing as 6dB/octave), in accordance with the formula:
f = 1/(2Pii x R x C)
f = frequency 1/2-boost point in Hz
2Pii = 44/7
R = resistance (in Ohms)
C = capacitance (in Farrads) hence 1F = 1,000mF = 1,000,000uF = 1,000,000,000nF = 1,000,000,000,000pF (so you need to remember to shift the decimal place to the left the correct number of spaces when working with cap values in the uF to nF to pF range etc)
You can use this formula to calculate the expected frequency half-boost/rolloff point.
e.g. for a 1k5 resistor and a 1uF bypass cap:
1/(44/7 x 1500 x 0.000001) = 106Hz, which is between the 4th and 5th fret on the 6th String of a guitar in standard tuning. G# = 104Hz, A = 110Hz
Increasing R, whilst proportionately decreasing C, acts to keep the frequency boost point within the same ballpark
e.g. for a 3k resistor and a 0.5uF bypass cap:
1/(44/7 x 3000 x 0.0000005) = 106Hz. Which is handy for working out how you can change the tube's bias voltage point, but keep the frequency boost point the same.
The way bypass caps 'work' is in conjunction with the slight change in tube current that causes a slight change in cathode resistor voltage during the signal cycle. The voltage across an unbypassed cathode resistor would be constantly rising and falling in phase with the signal swing at the tube's control grid (although to a lesser extent that what occurs at the control grid). By putting a bypass capacitor in parallel with the cathode resistor, this causes the cap to charge up during the 'higher-current' portion of the signal cycle, and then alternately to 'discharge' during the 'lower-current' portion of the signal cycle. This cap charging/discharging cycle acts to hold the cathode voltage constant at a given frequency (in the same way that a filter cap shunts any AC, which may appear in the power supply rail, to ground.) The capacitance of the cap determines how fast it discharges. This is why smaller caps, which discharge faster, only remain effective at shunting the higher frequencies, and so you select small capacitances for progressively higher frequency boost.
Heh. *You* can mod care free. I can savor my 'vintage correct' beauty for a year or two (OK, maybe a month or two). I do like your realtime A:B testing approach. My amp-sensei brother can play his amp, solder in new stuff, play it again, and remember which sound he likes best. Me, 10 seconds of silence starts to mess with my perception.
IIRC, your original 6G2 became a test sled for a bunch of mods you liked. Or is that your current one? BTW., remind me, have we seen pics of your final, finished, cabbed and covered 6G2 head? Forgive my forgetting -- I took a really big bite and have had 6G2 brain freeze for months....
TW, you are on *fi-ah*. I'm going to clip and file this post -- cap effects on frequency have been way outside my ken. OK, I still don't remotely understand why 2pii = 44/7, and moving decimal points 6 or 9 digits makes my scalp hurt, but thank you very much, you rock....
Some amazing info there from @tubeswell for sure. So many fascinating little tweaks we can make!
Ok you asked for it. Not meaning to take anything away from King Fan's incredible 6G2 build thread. However I can't overlook the opportunity to post a pic of any of my amps anywhere.
Here is my completed clone-ish 6G2. Head format. Dual tap OT. NOS vintage Hytron 6V6 tubes. As mentioned no NFB and added second stage cathode cap mods.
I like playing with speaker and cab options. I can put this head on a 1x8 up to a 4X12 cab. Also enjoy virtually any speaker combination in between. Today, I have the head plugged into the combo cab of my 1959 5E3. It houses a Weber 12A125 30W. Holy moly. "Honey drips, I can't keep away."
Thanks @King Fan for your "tongue-in-cheek" enquiry about my build here. You knew I'd bite.
Take all the credit you want, Keith. She's a beaut. And hey, I like those corner protectors!!!
Expressed as a fraction Pii = 22/7. So 2Pii = 44/7
Expressed as an integer (to 15 decimal places), Pii = 3.142857142857143... The integer has a recurring infinitestimal interval, so its commonly rounded to 3.142 for short (so 2Pii = 6.2857 for short), which is close enough for science, and more than close enough for tube amps.
P, pronounced 'pie' or 'Pii', written as 'π' in the classical Greek alphabet, is the first letter in the ancient Greek word 'perimetros' (perimeter), meaning the circumference (of a circle), which is used in the formula 2.π.r to calculate the circumference of a circle, or π.r^2, to calculate the area of a circle.
I use the notation Pii, but many people use just Pi.
When used in calculating frequency, Pii (to quote the venerable wiki) is a scalar measure of rotation rate. It refers to the angular displacement per unit time (e.g., in rotation) or the rate of change of the phase of a sinusoidal waveform (e.g., in oscillations and waves), or as the rate of change of the argument of the sine function.
There are 6.2857 radians* in 1 circumference, or 6.2857 radii. (i.e. 6.2857 radii per cycle or 2Pii per cycle).
* 1 radian is the angle that defines 1 radius' worth of the circumference of a circle.
Edit - fixed up Pii
Aha, excellent. Click. You remind me we had to calculate pi to 20 places in school, and much later used it in all sorts of deg / rad math. What I mostly recall from those years: lyrics of Who songs...
Minor point but Pi is 3.14159 or 3.1416.
Indeed it is thanks. fixed.