Preamp stage of 6D30 simulated

I've been doing a lot of wiring on the amp and realizing some of the subtleties of circuit design, as well as the practical aspects of fitting all of this stuff into a combo cabinet. The final sound is really where the proof lies, but elegance can be recognized anywhere. In the spirit of learning about each stage of the amp, I will try to simulate as many of them as possible. Below is the schematic for the first preamp stage of the 6D30. I have employed here, the generic triode model from Duncan Amps.

In case you are interested, the file for the schematic is here.

The above circuit seems innocent enough: two inputs running into the circuit via the ubiquitous 68k resistor and separate 12AX7 stages mixed at the output with a gain of ~30 (15 dB). Thus, given a 300 mV sinusoid, the voltage across the 500kOhm resistor will be ~8.7V sinusoid. This can be seen below:

The image plots the input signal and the voltage across the 500k resistor on the output. 300 mV is a reasonable approximation to a guitar signal. As you can see, this is a pretty big amplification from a small signal. This is why the wiring from the input jack should be both SHORT and SHIELDED. Due to the low signal level at the input, a smaller amount of noise can drive your signal to noise ratio (SNR) very low fast. Then every stage downstream amplifies your noisy signal resulting in .. loud noise mucking up your sweet guitar tone! :(

The other thing worth noting in this circuit is the high-pass filter that couples this stage to the next stage. This consists of C2 and R6 and is a typical RC circuit. The -3 dB (half-amplitude) point for this circuit is ~600 Hz (found by using 1/(2*pi*R*C)). Why am I mentioning this? The fundamental frequency range for guitar is from 80 Hz to 1 kHz. Since 600 Hz and below are attenuated by more than half in the first stage of this circuit, the guitar will certainly be affected. Frequencies above 1 kHz account for the "timbre" of a guitar, so none of that is lost. If channel 1 has too many highs, this might not be a bad place to throw in some more low frequencies by increasing the value of the C2, and dropping the -3 dB to more like 400 Hz.

As a note to anyone building this amp, the resistor on the layout says 1M, but the package contains a 500K resistor, and 500K is what the schematics say. I have noticed one should trust the schematic more than anything with the kit. The wiring layout is really confusing to me, since it seems to be mirrored and upside down. Maybe I'm just not thinking about it right.

Amp arrived

Sweet. My amp parts arrived. It's a Weber 6D30. If you're interested, you can check the schematic/layout there. I will try to take pictures as it is built.

(note: also switched the red ale to secondary and added oak chips and 2 oz hops!)

National Homebrew Day

man + drill + bucket = mandrilbuket

We rang in National Homebrew day by making 2 5 gallon batches of beer and some BBQ'ing in the sun. One of the recipes is a delicious red ale recipe from MoreBeer (http://www.morebeer.com). It's called Fire In The Hole, and the ingredients are listed below:

Malts:
7 lbs Ultralight Extract
Grain:
2 lbs 2-Row
1 lbs Flaked Wheat
1 lbs Ctystal 75 L
.5 lbs Rye
.5 lbs Munich
.18 lbs Carafa

Hops:
1 Oz Magnum Hops 60 mins
2 oz Centennial Hops 5 mins

Water Treatment:
Wirfloc Tab 20 mins

Secondary
2 oz Columbus
1 oz Oak Chips

I think it is a really excellent red ale, so you should get their kit if you're looking for a new brew to try, but not wanting to write up a new original recipe. It's my second time to make it. The first time, it was delicious, but I came away with only 3.5 gallons of beer due to a slight mishap with the wort and my butter fingers directly after chilling. I will just be thankful the wort wasn't hot when over a gallon spilled on my floor :) Anyhow, that experience inspired this second batch, so it's kind of hard to complain. The other recipe du jour brewed by my buddy was Vagabond Gingered Beer:

1lb of crystal

1lb of chocolate

dark malt extract (7lbs-ish)

one chuck of finely chopped ginger (recipe called for between 2 and 4oz by weight)

domestic nugget for bittering

fuggles for aroma

white labs california ale yeast

Based on the Charlie book recipe. The ginger looked delicious, and I am excited to try it. The only time I've ever placed such a strong flavor agent into a beer was with a bourbon vanilla porter (BVP). Basically prior to brewing, people were warning me, "don't put too much bourbon or vanilla beans into it!!!" My BVP turned out to be pretty vanilla-ey, despite this advice, and I've heard of more than one story about other strong flavoring agents that take a long time to mellow; jalepeno peppers and sarsaparilla roots to name a couple. I relayed this information to Chas when he was cutting up his organic ginger, so we'll see. I'm sure it will be good.

Brew Gear

One fun thing about brewing to me is that it's always fun to work with and optimize the setup. In the photo below, Chas creates a mash tun by drilling 1/8" holes into a bucket. This will operate as a removable false bottom when making a full-mash brew. I'm pretty stoked to see how it will work. I use a Gott cooler mash tun, which is reasonable, but I have my suspicions about the fluid circulation, since it does not have wonderful efficiency.



The day also saw my second attempt with counter-flow chilling, and an inadvertent experiment of concurrent flow chilling (woops). The chiller I built is from the following design:

http://texanbrew.com/article.php?story=BuildACFC

Counterflow is no doubt more efficient, although the the arrangement is a little more tedious. I think fixing the chiller in a specific location on a brewing rack would be ideal. Another option would be the addition of pumps to the system, which would eliminate the danger of placing a large volume of fluid at 212 F six feet in the air. All things to consider for next time.