Eighteen.

[Vaum]

I built a ROG Eighteen and originally used a 47K pot for gain control with 450K between it and ground, I figured I'd always leave it wide open ... I really, really liked the tone but it was a little muddied with the gain set that high so I replaced the pot with a 1M (removing the 450K) and now the sound is pure mud anywhere on the dial. Highs are pretty much non-existent and drive went way down. It also became very hissy after the change and it was dead quiet before. I measured the pot and it seems fine. All voltages are spot on. 2SK373 in Q1 and Q2, 2SK117 in Q3. Sorry, I wrote this in a rush.

Thanks.

[Vaum]

Fixed (for now).

I have a jumper completing the tone circuit (I substituted fixed resistors setting the tone at approximately 60%) and this is the second time it has given me trouble. I had a similar problem earlier that, using an audio probe, I traced to that area.

Each time I've checked for continuity from the jumper to possible points it could be shorting to and never gotten anything. I also measured resistance through the jumper and the resistor it connects to and it reads as it should. Disconnecting the jumper has fixed the problem both times, and reconnecting the jumper no longer has the negative effect.

The only thing I can think of is that I used a CRL film cap from an old organ in the tone circuit, one of those clear cylinders with the foil showing through and red at one end ... I just assumed the red was arbitrary and that the cap has no polarity, so I installed the cap with the red side connecting at the junction formed by it, the 4n7 cap and the drain of Q1 ... Are those caps polarized? What else could be causing this? It's fixed now, but I fear that it will slowly break itself again.

Thanks again.

Also, I want to simulate a power section at the back end using a CMOS chip. I know that CD4049UB is the standard but I happen to have this: http://www.datasheetcatalog.org/datasheet/toshiba/3923.pdf on hand. Anything to take into consideration?

[grolschie]

What do 2SK117's sound like in the Eighteen? How would these compare to J201s which are the norm right?

[frankclarke]

The 8k2 resistor has a big effect on gain, if gain is the issue. The caps on the output do remove a lot of treble.

[Vaum]

The 2SK117 and 2SK373 sound really good in the Eighteen. Though, I have to admit, I've never heard J201 in the circuit other than in clips. I much prefer this configuration.

The 1M pot really cleared it up even at maximum gain, so that's no longer an issue. As I said, the whole problem comes down to the tone section, but I have no idea what the problem is. It's fixed by disconnecting and reconnecting the jumper completing the tone circuit.

Anyways, I also have a switchable boost (Fetzer Valve) on the board and for the power amp "simulation" I ended up going with a LM386 instead of the CMOS route. It does what I'd hoped it would, even though I realize now that the ROG Eighteen already includes the power amp.

[grolschie]

I have a 2SK117 somewhere. What stage is responsible for the most gain? I am guessing Q1 is the best position for it?

So what exactly was the effect of changing to a 1M pot? I find the the gain sounds farty at full gain.

The tone circuit is not really that extreme I find. Which is fine. I quite like the Rat's filter knob though. Hmmm.... 

[drewl]

Damn....stupid me.....I had no idea what an "eighteen" was.....I could have built this pedal instead of a few 18w tube amps I built