Author Topic: Making a Rat more like a Top Fuel?  (Read 1568 times)

benallison

Making a Rat more like a Top Fuel?
« on: April 07, 2009, 11:28:24 PM »
Yeah, I know: Rat = LM308, Top Fuel = transistor-mostfet-whatchmacallit.

But I'm just wondering if anyone has any good Rat mods (beyond the standard ones) that could make it more "like" the Top Fuel (translation: smoother, fatter, and able to copy the tones from Comfortably Numb's 2nd solo).

I've done the Reutz thing already.

DWBH

Re: Making a Rat more like a Top Fuel?
« Reply #1 on: April 08, 2009, 06:50:15 AM »
How are you using the Rat's gain control. Turn that sucker up, it gets fatter and squishier.

benallison

Re: Making a Rat more like a Top Fuel?
« Reply #2 on: April 08, 2009, 07:07:38 AM »
I'm afraid to because of the fizzzzz and the too much distortion.  :(

DWBH

Re: Making a Rat more like a Top Fuel?
« Reply #3 on: April 08, 2009, 08:27:29 AM »
I think if you want that sustain you might need that extra distortion. There isn't much fizz if you use the filter knob wisely.

But, if you want to fatten up your rat you might want to sub those clipping diodes with Leds.

Mark Hammer

Re: Making a Rat more like a Top Fuel?
« Reply #4 on: April 08, 2009, 10:01:21 AM »
1) Hi Ben. Nice to see you here and on the Canadian forum.

2) The Rat uses a network from the ground leg to provide MUCH more gain for content above 1.5khz than for content below.  You can change the degree of extra gain and where it happens by tinkering with the 47R/2u2 network.

3) The Rat adjusts gain via the feedback loop, with a feedback cap in parallel.  As the feedback resistance is increased, the gain goes up, but at the same time the impact of the feedback cap is increased and more top end is rolled off.  Smoothing out the top end can be achieved by adding a bit more capacitance in the feedback loop.  Not too much, mind you, but tacking a 22pf cap on top of the existing 100pf will provide more "smoothing" at medium to high gain settings.

4)  The "filter" control rolls off treble at a 6db/oct slope at over 31khz when the filter is wide open, and at around 475hz when the filter is turned all the way down.  However, with that shallow a slope, and with that much goosing of the top end earlier in the circuit, a fair amount of top end will still sneak through.  One solution is to use a steeper filter, such as a 2-pole lowpass formed by a dual-ganged 50k pot with each section having a 1k resistor in series with it just before the cap to ground.  Two-pole filters  do a better job of keeping the fizz out while retainer the character and pick attack.  If you used a dual-ganged 50k (or a dual-ganged 100k with a 51k resistor in parallel with each section) and a 3900pf cap to ground, the filter would range from a rolloff starting at 40khz to a rolloff starting around 800hz.  While 800hz may sound higher than the stock 475hz, because the rolloff is steeper you can live with a higher rolloff frequency.  To whit, using a 6db/oct rolloff and the stock filter, content at 1900hz is an estimated 12db down and at 3800hz, only 18db down.  In contrast, using our described 2-pole filter, content at 1600hz is 12db down, and at 3200hz is 24db down.  So, like I say, better fizz protection.  You may find that simply modding the filter control for 2-pole functioning overrides the need for the other suggested adjustments in #2 and 3.

benallison

Re: Making a Rat more like a Top Fuel?
« Reply #5 on: April 08, 2009, 10:15:43 AM »
1) Hi Ben. Nice to see you here and on the Canadian forum.

Hi Mark!

Wow, tons of info here. You blew my mind. Thanks so much!

Point about where in the spectrum the gain happens is not really appreciated by a lot of people. For example, one thing I love about the Barber LTD is that the "distortion" seems to happen lower down, so the top end stays more clear, but you get a nice, low-mid "grunt."

Thanks again. Looking forward to trying some of these things out!

I thought about throwing a cap over the outer volume pot lugs... that seems to be a fairly standard "fizz" tamer.
« Last Edit: April 08, 2009, 10:21:04 AM by benallison »

biggy boy

Re: Making a Rat more like a Top Fuel?
« Reply #6 on: April 08, 2009, 10:24:42 AM »
1) Hi Ben. Nice to see you here and on the Canadian forum.

 

Oooo Canadian forum??
Wheres that located?


Glen

benallison

Re: Making a Rat more like a Top Fuel?
« Reply #7 on: April 08, 2009, 10:36:56 AM »