I wonder if there could be variances in transistors between brands. I normally use Fairchild transistors but went with Central Semiconductor for the GK pedals as I noticed they were made in USA.
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GK rails in a pedal?
- Thread starter sprag
- Start date
Central Semi 2N4401's and 4403's have the same gain as other manufacturers of these transistors. The bulk of the circuit gain comes from the U1a opamp set with R14 and R16 for approx 56X. There is also the gain around the transistors (for the original HV circuit) that is set by R6 and R8 for approx 17X. But because opamp U1a is the master "controller" for the circuit, the overall gain is set by R14 and R16 for approx 56X.
-Frank
- Feb 16, 2008
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- John K Custom Basses
+1.
IMO, you could use practically any general purpose NPN & PNP transistors in it and get similar results.
that said, I prefer to use Fairchild transistors whenever possible.
IMO, you could use practically any general purpose NPN & PNP transistors in it and get similar results.
that said, I prefer to use Fairchild transistors whenever possible.
If people want to build this circuit with less gain, you can easily increase the value of R16 to lower it.
Here is an alternate Q89 boost circuit that keeps it in the circuit and switches the same way as the original 800RB:
It has one less capacitor and S1 has been moved to the gate of Q1.
-Frank
Here is an alternate Q89 boost circuit that keeps it in the circuit and switches the same way as the original 800RB:
It has one less capacitor and S1 has been moved to the gate of Q1.
-Frank
- Feb 16, 2008
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- John K Custom Basses
since you don't have the actual pedal in hand for testing Frank, i'll build you one (obviously for free, since you designed it 
) so you can play with it & hear it for yourself. I just have a few builds ahead of it.
) so you can play with it & hear it for yourself. I just have a few builds ahead of it.
since you don't have the actual pedal in hand for testing Frank, i'll build you one (obviously for free, since you designed it
Thanks John, you're too kind!!!!
I don't think that I really need to "test" it, because your measurements while prototyping it were matching my calculations and simulations almost perfectly. I am 100% confident that this circuit works as planned.
Remember, we left out the boost in the original design to see if the "rails" tone was from the power amp. Then we added the boost, so that we could see if the rails tone was from the power amp with the boost circuit... that's why the boost was designed to be either completely in-circuit or out-of-circuit. I drew up the V2 boost so that people could better match the way the original 800RB boost switched in and out if desired.
-Frank
- Feb 16, 2008
- 21,240
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- John K Custom Basses
yeah, but you really SHOULD have one and I think that you'll love it!
- Feb 16, 2008
- 21,240
- 23,638
- Disclosures
- John K Custom Basses
Here is what the Q89 boost freq response looks like for both versions:
You can see that the low freq -3dB point is approx 60Hz. If you would like to increase the low frequency response, then increase the C1 value on the boost ckt. The values of the boost components are original 800RB values.
-Frank
You can see that the low freq -3dB point is approx 60Hz. If you would like to increase the low frequency response, then increase the C1 value on the boost ckt. The values of the boost components are original 800RB values.
-Frank
I just fixed one of the pedals from my last batch. The transistors were the issue. I replaced the Central Semiconductor's with Fairchild's and now it's working as it should.
Either the Central Semi transistors you have came from an out of spec lot or there is some performance characteristic that is not listed on their data sheets. If you compare the Fairchild data sheet to the Central Semi data sheet, they match almost perfectly.
Where did you buy your Central Semi transistors?
-Frank
Mouser has a great reputation for sourcing reliable parts directly from the factory, so I'm clueless as to why the Central Semi transistors didn't work properly.
It's great news that you got it figured out!
-Frank
- Aug 21, 2003
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- A-Designs Mike Lull GK Tsunami Cables GHS Strings RMI Basswitch Nordstrand Pickups Darkglass
I just got a hold of early 80's 800RB.
Once I get my pedal I will do some A/B Testing.
Looking forward towards it.
Once I get my pedal I will do some A/B Testing.
Looking forward towards it.
- Aug 21, 2003
- 11,609
- 18,664
- Disclosures
- A-Designs Mike Lull GK Tsunami Cables GHS Strings RMI Basswitch Nordstrand Pickups Darkglass
hey everybody, I had some time and made a little test audio.
first - a riff - that is played clean, then with the effect on a moderate setting, then with the boost on
second - another riff - clean, on low setting, on higher setting, boost
third - a dynamics test - basically playing soft then heavy with the effet on
fourth - a pickup test - Both, bridge, neck, both with tone rolled off all the way
fifth - high switch - mid position, down position, up position
sixth - madness - input all the way up, then boost all the way up
gear used - Fender jazz with Dimarzio model j's, the pedal, focusrite scarlett 2i2, ableton live 9
i did not add anything to the track. no eq's, no compression, nothing.
the first two riffs have a simple drum sample behind them.
i used a pick for all of these.
the setting i like most so far are with the input and output at about 1-2, and when using the boost that at about 9 (on a clock face).
since ive gotten the pedal back, its exactly what i thought it would be, and its great! you dont have to push it to get unity, its not nearly as compressed as i thought before (unless to crank it, and then it starts to compress). i really wanted an always on "low" gain settings pedal that still let me keep most of my dynamics and just add a little magic. now this pedal does exactly that, and way more! if you crank it you can easily get into fuzzy territory and with the boost you always have that high gain setting just a click away if you need it. it sounds GK ish to me, again similar to my mb500's boost channel, but different. i havent had the pleasure of playing any other gk's except a couple other mb models and a 700rbII in stores. either way, GK sound or not, its an awesome pedal.
first - a riff - that is played clean, then with the effect on a moderate setting, then with the boost on
second - another riff - clean, on low setting, on higher setting, boost
third - a dynamics test - basically playing soft then heavy with the effet on
fourth - a pickup test - Both, bridge, neck, both with tone rolled off all the way
fifth - high switch - mid position, down position, up position
sixth - madness - input all the way up, then boost all the way up
gear used - Fender jazz with Dimarzio model j's, the pedal, focusrite scarlett 2i2, ableton live 9
i did not add anything to the track. no eq's, no compression, nothing.
the first two riffs have a simple drum sample behind them.
i used a pick for all of these.
the setting i like most so far are with the input and output at about 1-2, and when using the boost that at about 9 (on a clock face).
since ive gotten the pedal back, its exactly what i thought it would be, and its great! you dont have to push it to get unity, its not nearly as compressed as i thought before (unless to crank it, and then it starts to compress). i really wanted an always on "low" gain settings pedal that still let me keep most of my dynamics and just add a little magic. now this pedal does exactly that, and way more! if you crank it you can easily get into fuzzy territory and with the boost you always have that high gain setting just a click away if you need it. it sounds GK ish to me, again similar to my mb500's boost channel, but different. i havent had the pleasure of playing any other gk's except a couple other mb models and a 700rbII in stores. either way, GK sound or not, its an awesome pedal.
