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GK rails in a pedal?
- Thread starter sprag
- Start date
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
yep. that's it.
okay, i'll try a 10K for VR2.
i'm hoping that the 10u help with the loss of top end too?
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
I settled on a 1K for R7 on mine and it has a TON of output, but I love it that way.
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With that cap you may see a bass boost due to resonances. I might have to look at also adding a Zobel network on the tranny output to control response with the loading. So we may have to play around with the tranny input and output to get a freq response that works well. With purely resistive loads this tranny rocks for being so inexpensive.
On V5 I am adding an optional cap across VR1 so that the high end response can be tweaked. With R7 at 1K, a 10nF cap has a nice gradual roll-off at approx 15KHz.
With the changes to the output driver circuitry, we are loading the high frequency response with R3 (1K) AND tranny primary, so we are back to removing R3 when the tranny is connected to the circuit. V5 is starting to look a lot like V1
-Frank
Here are two questions for everyone who has rocked their GK amps into 'rails' distortion.
Do you dial back the highs on the preamp EQ?
Does your speaker cab have a tweeter?
-Frank
Do you dial back the highs on the preamp EQ?
Does your speaker cab have a tweeter?
-Frank
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
whenever I push a GK amp to the rails I don't dial back the top end, but I also very rarely use tweeters.
I usually roll off the treble quite a bit and boost the high mids a little. I don't change the EQ especially for pushing the amp, though.
My GK cabs have tweeters, but I don't normally use them.
***IMAGE REMOVED- WE HAVE A NEWER VERSION LATER IN THIS THREAD***
I believe that I have all of the changes in this one. Please look it over.
Instead of AC coupling the tranny primary and having to deal with resonances, I show a jumper on here that either grounds the R3 load OR places R3 in series with the tranny primary. With this R3 resistor in series with the prim coil the input will be attenuated by 6dB, but the amp should be very happy with the load and this circuit has more than enough juice to make up for the 6dB attenuation.
C14 and R27 are needed to keep the amp from eating itself with ultra sonic oscillations.
D7 and D6 5V1 zeners keep that LF353 from ever seeing an input greater than the supplies.
The heavy filtering on U1 power supply pins keeps any "power" amp noise from the input circuit that is propagated thru the supplies. Opamp PSRR (power supply rejection ratio) numbers are really good, but those rejection ratios diminish quite a bit with high frequencies. So this filtering will be a great asset for this circuit.
Also, a lot of the reference numbers have changed, there were so many parts removed that I had my schematic editor reassign sequential numbers.
-Frank
I believe that I have all of the changes in this one. Please look it over.
Instead of AC coupling the tranny primary and having to deal with resonances, I show a jumper on here that either grounds the R3 load OR places R3 in series with the tranny primary. With this R3 resistor in series with the prim coil the input will be attenuated by 6dB, but the amp should be very happy with the load and this circuit has more than enough juice to make up for the 6dB attenuation.
C14 and R27 are needed to keep the amp from eating itself with ultra sonic oscillations.
D7 and D6 5V1 zeners keep that LF353 from ever seeing an input greater than the supplies.
The heavy filtering on U1 power supply pins keeps any "power" amp noise from the input circuit that is propagated thru the supplies. Opamp PSRR (power supply rejection ratio) numbers are really good, but those rejection ratios diminish quite a bit with high frequencies. So this filtering will be a great asset for this circuit.
Also, a lot of the reference numbers have changed, there were so many parts removed that I had my schematic editor reassign sequential numbers.
-Frank
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- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
John, you will see that I changed the configuration of the components for OUTPUT 1. I wanted to keep any device connected to this output from allowing DC on the pot wiper.
-Frank
-Frank
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
got it. i'll start drawing up a new PCB for it tonight.
it looks simpler and easier without the 2 extra transistors in the poweramp section.
it looks simpler and easier without the 2 extra transistors in the poweramp section.
I thought you would appreciate less parts.
BTW, try the tranny with V5 configuration and let me know what you hear before you lay down the copper.
-Frank
Maybe put C15 on a switch for simulating a cab with and without tweeters?
BTW, C15 at 10nF was just something I tried. This cap can be any value that you like. Maybe have a switch with tweeter, no tweeter light and no tweeter dark???
-Frank
BTW, C15 at 10nF was just something I tried. This cap can be any value that you like. Maybe have a switch with tweeter, no tweeter light and no tweeter dark???
-Frank
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***IMAGE REMOVED- WE HAVE A NEWER VERSION LATER IN THIS THREAD***
Updated boost schematic.
-Frank
Updated boost schematic.
-Frank
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I have a 700rb-II and a GK Neo410 cab here at my place. Would anybody be interested in me recording some clips to compare with the ongoing project here? I don't have the best microphone for recording bass (it's a Blue Microphones Yeti) but I *think* I could probably get a reasonable recording of the different types of grit that various settings on the amp can produce. I have my own opinions about where my amp's growl comes from but I don't want to spoil any potential surprises. 
In theory I would start by recording clips with these types of settings:
1. Volume (preamp volume) set HIGH, Master Volume set LOW, Boost at a few settings.
2. Volume set LOW, Master Volume set HIGH, Boost at a few settings.
I wouldn't be able to crank all the knobs but this should hopefully give us a better idea of where the growl is coming from, no? Let me know if there's any other possible setting I should record.
In theory I would start by recording clips with these types of settings:
1. Volume (preamp volume) set HIGH, Master Volume set LOW, Boost at a few settings.
2. Volume set LOW, Master Volume set HIGH, Boost at a few settings.
I wouldn't be able to crank all the knobs but this should hopefully give us a better idea of where the growl is coming from, no? Let me know if there's any other possible setting I should record.
I have a 700rb-II and a GK Neo410 cab here at my place. Would anybody be interested in me recording some clips to compare with the ongoing project here? I don't have the best microphone for recording bass (it's a Blue Microphones Yeti) but I *think* I could probably get a reasonable recording of the different types of grit that various settings on the amp can produce. I have my own opinions about where my amp's growl comes from but I don't want to spoil any potential surprises.
In theory I would start by recording clips with these types of settings:
1. Volume (preamp volume) set HIGH, Master Volume set LOW, Boost at a few settings.
2. Volume set LOW, Master Volume set HIGH, Boost at a few settings.
I wouldn't be able to crank all the knobs but this should hopefully give us a better idea of where the growl is coming from, no? Let me know if there's any other possible setting I should record.
Absolutely! Go for it.
I have never seen a definitive explanation of where this 'rails' sound comes from. Most people are saying it's the power amp, but there are two scenarios that I see here:
If one were to take the effects send jack of the amplifier and feed another power amp, they are bypassing the Q89 boost circuit on the preamp board. The other scenario is if they fed another preamp into the effects return jack to use the GK power amp they are getting the Q89 boost circuit from the preamp along with the power amp.
So with those two scenarios, would the GK power amp have that 'rails' tone if the Q89 boost circuit was removed? Which leads to, could any clipping power amp sound like the GK with a Q89 boost in front of it? Questions that we NEED answers for.
-Frank
I have a 700rb-II and a GK Neo410 cab here at my place. Would anybody be interested in me recording some clips to compare with the ongoing project here? I don't have the best microphone for recording bass (it's a Blue Microphones Yeti) but I *think* I could probably get a reasonable recording of the different types of grit that various settings on the amp can produce. I have my own opinions about where my amp's growl comes from but I don't want to spoil any potential surprises.
In theory I would start by recording clips with these types of settings:
1. Volume (preamp volume) set HIGH, Master Volume set LOW, Boost at a few settings.
2. Volume set LOW, Master Volume set HIGH, Boost at a few settings.
I wouldn't be able to crank all the knobs but this should hopefully give us a better idea of where the growl is coming from, no? Let me know if there's any other possible setting I should record.
BTW, If you have a theory on this 'rails' tone, then please share it!
-Frank
- Feb 16, 2008
- 21,240
- 23,634
- Disclosures
- John K Custom Basses
with R3 (1K) in series with the output transformer's primary to lug 4 and using a 10K pot for VR2, there's definitely more level, but it still has a severe top end roll off, at from 800hz-1K (just a guess on the freqs) as if a Pbass's tone control is rolled all the way down.
Hey Frank,
Just as a note the GK 700RB-II and 1001RB-II have a separate 50W amplifier that drives the horns on GK cabs with the crossover at 5kHz. This keeps the highs clean and free of the clipping that comes from pushing the power amp.
And for what it's worth I thought my 700RB sounded better when pushed with GK cabs than other tweeter loaded cabs.
