• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

GK rails in a pedal?

#41
mbelue said:
Its the power amp running all-out, as in its output transistors running completely non-linear.
I believe it's possible to do by bypassing the preamp entirely. Substituted by another preamp of course
Click to expand...

Right. But how can you push the power amp "all-out" without running a hot signal into the power amp? You're not going to get near the rails without the "Master" volume turned up high enough to push the power section near it's limits. Is that not correct? BTW, I'm not trying to be argumentative. I really want to understand this.
 
#45
Dave W said:
The Master isn't part of the preamp
Click to expand...

Yes it is. I've studied the schematic for the 800RB for months. The "Hi" and "Lo" master volumes are the end of the preamp section. They only control preamp-level signals. The power amp essentially runs full-on with the volume being controlled by the signal going into the power amp.

Here's the schematic for the preamp:

800rbpre.jpg

The master volumes are in the bottom right, R115 and R118. The signal goes through the pots, into a capacitor each, and then that's where they leave the preamp board. It's a 1.1V signal there, which is preamp-level.
 
Last edited:
#47
The volume control is irrelevant.
That "hitting the rails" sound is the output transistors clipping.
The preamp has sufficient gain to to do this. You could also do this by tapping in the effects return.
 
#48
So I understand, saying the master volume really means the woofer volume correct? The actual VOLUME knob is actually a gain? So, turn woofer volume all the way up and use the other one on far left (800rb) to control your volume? I actually went the opposite on my 1001ii turned the gain (volume) to around 1 and used woofer for my overall volume.
 
#49
Dominic D said:
Yes it is. I've studied the schematic for the 800RB for months. The "Hi" and "Lo" master volumes are the end of the preamp section. They only control preamp-level signals. The power amp essentially runs full-on with the volume being controlled by the signal going into the power amp.

Here's the schematic for the preamp:

View attachment 527904

The master volumes are in the bottom right, R115 and R118. The signal goes through the pots, into a capacitor each, and then that's where they leave the preamp board. It's a 1.1V signal there, which is preamp-level.
Click to expand...


If you look at the test voltages on the schematic with the specified test conditions, you will see that the preamp has a gain of 550 from the input to the output of Q89 (J113 FET)… that’s a gain of approx 55dB. The 2mV input test signal is very, very small and with a normal output bass guitar this circuit would easily clip the preamp power rails with the controls set to these test conditions.

-Frank
 
#50
"The rails" are the power supply peak voltages, at which the power amp distorts. "Hitting the rails" means forcing the amp to try to exceed those power supply levels. The power amp is doing the distorting, which is very different from preamp distortion, which is why normal pedal-format distortion pedals usually can't make that particular type of distortion. Most analog dirt pedals are basically preamps.

Regarding the master volume and the preamp output level, the master volume is the last stage of the preamp, right before the power amp. It controls how high the preamp signal is, going into the power section. It does not directly control the power section. The only way to make the power section hit the rails is to send in a high-enough input signal--for example by turning the master volume up all the way, so that the preamp's maximum output level is achieved. The same result could come from sending any hot-enough signal, e.g. from a mic preamp or other high-output gain stage, into the power section.

So while the "rails" distortion is entirely from the power section, it only gets that way because the preamp's master volume is turned up high enough (relative to the signal at the preamp's input).

In order to make a pedal version, the designer would need to figure out how to emulate the exact sound of the power stage distorting, and not rely on any portion of the preamp schematic, or any similar preamp schematic. That schematic will not provide the "rails" distortion.
 
  • Like
Reactions: odarellmc, Andy V., Ethereal Thorn and 9 others
#51
Dave W said:
The 800RB sounds cool but the power section is what he's looking to cop.
Click to expand...
+1!

bongomania said:
In order to make a pedal version, the designer would need to figure out how to emulate the exact sound of the power stage distorting, and not rely on any portion of the preamp schematic, or any similar preamp schematic. That schematic will not provide the "rails" distortion.
Click to expand...

I feel like confusion about this point is perhaps what has prevented such a pedal from being made. There have been several threads like this over the years, and failing to understand this point seems to always be what gets them side-tracked.
 
Last edited:
  • Like
Reactions: mbelue
#54
In other words, you don't need s pedal, you need a rack or floor unit that you can inject between the send and return that would accurately emulate the overdrive, and be able to do it at a lower volume.
 
#55
bongomania said:
"The rails" are the power supply peak voltages, at which the power amp distorts. "Hitting the rails" means forcing the amp to try to exceed those power supply levels. The power amp is doing the distorting, which is very different from preamp distortion, which is why normal pedal-format distortion pedals usually can't make that particular type of distortion. Most analog dirt pedals are basically preamps.

...
Click to expand...

Thank you so much for your post. That's exactly what I was trying to say but failed to explain in a way that made any sense.
 
#56
Not to burst bubbles here, I too would love a gk In a pedal (sans amp style w/di) but it seems like its all about the interaction between pre and gk power amp. That attack, tightness, those gk trademarks. I can't see how that could be replicated using just pre. The RBP apparently doesn't get the same sound so how would anything else? But hey, I'll buy one if it happens. For now I have NO problem running my 87 800rb!
 
#57
wolfkeller said:
Not to burst bubbles here, I too would love a gk In a pedal (sans amp style w/di) but it seems like its all about the interaction between pre and gk power amp. That attack, tightness, those gk trademarks. I can't see how that could be replicated using just pre. The RBP apparently doesn't get the same sound so how would anything else? But hey, I'll buy one if it happens. For now I have NO problem running my 87 800rb!
Click to expand...
You know what this means. We need a gk solo power section
 
  • Like
Reactions: ryansalmond
#59
Given the amount of stuff FEA, Darkglass, [SFX], or Source Audio can accomplish in a pedal, it seems like it should be do-able. I don't know anything about making pedals or amps, but if it comes down to distorting power amp transistors, it's easy to at least imagine one of these guys making a pedal where a quieter signal distorts a smaller set of transistors or something along those lines. ...I am talking out of my ass at this point, but then again, look at CustomTonesInc- they make a sansamp-sized pedal version of a Dumble amplifier. What's unique about them is that rather than starting from scratch (like SansAmp et al.), the guy simply took the existing amplifier design as his starting point and worked backwards until it was something that fit into a pedal while still sounding the same.
 
  • Like
Reactions: mbelue

Latest posts

Top Bottom
Back