GK 800RB.. Should I?

[Astronaut]

I'm eyeing one of these unicorns for little above 500 bucks and am wondering whether or not, to pull the trigger.
The amp is inside a rack, some buttons miss the indicator cover and the front panel seem to have a global discolouration (as in yellow tinted).
No pictures of the back panel.
The main question is; does anyone knows if the 100W output is somehow High passed? Is it wise to connect anything other than a compression tweeter?

Thanks.

 

[agedhorse]

The pots are sealed and impossible to clean.

 

[dave_p]

500$ for a marginal 800rb is too much.

 

[shirojiro]

Ok. I brought out my 800RB and connected it to 2 Bag End S15 cabs. It’s awesome. Love the tone and the grind of the Drive circuit.

Find a better one if you want one. Yeah, if it breaks, it will be difficult to fix. But if it’s working, it will be good for an awhile, and if you take care of it , it should run for a long while.

Oh, and a 400RB also sounds amazing. You won’t lose much with one of those as well. The later ones have more power.

 

[Michael Eisenman]

What is the best way to get the “rails” sound on a 400RB Mark-IV (280 watts)?

 

[stockplus]

I have had two 800RBs and they are great. Wait around and pick up one in good cosmetic and functional shape. GC used has them show up from time to time and the return policy is great.

I have a softer spot in my heart for the 400RB, though. I really love that amp and it's my favorite GK. Honorable mention goes to the newer 700RB-II and 1001RB-II, of course.

 

[ProbablyTooLoud]

What is the best way to get the “rails” sound on a 400RB Mark-IV (280 watts)?

High boost low master.

 

[agedhorse]

High boost low master.

Or maybe you mean the opposite?

The master has to be high enough for the signal to “hit the rails” (power amp clipping).

 

[ProbablyTooLoud]

Or maybe you mean the opposite?

The master has to be high enough for the signal to “hit the rails” (power amp clipping).

No, I should have said "higher boost than master." When I had this amp that's what worked.

 

[73_Blazer]

I have the 800RB, Mesa Carbine M3, Genz Benz GBE 600 and Mesa Walkabout and they all sound great. There is no reason to get crazy about it. I could be happy with just either one. My opinion is the cabinets make more of a difference in your tone

 

[Kindness]

Or maybe you mean the opposite?

The master has to be high enough for the signal to “hit the rails” (power amp clipping).

Remember, there are three gain controls on the face of the amp: volume, boost, and master. You use the volume to set the appropriate preamp gain staging, use boost to hit the rails, and use the master to control the overall output. If you want that distorted sound, you favor pushing the boost more than pushing the master.

 

[agedhorse]

No, I should have said "higher boost than master." When I had this amp that's what worked.

That’s preamp distortion, not power amp distortion (which is what “hitting the rails” means in this context).

 

[Wasnex]

You use the volume to set the appropriate preamp gain staging, use boost to hit the rails, and use the master to control the overall output.

The Boost stage is a FET that is designed to emulate a pushed tube. Adjusting either the Volume or Boost will effect the level at the FET. If the signal entering the FET is hot enough the circuit will compress. An even hotter signal will cause the circuit to break into drive.

Although people may call pushing the Boost circuit into drive territory hitting the rails, that is not correct. The rails are the + and - DC power supply lines. Hitting the rails occurs when the signal swing in the output section approaches the voltage of the power supply rails and can go no higher. In other words it only occurs when the output section is pushed beyond it's limits.

To set the input level we assume one would turn up the Volume until distortion occurs in the preamp and/or tone stack. IME the resulting distorting tends to come on quickly and is harsh. Most consider it undesirable. So the normal process is to back the Volume of just a touch so the signal passing through the preamp and tone stack to the Boost circuit is always clean.

The 800RB I used had some problems with signal to noise ratio. So it was necessary to adjust the Volume, Boost, and Master to keep the hiss as low as possible. In other words, I wasn't able to simply turn up of the Volume till distortion occurred and then back it down a hair. Instead the ideal setting varied with how the tone controls were adjusted.

Here is the block diagram of the 800RB. Adjusting the Volume effects the signal at all points after the Volume control. Adjusting the Boost Control effects the signal at all points after the Boost Control

The Boost switch is drawn is not accurately. Here is the actual schematic of the Boost circuit. There may be some variation of different production runs of the amp. The Boost switch is connected to A. The switch toggles a Relay (U5). In one position the signal is taken off the bottom of the Boost control (R82) at pin 4 of U5. I.E. the signal is dropped by the entire 50K resistance of the Boost control. In the other position, the signal is taken off the wiper of the Boost control at pin 5 of U5, so the resistance seen by the signal is variable from ~0 ohms to ~50K.

J113 is the FET.

 

[ProbablyTooLoud]

That’s preamp distortion, not power amp distortion (which is what “hitting the rails” means in this context).

really

 

[agedhorse]

really

Yes, really.

 

[Wasnex]

In the other position, the signal is taken off the wiper of the Boost control at pin 5 of U5, so the resistance seen by the signal is variable from ~0 ohms to ~50K.

Sorry...it's pin 3 of U5 .

 

[Kindness]

Thanks Wasnex. I was mistaken about where the boost circuit sat in the circuit!

 

[ProbablyTooLoud]

Yes, really.

I never knew the distinction