Gallien Krueger 250RL Setup procedure questions.

[RogerT]

Anyone that can lend some clarity to a setup procedure for the above amp. Was blown, and missing parts, and each potentiometer has been mucked with, all maxxed on ten. Have parts in, and all pots turned back down, to prevent blow ups. As mentioned each were maxxed. It's been to another, before it came to me. My question: Says set the following: Loads A and B on 8 ohms. Got that. 8 ohm spks are plugged in. Next-sine wave input w/sig gen.@ (400Hz 160 Vrms)-16.0dB. Then am to turn up vol. A until output clips. My question, is the equation there. Never really dealt with much rms and dB since Devry, in 91, and was never clear on that, to be honest. Not understanding what they are asking, and how that -16dB comes into play. Been doing fixes for years, but any instruction, of understanding, what the actual input voltage at 400Hz, that they are asking for, would be greatly appreciated. There are additional instructions, but I think I can get that. Researched the web, and not getting a clear answer. I may be asking it wrong. Eventually I'm to set the output, at a clipping point. Says it will =18Vrms, at clip. Says continue to turn up volume. Output should slam into the rails. Check R and L amp. Then I'm to change it to 4 ohms loads, and set it = to 14 Vrms. I used to be Fender authorized, as a Silver tech, and have fixed amps for many years, but am no super tech, and this two page setup procedure has me baffled. Any help, would be appreciated. Thank you, much. ]if that helps. I think that it's listed already. Thanks again, guys.

[Enzo]

Hi Robert, welcome.

Please don't put your personal email in your posts, it can result in your address on spam lists the rest of your life. If you feel the need, spell out the dot and com or something so spam bots don;t cue in on it. There should be something in your profile page about receiving email from members.

It would be good form to post or link the service material you are working from, I for one do not have 250RL in my files.

Well, whadaya know, I opened my 250 ML file just for reference, and apparently the same manual covers the RL version. What the differences are I know not.

The stage gain trimmers maxed ought to make it unplayable for sure. What happens if you just center each one? I will look at the setup procedure, but never lose sight of the fact this is just a guitar amp, not a piece of lab gear. If setting things in the middle works, then it works. Close enough rules.

[g1]

I think you meant 160mV, not 160Vrms. That is also approx. -16dbV, so they probably just added that figure for people who have signal generators labelled in db. You can use either one.

[J M Fahey]

That amplifier uses Fets for clipping, difference to common use is that they clip *current* instead of voltage ( , it took me 20 years to realize wtf they were doing) , so each one has to feed a current-to-voltage converter to be back in the game.

These in practice are much simpler than they sound, just feed them into the - input of an Op Amp and read the Op Amp output.

Now unselected Fets which they seem to have used have characteristics spread all over the map, and that´s an understatement, so each following stage needs a trimmer to set its gain , not too different in concept to those kludgy pedals which "clone" tube schematics but replace each tube with a FET ... and need a trimmer on each drain.

So I guess that the general purpose adjustment for each stage is to feed them a clean sinewave the approximate level expected on each stage (which is shown in the schematic) and set the individual trimmer to the expected output level.

[Jazz P Bass]

I set the 'gain stage trimmers' while playing a guitar through the amp.

[RogerT]

Thank you, Enzo. Been reading your knowledge for a few years, now. Believe it, or not, that has fixed many things for me, over the years. Wish I worked right beside you. I love to learn things, am meticulous about procedures, etc. Have never had a good audio technician to ask, etc. and it means the world, to speak with you. Thank you. Going to set for mid points, and the sound does get better. Should the volume controls be cranked, or halfway open, etc? I'm going to plug in a guitar and go for nice sound, like some of the others have mentioned, also. I did get sound going out of it last night, by moving the pots, from max, but the back/heat sink of the amp started heating up pretty good. Figured things were still a bit set too hot. I did get the bias set on the amplifier circuit. Now, just to conquer what gets sent that direction. Thank you, sir.

[RogerT]

Thank you, for the clarity. It is a type on the instructions. They omitted the m, for millis. Much appreciate.

[52 Bill]

I did get sound going out of it last night, by moving the pots, from max, but the back/heat sink of the amp started heating up pretty good. Figured things were still a bit set too hot. I did get the bias set on the amplifier circuit.

Yes, don't confuse the two bias trimmers with the distortion and compression trimmers.

[RogerT]

Thank you for your responses. I have the same sheet you have with the "250ML/RL T/O Procedure," from the sound of things. Should I be converting these RMS values to peak voltages, for simplicity's sake-also mine? Example. Am using scope, and also Fluke 83 multi. Down on 7B on sheet-Sine input 400 hz., 160mVrms,) then the minus sign 16.0dB. I have converted the RMS value to 160mVrms using the equation Vrms=.3535X ptop voltage. So I had my Rms 160mVrms divided by the .3535, to get my peak to peak value. Is this the way you would approach this; getting it into voltages, like I'm used to reading on the scope? When I get this accomplished I will enlighten the board. Figured with you having the procedures in hand, I'd speak with you directly. I'm used to getting in, fixing it, and getting away from it, and not having to go through so many procedures. This is so close to our labs back in school, it's not funny. Always had a hard time, seeing, and understanding what I was supposed to be seeing, after we built the circuits on the breadboards. I tried another GK amp years ago, and it ate my lunch too, but nothing had been tampered with on all those pots. Thank you. Feeling like a pup, again. I swear to goodness. Ha.. sort of..

[RogerT]

Yes sir, That is mentioned in the steps ahead. Thank you.

[Enzo]

I wish I was as meticulous, I called you Robert instead of Roger.

[drewl]

Your meter measures in RMS and is more accurate than a scope.

The difference probably won't matter much.
The last one of these I fixed I followed the manual but tweaked a bit using my ears for final settings.

[RogerT]

Thank you all for the knowledge. I have the Fluke 83, and there is a retrofit for RMS. The 87 Fluke had that function, and they look identical. I did get some waveforms, on the scope, last night. There is a dead spot, in Vol. control A, and when it hits, it must go directly to ground. I'm sure it would love to blow my outputs. Not happening. (very determined, argh.)Channel B, is not sounding bad, but as for nice waveforms on the scope, they are distorted, similar to a youtube video, when the guy explains RMS voltages, etc. So, I know I'm on the right track. Going to continue today, after a nights sleep, and see, what I see. I'd like to get these instructions up here, for others, like Enzo was referring. I'm looking, on how. As for me, and Galien Krueger, grrrrrrr.. Thank you all, guys. Much appreciate. Were not done yet.

[Dave H]

I'd like to get these instructions up here, for others

Is this it?

Gallien-krueger_250ML,_250RL,_212SCL_Service_Manual.pdf

[RogerT]

Yes, sir, it sure is. My nemesis. Waiting to dive back in tonight. G1 is correct, there is a type O, and it is 160mvrms verses what it says as 160Vrms. I wish they were a bit more clearer, in all of their set up procedures. Going to whip this thing yet.