Gallien Krueger MB500 "Master" pot not working?

[Airfish]

Hello.

I have purchased a used GK MB500 head, seems to be in excellent condition, but I have a control issue.

When I turn the GIVE circuit OFF, the volume should be controlled by the "Master" pot, according to the original GK user manual.

But the Master pot on my amp does absolutely nothing and I can only control the volume with the input Gain pot.

N.B. I have the amp connected via the DI out, in "post-EQ" setting.

Is this a technical issue and should I ask for a repair? I have a one-year warranty from the seller.

Or is this just some control panel specifics that I do not understand and you could give me advice on?

Thank you very much in advance for your help.

 

[darkwasthenight]

Not affiliated with GK so there will be others with more insight into this, but based on your post you are running the signal from the DI only and not into a cabinet? If that's the case the Master will probably not affect the DI output so you are able to alter your stage monitoring volume without changing levels going to FoH.

Hopefully I haven't misunderstood your post.

 

[Redbrangus]

The DI signal output is before the master volume control, regardless of whether it's switched into 'pre' or 'post'. If you had a speaker connected to the speaker output, the amp should perform as the manual states. This is the way it's supposed to work, not a failure or problem.

Edit: The last page of the manual has a block diagram showing the signal path -- that should help explain how the direct output relates to the amp output.

 

[Airfish]

@darkwasthenight & @Redbrangus: Thank you, guys! That is exactly what I wanted to hear! That means my amp should be OK. Will check it with a cab later this week, just to be sure.
Just some additional explanation: I got confused about that Master not working, because the "Master-B" is working, even with the DI outuput. But I guess "Master-B" is probably before the DI, not after it, like the "Master".

 

[Airfish]

@Redbrangus The block diagram.... good point, thank you. But I´m sorry, I don´t understand those, I am a technical village idiot... Do you understand that diagram? And can you please confirm that Master-B does affect the DI signal, but Master does not? Thank you again!

 

[Redbrangus]

Which master pot affects the amp/speaker signal depends on whether the GIVE circuit is switched in or not, but my interpretation of the block diagram is that neither is supposed to affect the direct out. If your master pots -- either of them -- are changing the DI output level, it's possible the diagram isn't correct, or maybe I'm misreading it. The likelihood that this is some sort of defect or wiring error in your particular unit is essentially nil.

 

[Airfish]

Which master pot affects the amp/speaker signal depends on whether the GIVE circuit is switched in or not, but my interpretation of the block diagram is that neither is supposed to affect the direct out. If your master pots -- either of them -- are changing the DI output level, it's possible the diagram isn't correct, or maybe I'm misreading it. The likelihood that this is some sort of defect or wiring error in your particular unit is essentially nil.

Master B is changing the DI level (if GIVE is switched on), but Master is not. Anyway, your trust in the technical state of my unit is encouraging, so thank you for that.

 

[darkwasthenight]

From the block diagram Master B shouldn't change the DI Level, although Boost should. As the Master B acts to keep the volume under control when you add the GIVE it's possible it was kept in the circuit to control jumps in level from the DI, but I don't have the amp and can only read off the block diagram and manual.

Agree with @Redbrangus that it's very unlikely anything is 'wrong' with your amp and would recommend you check through a cabinet at volume to understand how the various Master controls interact, after which it may seem more intuitive for you.

 

[Al Kraft]

I'd give Jason a call at GK to get the definitive answer. Based on my read of the manual (which admittedly looks like it could have benefitted from better proofreading) I think the signal path diagram may be incorrect. This is from the manual:

Changing the Pre/Post EQ switch to “Post” ( In),sends the "GK Sound" (EQ settings etc.) andpreamp response to the Direct Out jack. This gives you complete control over the Direct Out’s tone and volume.

From the signal flow diagram, it appears that the headphone output and unbalanced direct out both come after the Boost circuitry...which is where you can add some "signature" GK growl to your tone (see page 9). Part of the "GK sound" (which is is more than just EQ but part of the "etc." underlined above) is provided by the Boost circuitry. Therefore I would conclude that the balanced DI output would include the Boost circuit functionality and you would achieve that by working with the Boost and Level B controls.

You can test my theory by pressing the Boost switch and fiddling with the boost and master B controls. If it works the same way as it does in the headphone out or unbalanced (1/4 ") line out we know that the signal flow diagram is not accurate in all areas.

That should work until you can get in touch with Jason. I hope that's helpful.

 

[Al Kraft]

I just found another version of the MB500 manual and it has an even less detailed signal flow diagram than the one I was looking at when I made the above post.

 

[Airfish]

I'd give Jason a call at GK to get the definitive answer. Based on my read of the manual (which admittedly looks like it could have benefitted from better proofreading) I think the signal path diagram may be incorrect. This is from the manual:

Changing the Pre/Post EQ switch to “Post” ( In),sends the "GK Sound" (EQ settings etc.) andpreamp response to the Direct Out jack. This gives you complete control over the Direct Out’s tone and volume.

From the signal flow diagram, it appears that the headphone output and unbalanced direct out both come after the Boost circuitry...which is where you can add some "signature" GK growl to your tone (see page 9). Part of the "GK sound" (which is is more than just EQ but part of the "etc." underlined above) is provided by the Boost circuitry. Therefore I would conclude that the balanced DI output would include the Boost circuit functionality and you would achieve that by working with the Boost and Level B controls.

You can test my theory by pressing the Boost switch and fiddling with the boost and master B controls. If it works the same way as it does in the headphone out or unbalanced (1.4 ") line out we know that the signal flow diagram is not accurate in all areas.

That should work until you can get in touch with Jason. I hope that's helpful.

Thank you for your detailed post. Meanwhile I received a reply from GK tech support that I contacted previously. The guy who wrote the reply was called "Jose Reveles", he confirmed that both Master pots are AFTER the DI out, he even sent me detailed schematics of the preamp. So the only thing left for me to do is check, if the signal is OK if I connect the amp to a cab, which I will only be able to try later this week or next week.

 

[Airfish]

From the block diagram Master B shouldn't change the DI Level, although Boost should. As the Master B acts to keep the volume under control when you add the GIVE it's possible it was kept in the circuit to control jumps in level from the DI, but I don't have the amp and can only read off the block diagram and manual.

Agree with @Redbrangus that it's very unlikely anything is 'wrong' with your amp and would recommend you check through a cabinet at volume to understand how the various Master controls interact, after which it may seem more intuitive for you.

Yes, you are right, from the whole "GIVE" circuit only the Boost pot influences the DI volume, the Master B pot does not influence it - meanwhile I received an answer from the GK tech support, they confirmed it too.

 

[Al Kraft]

Thank you for your detailed post. Meanwhile I received a reply from GK tech support that I contacted previously. The guy who wrote the reply was called "Jose Reveles", he confirmed that both Master pots are AFTER the DI out, he even sent me detailed schematics of the preamp. So the only thing left for me to do is check, if the signal is OK if I connect the amp to a cab, which I will only be able to try later this week or next week.

Then it would appear that the signal flow diagram is correct according to the GK factory tech's answer.