Double Bass GK 200mb repair

[fdeck]

I just disassembled my MB200-112E, that's around 17 years old, due to scratchy pots. Now it's like new. It's pretty beat up, as it saw a lot of gigs and I never got the carrying case for it.

I'd love to see a Class-D version. Today, I think there are some possibilities for ported speaker designs with smaller cones that could outperform the sealed 12"er, in the same space. But Bob Gallien's design is an extremely well thought-out solution to a hard design problem -- getting gig worthy bass sound out of a cubic foot of amp.

 

[Ric Vice]

I just disassembled my MB200-112E, that's around 17 years old, due to scratchy pots. Now it's like new. It's pretty beat up, as it saw a lot of gigs and I never got the carrying case for it.

I'd love to see a Class-D version. Today, I think there are some possibilities for ported speaker designs with smaller cones that could outperform the sealed 12"er, in the same space. But Bob Gallien's design is an extremely well thought-out solution to a hard design problem -- getting gig worthy bass sound out of a cubic foot of amp.

Francis,
Just what is the topology of the power amp and power supply used in the MBS and MBE? My 200MB's had a small transformer that was bolted to the underside of the cabinet. When the MBE was released it had a different power supply, which wasn't Class D. Rail Switching was mentioned in the GK literature.

Ric

 

[MR PC]

Besides the eq, my favorite features of the 200MB combo is the analog power and the sealed cabinet. The new GK MB mini combos are great, but they sound much different then the old metal ones.

 

[fdeck]

Francis,
Just what is the topology of the power amp and power supply used in the MBS and MBE? My 200MB's had a small transformer that was bolted to the underside of the cabinet. When the MBE was released it had a different power supply, which wasn't Class D. Rail Switching was mentioned in the GK literature.

Ric

My 200MBE has the transformer under the cabinet, and a conventional Class-AB power amp. Historically, the earliest 200MB head had a switchmode power supply, with a reputation of being unreliable, and the head was discontinued. Literature on a later revision of those amps (so called series 3?) suggested that they had gone to rail switching, but I don't have one of those.

Of course TB'er @rag would remember all of this better than me.

 

[Ric Vice]

My 200MBE has the transformer under the cabinet, and a conventional Class-AB power amp. Historically, the earliest 200MB head had a switchmode power supply, with a reputation of being unreliable, and the head was discontinued. Literature on a later revision of those amps (so called series 3?) suggested that they had gone to rail switching, but I don't have one of those.

Of course TB'er @rag would remember all of this better than me.

Francis,

Very interesting, I had both incarnations of the 200MB a Series I and later a Series II. Personally, I never had any problems with my Series I (SMPS?) or my Series II.

As I didn't have the Series I for a long time, perhaps there wasn't time for me to have issues with the SMPS version. Though if memory serves me, it had a transformer just like the Series II,

but that was a long time ago. Are your referring to the 200RCB which was the head only version? I still have the RCB which I loaned to a friend when his Aguilar went down.

Ric

 

[robgrow]

Until relatively recently, not many amps (besides WW) had switch mode power supplies. I've used various GK amps since the mid 80's, and have never seen one with a SMPS. Perhaps I missed one.

There are actually multiple versions of the venerable GK MB150 combo amp: Early models had class AB amps with a standard EI power transformer in the speaker cabinet, which was constructed of multiple aluminum panels that were screwed together like the 200MB. The current MB150 Series III is very different using a class H amp (with a rail-switching power supply), a toroidal power transformer mounted up top with the amp, and the speaker cabinet is one piece of welded aluminum construction. There are also multiple versions of the preamps.

 

[Ric Vice]

Until relatively recently, not many amps (besides WW) had switch mode power supplies. I've used various GK amps since the mid 80's, and have never seen one with a SMPS. Perhaps I missed one.

There are actually multiple versions of the venerable GK MB150 combo amp: Early models had class AB amps with a standard EI power transformer in the speaker cabinet, which was constructed of multiple aluminum panels that were screwed together like the 200MB. The current MB150 Series III is very different using a class H amp (with a rail-switching power supply), a toroidal power transformer mounted up top with the amp, and the speaker cabinet is one piece of welded aluminum construction. There are also multiple versions of the preamps.

Bob,
By my count there were 4 versions
of the MB preamps 200MB Series I and Series II, MBE and MBS. My favorites were the 200MB's.

Ric

 

[fdeck]

Francis,

Very interesting, I had both incarnations of the 200MB a Series I and later a Series II. Personally, I never had any problems with my Series I (SMPS?) or my Series II.

As I didn't have the Series I for a long time, perhaps there wasn't time for me to have issues with the SMPS version. Though if memory serves me, it had a transformer just like the Series II,

but that was a long time ago. Are your referring to the 200RCB which was the head only version? I still have the RCB which I loaned to a friend when his Aguilar went down.

Ric

Maybe the head-only version was the only one with the switching supply. Now, the reliability thing could have been just a few bad apples, so I'd say that if your unit is fine, then it's fine.

My 200MB schematics from GK include the switching supply, but that doesn't make it clear which subassemblies went into which models.

 

[Ric Vice]

Maybe the head-only version was the only one with the switching supply. Now, the reliability thing could have been just a few bad apples, so I'd say that if your unit is fine, then it's fine.

My 200MB schematics from GK include the switching supply, but that doesn't make it clear which subassemblies went into which models.

Very interesting,

Could you post a picture of that particular schematic. I'd love to take a look at it and compare it to the Walter Woods MI-100-8 schematic. I own a 200RCB that we could compare it to.
There were apparently early switchmode
power supplies used in automobile radios.

Ric

 

[fdeck]

Maybe the head-only version was the only one with the switching supply. Now, the reliability thing could have been just a few bad apples, so I'd say that if your unit is fine, then it's fine.

Very interesting,

Could you post a picture of that particular schematic. I'd love to take a look at it and compare it to the Walter Woods MI-100-8 schematic. I own a 200RCB that we could compare it to.
There were apparently early switchmode
power supplies used in automobile radios.

Ric

Time to dig into my files.

I once took apart an old car radio, probably from the 40's or 50's, that used tubes, and contained a "vibrator" which was like an electromagnetic buzzer that produced an alternating current for generating the high voltage supplies.

 

[Ric Vice]

Time to dig into my files.

I once took apart an old car radio, probably from the 40's or 50's, that used tubes, and contained a "vibrator" which was like an electromagnetic buzzer that produced an alternating current for generating the high voltage supplies.

Remarkable, so sometimes there truly is "nothing new under the sun". I had thought, that practical switchmode power supplies were likely something that NASA used in space capsules. Walter Woods alluded to this in his sales literature for his MI-100 and MI 225 series amplifiers.

 

[fdeck]

Remarkable, so sometimes there truly is "nothing new under the sun". I had thought, that practical switchmode power supplies were likely something that NASA used in space capsules. Walter Woods alluded to this in his sales literature for his MI-100 and MI 225 series amplifiers.

It's true that switchmode has a long history. Another example is the spark coil in a car, and televisions have some high frequency circuitry to develop their high voltages. But I'll bet that reliable regulated SMPS's are a newer technology, and they have experienced some pretty impressive development. I've seen some giant line frequency power transformers in early computers, and they wouldn't have done that if they could have used SMPS's reliably.

I once pulled up one of WW's patents, and he was doing some stuff that most people would say: "That can never possibly work," such as switchmode audio amplifiers using bipolar transistors. Things have improved greatly since then, perhaps in no small part due to the development of modern MOSFETs, magnetic components, and the ability to model and test high speed circuits. I think @agedhorse knows this history pretty well.

 

[Klotzbassman]

I have a GK 200mb that started distorting/crackling/buzzing. Took it to a local GK authorized service center - the first time it came back with the same problem, the second time they said they tightened a screw on the woofer. It is slightly better but still distorts especially on low notes. I called GK and they said they could not repair it because it is too old (1987).

Does anyone have any suggestions or experience with repairing this amp?

Probably a tired old speaker.

 

[dylanjohnson]

Probably a tired old speaker.

After gigging an early MB150S for about 20 years, the cab literally fell apart. It rattled at medium volume and I couldn't solve it. I even had GK send me a new empty cab, which didn't work because mine was the early version with the transformer in the cab like the 200mb. I stupidly tried to cut a hole in the cab to make it work and ended up messing the whole thing up. I will say that the early MB150S sounded good to me. Much better than the Series 3 or the later MB150S Series 2 amps. Maybe that different power section helped the tone?

Based on Ric and other peoples' posts here, I ended up with the 200mb and love it. It is starting to rattle again now about 6 years later. Time to go down the rabbit hole and see what's up with it.

 

[Ric Vice]

After gigging an early MB150S for about 20 years, the cab literally fell apart. It rattled at medium volume and I couldn't solve it. I even had GK send me a new empty cab, which didn't work because mine was the early version with the transformer in the cab like the 200mb. I stupidly tried to cut a hole in the cab to make it work and ended up messing the whole thing up. I will say that the early MB150S sounded good to me. Much better than the Series 3 or the later MB150S Series 2 amps. Maybe that different power section helped the tone?

Based on Ric and other peoples' posts here, I ended up with the 200mb and love it. It is starting to rattle again now about 6 years later. Time to go down the rabbit hole and see what's up with it.

Glad you were able to get your hands on one. I missed a particularly nice one on Reverb 2 years ago. It was offered with a 200MG.

 

[CanaDave]

Came across this thread and am now frightened since my MB200, reconed years ago, is now cutting to half volume with static after about an hour of playing at rehearsal volume, and the problem comes and goes from there.

I'll try the external speaker test and also using the effects return... but she's going to the amp tech, hopefully not her final journey!

 

[dylanjohnson]

Came across this thread and am now frightened since my MB200, reconed years ago, is now cutting to half volume with static after about an hour of playing at rehearsal volume, and the problem comes and goes from there.

I'll try the external speaker test and also using the effects return... but she's going to the amp tech, hopefully not her final journey!

I'm having that issue now too. Static and dropping in volume by about 3/4 and sounds compressed and fuzzy. My input jack is super noisy, too. I think it must have something to do with the input.

 

[Earl]

Try putting one end of a cable into send and the other end into return.

 

[dylanjohnson]

Try putting one end of a cable into send and the other end into return.

Will do. I had that problem with my 800RB years ago. Should have thought of that! Thanks, Earl!

 

[Ric Vice]

Came across this thread and am now frightened since my MB200, reconed years ago, is now cutting to half volume with static after about an hour of playing at rehearsal volume, and the problem comes and goes from there.

I'll try the external speaker test and also using the effects return... but she's going to the amp tech, hopefully not her final journey!

Try cleaning all the jacks with Deoxit 5
that could help. If you haven’t already done this.