Solid State Power Amp Section Question(s)

[Sleet]

I am a hobbyist working on a repair and restoration of a Gallien Krueger GK-210 guitar amplifier. I have attached a schematic and several pictures of the actual amplifier for reference. I am trying to work out a problem with the amp randomly breaking up while playing at all volume levels. As for the restoration part, I'm planning on replacing the electrolytics throughout the amp as they seem to be period correct and most likely original.

One thing I thought that was curious was that the leads connecting the power transistors use spade lugs (see Q9, Q10 and #4 orange and #6 green leads on the schematic) to connect to the output transformer. Would it be wise to perhaps solder these leads to the spade connectors on the transistors? I was also considering renewing the thermal paste on the heat sinks and output transistors.

I have yet to do any live measurements on any of the circuits. I have a Fluke Multimeter, but nothing else for test gear at the moment.

This amp seems to be well made and unhacked by anyone. I want to use it as learning experience and end up with a fun amp to play with in the future.

Thanks for any feedback.





210G Power Amp 60015.pdf

[Randall]

I doubt the electrolytics or heat paste have anything to do with your problem, and are probably just fine. You don't need to solder those wires to the terminal, either. Give them each a firm but measured tug with some pliers gripping the female connector. Does it slip right off? I so, give it a little crimp and firmly push it back on. The connector ends dig into the terminal and makes a suitable connection. If it is not easy to pull off, then you could put a little drop of DeOxit on each connector and work it up and down a bit to reseat it, but solder them? No.

Look at the schematic at the lower right. See the power amp in jack? That's the first place to go. Notice how in the upper left the preamp cable delivers the signal down to the Amp In jack. That little arrow is a switch, which the signal must pass through when no plug is inserted in that jack. If that connection in that switch is perfect, the signal then goes back up to C1 and on it's merry way. BUT, if that connection is less than perfect, intermittant say, then the signal gets compromised, scratchy, or even goes away. Clean that jack, or more accurately the blade switch on that jack. I like to spray some DeOxit on a sliver of coarse business card or other thick paper stock and get it in between those blades with no jack inserted, and work it back and forth.

There is a thread at the top of this forum called The Dreaded Switching Jack, I suggest reading it.

[TomCarlos]

Echoing what Randall said. Get yourself some DeoxIT and clean the jacks. For the pots, use DeoxIT F5 - that is the can which is a combination cleaner and lube.

Take a good look at those connections and solder joints. Make sure you do not see any cracks anywhere in the traces.

The good news is that you have output! The next step is to figure out why it is breaking up. The hints thus far is step 1. (That is what I have learned to do).

[Sleet]

I doubt the electrolytics or heat paste have anything to do with your problem, and are probably just fine. You don't need to solder those wires to the terminal, either. Give them each a firm but measured tug with some pliers gripping the female connector. Does it slip right off? I so, give it a little crimp and firmly push it back on. The connector ends dig into the terminal and makes a suitable connection. If it is not easy to pull off, then you could put a little drop of DeOxit on each connector and work it up and down a bit to reseat it, but solder them? No.

Look at the schematic at the lower right. See the power amp in jack? That's the first place to go. Notice how in the upper left the preamp cable delivers the signal down to the Amp In jack. That little arrow is a switch, which the signal must pass through when no plug is inserted in that jack. If that connection in that switch is perfect, the signal then goes back up to C1 and on it's merry way. BUT, if that connection is less than perfect, intermittant say, then the signal gets compromised, scratchy, or even goes away. Clean that jack, or more accurately the blade switch on that jack. I like to spray some DeOxit on a sliver of coarse business card or other thick paper stock and get it in between those blades with no jack inserted, and work it back and forth.

There is a thread at the top of this forum called The Dreaded Switching Jack, I suggest reading it.

Thanks, to both of you for your replies. I have read the FAQ on the "dreaded switching jack" and will clean things up paying close attention to the blade switches of the jacks. I have some DeOxit so I can see if it helps.

[Mick Bailey]

My advice with SS amps is unless there's a clear fault, leave it alone. Electrolytic caps have a far easier life in SS gear than in tube amps. OK, the odd one may go high ESR (maybe build an ESR meter and check them?) but they generally last. You risk lifting pads and there are only so many heat cycles those PCBs can stand before you get into trouble. The thermal paste is there for the lifetime of the device and isn't a service item. If you ever need to replace the devices, that's the time for fresh paste.

Each time you work on an amp there's a risk. No matter how many you work on, and whether you're a full-time pro or amateur, you can make a mistake or have an accident. Fix what's wrong with the GK and enjoy it. Maybe get a cheap amp that really needs some attention as a restoration project.

[g1]

The output transformer is interesting and fairly uncommon for SS power amps. I've not seen this GK model before.
Those screws and spade lugs on the power transistors look fairly corroded and could use a cleaning.

[J M Fahey]

Agree and add: those spade lugs are there for a purpose: this amp has most electronics parts mounted on a PCB, which might be removed for testing/replacement, while the large heavy output transformer is firmly bolted to amp chassis, so some kind of connector between them is advisable .

Power transistors might have TO3 metallic collectors connected to some PCB pad, and the board might have some kind of connector where transformer wires are plugged using their own mating ones.

GK simplified this by mounting terminals straight to the cases: brilliant.

Agree those screw heads are corroded; I´d pull them, squirt some Deoxit or wd 40 on them, let it act a little and wipe them withy some clean cloth. Rinse and repeat (literally)-

Leave a thin film of oil on the metal or simply wipe them after mounting with an oil soaked Q Tip.
As good as new for the next 30 years.

[Sleet]

Juan,

Thanks for the input. I will make sure to clean the screw heads and spade lugs before reassembling the amp. I'm hoping this along with cleaning the input and speaker jacks will eliminate my "break up" problem.

[Mick Bailey]

From the pics it looks like those screws have a greenish tint to the oxide. Zinc usually goes grey/white. I'm always careful not to create or breathe dust off old corroded parts when cleaning them and wear disposable gloves, just in case they used cadmium for the plating. Greenish may indicate nickel. Still not good to breathe in.

Old 60s AC30s are one of the worst - they often have a thick furry coating of yellow cadmium oxide on the back of the pot cases and lamp holder. Really nasty.

[Sleet]

Mick,
I will be sure to clean this off outside, heavy metals are very bad to ingest. Thanks for the heads up.

[teemuk]

I think the corrosion of screws and bolts is largely just a cosmetic issue. A nice finishing touch when you don't have to worry about everything else working. I would excessively worry about it only if the screw actually carries an important electric contact (e.g. ground, "bolt" capacitor terminals, etc). In case of those output transistors a more worrying factor should be proper thermal coupling between device cases and heatsinks. e.g. Is connection solid, is pressure applied evenly.

Correct me if I'm wrong but isn't corrosion sometimes even accelerated by close vicinity of different kinds of metals. e.g. I think that little copper "thermal spreader" (which aids thermal coupling of the bias transistor and output transistor), for example, makes that mounting screw corrode faster. Anyway, that really shouldn't affect operation of the amp.

If - and that's if - the issue is caused by an intermittent contact then the contact could be pretty much anywhere in the amp - jacks of the "FX loop" are one suspect, and indeed very common cause of intermittency. So are dirty and worn potentiometers, switches, connectors and failed solder joints or lifted pads. If you want to limit your troubleshooting area then you can exploit the FX loop feature to find out whether its a preamp or power amp -based issue.

It's a bit different power amp design allright. More like a typical tube power amp, except devices are solid-state, which allows the circuit to operate at much lower impedances and operating voltages. The circuit architecture has nice and fairly gradual overdrive characteristics and moderately poor damping factor. All of this is enhanced by applying very low magnitude of NFB, which - like in tube amps - is probably restricted by transformer coupling.

There is also a nice little detail, which is quite poorly illustrated by that circuit diagram only: These amplifiers have a "Current Limiter" feature! No, I'm not talking about that generic "VI limiting" circuit of the output transistors, I'm talking about current limiting of that phase inverter stage. See the tail circuit? See how it eventually hooks to connector terminal labeled "C.L", from which it receives negative DC bias voltage for the LTP? If I remember correctly this goes to preamp board where you have a potentiometer that adjusts the DC bias voltage. This control essentially adjusts "headroom" of the phase inverter stage. Without much NFB the differential circuit also overdrives gradually and provides impressively soft clipping. The actual power stage will have much "harder" clipping characteristics, albeit impressively "soft" for a solid-state amp.

[Sleet]

Teemuk,

I've attached a copy of the preamp schematic for the amp. If there is anything else I should consider based on my problems with output breaking up while playing, I would appreciate your thoughts. Again thanks to all of the respondents to this thread, I have learned a lot in a short period of time.

210G Preamp 60025A.pdf

[The Dude]

I noticed that the output transistor in the picture is an SK generic substitute, which means the amp has likely been worked on before. It's possible there are other substitutes in there as well and possibly not fully compatible. It's also possible that some resistors may be out of tolerance and missed the first time it was repaired. I would give it a good once over and make sure everything is as it should be. Also, you might try biasing the amp per the directions on the schematic for a couple of reasons. First, it may not have been done when the first repair took place. Second, it may lead you to finding a problem if the bias procedure can't get you the current measurements specified.

[Sleet]

I noticed that the output transistor in the picture is an SK generic substitute, which means the amp has likely been worked on before. It's possible there are other substitutes in there as well and possibly not fully compatible. It's also possible that some resistors may be out of tolerance and missed the first time it was repaired. I would give it a good once over and make sure everything is as it should be. Also, you might try biasing the amp per the directions on the schematic for a couple of reasons. First, it may not have been done when the first repair took place. Second, it may lead you to finding a problem if the bias procedure can't get you the current measurements specified.

I've been thinking about what you said about someone may have previously replaced the output transistor(s). There is evidence of rework as seen by the amount of flux residue around the output and bias transistors. Also the screws fastening the power transistors seem slightly damaged by someone removing them. I could be overthinking this but I'm now curious about this.