KMD GV-100S Problems

[dpm309]

I have a KMD GV-100S head on my bench that is experiencing a couple of problems.  This is actually a hybrid amp with solid state pre-amp and tube power amp.  The first is after each note there is a compressed/distorted/clipped sound. I read that this could be a bias problem and I will check and re-adjust the bias if necessary. The second is that when you turn on the amp, sometimes there is no sound coming out on either of the standby settings (high or low). When I touched the 1st filter cap with my multi-meter probe, there was a loud noise and then the amp worked perfectly. Here are my voltages with the standby switch on the high setting:
 
                      1       2        3        4        5         6         7          8
V1 6L6            -       -      772      386    22         -      6.8 AC    68
V2 6L6          22*     -      772      386    22        -     6.85 AC     68
 * - Pins 1 and 5 on V2 connected together.
1st filter cap - 772 2nd filter cap - 386

These amps use a dual-rail power supply to get 100 watts output from 2-6L6s so that is why the plate and B+ voltages are so high
 
On the low standby setting (I assume some kind of output attention) I get 514 V on the first filter cap and 256 on the 2nd. Pin 3 is 514V and pin 4 is 256V.

I was able to re-create this several times and it came back each time I touched my probe to the first cap. What could be causing this?

I have attached a copy of the schematic from Kaman. They were very helpful and apologized for the poor quality of the schematic as it has been scanned an re-printed several times. Would anyone have a clearer copy.
 
Thanks,
 
Dan
 

[Willabe]

The OP stage is along the lines of a Music Man amp. It is hard to see. The plate is not really seeing 700+dcv. The cathode is elevated, (about 60+dcv at idle ?), which is subtracted from the plate voltage. Also the OP stage is biased differently than what is usual (class B?), the control grid has a +dcv on it to go with an elevated cathode from cathode driven (not grid driven) OP tubes. Note also that the screen dcv on both of these amps is about half of the plate dcv.

Look at this MM schematic, it may help you, much easier to see;

http://www.el34world.com/charts/Schematics/files/musicman/musicman_2165-rp_&_2100-rp.pdf

And no I don't understand any more of this than what I wrote, which could be wrong.    

Clapton and Johnny Winter were among those who were using MM amps when they first came out, but stopped using them live after the MM amps went up in flames a few times in the middle of a show.          


         Brad      

[dpm309]

I went over all of the soldered connections and re-soldered the 2 filter caps.  I am still getting the problem of no sound until I either touch my MM probe to the caps or plate pins on the power tubes or touch the tip end of a guitar cord plugged into the amp.  When this happens, I get a loud pop and I get sound, instead of a hum, through the speaker.  Once in a while, the amp will power up fine.  When the amp comes back up it sounds good on both channels.  The filter caps have to be almost 30 years old but do not exhibit outward signs of failure.  Could the old filter caps be the cause of this problem?

Dan

[Rev D]

 I don't know if it'll fix it or not but after 30 years I'd say they (the caps) have done they're job and should be replaced anyway, it would at least eliminate that variable and it would be a expected repair of an amp this old. Just an opinion though. I'm sure others will weigh in with more exact prognosis on the problem. Good luck with it!

Best regards,

Don

[PRR]

There's an almost-touching contact. One big JUMP of signal micro-sparts it into working for a little while.

Start with loudspeaker jack and connections. Could just be tarnish.

Use clip-leads to monitor B+ before, during, and after your probe-start.

Again, only monitoring grid voltage. Then cathode voltage.

If it isn't jacks or jack-solder, it may come down to extracting the PCB and examining it with bright light and magnifier for cracked solder, solder not wetting leads, any darn thing.

[dpm309]

Well I installed new caps and touched up a few solder joints and the amp was working fine after several hours of running.  The client called back today and said it was doing it again.  I will probably have to go over the whole thing with a fine tooth comb to see if there are any other cold joints, etc as PRR suggested.  I will also clean up the speaker jacks.  He also said he is getting nothing out of the effects loop so that is where I will look first.  I did re-solder the effects loop jacks and cleaned them previously because when I tapped them with a chopstick, the amp cut out.  Did not happen again after I re-soldered and cleaned up both jacks.  Back to the drawing board.  The amp sounded and acted great when he picked it up.  Gotta love these intermittent problems.

Dan

[dpm309]

Got the amp back yesterday and there was no sound coming from channel 1.  The owner brought along his Boss GT 8 effects board and hooked it up and was able to get some sound out of channel 2 but it was a bit distorted/clipped and cut in and out.  He had the leads reversed on the effects previously so that is why he was not getting any sound at all.  Pulled the chassis and hooked up the speaker and everything is now working fine.  I tried a couple of my effects (Tube screamer clone, MXR phase 90 & Danelectro chorus) and these all worked using the effects loop or straight into the amp.  Let the amp run for several hours and chop-sticked everything and did not get any pops or cutting out.  I am wondering if the problem might be with the Boss unit.  This was hooked up using the 4 cable method (send, return, output of unit into the input jack, and guitar plugged directly into the unit).  When using the 4 cable method, the unit acts as the pre-amp until you switch it to bypass.  We also tried the unit directly into the input of the amp with no change.  Also want to mention that before the I plugged in the unit, I could not get anything sound out of the amp until I hit the reverb/channel footswitch a couple of times and then only channel 2 worked.  I previously cleaned the footswitch jacks and switches earlier. 

Since I have not been able to recreate the problem on my bench, I am at a loss as to what is going on.  Will run it for a few more hours today to try to get it to cut out so I can check voltages before and after.  Any ideas?  Have been on Boss' forum and cannot find anything there.

Thanks,

Dan

[dpm309]

I think I ruled out the Boss unit as the problem.  As soon as I pull out the 1/4" stereo jack out of the footswitch, the amp cut out until I touched or shook the end of the cord.  Removed a few inches of the cord and re-soldered the jack and also re-soldered the contacts on the board where the 3-conductor footswitch wire is attached.  I tested the cable and jack and there are no shorts or intermittent connections.  When the amp was in cutout mode, all of the B+, plate, and cathode voltages were unchanged and I get no sound or signal out of line out.  The problem seems to lie somewhere in the pre-amp or switching circuits which are all solid state.  

Have been running the amp for a while after re-soldering the footswitch connections with no cutting out.  Now when I pull out the footswitch cable, the amp cuts out sometimes when I touch the plug with my fingers and then comes back in when I touch it again.  Other times, it doesn't do it.  Could a small static electricity charge be causing something in the switching circuit to cut out?

Re-soldering the footswitch connections seems to have improved the problem a bit but I it is still cutting out.

Dan

[Willabe]

When the amp was in cutout mode, all of the B+, plate, and cathode voltages were unchanged and I get no sound or signal out of line out.  The problem seems to lie somewhere in the pre-amp or switching circuits which are all solid state.

I think you found a good clue.      

There's an older book that AES sells called Electric Guitar Amplifier Handbook, by Jack Darr. It has both SS and tube trouble shooting info. It may be helpful to you.

In Doug's library of info he there's info on how to us a small amp as a signal tracer. I seem to remember Darr doing that also? I'll take a look in there and get back to you.

Look in here for a signal tracing amp.

http://www.el34world.com/Hoffman/tools.htm


                           Brad        

[Willabe]

Hi Dan,

All right I read the book a little in the ss trouble shooting section. He's checking for signal gain/voltage at the input and output of each transistor gain stage with a scope.

I don't see why you couldn't do the same thing but using Doug's listening amp instead of a scope. Run a signal (CD player?) into the input and trace to where it stops, then figure out why that stage in not passing signal.    

It looks like the IC chips are mostly using pins 2/6 as input and pins 1/7 as output. There's coupling caps on/from the output, pins 1/7, to the next stages input, pins 2/6, just like a tube gain stage. You can test for your signal there.

Check the wiper on the treble control in the tone stack for signal. Also check for signal at the send and return of the Fx loop. I can't tell from the schemo but the jack's have to have a switch contact on them to pass the signal when nothing is pluged into them ? If so they could be the problem, contact could be corroded or not making good contact, staying open. You can use an aligator clip test wire to jumper across them on/from the tip to tip of the 2 jacks.

Edit; I just went back and reread the thread and see you've already fixed the Fx loop jacks.
                                          
                                Brad        

[dpm309]

Willabe, thanks for the responses and the suggestion for the listening amp.  I have built one of these and have used on several occasions.  Have not been able to re-create the problem since I re-soldered the footswitch leads so I am keeping my fingers crossed that this solved the problem.  If it does cut out again, I will use the listening amp to trace out the circuit.

Dan

[ernisan]

can somebody please help me... i have squealing amp problem.. already replaced the tubes...

[PRR]

What make/model?

Had it been modified?

When did it start?