Gibson GA-8T tremolo fault - diyAudio
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Old 23rd July 2013, 07:58 AM   #1
osci is offline osci  Australia
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Default Gibson GA-8T tremolo fault

Hi all,

Just finished buidling a GA-8T amp,GIBSON GA-8T SCHEMATIC Service Manual free download, schematics, eeprom, repair info for electronics

the version I built uses the
1 x 12AX7
2 x 6BM8
1 x 5Y3GT

I've made some changes to the circuit namely that I used a 6V4 rectfier,

The issue I have is everything is working great except you can still hear the tremolo oscillate at a fast rate when the footswitch is clicked to off......The only way to stop oscillation is to turn the depth pot all the way down.

When the trem is switched on it works perfectly both the frequency & depth.

I've used two wire shielded cable from the footswitch to chassis, and use a 6 pin swtch with only two pins of the switch on one side used to switch the trem on/off.

Is this normal for a circuit of this vintage or is there a fault somewhere in the trem circuit?

I have made some changes to the circuit and they are starting with the trem, the 3rd cap from the plate of V3a I used a .022 in place of the .047.

I used a .027 cuppling cap between V1b & V2a

and used a 6V4 rectfier those are the only changes made


any advice is greatly appreciated,


cheers Ron
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Old 23rd July 2013, 09:17 AM   #2
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Lift the cold end of the 2k7 resistor. (cathode of V3a pin 8, not the cathode end), disconnect the switch end of the speed pot and connect the pot to ground. Connect the switched side of the foot switch (now disconnected from the speed pot) to the lifted 2k7 resistor. This will ensure V3a is biased off and will stop the oscillator.
This mod was used on later models.
Unfortunately the 6V4 rectifier will not last long, the cathodes will strip as it is being under run, 6V4 means 6volt heaters and the 5Y3 has 5volt heaters!
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Old 23rd July 2013, 09:36 AM   #3
Enzo is offline Enzo  United States
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Here is another place to download, and if you go back to the directory page, MANY other Gibson drawings are there.

http://www.webphix.com/schematic%20h...namps/ga8t.pdf

Interesting you would decrease one of the feedback caps, I almost always increase them because most trems don't go slow enough.

Isolate the problem. When you have this condition, does the speed control affect the speed of what you hear? We need to know the trem circuit is REALLY still tremming, as opposed to just adding some noise.

CLip a test wire to ground. Usually opening the switch should STOP the trem oscillator. SO have it running and verify proper operation, then click the trem switch off. Now with your grounded test wire, short to ground pin 1 of V3a - the grid of the trem tube. Does that stop this sound? Pin 1 is also the right side of the third feedback cap as wired. Just for science, ground the left side instead, That appears to be a 240k resistor to ground, we are shunting that. Same question, stop the sound? In fact if this does turn out to be trem still going, then maybe the footswitch shunting the 240k to ground might be more effective than opening the speed pot circuit as a trem kill.


Can you scope this circuit?
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Old 24th July 2013, 02:36 AM   #4
osci is offline osci  Australia
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Quote:
Originally Posted by JonSnell Electronic View Post
Lift the cold end of the 2k7 resistor. (cathode of V3a pin 8, not the cathode end), disconnect the switch end of the speed pot and connect the pot to ground. Connect the switched side of the foot switch (now disconnected from the speed pot) to the lifted 2k7 resistor. This will ensure V3a is biased off and will stop the oscillator.
This mod was used on later models.
Unfortunately the 6V4 rectifier will not last long, the cathodes will strip as it is being under run, 6V4 means 6volt heaters and the 5Y3 has 5volt heaters!

Hi Jon,

Thanks heaps for the response, I have a couple of questions about the mod you’ve described, I can be thick at times but my thickness is quite severe today J however,

1 – does this mod completely disable the tremolo, meaning the amp won’t have tremolo?

2 – Does the 2k7 resistor @V3a “still” go to ground with this mod”
or it doesn’t go to ground at all with this mod?

3 – I’m also assuming with this mod that only the middle pin of the speed pot is connected to the circuit?

I’ll have to wait till I get home from work tonight before I can make the mods.

Btw, I’m using the 6V4 rectifier simply because I only had a PT with the 6.3v heater winding, all up this amp has cost me virtually less than $30 as I already had parts laying at home looking for a use.
Cheers Ron.
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Old 24th July 2013, 02:37 AM   #5
osci is offline osci  Australia
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Quote:
Originally Posted by Enzo View Post
Here is another place to download, and if you go back to the directory page, MANY other Gibson drawings are there.

http://www.webphix.com/schematic%20h...namps/ga8t.pdf

Interesting you would decrease one of the feedback caps, I almost always increase them because most trems don't go slow enough.

Isolate the problem. When you have this condition, does the speed control affect the speed of what you hear? We need to know the trem circuit is REALLY still tremming, as opposed to just adding some noise.

CLip a test wire to ground. Usually opening the switch should STOP the trem oscillator. SO have it running and verify proper operation, then click the trem switch off. Now with your grounded test wire, short to ground pin 1 of V3a - the grid of the trem tube. Does that stop this sound? Pin 1 is also the right side of the third feedback cap as wired. Just for science, ground the left side instead, That appears to be a 240k resistor to ground, we are shunting that. Same question, stop the sound? In fact if this does turn out to be trem still going, then maybe the footswitch shunting the 240k to ground might be more effective than opening the speed pot circuit as a trem kill.


Can you scope this circuit?

Hi Enzo,

Thanks heaps for your response also,
I changed the 3rd .047uf cap from the plate to a .022 cap as I didn’t want the trem speeding off into oblivion, having done that it has a very good speed range throughout the entire rotation of the speed pot, not too slow and not too fast.
Apart from the fault I’ve described in the first post, the trem works perfectly, it is ‘only’ when I switch off the trem with the footswitch that I can still hear the oscillation at a fast rate, the speed pot has no affect, it is only the depth pot when turned all the way off that the oscillation completely stops or can’t be heard anymore.. I do have access to a scope but I’d have to wait a few days to be able to use, I’ll try the mod Jon posted tonight and take from there
cheers Ron.
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Old 24th July 2013, 03:01 AM   #6
Enzo is offline Enzo  United States
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The point of the scope and stuff was to make certain it was actually still the trem circuit oscillating and not some other interference.

Jon's mod ought to stop the trem, but so should the stock circuit, hence my desire to find out exactly what is REALLY going on. Once we know that we can cure it.

In the stock circuit, your cathode resistor is grounded, and a switch connects the speed control to ground. In Jon's mod, he just moved the switch to the cathode resistor. SO he asks you to ground the speed control and switch the cathode resistor ground connection.


My suggestion of simply grounding the top end of that 240k resistor should stop any oscillation too, and simple to do as a test.


By changing the feedback cap from 0.047 to 0.022, you are speeding up your trem. The larger the caps, the slower it goes. Just my own opinion, but most fender amp, hell most amps, are ranged too fast. The slowest setting is too fast still, and by the time you are half way up, it is so fast the sound is a blur. I enlarge the three caps to slow it down. I like the slowest setting to be a slow sexy throb, and then the useful range extends further up the control.
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Old 24th July 2013, 07:11 AM   #7
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Doing this mod will ensure the tremolo will stop oscillating completely when the 2k7 is lifted from ground through the foot switch. The connection of the pot that is at present connected to the switched ground goes to ground.
This is fail safe as the switch literally allows the oscillator to work or not.
The colpitts oscillator circuit should have all coupling capacitors of the same value, otherwise the sine wave becomes non linear.
The 6X4 is fine with 6.3v heaters.
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Old 24th July 2013, 09:38 AM   #8
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Hi Jon & Enzo,

just went to do the mod like right now and my soldering iron crapped itself I'll to wait to tomorrow to get another then have a crack at it.

cheers Ron
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Old 25th July 2013, 11:20 PM   #9
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Hi Jon,
I was able to try the mod last night and unfortunately it didn’t work, I can still hear the tremolo oscillating when the footswitch is off.
I’m certain that I’ve done the mod correct, I still have good range of tremolo speed on the entire rotation of the speed pot but when I turn off the trem, the trem still oscillates at the one fast speed/rate, it is only when I turn the depth pot all the way down that the oscillating stops/can’t hear it anymore.
Just one question though, the ground wire connecting the bottom the 510k & 240k resistors, should this ground connection be made at star ground or at ground of V3a?
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Old 25th July 2013, 11:21 PM   #10
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Hi Enzo,
After trying Jon’s mod, I followed your suggestion, but I’ll clarify a few things first.
When I “engage” the tremolo the speed controls the speed perfectly for the entire rotation of the speed and still with tremolo engaged/switched on, if I turn the depth pot all the way down it stops the trem & oscillating completely, and when I increase the depth pot the trem gets more body as I would expect.
Now, I connected a test wire to signal pin 1 of V3a and used that short/shunt first, the 240k resistor and the 510k resistor of the trem circuit.
With the trem engaged/on I touched the ground side of the 240k resistor, doing this stopped the trem completely/no oscillating at all, the speed & depth pot had no affect when turned them up or down.
I did the same for the 510k resistor and again the trem completely stopped with no affect from the speed or depth pots when turned.

I then disengaged/switched off the trem via the footswitch, now the fast oscillation was there almost like its at full speed, I touched the ground side of the 240k resistor the oscillating stopped completely exactly the same happened when I touched the ground side of the 510k resistor.
An interesting observation I made during this test Enzo was each time the end of both resistors was touched with the test wire the trem obviously stopped oscillating, it would take at least 1.5 - 2 sec for the oscillating to start again…..I don’t know if that has any bearing or not?
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