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
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
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!
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!
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 heaven/www.schematicheaven.com/gibsonamps/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?
http://www.webphix.com/schematic heaven/www.schematicheaven.com/gibsonamps/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 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.
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.
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.
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.
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.
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.
I'll to wait to tomorrow to get another then have a crack at it.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?
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?
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?
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?
My thought was I had that cap connected incorrectly, and I know last night I checked that cap at least 7 times, but for piece mind I'll grab a couple of the caps from Jaycar today and replace it.
cheers Ron
That cap is there to prop up the low freq response, low freqs being what the LFO is after all. If you left it connected and switched only the 2.7k resistor, then a leaky cap could indeed keep the oscillator alive. Just disconnect one end of the cap. Now does the unwanted trem still happen?
You can check a cap 7 times, but what are you checking it FOR? Measuring its capacitance on a hand meter tells you very little. And one important thing it won't tell you is leakage at operating voltage. Especially in tube circuits, I can test a common 20uf 450v cap on my meter and it measures fine, but in a real world circuit it might leak like a screen door at anything over 20v.
You did the mod, which SHOULD stop the oscillation, but your unwanted trem continues. BY doing the test I suggested, grounding the grid or that 240k, it stopped the symptom. That verifies it is in fact an oscillation and not some other thing. I think Jon may be right, that cathode bypass cap is leaky enough to allow oscillation.
These oscillator circuits are pretty simple, resistors, caps, and a tube. The oscillator has to be self starting, and sometimes they get sluggish. That is almost always weak caps. AFTER you solve the unwanted trem, we can cure the slow start. If the triode is weak,try swapping places with the two 6BM8s. In cases where my trem doesn't want to work, I usually just recap it and check for WAY off resistors. Resistors off 20% or less are probably fine, but tired caps won't feed back as well. SO a fresh bypass cap, three fresh feedback caps, and don't forget the large .25uf output cap to the depth control. A leaky one there can drag your oscillator down.
I don't recall, did we ever check your B+ for ripple? The trem runs off the screen node, is that clean of all ripple?
You can check a cap 7 times, but what are you checking it FOR? Measuring its capacitance on a hand meter tells you very little. And one important thing it won't tell you is leakage at operating voltage. Especially in tube circuits, I can test a common 20uf 450v cap on my meter and it measures fine, but in a real world circuit it might leak like a screen door at anything over 20v.
You did the mod, which SHOULD stop the oscillation, but your unwanted trem continues. BY doing the test I suggested, grounding the grid or that 240k, it stopped the symptom. That verifies it is in fact an oscillation and not some other thing. I think Jon may be right, that cathode bypass cap is leaky enough to allow oscillation.
These oscillator circuits are pretty simple, resistors, caps, and a tube. The oscillator has to be self starting, and sometimes they get sluggish. That is almost always weak caps. AFTER you solve the unwanted trem, we can cure the slow start. If the triode is weak,try swapping places with the two 6BM8s. In cases where my trem doesn't want to work, I usually just recap it and check for WAY off resistors. Resistors off 20% or less are probably fine, but tired caps won't feed back as well. SO a fresh bypass cap, three fresh feedback caps, and don't forget the large .25uf output cap to the depth control. A leaky one there can drag your oscillator down.
I don't recall, did we ever check your B+ for ripple? The trem runs off the screen node, is that clean of all ripple?
Hi Enzo & Jon
ok, I changed the 20uf cap at pin8 of V3a and it has made no difference, trem works fine with switch on but with switch off I can still hear the fast oscillation.
I also changed the 3 cap's to the plate of V3a without success, I've virtually rebuilt the tremolo circuit.
I've spent the past 4 hours tonight making sure the entire circuit is wired correctly and I'm absolutely certain its correct.
ok, I changed the 20uf cap at pin8 of V3a and it has made no difference, trem works fine with switch on but with switch off I can still hear the fast oscillation.
I also changed the 3 cap's to the plate of V3a without success, I've virtually rebuilt the tremolo circuit.
I've spent the past 4 hours tonight making sure the entire circuit is wired correctly and I'm absolutely certain its correct.
OK. With your switch unplugged. Lift the resistor from pin 8 to ground at pin 8. Does the oscillator stop?
If it does check your cable, plug and switch.
The oscillator cannot run with no gain from the valve caused by no bias.
Just say to yourself, I am human and that is just a simple circuit. It won't help in fault finding but it will make you feel better and don't forget, keep a clear head, have a break and do something else for 5minutes.
If it does check your cable, plug and switch.
The oscillator cannot run with no gain from the valve caused by no bias.
Just say to yourself, I am human and that is just a simple circuit. It won't help in fault finding but it will make you feel better and don't forget, keep a clear head, have a break and do something else for 5minutes.
Cool machine, Nigel.
Osci, do you have that scope, and know how to get an image on its screen? When your trem is ON, look at the plate of the trem tube with the scope set on AC coupling. You should see a very low freq wave form. Probably too slow to resolve into a stable pattern on the screen, at least at slow speeds. That is the baseline reading.
Now turn OFF the tem and see what is on that same plate. It SHOULD go to no waveform, just DC. There are only so many ways a circuit can oscillate. We need to verify it is really oscillating and not doing something else.
Osci, do you have that scope, and know how to get an image on its screen? When your trem is ON, look at the plate of the trem tube with the scope set on AC coupling. You should see a very low freq wave form. Probably too slow to resolve into a stable pattern on the screen, at least at slow speeds. That is the baseline reading.
Now turn OFF the tem and see what is on that same plate. It SHOULD go to no waveform, just DC. There are only so many ways a circuit can oscillate. We need to verify it is really oscillating and not doing something else.
Hi Jon,
I lifted the resistor at pin 8 again, powered up without the footswitch and no oscillation, I powered down, with the resistor at pin 8 still lifted, I connected the footswitch and powered up again and again no oscillation.
I don't know if this is significant, but I measured voltages at pin 9 V3a and was surprised to see approx 20 VAC as well as DC voltage, I measured voltage both sides of the .047 cap closest to pin 9 and the .25 cap which leads to the depth pot, should this AC voltage be there and particlarly that much AC?
cheers Ron
Hi Enzo,
I managed to borrow a scope but honestly I really don't know how to use it apart from turning the power on and connecting the probe.....I'll have a go at it????
Do I connect the probe directly to pin9 V3a the plate?
and i don't know what settings to use, if its any help the scope is a
BWD 525 50mhz two channel.
cheers Ron
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