Garnet Mini Bass tremolo repair

[thehoj]

I'm looking at a friend's Garnet Mini Bass G90T. I was able to get a schematic for this. Here it is.

Power section: http://i.imgur.com/CZsiFBQ.jpg

Preamp: http://i.imgur.com/9K25yF2.jpg


So far I've replaced a few things to get it sounding better. The amp was very very quiet, I thought maybe the OT was blown or something.. Turned out that after replacing preamp and power tubes it sounds significantly better.. Pretty much how I'd expect actually.

The capacitor in the bias circuit had also popped, so I replaced that. In the schematic it's 64uf / 64V, in the actual amp circuit it was a 100uf / 64V capacitor, which I replaced with a 120uf / 330V.. It's all I had around. That should work fine right?

The last issue with this amp is that the tremolo doesn't work.. I can see that the bias runs past the tremolo depth and intensity controls on the front before making it to the grid resistors on the power tubes. I don't really understand how adjusting the bias will induce the tremolo effect.

Can someone explain to me how this tremolo works, and what might possibly be wrong with this? I've measured values and everything seems fine.


Here are a few pics of that tremolo section of the amp.

The 64uf/64 cap is replaced with a 120uf/330V cap. Everything else looks just like the schematic.
http://i.imgur.com/ArtMM6S.jpg


Here's the back of that board
http://i.imgur.com/BuxIFLh.jpg


Front tremolo controls
http://i.imgur.com/RzbpjeB.jpg

[Jazz P Bass]

So what is happening at the plate of the trem tube?
You should see an ac waveform: the Low Frequency Oscillator.

[thehoj]

So what is happening at the plate of the trem tube?
You should see an ac waveform: the Low Frequency Oscillator.

If I measure with the multimeter at that point, DC I get 280V steady.. AC 529V..

If I hook up my oscilloscope (which I must admit I'm not very adept at using).. I don't see a waveform at that point.. I connect the ground on my lead to the same ground as the cathode, and I connect the x10 probe I have right on the plate, so below that 150K resistor. Just a flat line. Do I need any kind of input signal?

[g1]

Pretty fancy writing on the schem., I don't think Gar drew that .
It's not shown on the schem., but there is a footswitch jack, yes? Are you using a footswitch or have you tried one or a shorting plug? If it uses a switching jack that turns on the circuit when no footswitch is used, it could just be dirty switch contacts on the jack.

Your 120uf/330V cap should be fine for the bias, the voltage rating is overkill, but any voltage rating over what you had should be good.

I don't really understand how adjusting the bias will induce the tremolo effect.

Think of this, the main sound that comes out of the amp (aside from the tremolo) is created by the original signal modulating the bias. It's like the signal is adjusting the bias rapidly back and forth. The frequency of the note you play modulates the bias faster or slower depending on the note. So when you play 440hz, the bias is modulated at that frequency and you hear it through the speaker.
The tremolo works on the same principle, but at very low frequencies. So you hear it as a pulsing which modulates the original modulation. Kind of like a beat note when you use a tuning fork.

[thehoj]

Yea, I didn't think it was an original schematic.. Very easy to read . I just don't know enough about the specifics of the interworkings of that trem circuit.

Also, yes there is a shorting jack there which seems to break the connection between the 1M resistor and the bottom lug on the 1M linear pot. I have tried connecting a wire over the middle lug and the right lug on that jack though to make sure it's shorted.

As I'm looking at the amp though, for some reason I don't seem to see that lug on the 1M linear pot being connected to ground.. Even though it seems like it should be from the schematic to me.

Here's a pic, maybe you can see if I'm missing something.. http://i.imgur.com/cMYIqev.jpg The two pots on the left are the trem pots.

Pretty fancy writing on the schem., I don't think Gar drew that .
It's not shown on the schem., but there is a footswitch jack, yes? Are you using a footswitch or have you tried one or a shorting plug? If it uses a switching jack that turns on the circuit when no footswitch is used, it could just be dirty switch contacts on the jack.

Your 120uf/330V cap should be fine for the bias, the voltage rating is overkill, but any voltage rating over what you had should be good.


Think of this, the main sound that comes out of the amp (aside from the tremolo) is created by the original signal modulating the bias. It's like the signal is adjusting the bias rapidly back and forth. The frequency of the note you play modulates the bias faster or slower depending on the note. So when you play 440hz, the bias is modulated at that frequency and you hear it through the speaker.
The tremolo works on the same principle, but at very low frequencies. So you hear it as a pulsing which modulates the original modulation. Kind of like a beat note when you use a tuning fork.

[Jazz P Bass]

If I measure with the multimeter at that point, DC I get 280V steady.. AC 529V..

If I hook up my oscilloscope (which I must admit I'm not very adept at using).. I don't see a waveform at that point.. I connect the ground on my lead to the same ground as the cathode, and I connect the x10 probe I have right on the plate, so below that 150K resistor. Just a flat line. Do I need any kind of input signal?

I don't have a clue what is up with that 529Vac reading on the Trem tube plate.
As to the scope reading a flat line (which does not jive at all with the 529Vac reading), a flat line means the LFO is not working.
Is there any change if you rotate the speed knob?
What reading (Vdc) do you get at th3e cathode?

[g1]

Some very basic meters don't like reading AC volts when DC is present, I think this may be the cause of the funny 529VAC reading.
The footswitch jack is grounded to the chassis mechanically. The black wire from it grounds the 1M pot. The tremolo is turned on and off by grounding the 1M resistor.
There should be a wire jumper between the ground terminal and the center terminal of the footswitch jack, I can't tell from the picture, is it there? Try your jumper across the 2 outside terminals of the jack.
Aside from all that, you said replacing the tube didn't help, so the most likely suspects are the caps. in the tremolo circuit. Measuring them will not likely show you leakage, especially if measured in circuit. Replacing them may be the easiest test.

[thehoj]

Some very basic meters don't like reading AC volts when DC is present, I think this may be the cause of the funny 529VAC reading.
The footswitch jack is grounded to the chassis mechanically. The black wire from it grounds the 1M pot. The tremolo is turned on and off by grounding the 1M resistor.
There should be a wire jumper between the ground terminal and the center terminal of the footswitch jack, I can't tell from the picture, is it there? Try your jumper across the 2 outside terminals of the jack.
Aside from all that, you said replacing the tube didn't help, so the most likely suspects are the caps. in the tremolo circuit. Measuring them will not likely show you leakage, especially if measured in circuit. Replacing them may be the easiest test.

There is a wire jumper between the ground terminal and center terminal. I actually replaced that jack because the one that was in there was damaged, and wouldn't even accept a 1/4" jack. I noticed that there is one of those big washers with the teeth on it between the jack and the chassis, maybe it is interrupting the connection to ground some how. I'll take that out and see if it works any different. I guess I can just measure for continuity between those lugs on the jack and the chassis to confirm.

Otherwise I'll replace those caps, easy enough to try.
Thanks for the tips!


On a side note, you're in Canada, north of Fargo?.. I don't suppose you're in Manitoba are you?.. I'm in Brandon actually .

[g1]

Well, almost the whole country is north of Fargo , but good call, we're fairly close, I'm just outside Miami. Glad we didn't get the rain you just got, we just got over our flooding a couple weeks ago.
The tooth washer should help ground the jack, unless there is bad corrosion or oxidization, which you probably would have noticed when you changed the jack.
Also (if you haven't already did this) check the 5 resistors and the 1M pot before changing caps.

[thehoj]

Nice! We're neighbours. Good call on the caps. I replaced all of them and also noticed that at some point someone had wired them incorrectly. Wired them as per schem and bam, awesome sounding tremolo.

Thanks for all the help everyone.

Well, almost the whole country is north of Fargo , but good call, we're fairly close, I'm just outside Miami. Glad we didn't get the rain you just got, we just got over our flooding a couple weeks ago.
The tooth washer should help ground the jack, unless there is bad corrosion or oxidization, which you probably would have noticed when you changed the jack.
Also (if you haven't already did this) check the 5 resistors and the 1M pot before changing caps.