Repairing Acoustic G60T Amp Head

[moondragon]

This amp had a nasty Channel 2 gain squeal that changed smoothly with the Channel 2 gain pot.

Ok, I just finished swapping out all the capacitors. Got some new fuses from Rat Shack, since I blew one testing the amp right after replacing the large dual filter cap. The fuse fit the holder like a glove, whereas the old one was quite loose. Tested the amp by turning it on without tubes. No problem. Turned it on by adding preamp tubes one by one. No problem. Put in the two 6L6's. Lights come on for both channels (individually when the switch is flipped). No fuse issue. Turned the amp off, connected a speaker and a guitar. I get a low level normal guitar amp noise with all pots at zero except master volume. Moment of truth, channel 2 turning up the gain and the volume. No squeal. Testing the guitar, no sound from the guitar... Uh oh.

Could it be a bad tube? If so, is it more likely to be a preamp or output tube? I've got a few preamp tubes to swap out, but I don't have any spare 6L6's. I'll have to cannibalize an amp we're using. I don't want to sacrifice good 6L6's just repairing an amp if I don't have to, but I'm willing to give it a go if anybody thinks it's unlikely to hurt them.

[Edit] Put three known working 12AX7's in. Now channel 2 works perfectly. All the tone controls and the reverb control responds as expected. But... channel 1 stopped responding. I still get nothing from it. Confused again...

For the rest of the back story, check this thread. I felt it made more sense to have a thread about repairing the G60T, since I'm no longer looking for an amp tech...

[moondragon]

Here are a few pictures of the innards. I'm not quite getting it yet, but it seems like the switch for the channels is wired strangely. The wire to the middle connector of the switch and one of the wires from the bottom connector of the switch connects to the "Volume Footswitch" jack on the back. Makes sense, since a footswitch would be acting instead of the amp switch. Now, the third wire connects to the small circuit board with all the pots on it. Shouldn't there be something connected to the top connector of the switch? Wouldn't it be functioning as it's connected now essentially as a "on/off" switch rather than an "either/or" switch?

You can see all my photos in this Photobucket album. These photos are after the capacitor swap.

[52 Bill]

The channel switching is electronic on that amp. The switch turns on and off 2 optoisolators that switch the channels, so the footswitch or the front panel switch only needs to be a single pole unit, so it is wired correctly. Do the channel LEDs change with the switch?

Both channels share the tone controls and the input stage, so if the clean channel doesn't work, the channel one volume pot or the optoisolator OI-2 are the only things in the circuit that would cause the symptoms that you describe. The optoisolator is the round metal thimble on the control board.

[moondragon]

Shoot, the optoisolator was my other thought. It surprises me that the volume pot might go bad in the time I swapped caps and fuses. The leds do change with the switch. If the optoisolator was bad, would the reason it worked before have been that the old capacitors were letting currents through that were enough to make it through somehow? That might explain the tacked on resistor there when I first got the thing.

Well, looks like I'm back to sourcing optoisolators that will work again and ordering a pot. Not that it's a bad thing to have extra pots laying around.

Edit: After checking with my multimeter, pot 1 is responding with normal resistance. If an optoisolator goes bad, why would it respond normally when the switch is closed and give no sound with the switch open?

[52 Bill]

If you open up the Vactrol, you will find a small light bulb and two cds cells. The cds cells are resistors that are sensitive to the light that hits their front surface. Meaning that their resistance changes when the light bulb is turned on.

The channel switch turns on and off the light bulb, which raises and lowers the resistance of the cds cells. Think of this as a sort of switch: when the light is off the resistance is high, when the light is on the resistance is low.

If the bulb in the Vactrol has burned out, then the resistance of the cells will always remain high, or the switch that it represents will always remain off.

I think that your Vactrol's bulb has burned out and the high resistance of the cds cells are blocking the signal from the channel one volume control.

The Vactrol that you need hasn't been made in 20 years so the only solution is to find a NOS one or open up the one you have and replace the bulb.

[moondragon]

Ok, the old Vactec specs show a 10V 14mA bulb inside the VTL9A10 (which should be the same for the VTL9A10/2). I can get a 10V 30mA bulb (made for train sets?) to replace the grain inside the Vactrol. Forgive me, but I have no idea whether mA rating makes a difference. Could a 12V bulb be used with similar results?

I assume it's best to get a bulb with the two leads exposed, but we all know what happens when we assume...

[52 Bill]

The current rating is just that, how much current the bulb draws or consumes when it is lit up. If the new one draws 30mA, it will pull more than twice the current than the original one.

The circuit for your amp is about as simple as can be. The bulbs are connected to a 10vdc source through a 47 ohm resistor. So I doubt that the higher current draw will be a problem. That being said, the closer you get to the original spec the better, as there are two of these Vactrols drawing from the same source.

As for the 12 volt bulb, it may work just fine, as long as it gets bright enough to lower the cds cells resistance and let the signal from the clean volume control get to the next stage.

Have you tested the bulb in your Vactrol with your ohmmeter? You should get a low resistance reading across the bulb's terminals. Unfortunately because it is paralleled by the second Vactrol, it must be removed from the circuit to be tested.

If you do need to replace the bulb, the original bulb has wire leads going down to the base socket. If you remove the metal cap from the base, you will be able to snip off the leads close to the glass of the old bulb. Then you can solder the new bulb's leads to these leads.

[moondragon]

Amazingly, a HobbyTown opened here in town two weeks ago, so I can get a hold of grain of wheat bulbs fairly easily. 12V, but I'll see about making it work. I'll take out and test the Vactrols this weekend and see if I have to Frankenstein either of them. IT... COULD... WORK!

[52 Bill]

Here's a fast way to check the bulb in the Vactrol. Carefully drill a small hole in the top of the metal cap. If you see the when it it switched on, then you know it's ok. If there is no light, you need to open it up and replace the bulb.

Be careful if you drill, cause if you slip and the bit goes too deep into the case, you will do damage to the bulb and or the photocells.

If the distortion channel shuts off when you switch to the clean channel, then you know that the Vactrol that grounds out the extra gain stage is ok and doesn't need to be messed with.

[moondragon]

If I gather what you're saying, the Vactrol on the main circuit board is fine (since Channel 2 definitely goes out when flipped to Channel 1), but the one by the pots on the secondary circuit board is probably out.

If I drill into the top of the Vactrol, what do I do to close it back up afterwards, electrical tape?

[moondragon]

It's alive! Well, breathing anyway. I'll have to wait until tomorrow to put any real volume through it, since I don't want to wake up the house. You were spot on with the bulb in the Vactrol being out, 52 Bill. Thanks for walking me through all this stuff.

I wanted to save the can itself and reuse it, but I wasn't quite handy enough to pull it off. I ended up replacing the bulb with a 12V 30mA GOW and wrapping electrical tape, sticky-side-out around the photoresistors and the bulb followed by a layer sticky-side-in to close it off. Maybe I'll come up with something better later if it seems too flimsy.

Now, barring any immediate silliness with the Acoustic, it's time to recap the SUNN Concert Bass I got at a pawn shop a few months ago!

**happy dance**

[moondragon]

It officially works. Now I have to do a cap replacement on my Dad's G60 combo, since his second channel just made with the squealing. Boy, am I glad I bought parts in case he would need a recap job!

Hmm, now to decide between the SUNN and the other Acoustic...

Edit: Oh, and everyone is thoroughly amazed that I got what appeared to be a hunk of junk to work. Hooray!

[Euthymia]

Man, those squealing G60T's. Every G60T I've ever had in here has done that. The first one I worked on years ago was my big education on the concept of stage decoupling. New caps does the trick.

[chrisofoz]

Hi Bill,

I'm having a very similar problem with my acoustic g60t combo.

Channel 2 seems to be stuck on, i have no channel lights changing,
I tried a quick fix by changing the bulbs in the opto isolators but no go,
however it did seem to improve the reverb, although it isnt quite as
strong as it should be, but I am hearing it now, I wasnt before.

I am going to take it to a friend tomorrow so we can attack it with
a DMM, but it seemed like you had an idea earlier with moondragon,
when you mentioned the channel lights not coming on,

Cheers,
Chris

[52 Bill]

Welcome to the place Chris. As a starting note, you should probably start a new thread for your problem, because as you have already found out, your problem is quite different the one that was discussed here.

The default channel when the amp powers up is the distortion channel. The channel switching circuit turns off the distortion channel and turns on the clean channel when the circuit is activated. If you have no lights or leds, then I suspect that the 10 volt power supply that operates the lamps and the leds is dead or missing.

First thing to check is the footswitch jack. It must be isolated from the chassis. The shield connection is not connected to a ground circuit and if the jack is mounted directly to the chassis, it will kill the lamps and the leds.

If the jack is mounted correctly, then check the voltage at the tip connection. You should get a 10 volt dc reading. Try switching the front panel switch and see if the voltage changes.

If there is no voltage or a low voltage there, look for and test R38 150 ohm/1 watt and D1 a 10 volt Zener diode.

Let us know what you find.