Amp repairs anywhere near Cheyenne, WY?

[moondragon]

Hey, I'm a newbie referred from the unofficial acoustic control corp forum.

Here's my story:

My dad picked up an acoustic G60T guitar head and a 464 cab for himself off Ebay. The speaker in the cab is an old Jensen. At any rate, it arrived with a cracked speaker cone, creating a superbuzz on the low frequencies. The mid to upper range tones still sound nice, though not as nice as my dad's G60t combo (all original but the tubes).

Also, the distortion pot on the head has been destroyed. You get a pretty hilarious squeal when you turn it to anything but zero. Would work great if we were into experimental electronic music from the sixties. Will that require a vintage knob, or are there newer replacements available?

Cheyenne, Wyoming is not the place to find instruments, let alone repair shops. We were hoping to find somebody within about 100 miles (Denver, for example) if anybody knows of a reputable repair guy (or gal) in this area.

Thanks,

Amber

[Enzo]

Think of it as two separate repairs. One is the torn speaker. You would take the wires off it, noting which went where, and send the speaker to a reconer. If you can send and receive via UPS where you live, then it doesn;t matter where the reconer might be. Bruce knows your part of the world a lot better than I.

Second is the amp chassis repair. if there is no one in your area competent, then send the amp chassis out of town. Once you are doing that, the shop can be anywhere.

[moondragon]

Well, currently, I've taken apart the amp. It looks like someone did a crappy patch job to bypass something called a Vactec Vactrol VTL9A10/2. I'm going to snip the patch wire, put the thing back together and test it out.

Could the squealy noise be caused by poorly matched tubes? It looks like whoever had this amp last broke one and just threw in a new Mesa Boogie tube so that it made sound.

[52 Bill]

Well, currently, I've taken apart the amp. It looks like someone did a crappy patch job to bypass something called a Vactec Vactrol VTL9A10/2. I'm going to snip the patch wire, put the thing back together and test it out.

The Vactrol is an optoisolator (a light bulb with 2 photocells). It is part of the switching lead/rhythm circuit. The squeal you hear when you turn up the distortion control is probably circuit feedback caused by the bypassed Vactrol.
I would suggest that you get a copy of the schematic and see what else has been changed by the former owner before proceeding with any other changes or tests.

Could the squealy noise be caused by poorly matched tubes? It looks like whoever had this amp last broke one and just threw in a new Mesa Boogie tube so that it made sound.

I guess that you are talking about the power tubes here. Mis-matched tubes will not cause the problem with the squeal.

[moondragon]

Snipping the wire didn't work. I don't think the wire had bypassed the opto-isolator completely, anyway. I have a copy of the schematic. That extra wire that I snipped appears to be the only change. I think the opto-isolator is bad. Aren't those things supposed to help control electrical feedback?

Anyhow, I have no idea how to get a replacement part for that, since the model number doesn't seem to come up on the company's current list of products.

Vactrol Vactec
VTL9A10/2

[52 Bill]

...I think the opto-isolator is bad. Aren't those things supposed to help control electrical feedback?

I don't think that part has been made for probably ten years now.

If you look at the schematic, there are two of these in your amp, OI1 and OI2. Even though they are photocells, think of them as switches. Instead of a mehanical lever to turn it on and off, the light bulb controls the resistance of the photocells, basically turning them on and off. They are used to silently switch the distortion channel on and off without the noise of a relay.

One is used to ground out the input and output of the distortion tube circuit and the other one is used to cut off the clean signal from the volume control going into the tone controls.

If one has failed, you can either fix it or replace it with some other optoisolator. Inside the little metal can is a light bulb and two photocells. If the bulb has burned out, you could try to replace it with a small "grain of wheat" bulb from Radio Shack or some other source. If the photocells have gone bad, you could replace them as well.

[moondragon]

Well, I suppose I'll wait until my new DMM comes in the mail. It's probably a good idea to make sure that's where the problem is (though it seems like it makes the most sense). I just hope I don't mess anything around it up when I take it off/put it back on. My sautering skills are definitely in the amateur stage.

Thanks for all the input, by the way. It's very nice to get help from a knowledgeable person.

[moondragon]

Been a while, but...

After having a ton of other projects get in the way, I'm back to working on this G60T (and a SUNN bass amp, but that's an easy fix once the parts arrive).

After a little more research (and getting used to reading schematics and looking at electronic components), the most likely cause of the squeal is the big dual filter cap hanging out by the output tubes. There are also a few bloated capacitors inside that need replacing. Once the parts arrive, I'll do a recap job and see if that deals with it.

At least none of the 12AX7's turned out to be microphonic.

[moondragon]

I replaced a capacitor that had been bloated and connected with a resistor outside the schematic. Turned on the amp. Same squeal in the second channel.

Started on replacing the big dual filter cap. It was pretty easy to see which was positive and which was negative, even though this isn't an exact replica of the part. Now, turned on the amp. No real problem. Took it off standby, got a little bit of a soft buzz from the equipment itself, then the fuse blew.

Now, what could cause a fuse to blow? I did drop one of the preamp tubes on the floor (yikes, but didn't look like it had any damage) when I was putting them back in after connecting the new dual cap. Could the preamp tube cause a fuse blowout?

Also, which fuse should I pick up? (There wasn't a spare.) The outside of the chassi says 11/2ASB 120V Fuse. How much of that is a manufacturer part number, and how much of it is a spec I can use to buy the right fuse?

The old filter cap said 30-40uF 500V (twice) and the new one just says 40uF 500V (twice). The old cap also had a metal ring with four ground connectors while the new cap has only one ground connector in the middle (and no ring). It doesn't seem as though connecting the two ground wires to the same connector would make any difference over the old cap, but I am new to electronics, so I may be missing something.

I've got several more capacitors in the amp that look bloated, so I'll replace them. Unfortunately, I won't be able to do any testing until I get some new fuses.

Oh, is it really common to blow fuses when working on tube amps?