70's Ampeg V2: need some maintenance help

[Jonny toetags]

Have a nice old Ampeg V2 that looks completely stock. I'm guessing it's a early/mid 70's model.

I'm used to Fenders and Marshalls, so I'm seeking some advice.
Here are my issues:
1- I want to add a grounded 3-prong cord: what's the best way to do this with an Ampeg (i don't see a death cap)
2- The output on the top speaker jack (circled in pic 5) is almost non-existant (amp on 10, sounds distorted and as loud as a cell phone): what could be the issue?
*the amp works fine with the other (bottom) speaker jack
3- How do I bias this thing?
4- All caps seem to be original: should I replace? (if so, do i need to replace all or just the filter caps)
5- Anything else i should do to this thing?

Here are some pics:

[pontiacpete]

Isn't the 'death' cap one of those in the first pix? The blue or the white one, I forgot.
Change the electrolytics.
That speaker jack is, I think, for an ext speaker cab, lik e you see in fenders.

[pontiacpete]

Here's the schematic

[Leo_Gnardo]

Lucky you Johnny, the V2 is one of my favorite amps. Sound of the Stones in the seventies - it's right there.

1. 3-wire cord, no biggie. You can just drill a small hole in the chassis, and bolt down a solder tab with #6 or 8 hardware. Be sure to scrape away a little paint under the solder tab so it will make good contact witht the chassis metal. Yes it's a good idea to clip out that death/shocker cap right away. One day after I replaced the cable, next power up I flipped the "ground" switch and that ceramic-tube cap shot its insides out, sparking all the way, and left a coil of smoldering aluminum foil arcing across the circuit board. The kind of party popper we can all do without. Shocker cap indeed! Clip it out without a second thought.

2. There's a shorting bar on the main speaker out, much the same as Fenders. Unless you have something plugged into the main speaker jack, it's shorting the amp's output and yes you'll hear practically nothing at the extension speaker output. A good way to stress your output tubes and transformer. ALSO it's a good idea to make sure the speaker impedance selector is working right - they often get corroded and don't pass signal - more stress your amp doesn't need. A short shot of Caig D5, work the switch back n forth a couple dozen times, then a shot of non-residue cleaner, most of the time that's all it takes.

3. I usually replace the load resistor in the bias supply with a combination series fixed resistor plus trimpot. IIRC it's usually a 56K or so resistor and 20K trimpot. I use the Bourns "blue cube" trimmers because they're small enough to fit comfortably in situations like these. Being adventurous I check the bias current with the "shunt" method, applying an ammeter between B+ and plate connexions. If you're not feeling so adventurous you can install 1 ohm current sensing resistors between cathode and ground. I recommend the 3W 1% Dale wirewounds from Mouser, less than a buck apiece. Haven't blown one up yet.

4. The filter caps Ampeg used seem to outlast just about everyone else's. Even at 40 years old many are perfectly fine. I'd say leave 'em alone unless obviously causing trouble. To replace them is an expensive time consuming project. Eventually you'll have to do it. OTOH I'd recommend changing the bias supply filter caps, for safety's sake. Cheap insurance.

5. Anything else? Yes just one thing, I'd replace the caps bringing signal to the output tube grids. Ampeg generally put way big caps here 0.33 uF IIRC, and if they're getting leaky bang go the output tubes. 0.1 is plenty big enough. I use Mallory 150 630V rated but you can pick your favorite brand if you like. Of course clean the pots, & scrub corrosion off the preamp tube pins & you should be good to go. You'll be hearing tones from Exile on Main Street all the way to Some Girls. Mmm good - ear candy!

[Jonny toetags]

Lucky you Johnny, the V2 is one of my favorite amps. Sound of the Stones in the seventies - it's right there.

1. 3-wire cord, no biggie. You can just drill a small hole in the chassis, and bolt down a solder tab with #6 or 8 hardware. Be sure to scrape away a little paint under the solder tab so it will make good contact witht the chassis metal. Yes it's a good idea to clip out that death/shocker cap right away. One day after I replaced the cable, next power up I flipped the "ground" switch and that ceramic-tube cap shot its insides out, sparking all the way, and left a coil of smoldering aluminum foil arcing across the circuit board. The kind of party popper we can all do without. Shocker cap indeed! Clip it out without a second thought.

2. There's a shorting bar on the main speaker out, much the same as Fenders. Unless you have something plugged into the main speaker jack, it's shorting the amp's output and yes you'll hear practically nothing at the extension speaker output. A good way to stress your output tubes and transformer. ALSO it's a good idea to make sure the speaker impedance selector is working right - they often get corroded and don't pass signal - more stress your amp doesn't need. A short shot of Caig D5, work the switch back n forth a couple dozen times, then a shot of non-residue cleaner, most of the time that's all it takes.

3. I usually replace the load resistor in the bias supply with a combination series fixed resistor plus trimpot. IIRC it's usually a 56K or so resistor and 20K trimpot. I use the Bourns "blue cube" trimmers because they're small enough to fit comfortably in situations like these. Being adventurous I check the bias current with the "shunt" method, applying an ammeter between B+ and plate connexions. If you're not feeling so adventurous you can install 1 ohm current sensing resistors between cathode and ground. I recommend the 3W 1% Dale wirewounds from Mouser, less than a buck apiece. Haven't blown one up yet.

4. The filter caps Ampeg used seem to outlast just about everyone else's. Even at 40 years old many are perfectly fine. I'd say leave 'em alone unless obviously causing trouble. To replace them is an expensive time consuming project. Eventually you'll have to do it. OTOH I'd recommend changing the bias supply filter caps, for safety's sake. Cheap insurance.

5. Anything else? Yes just one thing, I'd replace the caps bringing signal to the output tube grids. Ampeg generally put way big caps here 0.33 uF IIRC, and if they're getting leaky bang go the output tubes. 0.1 is plenty big enough. I use Mallory 150 630V rated but you can pick your favorite brand if you like. Of course clean the pots, & scrub corrosion off the preamp tube pins & you should be good to go. You'll be hearing tones from Exile on Main Street all the way to Some Girls. Mmm good - ear candy!

Thks for the reply Leo!
Quick follow up:
1- I'm guessing i can just use a PT bolt like i'm used to doing.
Am I good following this diagram: Ampeg 3Prong
Which cap is the death cap on my first picture?
2- Gotcha!
3- Any chance you would have the time to let me know the details as to what resistor i change and whatnot? I can follow wiring diagrams and all but I'm not that good with schematics yet.
4- Which are the bias supply caps?
5- Which is my output tube cap?

Sorry for all the questions....I'm still learnin'...only built a bunch of Fenders and Marshall to date. Not used to repairing yet

EDIT: Forgot to ask...is it possible these caps have no voltage in them? I'm getting ready to drain them and was testing voltage like I normally do and it's pretty close to 0.
I'm a bit sceptical and want to make sure before I stick my hand

[52 Bill]

1- I'm guessing i can just use a PT bolt like i'm used to doing.

Officially, No. Lots of people do it, but the ground wire should be connected to the chassis by itself, with its own hardware.

Which cap is the death cap on my first picture?

I think that it's the white one, but you should learn to check this yourself. There are three switches on the front panel. One is the power switch, another is the standby switch and the third is the polarity or ground switch. The so-called death cap is directly connected to the polarity switch. Follow the wires from the back of the polarity switch and see where it goes. If you find a 0.047uF cap that connects to ground connected to the switch wiper, then that's the cap you want to remove.

3- Any chance you would have the time to let me know the details as to what resistor i change and whatnot? I can follow wiring diagrams and all but I'm not that good with schematics yet.

The bias supply resistor that you want to remove and replace with a resistor and trimpot is R49, a 75K resistor. As Leo suggested, a 56K resistor in series with a 20K-25K trimpot will work as a bias adjustment circuit.

4- Which are the bias supply caps?

They are caps C15 and C23 10uF at 100v, which will be near the bias resistor mentioned above.

5- Which is my output tube cap?

Leo was referring to the two caps that connect the phase inverter to the inputs of the power tubes, C10 and C13 (hard to read these numbers on the schematic). They are both 0.33uF caps.

Forgot to ask...is it possible these caps have no voltage in them? I'm getting ready to drain them and was testing voltage like I normally do and it's pretty close to 0.

Yes just like most Fender amps, the power supply has built-in bleeder resistors to drain off the voltage after the power is turned off.

I'm a bit sceptical and want to make sure before I stick my hand

Checking the voltages before you start work is always a smart thing to do!

[J M Fahey]

Which cap is the death cap on my first picture?

If you have to ask, don't touch this amp and send it to some real Tech.

I mean it.

I'm not that good with schematics yet.

4- Which are the bias supply caps?

5- Which is my output tube cap?

All 3 above confirm my idea.
I'm worried about your safety, to be more precise your life.

I'm still learnin'...only built a bunch of Fenders and Marshall to date. Not used to repairing yet

Cool, congratulations.
For now stick to "follow the instructions to the letter" builds and continue studying.

But do no, repeat do not mess with live power wiring.

C'mon, you can't read a schematic nor recognize the parts !!!!!

Knowing this, anybody who, in good faith, encourages you to go along that dangerous path, is co-responsible of whatever happens.

[Jonny toetags]

OK, for the ground plug:
-the polarity switch passes to the blue .047 cap which then passes through the white cap to the tab screwed to the chassis. Should i remove both caps?
-in pic #1, i want to bypass the aux plug. If you notice, there is a flying fuse between the fuse holder and the aux plug. Do i remove this and solder the wire directly to the fuse holder?

[Jonny toetags]

Why there may be some things I'm still learning, I'm pretty sure I can do a few simple mods like a ground plug with the help of people on the forum.
That's how I learn.

[52 Bill]

...-the polarity switch passes to the blue .047 cap which then passes through the white cap to the tab screwed to the chassis. Should i remove both caps?

The blue cap is marked 150v AC, so that is the ground cap. I've not seen a series wired ground cap before. What else is the white cap connected to besides ground?

-in pic #1, i want to bypass the aux plug. If you notice, there is a flying fuse between the fuse holder and the aux plug. Do i remove this and solder the wire directly to the fuse holder?

By bypass do you mean remove the ac socket?

The pigtail fuse is the idiot saver fuse. It is there to save the amp when an idiot sticks in a 20 amp fuse or tin foil into the fuse holder to fix the amp. Do not remove it. Ampeg used the back of the aux. outlet as a convenient tie point to connect the amp's circuitry to the ac power cord. So depending upon what you plan to do with the outlet, you will need to add some wiring to replace the original wiring safely. There is a correct way to wire the ac primary of an amp. Please be sure that you understand it and follow it when you redo your amp.

Juan makes a few good points in his post. I'm not against the idea of you learning, but you should approach these changes very carefully and you should plan them out before acting on them.

[Jonny toetags]

The blue cap is marked 150v AC, so that is the ground cap. I've not seen a series wired ground cap before. What else is the white cap connected to besides ground?

Here's the path:

[52 Bill]

If that is the case, then remove both caps, disconnect the switch and tape off the switch wires.

[Jonny toetags]

If that is the case, then remove both caps, disconnect the switch and tape off the switch wires.

Thanks Bill. That's what I was thinking had to be done.
I decided to leave the ac plug wired in at the back of the amp.

[pontiacpete]

Whenever I change AC cords I like to put the black which should be the hot wire to the fuse not the neutral(white) wire.
Lots of older amps have the opposite.

[Jonny toetags]

Whenever I change AC cords I like to put the black which should be the hot wire to the fuse not the neutral(white) wire.
Lots of older amps have the opposite.

If I do it that way, don't I have to do a bit more wiring?