Ampeg V2

[Smokebreak]

I'm fixing up my buddy's V2. Everything looks original. I just got done replacing the cap cans, and am about to do the bias caps, and 3-prong. I have not fired it up.

Backstory:
5 years ago it "stopped working in the middle of a show", and he put it in a closet until he gave it to me today. He doesn't remember any other details of the failure, so I'm assuming nothing catastrophic happened. Nothing exploded, that is.

The attached schematic doesn't show it (?), but the one on the inside of the chassis('71) does: There are actually 2 fuses- a 6A coming right off the AC, soldered directly on it's ends, to a 4A in the fuseholder. Both fuses are good.

First question : Why the discrepancy in the schematics and why would this amp need 10A of mains fusing? I know the B+ is high but wow.

Next, this is the first PCB I've worked on, so I spent a couple hours tracing things out, doing continuity tests, familiarizing myself with the circuit, etc. The component side faces down, and they obviously used this board for another amp, as the flyback diodes are mounted in some Resistor positions that aren't even on the schematic.
Also, the "A" spot on the rectifier has a wire running from it(on the board) to the first capcan "+", which is not reflective of the schematic...quite the opposite. There are a few other discrepancies like that that are making things a bit difficult.
I don't really have a point here, other than it's quite confusing.

The first thing I did in this amp was check all ground continuity. Capcan C20 was not reading to ground, then I saw the standby switch. So I flicked it, and still nothing. Then I saw D5.

What is the purpose of a reverse diode there?

I assumed that had everything to do with the grounds not reading, as every other ground on the other side of the diode checks out fine.
Then, I noticed that there is no continuity on the standby switch, in either position. It's enclosed in plastic w/ leads, so I was measuring at the board.

I called him and asked if there was ever anything wrong with the standby, and he said "oh yea, it used to pop really bad coming in and out of standby).

All this to say, I think the standby switch is bad , which would explain the show-stopper
So I'm looking for advice as to why this happened(short of just basic, non-causal mechanical failure), as perhaps a bigger issue caused this in the first place, and I'd like to prevent it from happening again, if that's the case . Again, both fuses are good, but I'm not sure what it would take to blow 10A.

Anything else I should look out for in these amps?

Thanks,
Jeremy

[Leo_Gnardo]

First answer @ fuses, Ampeg put 'em in series, so the operating fuse rating is that of the smallest fuse, 4 amps. They put a second fuse in series, just in case some @#$% put a larger than specified fuse in the fuseholder, then if there was serious trouble hopefully the 6A would pop and it would have to get "professional help."

Yes replace the standby switch.

Best to remove the shocker cap. I've had 'em go flying right out of Ampeg V4 and V2 on power-up. If your AC cable grounds the chassis, you don't need it. Just cut it out & throw it away.

Love that V2 tone! Great amp.

[Smokebreak]

Thanks Leo. That fusing is pretty clever.

So what's the proper way to replace components on on board where the component side is facing down? I mean, I know it would be preferable to insert components on the component side, but every wire from the board looks to be 18ga solid, and there's no "kinda flipping it over ". It would involve removing about 20 leads to pull the board.
It looks like the only mod to this amp was changing resistors in the bias supply at some point, and they are...on the solder side facing up.
Is this acceptable , or should I really pull the board to change 2 electrolytics?

[Smokebreak]

Just making notes...The death cap arrangement off the polarity is interesting as well, in the amp. There are two 47n series caps to ground. One is rated 150V and the other 630V.

[JazzGuitarGimp]

If it's like the VT-22 I had in the shop last year, there will be plates on the other side of the chassis with holes in them for the pcb-mounted tubes to pop up through. These plates are held on to the chassis with six screws each. Remove the plates, and you will have access to all of the components on the boards.

[Leo_Gnardo]

So what's the proper way to replace components on on board where the component side is facing down?

To maintain my sanity and laziness factor I put most replacement parts on the foil side of the board in these amps. You'll still work up some muscles flipping it over a few times.

[AL]

What year V2 is that? All the V-Series amps I've been in (I have several) have a removable metal plate. Take off the plate to access the circuit board. You've still gotta flip over an amp that weighs as much as a small car but you don't have to remove the board. Does that V2 not have a plate?

AL

**edit** whoops, sorry JazzGuitarGimp beat me to it.

[Smokebreak]

Thanks guys I'm not sure how I missed the massive removable plate!
Just when I was getting good with my dental mirror
Not sure what year, schem on plate is labeled 71 but it doesn't exactly match the board

[Smokebreak]

I got this thing fired up last night and oh my it's loud!

I'm having a problem biasing any tubes I put in it. I think I'm having issues with my 1W resistor method, and understanding the ground scheme. I've attached a drawing of the original wiring, and the way I have my bias test set up.

Here's what I've got:

All voltages are really close with the schematic. 585V plates, 586V screens, -65V at the bias feed, and at the grids.

I've tried the 7027s that came to me with the amp, as well as two different matched pairs of new prod Sovtek 6L6WXT and TAD 6L6STR.
Per the picture, I can't get the 6L6 on the left to pull more than 4mA. This is from reading cathode to chassis.
The other reads 22mA, and my bias resistor R49 is 73K right now. I'll go lower there once I get things sorted out.
Now, if I measure from cathode to the junction of the 1Rs at the term strip, the 4mA tube reads 11mA, and the 22mA tube reads 27mA

The issue follows the socket.

Reading across the 1Rs 1%, I'm showing .8R on the left and 1.6R on the right, and 1.8R from cathode to cathode, which could be imperfections in my meter, but something isn't right here. It's like I'm reading across 2 resistors on the right socket.

I'm not sure where chassis ground is. I'm assuming at the input, as the output jacks, capcans are isolated. Apparently, using the ground lead from the main board, that attaches to the board after the reverse diode on the ground trace, is not working.

I'm thinking of putting the 1Rs right to the chassis, but not sure if I should float that board ground lead.
For all I know the amp could be in perfect order, but I'm just not reading it correctly.

What do you guys think?

[xtian]

First lift one end of both 1Rs, and try to get an accurate measurement of their resistance. Then ground them securely to the chassis near the power tubes.

If you see the same disparity between power tubes (11mA vs 27mA) try swapping the position of the power tubes and see if the current measurements follow the tubes.

[cbass]

Check the bias by measuring the voltage drop between your "A" node the ot centretap and the power tube plates. Then divide by the resistance measured between the center tap and the primarie wires.
I never cared for the 1 ohm method

[martin manning]

No doubt measuring voltage across the 1R's (not to chassis if there is much of anything between the terminal strip and ground) is the most accurate way to get cathode current. You can't measure 1R accurately with an ordinary meter. What voltage do you get from grid to cathode, and across the 100k's, on the power tubes? Is it the same on both sides? If not perhaps you have a leaky coupling cap.

[Smokebreak]

Granted with my crappy meter, lifted each 1R reads 1.6ohms which is at least consistent with the 1.6 I was getting on the right one. Looks like I was reading the one on the left in parallel, somehow, but I can't see how.

Problem stays with the socket on tube switch.

Martin, I'm a little confused. Reading across the 100K bias feeds reads negligible V drop(~.01v), but that's because there's no current in the grids right? Reading straight from grid to cathode, with the 1Rs in reads the -65V bias voltage.

I've never used the shunt method, and I'm skiddish about learning the process on this amp.

So the chassis ground is indeed at the input, so the ground wire from the main board only connects OT, cathodes, and last capcan grounds to the ground network on the board, on the other side of the reversed diode.

So if OT,capcan is connected to cathodes, and cathodes go 1R straight to chassis, I should be fine floating that board ground wire eh?

[Smokebreak]

Granted with my crappy meter, lifted each 1R reads 1.6ohms which is at least consistent with the 1.6 I was getting on the right one. Looks like I was reading the one on the left in parallel, somehow, but I can't see how.

Cancel this bit. Like Martin said I can't draw any conclusions with this meter on the 1Rs. Readings change every time I solder/desolder

[cbass]

[quote="Smokebreak"]

I've never used the shunt method, and I'm skiddish about learning the process on this amp.
to /quote]
I don't think what I described is the shunt method .its just measuring the voltage drop across the transformer windings then dividing that by resistance of the windings to calculate current.should be safe and easy to do with some clips.
Anyway you should probably be listening to everyone but me . don't let me get you sidetracked.