Ampeg V2

[lowell]

This head came in with some bad overtones on top of notes. I found that the initial filter cap off the rectifier was GONE... yes, gone. Also the screen cap was a single 50uf/450v cap instead of the stacked series arrangement for a higher voltage rating. After fixing these issues the amp was louder and the overtones gone. The HT also came back up to the 594v per the schematic. The amp now has some hum issues.

With all controls turned off - when the amp is first turned to play position there is an audible hum that fades over 4 seconds. It's still audible after that but is far less. I cannot tell if the hum is 60hz or 120hz. It sounds like a B on the D string. If I disconnect C10 the hum is gone when the amp is turned to play position so it's coming from before that point in the circuit. I tried pulling all the other preamp tubes before that point and the hum still happens.

If channel 2's volume is turned up the hum gets worse. This does not happen on channel 1. I tried a new preamp tube for channel 2 and it didn't help.

Any ideas? Also this amp breaks up and gets dirty if the volume is brought above 10 o'clock. Is that normal for this amp?

http://www.drtube.com/schematics/ampeg/v2.gif

[Enzo]

You got a scope? Touch the tip of the probe with your finger and you'll get a 60Hz signal on the scope. it will be ugly and distorted, but 60 Hz nonetheless. Now set the H sweep so you get one complete cycle across the screen. Now clip your probe to some point after the rectifiers where there will be some 120Hz ripple. adjust the vertical for size but leave the H sweep alone. See how you have twice as many cycles across the screen?

60Hz and 120Hz are the same "note", one is an octave higher than the other. Learn to tell them by ear, you'll thank yourself later. And if ears can;t learn that, you can always just touch your scope probe as a 60Hz reference.

You can change tubes all day, but my money is on a shared ground return path. I can't read the drawing you linked, wher is C10?

Hum can come from a million places, and it all will be of the 60/120Hz variety, so it all sounds the same. But your channel 2 only hum could be from a totally different source from the background hum. They may be all related, but don;t assume that offhand.

[lowell]

Thanks Enzo. Try this one http://www.freeinfosociety.com/elect...es/ampegv2.gif it's a bit easier to see. C10 is the coupling cap from the last gain stage's plate to the PI grid.

I just moved the preamp filter cap to the input jack, it was tied to where the screen filter ground was. Whoever was in here before was out of their mind. Seems a bit better but still there.

What is F10/D5 for? It is between the rectifier and chassis ground.

I should mention that the amp is pretty dirty and kinda has a farty sound to it. It doesn't sound right to me.

[Jazz P Bass]

Ampeg V2 Schematic

Here is the schemes from a 1971.

[Enzo]

Aha. Thanks, but those are those damned Joe Piazza drawings. Schematic heaven is full of them. Not only are they sometimes inaccurate, but they PISS ME OFF by omitting part numbers. Where is C10? We'll never know from Joe.

Did you determine the hum freq? That was the point of my trick.

Isolate the problem. You may have more than one source of hum here too.

C10, so the hum is before that point. OK, most of the amp is before that point.

Is V3 a 12DW7 as it should be and not a 12AX7?

What controls have ANY effect on the hum? We already know about VOL2, so leave it down. DO the EQ controls affect the EQ of the hum?

Pull V1, still hum? Pull V2 also, hum? Pull the 6K11, still hum? Pull the reverb recovery tube V203, still hum?

Clip lead to ground - see J3,4 EXT.AMP jacks, across R21? Ground that point. Hum stop? Move back a stage and ground the left end of C8. Hum stop?

Oh, and does the POLARITY switch make any difference? Do we have a three wire plug on the amp now?

Is the hum balance pot functional, and does it work?

[Jazz P Bass]

V2 pdf.

[g1]

I'll second that emotion regarding the Piazza drawings.
F10 is the diode no one seems to want to talk about, seem to recall reading something relating it to the bias supply but never anything definitive.
As far as the distortion, the first stage (pre volume pot) overdrives quite easily unless the sensitivity switch is set to lowest position. Or is the distortion from the power amp?

[lowell]

Oh yeah sorry... it's 120hz hum

V3 is a 12DW7, yes.

When stby switch is thrown to play and all controls are off there IS that startup hum for 3-4 seconds, regardless of controls being off. However if all controls are up all the way the hum is ridiculously loud. All controls make the hum worse if they're turned up... especially the mid knob.

V1 pulled, no change.

V2 pulled, hum is far less. With V2 pulled and all controls up all the way, the hum is as loud as if all tubes were in but all controls were off.

6K11 pulled, no change.

V203 pulled, no change.

Hum gets worse when I ground top of R21. Grounding left end of C8 does not help.

Polarity switch has no audible effect and yes on the 3-prong plug.

Hum balance pot is working however there is no audible effect. It is however adjusting the heater voltages.

[lowell]

Enzo do my responses tip you off to anything? It seems to me that the stage V3 is the issue, in that pulling V3 fixes the issue as well as grounding the grid side of C10. If all tubes before V3 are pulled the noise is still there. However, the noise is worse with all tubes in and controls turned up. So maybe there's 2 issues? And what the heck is D5/F10 in the ground path between the rectifier/stby switch and chassis ground? Is it some kind of ground loop elimination technique?

[Enzo]

I sat down and fathomed out the standby switch and diode arrangement at some point, and promptly forgot it all. The design works for whatever reason, and As far as me repairing the things, I have no reason to change it, so I ain't worried about it. I am not suggesting you shouldn;t be curious, I just don't recall the rationale I had for it.

I have no particular ideas other than those stated. Since it is 120Hz, it is power supply ripple related. It could be this is normal for this amp. It may involve moving a ground attachment somewhere. I have encountered AMpeg amps that had hummed from Day one because the repair was to move the ground wires.

[lowell]

Okay this is what I've done so far. If there's another suggested grounding mod please let me know.

Moved very last preamp filter cap and reverb stage filter cap grounds to the input jack. I also moved the PI filter cap ground to where the Screen grid supply cap is grounded - which is physically about 2 inches from power cord earth connection. The initial power tube plate supply filter cap was and is still grounded at another point near the power transformer, but not directly on the earth connection to the chassis.

[lowell]

Ok so I started pushing on the input jacks and noticed that the hum would get better or worse. So I pulled the jacks and ran a clip wire to where the sheilding on the input wires went. That fixed it so I added insulation washers to the input jacks and that fixed the hum issue that was affected by the preamp volumes.

I also found an iffy wire on the power amp in jack that would change the hum when I pushed it around. I redressed it and that helped alot. Basically the hum is gone now with all the controls off. With nothing plugged into the input there is no hum when all volumes and master are up. So that's really great.

Now, when I plugged my guitar into the power amp in jack there IS hum. It is only affected by the master volume which makes sense. However the amp just sounds bad to me. There is an odd overtone and the sustain is real bad on some notes... some notes just sound dead and don't hold out. I just found that V4 the PI has odd voltages on its plates.

Pin1: 178v
Pin6: 330v
Vgk: 6.4v

All other pins' voltages measure correctly. According to the schem these are both supposed to read 218v. I've measured ALL components in this stage and checked that all components are indeed connected and there is nothing wrong. I'm lost Any ideas on this?

[g1]

Assuming the tube and socket are good, could you have a leaky coupling cap? Are the PI grid voltages both the same? Is the bias voltage the same for both power tube grids?

[lowell]

I'm an idiot. Somewhere along the way I had pulled an ecc832 out of an ecc82 box... whoops. PI voltages are good now and amp sounds much better.

[Jazz P Bass]

Glad that you caught it, Lowell.
ECC832:This is a suitable replacement for any 12DW7 / ECC832. One side has a gain
factor of 100 - the other side has a gain factor of 20.