Ampeg V4 Correct Grounding

[roadkingbluzs]

Hello to all with Ampeg experience.I am preparing to add 3 prong cord to a V4 before I do any additional work to this amp (geez this thing weights a ton) and am a bit confused with regards to the hum balance VR. Can someone help in clarifying proper grounding schematic for V4 with the 100 ohm hum balance VR. Is it as simple as eliminating Death cap and adding 3 prong cord, in turn making the polarity switch non funtional. At first look I thought this amp had a correct 3 prong cord ,but someone just added a 3 wire cord cap, polarity correct but no green ground wire. I have yet to here this thing which BTW belongs to a friend. needs cap job tubes power switch etc. I understand this is a real clean machine and loud muttha. Bob

[Structo]

I'm not sure if I am right but I would think that you would ground the wiper of the hum balance pot, get rid of the death cap.

Here are a couple drawings about converting to a three prong cord.

[KT66]

Ampeg probably used a good quality hum balance pot, but I don't think it's not that useful of a feature and on old amps it's a typical thing to fail, so I would just take it out of the circuit and use 2 X 100 Ohm resistors to make it more solid.

I understand this is a real clean machine and loud muttha

Ampeg is known for making clean amps, but when you crank a V4 it's one of the greasiest, raw sounding amps. For example : http://www.youtube.com/watch?v=b0T9Ag7C ... L&index=18

[paulster]

Ampeg is known for making clean amps, but when you crank a V4 it's one of the greasiest, raw sounding amps.

Yep, awesome tone machines.

Unfortunately they (and the equivalent VT-22 combo) are exceptionally rare this side of the pond and I'd dread to imagine what the freight cost would be if I bought a US one!

[jaysg]

I don't think it's not that useful of a feature and on old amps it's a typical thing to fail, so I would just take it out of the circuit and use 2 X 100 Ohm resistors to make it more solid.

If you go that route, set the pot for the least hum, take it out, then measure. It may not be dead center.

[Structo]

I don't understand why a balance pot would fail.
There isn't much current on it is there?

Not sure I would eliminate it, different tubes will have a different balance point.

What I would do though is install a trimmer pot in the bias circuit so you can adjust it instead of changing out a fixed resistor.

http://www.schematicheaven.com/ampegamps/v4poweramp.pdf

[jaysg]

I don't understand why a balance pot would fail.

Pots, like most switches and connectors, are self-cleaning. A hum balance pot is sort of the opposite of the front panel pots which might fail from over-use. They hardly ever get touched, so the wiper can go open.

[roadkingbluzs]

Thanks for all the info. and I have checked hum balance pot and it is good. I will most likely leave it in and do the 3 prong cord mod. Chassis is from a two twelve combo (Altecs). I still can't get over how much this thing weights. Any additional info. welcomed. Sure does have a Dumble SSS look to it. Bob

[Structo]

I don't understand why a balance pot would fail.

Pots, like most switches and connectors, are self-cleaning. A hum balance pot is sort of the opposite of the front panel pots which might fail from over-use. They hardly ever get touched, so the wiper can go open.

Yeah, that makes sense since the wiper is just sitting there in one spot.

Would a wire wound rheostat type work better there?

[mojotom]

A 74 V4 is my main amp for bass and guitars for almost 15 years, amazing cleans and punch but very balanced, it kicked out a lot of amps I tried including some fine classics. i always think about it when I heard about the SSS.

It got BASS for sure but nothing too boomy, the main issue is about neighbourhood because everyone will hear your bass notes, I got lots of issues...

Strangely I didn't study the circuit that much, a real mess inside but when I bought it (they are a bargain even at $1000) got loud hum. Send it for repairs but still got the hum. I then discovered that the hum balance could fail if an output tube goes bad, change the pot. Voilà. Looks like it works as a fuse just like the 10R resistors on a svt, once in a while you will have hum and the pot wouldn't change anything. Change the output tubes and pot (easy to access to) and you're done

I don't know if I would advise buying the amp though. At least you will need a good tech : lots of tubes, very sensitive to tube choices and lots of stranges parts and something like 700V on the plates add 30 years of abuses (who woudn't play the amp ?) and you will have something on the verge of instability.

[paulster]

I don't know if I would advise buying the amp though. At least you will need a good tech : lots of tubes, very sensitive to tube choices and lots of stranges parts and something like 700V on the plates add 30 years of abuses (who woudn't play the amp ?) and you will have something on the verge of instability.

But when they're working right, just how good do they sound? Awesome clean tone and the greasiest overdrive if you can ever stand to get it up that loud!

Would love some chassis shots if you ever get it open to see the cap and resistor values either side of the bass and treble controls as they are currently undocumented on all the schematics doing the rounds (presumably because they're on a custom Ampeg PCB).

[roadkingbluzs]

Unsoldered leads for testing and as soon as I resolder (tomorrow) I will grab digital camera. Not a high end camera but it works. Just took another quick look at the chassis and guess what, pots are PCB mounted caps sandwich between face plate and PC board. I see what you mean about documentation, Paulster, schematic does not show values. If it is not to much of a hassle,lots to unscrew I think I can lay PCB back and document. BTW schem. is dated 6-70 (Rev. E)

[mojotom]

But when they're working right, just how good do they sound? Awesome clean tone and the greasiest overdrive if you can ever stand to get it up that loud!

Totally agree. This is my fave even against 66 JTM45 and Super Rev. Also one could use it For guitar, bass and keyboards (Rhodes especially).

After two years of extensive use for almost everything in my shop it start to fall appart so I have to work on it (a lot) and I don't like the idea, just feeling lazy looking at it.

Mine is a guitar model featuring reverb and a useless Ch1 distortion circuit so I only use Ch2. I will work on it soon and documented it (already got the special filters caps) but it may take a while as it seems complicated to work on compare to Fender, marshall or even Vox.

If I remember correctly the components around the tonestacks were mounted inside some kind of epoxy modules


My point was with 13 tubes it would cost a lot to retube and service and you have to find a good tech

[roadkingbluzs]

This is my first go round with any Ampeg and after studying it a bit seems pretty clean and well laid out/compact. As Paulster stated some hidden cap values. I'm kinda taking a liking to it. Maybe I should make the kid an offer?

[paulster]

My point was with 13 tubes it would cost a lot to retube and service and you have to find a good tech

I've already simplified the schem taking it down to a single channel and removing the reverb and got the preamp down to 5 tubes, using 12AX7s and 12AU7s in place of the Compactron and the 12DW7 (which these are a mix of) to use readily-available tubes. It actually works really well because the order of the triodes in the Compactron and the DW7 means you have the triode types in sequence in groups of two, so perfect for using the conventional dual triodes.

Going to look at the reverb circuit and see if there's a practical way to sub out the 6G11 for something that's in current production so easier to obtain going forward and I might actually consider the practicalities of making a hand-wired version of one of these.

There is also the possibility of adding the V4B (bass) ultra-low switch as well to extend functionality since this was just a filter after the first gain stage which also had a bypass position, and a switchable bypass cap I think it was. Aside from this (and the reverb) the V4 guitar amp and V4B bass amp were one and the same.