V4 PCB component repair/updating

[beedoola]

I got a V4 project that I'm getting ready to order parts for. I already have plans to redo the filter caps, screen resistors, plate resistors.

I had been told that the coupling caps sometimes need replacing. Are the coupling caps in these pictures the small silver caps on the big PCB?

Also, you'll see a burned spot on the big PCB near the screen resistors, the resistors don't seem to be compromised but I wonder what else there is.

Anything else that should probably replaced/updated? Diodes.

I'm also going to replace C16.

[Hangman]

the small silver guys are the coupling caps. I'm not familiar with those particular caps, so I can't say how reliable they are. probably wise to replace. The small grey caps are cathode bypass caps, they are electrolytics and should be replaced. The larger silver one is the filter cap for the bias supply and should also be replaced. 100uf 100V in your amp. (later v4s had 2 10uf 100v)



The burned spot on the main pcb is not uncommon for these early V4s. in later version of the V4 they moved r53 over by the 3x40uf 500V capacitor that is on the other side of the chassis, near the Output transformer.
you can see if the board cleans up a bit there... but it is likely permanently discolored.
if it really bugs you... go to ebay
http://www.ebay.com/itm/Ampeg-V2-V4-VT22...3f361517ec
our friends at Orville amp technologies have reproduced the original board. it is a very high quality exact replica.

basically ampeg put too many hot components too close to one another. all of those resistors generate a fair amount of heat. my recommendation is to move R53 over by c17 (by the OT) when ampeg did this they added a small terminal strip.

if you are replacing the 10ohm resistors anyway... lower the resistance on them a bit... that will help keep the heat down there too. some people completely remove them... others change to 5.6ohm (like in later v4s) or even go as low as 1 ohm. I don't think the value is critical, but don't take it higher than 10 ohms.

some people update the diodes to faster diodes... but it isn't needed.

the striped film capacitors should probably get replaced. (often they crack)

hope that helps!
-steve

I got a V4 project that I'm getting ready to order parts for. I already have plans to redo the filter caps, screen resistors, plate resistors.

I had been told that the coupling caps sometimes need replacing. Are the coupling caps in these pictures the small silver caps on the big PCB?

Also, you'll see a burned spot on the big PCB near the screen resistors, the resistors don't seem to be compromised but I wonder what else there is.

Anything else that should probably replaced/updated? Diodes.

I'm also going to replace C16.

[Liquids]

Not everyone agrees with me, but I say, replace everything you are willing to. This is not an old Marshall/Fender where original parts vs. updated parts or "stock" makes it more valuable (and hence an piece of gear for home play, a collector).

You can figure out what cap is what by following a schematic, the parts numbers are labeled on the board accordingly.

One thing I don't think I've mentioned before--reflow the solder, or remove and replace the solder on all the PCB tubes. Not too long ago I was getting an occasional intermittent issue I couldn't figure out. When I finally got it to the bench I removed the ancient and cooroded solder on the pcb pins 3-6 pins at a time, cleaned the joints (as pure an isypropyl alcohol as can be found and a stiff brush, check tubedepot), then resoldered the preamp tube pins, then last but not least, applied pressure to the tube socket while I reheated each of thee solder pins to make sure the sockets were as close to the board as possible, given that they can get wiggled/cracked etc over time. I think it's good practice for pcb-mounted tube sockets, and whether it was coincidence or the issue, I haven't had anything cut out since either.

---

You also have some serious burn marks on that board. If you are willing and able to do the work, buy one of the new main boards available from orville amps and save it for the day you have more trouble with arcing, or just do that from scratch. You'll be glad you did if you use this amp regularly. Easier to populate a clean board than it is to desolder old components and resolder new components on the old board if you are going to do more than a handful of components.

Only trouble is being extra sure of where your wiring points go to/come from, easy to get lost unless you take the time needed for when you get a little lost.

[beedoola]

Should any of the larger wattage resistors also be replaced? - like those near the 100uf bias supply,

[Liquids]

Should any of the larger wattage resistors also be replaced? - like those near the 100uf bias supply,

It may not be necessary, and the 10W resistors tend to be larger and may fit more neatly on the other side of the board, but if you're ordering parts, I'd order one or two replacements even just to have on hand Or if you are handy, play the amp for a while (let it get hot as ususal), then turn it off, drain the voltage on the caps, and measure the resitors resistance in circuit while still warm but without voltage on it...see how close it is to spec. That may be a good indicator on how important it may be. Then again, it is another place, on a PCB amp, that if the old component falters, you'll get arcing and such...you *could* just replace it.

[Hangman]

As liquids mentioned, the larger resistors probably don't require changing. I have seen v4s where the did... I would just measure them, and if they measure within tolerance, then leave them. Although, The resistor I mentioned in my previous post that gets moved will probably require replacement just for the sake of lead length.

Should any of the larger wattage resistors also be replaced? - like those near the 100uf bias supply,

[beedoola]

Is it important/need to replace the bias resistors? - R49, 50?

[Liquids]

Is it important/need to replace the bias resistors? - R49, 50?

Really, again, why not?

I'd make it more critical to replace that diode and the bias capacitors (make the diode it the last part you replace in the bias network so it has the least soldering heat time).

You don't want your bias straying as the amp warms or over time, period. Note that carbon comps (if that's what any of them are) are most likely to drift in value when warm and current is flowing.

If you want to spend the time--my time is more valuable to me than this--measure the values, see how far out of spec they are especially when warm and the amp is hot, but be careful as always. Ok, if they measure within spec, fine, you saved a couple of cents on resistors. I just change them. I'm the "overhaul the amp while it's unbuttoned" guy through and through though. Once and for all, and you are being proactive...especially in a V-series where instability can take out tubes running at or above max spec already.

Some people change the stock resistor values for bias point tweaking. I'm a fan of cold bias here--fatter bass if you ask me. The Fender 60%-70% bias at idle rule will eat tubes here, and with my real-time experiments, warmer biasing adds a kind of midrange but makes the lows mushier. This amp's great features are that it has tons of midrange available, and the lows can as FAT or FATTER than any other amp I have played without mushing out. The cool bias clears up the mids to let the mid knob do what it does, and keeps the lows fat and powerful. So, why bias this one hot? Some loose sleep (like I once did) because some forum doods claimed that it is across the board better to have hottest bias... Don't do it, at least not with a V-series...but hey, for anyone that thinks the amp lacks mids(?) or the bass isn't mushy enough, and wants to replace power tubes as often as possible, hot bias is your ticket here. But maybe they should play a Fender tweed or blackface amp, if so (which are no doubt quite nice amps)... ;D

I prefer a really cool idle bias with this amp, tonally (as low as ~40% is quite good IMO). IT also, serendipitously, prolongs tube life. The stock values are wisely chosen and good IMO as well.

[beedoola]

C1 in my amp is .1uf, the schematic calls for 50PF or that some units won't have C1...

The rest of the possibly omitted components aren't there: R1, R7, C4.

Should I just leave C1 out?

And what is the value of D6?

EDIT*** - I saw on the old V4 forums that you (hangman) said 1N4007s will work for D6.

[Liquids]

Are you certain that it's C1 that 100n/.1uF? From zooming in on the image of your board it looks unpopulated, and maybe that .1uF is some other cap.

R1/C1 are an RF filter. Unlike most amps, there is no series input grid resistor on the first tube stage, your signal goes directly to the grid of the tube. This gives Radio Frequencies/stations a real chance at being involved in your gig/practice, but seems rare? Thought I've had other amps with RF issues and it's quite annoying.

It seems many V-series amps don't seem to have any R1/C1 (Channel 1) or R7/C4 (Channel 2) [hangman/Steve could probably offer info on how rarely he sees them fitted].

I think R1/R7 are good in practice/theory (and a little larger isn't an awful ideal), but the input resistors to ground (R2/R8) are absolutely enormous, probably beyond spec for most amps. though in realtime, whatever guitar or piece of gear right before your amp's input is probably running in parallel so the tube is probably not actually seeing 5.6M to ground but less...

Anyhow, 1k and a small value cap for one of the channels might bail you out of situations you could theoretically get in where you are sending such a low output impedance signal to the amp and/or in a such a RF prone area (ever play near a college radio station?!) that you get RF at the input grid of the amp and it's broadcasting through your amp(!).

If you add a resistor and/or cap on even just one of the two channels here, you might save yourself from the headache of that (albiet unlikely) situation ever occuring, or having a plan B if it does. I added either an R1 or R7 resistor and no cap myself, but can't remember which one.

Just be sure you look close and make sure the input wire is properly located if you do this, because obviously the input signal is routed to the tube side of the resistor if it's not there, unless there is a jumper in place of R1/R7....