'76 V4 Distortion, a couple of questions

[danb435]

Hey all, back from an extended recess. I pose a couple of questions for the experts here:
1. I'm aware of all the conversation about the "death cap" on these Ampeg amps. Can I confirm it is NOT required with the later 70's amps that use a three-prong polarized AC plug? My concern is hum and I assume this cap bypasses any AC potential between neutral and ground (although both are common within the chassis itself, seems redundant?)
2. My standby switch failed a while back, replaced it with a stock white rocker from Fliptops. I really prefer the stock look of the new switches although I'm not sure how well the standby switch holds up to the 600VDC it controls. Something inside me cringes every time I switch it on, imagining some level of arcing on the contacts. So here's the question: If I switch on the power long enough to get the tubes good and warm, then turn off the power long enough to turn on the standby with the power off, then immediately turn the power back on with the standby already on (tubes still warm), this will spare the harsh inrush to the contacts on the standby switch. Is there any reason this would not be an acceptable practice?
3. Again regarding hum issues, my amp was rewired at some point in the past using all white wire (nightmare to troubleshoot!). I question if the routing of the chassis wiring is causing some of the hum problems. I'm going back to rewire the chassis wiring again and need to know the best practices for keeping everything tidy and quiet. I've been unable to locate a wiring layout diagram which would be helpful. I will replace the heater wiring with tightly twisted wires observing consistent polarity on the tube heater pins as per the schematic, routing the wire pair as far away from everything else as possible. I was also considering using star grounding with the PS caps, preamp boards, power tubes and everything else with a ground wire going to one central point common with the chassis. Not sure this is worth the effort because the input jacks and pre out/power amp in jacks are all common with the chassis (possible ground loops?). Good idea or not? What other considerations do I need to observe routing the other power tube wires (control grid, screen, cathode and anode) and the HV DC supply wiring? At this point I know enough to interpret the schematic and work around the HV without killing myself; the finer points of induced hum and noise are still a mystery. Thanks in advance to the forum's brain- trustee's assistance, Dan B.

[Hangman]

hey Dan,

The death cap is not "needed" but if you want to have the benefit of having one, without the worry of it blowing up, or shorting, you can use Safety caps. The X2 type safety caps are a good choice. They are specifically made with the "death cap" application in mind, and they are very rugged.

the idea behind this is to remove any noise/hash on the power line. it can honestly help a lot if you've got some noisy old appliances on the same circuit.

The standby switch in the distortion V4s is a weak spot. They really could've used some nicer switches. you can actually see those switches arc when you switch them and the lights are low (or off). kinda scary. I think your idea of turning the power on first, letting it warm up, then turning it off, turning the standby on, and quickly turning the power back on is a great idea.

Remind me, did you post pictures of the wiring of your amp? I know from personal experience that the wiring of the distortion model can be tricky business. I rewired one (but used the original wire as much as possible) and there was definitely some issues with hum if things weren't just so.

[danb435]

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hey Dan,

The death cap is not "needed" but if you want to have the benefit of having one, without the worry of it blowing up, or shorting, you can use Safety caps. The X2 type safety caps are a good choice. They are specifically made with the "death cap" application in mind, and they are very rugged.

the idea behind this is to remove any noise/hash on the power line. it can honestly help a lot if you've got some noisy old appliances on the same circuit.

The standby switch in the distortion V4s is a weak spot. They really could've used some nicer switches. you can actually see those switches arc when you switch them and the lights are low (or off). kinda scary. I think your idea of turning the power on first, letting it warm up, then turning it off, turning the standby on, and quickly turning the power back on is a great idea.

Remind me, did you post pictures of the wiring of your amp? I know from personal experience that the wiring of the distortion model can be tricky business. I rewired one (but used the original wire as much as possible) and there was definitely some issues with hum if things weren't just so.

Hey Steve, back with a current picture, should be high enough res to get a good look. As noted previously most of the wiring at some point had been replaced with all white wire, 16AWG TFFN, the stiff bulky stuff electricians commonly use. What the??? As you will see I have gutted that stuff out and am going back from scratch with 20AWG 600V stranded. Thus far I have completed the AC mains thru the power switch/pilot and the HV supply wiring through the voltage dropping resistor network, PS caps and up to the driver board and standby switch/pilot. All HV leads beyond that going to the preamp board are free hanging temporarily tacked in place with solder until the final routing allows for a tidy permanent solder connection. I'm waiting on replacement tube sockets to continue. As of now I need to know the best way to route the grounding, heater wiring and control grid wires. Also the footswitch wiring going to the 56Kohm 2W resistors near the inputs. Hum was a big problem, just trying to get this amp to it's previous glory. Thank for your help.

[Hangman]

whoever owned it before probably convinced themselves that thicker wire = better sound or some goofy thing like that. no doubt encouraged by the marketing geniuses in the guitar cable industry.

If there is a hum and its been rewired, I'd keep an eye out for goofy redundant grounding, or filter caps that should be grounded but aren't. remember that redundant grounding can be through the chassis too. so if you have one PCB grounded to the chassis in two different spots, thats bad form.
since you are rewiring it, just make sure that every wire you are replacing is going to the right spot.

[danb435]

Well I finally got this thing put back together and everything seems to be "mostly" working. No explosions, no blown fuses or tripped breakers and and it didn't make any smoke...pretty much a miracle after rewiring it from the ground up (is that a pun??). Plugged in a guitar today for the first time and with the exception of some hum it sounded great, true Ampeg mojo. Almost brought a tear to me one good eye.
The excessive hum issue remains. All tubes and filter caps have been replaced. I have looked over the grounding several times, tried moving things around and can't seem to locate the cause. I tried star grounding, running everything to one single point common with the chassis and this did not help. I tried floating the shield on one end of the inputs and the pre/pwr amp cables from the pcb's to the jacks (with the star grounding intact) to avoid possible ground loops with no success.
Here is what I have determined:
1. The problem seems to be in the preamp section. I can plug a shorted cable into the power amp input and all goes quiet. I can plug a guitar with a stomp box directly into this jack and the hum is minimal.
2. Assuming that all tone controls are neutral at the noon position, the bass control acts normally. I can cut or boost the bass and there is no appreciable difference in the hum.
3. The midrange control is a different story. I get considerable hum when I use any boost, maybe starting at 2:00. It gets completely overwhelming fairly quickly. The frequency select appears to be working as the noise around the fundamental hum frequency seems to follow the selected frequency.
4. The treble is similar to the midrange, I get significant additional noise as I start to boost the treble control.
5. The reverb is dead, as I turn it up I get increasing noise until it starts to have a high frequency oscillation.
6. The hum balance seems strange to me. I would expect the sweet spot to be somewhere close to the middle of the sweep. Not so, it's quietest at one end of the rotation and picks up some treble noise as I rotate it the other direction.
7. All heater wiring is tightly twisted and routed away from all other wiring as much as possible.
8. I moved the 47k power tube control grid resistors to the tube sockets as previously suggested.

Question: The midrange inductors are all original. Could these possibly be part of the problem? I have no clue if the opto-isolators in channel one are good. If these were defective, what would be the result and how would I test them?
That's about all of it. Apologies if I wrote a book here, just trying to give some detailed info for others to digest. Any suggestions from the forum would be welcomed and much appreciated.

[Hangman]

Hmm, is the hum balance pot the original one? I have seen those pots fail and when they do, they behave exactly like you were saying:
"it's quietest at one end of the rotation and picks up some treble noise as I rotate it the other direction."
If you don't have another 100 ohm pot around, try using two resistors as fender did.

[danb435]

Hmm, is the hum balance pot the original one? I have seen those pots fail and when they do, they behave exactly like you were saying:
"it's quietest at one end of the rotation and picks up some treble noise as I rotate it the other direction."
If you don't have another 100 ohm pot around, try using two resistors as fender did.

Thanks Hangman. I installed a Fender replacement hum pot and it seemed to help a little but the hum is still pretty bad and the behavior is still the same. I've pulled each preamp tube one at a time to try to isolate the problem. The only way to quiet this thing is to plug something in at the power amp input jack -or- pull the PI tube. (related?)question: some of the caps according to the schematic (besides the PS filtering) appear to be polarized. C12, C14, C15 and C202 are good examples of what I'm seeing. Are these supposed to be polarized electrolytics or what? I'm grasping here, any help really appreciated.