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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Neutral wired to chassis, how to add a 3-prong cord?

Just picked up a nice little 60's amplifier, and noticed it had the dreaded unpolarized, ungrounded plug. I retrofitted a nice noise-filtered receptacle for a 3-prong computer/pro style plug. Thats when I notice that the power switch connects a hot power lead to ground through a ceramic cap.

Questions:

1. As long as the polarity is correct, and I wire chassis to ground, is this safe?
2. Will I wreak havoc with other grounded things if I plug this in? I seem to recall there being a good reason that Neutral was not directly connected to Ground. Is the cap doing enough?
3. Other parts of the circuit seem to pull power from a chassis connection. Would it be wise to just run the neutral wire to these and disconnect everything from chassis, and run chassis to ground or am I likely to run into issues?
 
1. Yes, there is a power transformer. The power is tied to ground before the transformer.

2. It's a no-name tube amp. Looks a bit too nice to be hand-built, but I could be wrong. The branding may have been on the original cabinet, but that is long gone, and i see no chassis markings on the simple folded sheet steel. The single-sided circuit board is laid out like an old Binding post style, with all components arranged neatly facing one direction, with the rest of the construction being p2p. Has some nice old rca 6l6's in the power section.
 
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FYI, other than watching the seller quickly demo that it made noise, it has not been plugged in. When I noticed the sorry state of the original power cord, i decided a new grounded setup was in order as to not blow anything unlucky enough to be plugged in when the non polarized plug ended up the wrong way. I also plan on replacing the cap can.
 
If my last message came off harsh, please consider:

1. I came here looking for DIY info. This board is DIYaudio. Seems pretty simple.

2. If you know enough to accuse me of not knowing enough, you are deliberately depriving me of information that could make the amp safe(er).

3. Please consider the following from the rules page:
" While most projects on this site deal with electricity and construction which inherently involve some risk, particularly dangerous topics and procedures should include a warning in the thread that adequately explains these risks. Certain inherently dangerous topics are not allowed. At this time they include but are not limited to: discussing power supplies directly fed by mains current without a transformer, and mucking about in CRT video monitors. Posts and projects are those of individual members of diyAudio. The forum itself is not in the business of vetting projects or posts for safety, accuracy, performance, reliability, function, or fitness for use. If you attempt to make something and it blows up, or turns expensive parts into charcoal, or just doesn't work the way you were hoping, that's between you and the person posting the project or idea. The forum is merely a bulletin board which allows anyone to post ideas, criticisms, or discussions. It is up to the individual to make the final determination of how appropriate a project is for them to attempt, based on their own experience. "

I think all the posts here have been not only aware of the possible dangers with mains voltage, but specifically to deal with that problem. If everyone got a coy reply of "go see an electrician noob" when they posted, then there would not be a forum in the first place.
 
Just picked up a nice little 60's amplifier, and noticed it had the dreaded unpolarized, ungrounded plug. I retrofitted a nice noise-filtered receptacle for a 3-prong computer/pro style plug. Thats when I notice that the power switch connects a hot power lead to ground through a ceramic cap.

Questions:

1. As long as the polarity is correct, and I wire chassis to ground, is this safe?
2. Will I wreak havoc with other grounded things if I plug this in? I seem to recall there being a good reason that Neutral was not directly connected to Ground. Is the cap doing enough?
3. Other parts of the circuit seem to pull power from a chassis connection. Would it be wise to just run the neutral wire to these and disconnect everything from chassis, and run chassis to ground or am I likely to run into issues?

The unpolarized plug is a problem. let me explain why. in your discription which is the neutral being wired to ground and an unpolerized plug.....that means...if you flip the plug inadvertently so that the chasis wired to neutral gets the hot side of the AC line and connect this amp to a preamp with a traditional 3 prong ground you will have a dead short, poping the breaker or flashing the gorunds on the RCA terminals. so def, replace the unploraized plug with a polarized one so that the chasis never becomes hot.

from simple to complex, change the plug to a polarized one, use a ground fualt service plug either in the wall or on a power strip, the question of changing the chasis architecture is difficult to answer with out schematics. Also, becareful of using any outlets on the chasis doing so in a hot chaiss senerio will extend the hot chasis to what ever is plugged into the socket.
 

prairieboy

Member
Paid Member
2010-11-22 2:31 am
You put in a 3 prong receptacle, so the ground from it should connect firmly to the chassis using a star washer and nut on a bolt. The hot and neutral should go to a double pole switch so both are switched, then the hot to the fuse, and then both to the power transformer.
There may be ground loops/hum issues, but that arrangement will make it 'safe'.
If you can access a variac, you should use it to slowly power the unit up. If you can't find a variac, then google to find the procedure to use a series of ordinary (not cfl) light bulbs.
 
@speakerfritz
You are correct, and addressing this is what prompted my question in the first place.

@prairieboy
I was planning to wire in a progressive double-pole to switch neutral and hot, as well as to add a heater only 'standby' (off-on-on) for extra safety and longer tube life.

I also plan on crimping and soldering a ring connector and mating it to a transformer bolt after roughing up the surface, good call on the star washer.

The unit has already been powered up with no issues, and all the caps (excepting the filter can caps, which will be replaced) are sprague orange dips. I might build a lightbulb rig to be safe, as I'm more afraid of frying my rca 6L6's and transformers than anything.

My original question, if running neutral to ground with a cap (per stock), rather than rewire everything tapped to ground is safe. Ground loops are yet another issue I'll deal with when I get there.

The amp had an empty tube socket where the rectifier tube would have been, but as noted above, it is using a solid-state rectifier (which I prefer). Serendipity would have it that the mounting holes were the same width apart as those for a 3-prong receptacle like the one in this thread http://www.diyaudio.com/forums/tubes-valves/211731-heater-wiring-good-bad-ugly.html and lucky for me, the rectifier socket is right next to where the power leads connect. This allows my power leads to be shorter and less susceptible to power-line rf.
 
@Andrew
I suppose the question is if the cap between neutral and chassis is enough isolation...

The cap in question is a axial ceramic 0.022uf 600v. The only non sprauge in the amp.

I don't have a schematic. it's a PP using 6L6GC's with HUGE output transformers. My life would be so much easier, and I suspect it would verify what looks like bias reference pins on the rectifier.
 
Just picked up a nice little 60's amplifier, and noticed it had the dreaded unpolarized, ungrounded plug. I retrofitted a nice noise-filtered receptacle for a 3-prong computer/pro style plug. Thats when I notice that the power switch connects a hot power lead to ground through a ceramic cap.

Questions:

1. As long as the polarity is correct, and I wire chassis to ground, is this safe?
2. Will I wreak havoc with other grounded things if I plug this in? I seem to recall there being a good reason that Neutral was not directly connected to Ground. Is the cap doing enough?
3. Other parts of the circuit seem to pull power from a chassis connection. Would it be wise to just run the neutral wire to these and disconnect everything from chassis, and run chassis to ground or am I likely to run into issues?

Greetings

1 - For this particular amp that is said to have a power transformer and 6L6 outputs, etc, I always eliminate the capacitor (sometimes referred to as "death cap") and proceed with a 3 prong AC cord and securely connect the ground right to the chassis in the vicinity of the cord or AC cord socket entry on the chassis. Many older amps have this situation and it does need corrected.

2 - The hot and neutral from the AC in, will not have any connection to the chassis. Those will only be running to a switch, a fuse and to the primary leads of the power transformer, basically.

3 - The chassis itself likely is the power supply "common" for the amp, and not abnormal to see various connections to the chassis. You could check to see if the cathode pins on the 6L6 sockets are essentially connected right to the chassis common. If so, then you will have some type of fixed negative bias on the control grids of your 6L6 tubes.

Which leads to...

MereTenacity said:
I don't know if it matters, but it appears to have a SS rectifier with adjustment pot, tied to chassis.

Without a schematic, my guess on this is you are seeing grid bias circuit components and there would be a potentiomenter (if adjustable bias) and a nearby diode and likely peripheral resistors and a filter cap. Yes, this would be referenced to the chassis common...
The electrolytic cap will have its positive lead to chassis and the negative lead after the diode, which the "banded" end of the diode will face away from this cap. (Unlike regular power supply diodes connected in a "normal" fashion to provide positive plate voltages, and using the chassis as common negative...)

Do always be careful, have the amp unplugged before any inside work, drain the main filter caps, and if unsure - is there any friends who could assist ?

Best wishes and carry on
Deric
 
@djcgtr
The whole reason i'm posting is that neutral was run to chassis from the switch and then (from chassis) to the transformer. I guess it didn't matter too much when there was no true ground, but it isn't going to work as-is.

I've found a solution, thats a bit involved to go into depth here, and potentially id'd the amp from the transformers as an old Honher. Basically I'm going to lift the neutral and feed it straight to the PT, trace for anything that might be carrying voltage or continuity that might be expecting neutral and not ground, and do the aforementioned switch mod (double pole, 3-pos for standby) and cap job.

It appears most of the amp was 'liberated' from and old instrument or PA style amp(s), and some half-a** bodging was done to make it a faux-hifi.

I feel like an idiot. I kept expecting things in the amp to have been done for a reason, but apparently that reason was the last guy knew even less about proper grounding than I. I thought I was looking at a 'death cap' but there is no virtual ground switch as on the fenders.

I'll update the thread with more info as I figure it out.

Any future issues, i'll post in the instrument amp section if you want to try to keep up. I may separate the amp back into what seems to be originally 2 mono amps. If all else fails I can still sell the tubes and iron for more than i paid for this magnificent headache. Closer inspection also seems to reveal 2 preamp stages that aren't actually hooked to anything but the heater pins. WTF.

Thanks to everyone who helped. Hope that all made sense.
 
Followup:

* Death Cap Removed!

* Chassis ground installed (Crimped, soldered, and star washered to the PT bolt after sanding chassis connection point)

* Rewired to put fuse and switch inline instead of on either end of the ac.

* PSU-style filtered plug installed in unused rectifier hole. (unit has a SS rect)

* Cleaned Pots and chassis.

* Rudimentary test of caps to rule out short's or opens.

TODO:

* Buy and Install Progressive Double pole switch /w 'standby' switching the b+
? Any opinions on whether to use a guitar amp style standby or a turn-on delay relay?

* Re-Bias 6L6GC's

* Recap - Using Sprauge's because that seems to be what the rest of the amp uses.
? I was planning on using stock values 10, 20, 30uf @ 450-500v, are there any advantages to going bigger on these?

* Calculate impedance on inputs. (instrument vs line)

I fired it up * BRIEFLY* to make sure all was good, and when warm I got a beautiful, loud, clean sound with a faint 60hz. Shut it off after I verified operation, in case tube bias is too far off.

Folks, this is what you get when you buy tube gear off craigslist from some hippies in a van in a supermarket parking lot. In the end, i still think i got a good deal at a hundred.
 
Is there a chassis mounted wirewound variable resistor connected to the filament winding? If so this is a hum neutralizer. (Duh, adjust it for minimum hum...)

Do check if switching the primary leads into the power transformer affects the hum level.

I would use a progressive switch and only change the electrolytic capacitors.

It's nice to see you do know enough not to test for high voltage with your tongue! :)
 
Followup:

* Death Cap Removed!

* Chassis ground installed (Crimped, soldered, and star washered to the PT bolt after sanding chassis connection point)

* Rewired to put fuse and switch inline instead of on either end of the ac.

* PSU-style filtered plug installed in unused rectifier hole. (unit has a SS rect)

* Cleaned Pots and chassis.

* Rudimentary test of caps to rule out short's or opens.

TODO:

* Buy and Install Progressive Double pole switch /w 'standby' switching the b+
? Any opinions on whether to use a guitar amp style standby or a turn-on delay relay?

* Re-Bias 6L6GC's

* Recap - Using Sprauge's because that seems to be what the rest of the amp uses.
? I was planning on using stock values 10, 20, 30uf @ 450-500v, are there any advantages to going bigger on these?

* Calculate impedance on inputs. (instrument vs line)

I fired it up * BRIEFLY* to make sure all was good, and when warm I got a beautiful, loud, clean sound with a faint 60hz. Shut it off after I verified operation, in case tube bias is too far off.

Folks, this is what you get when you buy tube gear off craigslist from some hippies in a van in a supermarket parking lot. In the end, i still think i got a good deal at a hundred.

Hello
Well it looks like progress is happening and good for you.
Things will be much better with that grounded AC supply and no cap to chassis. I have a repair bench horror story (of many) I might have to share with the forum sometime on those older amps getting plugged into a grounded mixer....oh boy..

Anyway, on the electrolytics you mentioned about replacing, with the age of the amp, they are all likely getting pretty dried up inside and losing capacitance besides potential of becoming leaky with regards to having literal resistance to ground common....
When they sometimes fail to the point of being very low resistance, a tube rectifier might exhibit some pretty purple flashes inside the tube, while it destroys the rectifier.

The original values on main filters should be a reasonable reference for replacement caps. I wouldn't stray too far from the current caps value but in your case, who knows what a previous person put in there, unless they all appear to be original parts.

The tolerance on the uF rating of these caps is pretty wide.
A 20 uF (likely 22uF) new cap might measure around 20 or considerably more and still be ok to use. Of course don't use too low of voltage rating...

It was a good idea when you only ran the amp briefly for the sake of the outputs possibly being underbiased or no bias at all. I have replaced outputs before that had actual holes where the plates melted. And likely a very large fuse was installed to "fix" the amp...more than one time I have found a 20 amp automotive fuse in the fuse holder...

A residual hum could be from dried out filter caps and other things are always possible. If the 6L6's are pulling a huge current from either internal shorting in the tube or very low or zero bias, that will pull the power supply down hard enough to have excessive ripple, regardless of the effect of PP cancelling minor ripple.

I have received amps that guys bought off of ebay and they were shocked at what work was needed to get the amp rolling again. I know what you mean about the 'unknown' when obtaining used gear.

Sounds like you will have some enjoyment working your way through this project.

Carry on
 
The output didn't sound squashed or muddy at all, so I'm really confident the tubes and likely the output caps are good, but I like to replace 'em every 10-15 years to be safe anyway. It's only $30 and can save $400 in tubes and xformers.

I have been told that many older amps used smaller caps to save money, space, and due to availability. And nowdays, caps are cheap, so i'm curious how to calculate optimum values. If i had a schematic it'd be miles ahead here.