Isolation Transformer an Option?

For safety reason I really need to find an alternate solution to cheater plugs. Almost embarrassed to admit this but I think its important to get the message out, I just got a shock off one of my amps. Talk about a wake up call.

What about an isolation transformer like this? Is 500w enough for 2 x M125? Are they equally effective or should I look at HumX? They are around $100 ea which seems a bit pricey.

https://www.amazon.ca/Tripp-Lite-IS500HG-Isolation-Transformer/dp/B00007KQKJ/ref=sr_1_1?ie=UTF8&qid=1506792556&sr=8-1&keywords=500+w+tripp+lite

I recently picked up an outlet tester and all my plugs check out fine. Only floating the grounds via cheaters so far has worked for totally eliminating hum.

Legrace - by chance is the amp that shocked you the same one that has consumed multiple tubes??


(Danger Will Robinson!!!)

Yes, while changing out blown fuse and installing new tube pair. Bias is more stable now, working good since.

LeGrace wrote:I recently picked up an outlet tester and all my plugs check out fine. Only floating the grounds via cheaters so far has worked for totally eliminating hum.

Earth to LeGrace:  You have a severe, significant problem *somewhere* within your system. The shock is a gentle warning of same - the next warning may not be as gentle. I really do not want to be snarky - but neither do I wish your SO to find you in some state of disorder, possibly permanent - or someone else in your household.

You need to do some significant, systematic and complete diagnostics as to *why* you are getting hum - I can think of half-a-dozen. And each one of those is potentially fatal.

a) Shorted cap.
b) Shorted transformer primary or secondary - shorting to the frame.
c) Pinched wire.
d) Miswiring.
e) Cap shorting to case.
f) Fudged connection where none should be.
g Any other suggestions? I am not going towards tubes as so many have been tried... But that is yet a few more possibilities.

Do not pass go, do not collect $200 - go directly to your VOM. On BOTH AC & DC volts, measure from the chassis to a good ground from each component you have, independently and collectively. If you are getting *ANYTHING* more than a few MV, you have a problem. Isolate where that problem is and both your shock potential and your hum WILL go away.

This is a most basic life-safety issue - having nothing to do with things Dynaco, VTA amps, tubes, nor anything else. FIX IT! If you cannot, or choose not, or will not, do not permit any living creature but yourself any contact with your system under any conditions. And think long and hard before touching it yourself.

+1
(What Peter says.......)

An isolation transformer may give you the same shock.  Issue seems to be the amp's chassis ground reference is "floating" and its ground potential is not always the same as other equipment.  Connecting the RCA connectors to the source should clamp the amp's ground to the same potential as what is feeding it.  If you have nothing connected and you touch the floating ground amp and some other grounded equipment is where the ground shock hazard may lie.  If your source equipment is transformer coupled on the RCA outputs and the RCA cable shield is not grounded at the source could be a problem and source of your woes.

But like others have stated you may have other issues to correct for your situation.  I burned up oscilloscope leads once probing a floating ground piece of equipment.  The oscilloscope was grounded but the floating ground potential difference eventually got so large it melted to oscope leads.  I had to use a cheater plug on the oscope to continue working.

What PeterW said, +2
My advice? Send the errant amp off to a reputable VTA guy like Bill Gratix (Audio Bill on this forum) and get it fixed. There are sometimes when you're just digging a deeper hole. We can't be all things to all people or gear. No way you should get nipped changing a fuse. Something's seriously wrong. A fresh and trained pair of eyes will probably spot what's been oblivious to you in a half hour. Yeah, they're heavy and expensive to ship, but there's no duty or customs charges on repair work, and your life and peace of mind are worth more than $75. And if you continue to poke and prod, be sure one hand is secured in your back pocket and don't wear jewelry.

Shut it down, wait 15 minutes, unplug the amp.  

Double check all of your wiring starting with all of the power supply connections.  Check for shorts, miswires, loose / unsoldered wires.  Ground connection mounting screws should be tight.  

If you feel confident in checking the amp live then use only one hand to probe the power supply connections.  Leave speakers attached but short the amp inputs.  With your vom attached to an earth ground see if there is any voltage or current on the chassis before going any further.  If there is then power down and check your wiring again.  Ideally there should be no hum with inputs shorted.  If the hum returns when the preamp is connected then the problem is probably coming from there.

If you feel it's safe to continue testing then with your ground lead attached to the chassis, power up and check the rectifier connections and then the quad cap connections for proper voltages.

Please be careful!

corndog71 wrote:Shut it down, wait 15 minutes, unplug the amp.  

Double check all of your wiring starting with all of the power supply connections.  Check for shorts, miswires, loose / unsoldered wires.  Ground connection mounting screws should be tight.  

If you feel confident in checking the amp live then use only one hand to probe the power supply connections.  Leave speakers attached but short the amp inputs.  With your vom attached to an earth ground see if there is any voltage or current on the chassis before going any further.  If there is then power down and check your wiring again.  Ideally there should be no hum with inputs shorted.  If the hum returns when the preamp is connected then the problem is probably coming from there.

If you feel it's safe to continue testing then with your ground lead attached to the chassis, power up and check the rectifier connections and then the quad cap connections for proper voltages.

Pleae be careful!

Corndog, is 15 minutes long enough to discharge the caps below lethal voltages? I know the raster chokes on old CRT colour TVs could kill you a day after you unplugged the set. Just because a cap or choke is not sufficiently juiced up to pass signal doesn't mean it's benign. Coward that I am, I'd wait a day, then short the leads with a screwdriver. Is that too conservative?

I agree with deepee also. It's just too dangerous to try and suss out a potentially lethal problem with sometimes obscure/conflicting and/or wrong internet advice.

deepee99 wrote: Coward that I am, I'd wait a day, then short the leads with a screwdriver.

Would that be for fun because I wouldn't want to try that.  

Thanks for the responses. I only checked the power outlets my system is plugged into. Now I see that 2 downstream outlets that are on the same circuit are being indicated by the tester as having reversed wiring (hot/neutral reversed).

LeGrace wrote:Thanks for the responses. I only checked the power outlets my system is plugged into. Now I see that 2 downstream outlets that are on the same circuit are being indicated by the tester as having reversed wiring (hot/neutral reversed).

When we moved into our current home (23 years ago) my wife complained repeatedly about getting shocks ever time she went into the fridge in the basement. I always wore sneakers, she went around barefooted, so I just assumed she was crazy cause I never got zapped. Until one day I touched the metal sink next to the fridge and the door of the fridge and got knocked on my butt. Put a meter to the fridge and a nearby water pipe and measured 120vac, the outlet was reversed. Good stuff what people do right.

LeGrace wrote:Thanks for the responses. I only checked the power outlets my system is plugged into. Now I see that 2 downstream outlets that are on the same circuit are being indicated by the tester as having reversed wiring (hot/neutral reversed).

1. Get out your VOM.
2. In both AC and DC, measure from chassis to a known-good ground for each piece of equipment you have, and for the system as a whole. That is - remove each item from service, plug it in, turn it on and measure from the chassis to ground. Then assemble the entire system and measure again, each to ground.
3. If you get more than a few millivolts in either AC or DC, you have a problem.

That you have miswired receptacles down-line of your system would not be a contributing cause unless more than just the neutral and the hot is reversed. Please do not be stubborn - just go ahead and measure your equipment. And please let us know the results.

What Peter says! Make sure ground/ neutral are not swapped. I had a shocking experience in my youth where a Sno-Cone machine had them backward. And standing in melting ice water wasn't the smartest thing to do...

Measured component potentials as suggested. Used a copper water pipe as ground. First shorted leads to check the meter = 0.0. DC readings were pretty similar across the board, 0.8 to max of 1.0 mV. On AC range all measuring 0.0. Checked line voltage = 121.4 v.

Note: readings taken with cheaters on both 125's. Feeling a bit better.

I will redo measurements without!

Note: readings taken with cheaters on both 125's. Feeling a bit better.[/quote]

NO.

The cheaters are masking the problem. Start over from the beginning. No Cheaters. Test everything - alone and as a complete system. Please report results.

Peter W. wrote:Note: readings taken with cheaters on both 125's. Feeling a bit better.

NO.

The cheaters are masking the problem. Start over from the beginning. No Cheaters. Test everything - alone and as a complete system. Please report results.[/quote]
Yeah, no cheaters. Check for voltage potential (AC &DC) between the two metal chassis.
Again, and sorry for repeating myself, Pierre, please ship the errant amp off to Bill Gratix or maybe PeterW would take it on. Something is amiss.

An alternative to shipping an amp out might be to ask around at local guitar stores. Many provide service and repair for tube based guitar amps which get abused and broken on a regular basis. My friend takes his Chinese built tube audio amp to the local guitar tech and is happy.