Why am I blowing fuses when connecting to next component?

[homemade]

As you might have guessed ,I'm new to tube circuits. I purchased a few pre built tube preamp boards for use as a gain stage in resistor i/v on some TDA1541 cd players I have. The boards are straight forward , with very easy hook-up. 120vac , 6.3vac , input , output. Bridge diodes are included onboard for each power supply. I'm currnetly using Line voltage 120vac and a 6.3vac transformer for heaters. The circuits works fine. In my shop , I use a 2 channel reciever for testing. Everything worked fine, sounded very good , and I was quite proud of myself. HOWEVER , I brought the player into my house and connected to my main system. This includes a good quality tube pre amp , that I have owned for 15yrs without issue. When I connected it to my tube pre , upon turning it 'on' , the player blow a fuse. i figured it was a glitch, so I replaced the fuse with a slow blow type. This also blew. Took it back to the shop , connected it to my reciever and NO problems, players fine. Back to the house , fuse blows. Anyone have insight? Pics below

[bob91343]

The problem is simple. You must isolate the 120 V input with a transformer.

[rknize]

Indeed. I have a large 100W amp in my basement that works this way. I have never plugged it in for this reason. Get a transformer!

[homemade]

Originally Posted by bob91343

The problem is simple. You must isolate the 120 V input with a transformer.

I see. How does the direct connection cause a problem when connected to some components and not others? It does not 'appear' to be a problem with SS pre amps , only on the 2 TUBE pre amps I have tried.
So I need a 120v-120v trans?

Thanks

[rknize]

You are tying the circuit to your home's mains. Depending which way you wired the hot and the neutral to the circuit, one of them becomes the common in the amp. If you then plug that into a component that ties its signal common to earth ground, you have just shorted that leg of your home's wiring to earth ground. In your case, it sounds like you have the hot side appearing as the common. You have been lucky that you have not been shocked.

It's possible that either the outlet in your shop or your home has the hot and neutral reversed, which is perhaps why you didn't receive a shock while in the shop.

[homemade]

Originally Posted by rknize

You are tying the circuit to your home's mains. Depending which way you wired the hot and the neutral to the circuit, one of them becomes the common in the amp. If you then plug that into a component that ties its signal common to earth ground, you have just shorted that leg of your home's wiring to earth ground. In your case, it sounds like you have the hot side appearing as the common. You have been lucky that you have not been shocked.

It's possible that either the outlet in your shop or your home has the hot and neutral reversed, which is perhaps why you didn't receive a shock while in the shop.

OK. It makes sense now. I did measure for voltage on the rca's ground side and it was 0v(line hot or neutral to output ground). Not that I'm trying to explain away the issue , but if the tube circuit is isolated from the chassis and the rca's are also , does this make any difference?

[stratus46]

Originally Posted by homemade

I see. How does the direct connection cause a problem when connected to some components and not others? It does not 'appear' to be a problem with SS pre amps , only on the 2 TUBE pre amps I have tried.
So I need a 120v-120v trans?

Thanks

When you use a bridge rectifier directly from the wall (you're looking to die - SOON), in essence your output negative ('ground') is connected to both sides of the AC line. It isn't really, just every other half cycle.

DO NOT SCREW AROUND WITH DIRECT CONNECTED SUPPLIES !!! (until you know exactly WHAT you're doing)

If you're not having trouble connecting to some units it because THOSE transformers are covering your butt. It you touch your 'ground' and a true earth ground, you will get a nasty potentially lethal shock.

If I seem harsh, it's to keep you around. Always always ALWAYS use a transformer for your projects and experiments.

G²

[homemade]

Originally Posted by stratus46

When you use a bridge rectifier directly from the wall (you're looking to die - SOON), in essence your output negative ('ground') is connected to both sides of the AC line. It isn't really, just every other half cycle.

DO NOT SCREW AROUND WITH DIRECT CONNECTED SUPPLIES !!! (until you know exactly WHAT you're doing)

If you're not having trouble connecting to some units it because THOSE transformers are covering your butt. It you touch your 'ground' and a true earth ground, you will get a nasty potentially lethal shock.

If I seem harsh, it's to keep you around. Always always ALWAYS use a transformer for your projects and experiments.

G²

I do appreciate this. I'm very glad I looked to this forum

[bob91343]

I would add at least two or three more ALWAYS's.

In the old days they had electronic stuff that wasn't isolated. They solved it by putting the gear into plastic cabinets, insulating knobs, and kept the chassis covered. If there was need for an external connection, they isolated it with a capacitor but there were lots of hum problems.

I had a cute one tube radio as a kid. It almost killed me a number of times.

[homemade]

It appears I was mislead by the seller , but I suppose I should have known better. What usually makes sense, should be obvious. Please see pic below.


Can anyone suggest a good transformer for this circuit? Maybe a link?

Thank you all for saving my butt.

Paul