Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on March 08, 2011, 03:00:46 pm
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I have a record player tube amp I'm converting to a reverb amp. Here's the thing. It doesn't have a PT, it runs off wall voltage. Do I need to use an isolation transformer? The amps input would be from a reverb tank, so if I got one with an isolated output my strings won't be connected in any way to the amp. If it's OK to run w/o do I disconnect the cap connecting the amps ground to chassis and connect the chassis only to the green wire of a 3 prong plug?
Also the amp uses two 50EH5s and takes the heater voltage from the wall. If I do need an iso tran do I need to run the heaters through it too or can those use the wall voltage directally to the heaters? I could use a smaller tran if I don't need to run the heater current through it.
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If it was me, I would get the iso tranny and run your heaters through it too just like it's designed (but w/out the iso tranny). You can get the iso tranny for like $10 or so @ Mouser under the Triad transformer category. Another thing you may want to think about is a ground lift when running two amps or pieces of equipment together at the same time in the way you're thinking of doing it. It's similar to Fender's ground switching...refer to Aiken or Merlin's site where they discuss this for more info on it.
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Please transformer it.
> tank, so if I got one with an isolated output my strings won't be connected
Insulation in a reverb tank is ample for small stray voltages and no body in line. Put 120V on that insulation and put your body on an escape path, you are begging for Murphy's Law to work against you.
> use the wall voltage directally to the heaters
Same deal except now you trust too-tight socket wiring and thin heater-cathode insulation. Yes, the heater is rated for full line voltage, but it DOES fail. The spec rating is about equipment safety-- if they are wrong you may burn a resistor or other replaceable part. Late in the AC/DC age UL banned trusting heater insulation for Life Safety (which was basically the end of cheap transformerless gear with accessible signal connectors).
5-cent insulation is OK when "goes wrong" means a $5 repair. You get mixed-up in a 120V breakdown, you can't buy a new heart for $5. Even a little fibrillation costs $5,000, by the time the parameds and docs do all their CYA tests and process all their billing. And it can be much worse.
Transformer the whole enchilada.
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I have an old RCA phono that will give me a tingle too. can I put a tranny between the plug
and wall or hard wire inside..? Do we have a diagram here somewhere? thanks.
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Please transformer it.
5-cent insulation is OK when "goes wrong" means a $5 repair. You get mixed-up in a 120V breakdown, you can't buy a new heart for $5. Even a little fibrillation costs $5,000, by the time the parameds and docs do all their CYA tests and process all their billing. And it can be much worse.
Transformer the whole enchilada.
Please listen to PRR.
It's not worth it for the price.
Brad :smiley:
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OK thanks. I won't use it as a reverb amp without a transformer. I have been using it with my MP3 player. I've marked the side of the plug that is tied to the chassis through the cap and plug it into the wider slot on the outlet(neutral). The knobs are plastic so I'm not comming in contact in any way with the chassis. Is this OK to do? I also have some table top transformerless tube radios, should I be using a transformer for these?
Just so I understand better, in what ways is the 120V from the wall more dangerous than the 120V from a 120V:120V transformer. Is the point to protect you in the event the outlet you plug into is wired incorectally?
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Making an analogy, think of it in terms of a coupling cap. It will block DC just like a cap but lets the AC pass. OK now let's consider what happens when a filament shorts to ground. With a transformer in place, the secondary becomes heavily loaded, the voltage approaches 0, and current soars. Hopefully there's a fuse and it blows. Without a transformer, wall power is directly coupled. Instead of a couple of amp transformer overloading and the voltage dropping, you're now relying on the fuse (if present) only, or the house circuit breaker. Voltage isn't going to drop but current is still gonna soar. 100A @ 1V = 100W but 100A @ 120V = 12kW. Electricity travels very fast. Under a dead short condition, 100A or more can realistically be reached for a few mS or longer if the oldie ain't got no fuse. Transformers are cool.
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Just so I understand better, in what ways is the 120V from the wall more dangerous than the 120V from a 120V:120V transformer. Is the point to protect you in the event the outlet you plug into is wired incorectally?
It isolates the circuit from the seemingly limitless current of the mains.
An isolation transformer's secondary windings will be rated in mili amps while the mains will be a minimum of 10 amps and possibly twice that.
Let us ASSUME we install a isolation transformer using good construction techniques.
1st we'll install a 3 prong cord and land the ground wire to a dedicated stud on the chassis. we now do not have to worry which side of the plug is neutral and the earth ground, neutral, and signal grounds are now at the same potential.
This eliminates the50/50 probability of you becoming the shunt between the guitar amp and another ground source like a mic plugged into a different amp.
a 3 prong plug without the isolation tranny does the same thing, but it does not limit the current that could develop in a number of mishaps. (like a mis-wired outlet)
2nd we'll put a properly sized fuse inline with the mains hot. if something shorts on the secondary side it cannot draw more than the milliamp rating of the secondaries in the iso tranny without popping the fuse. So if YOU become the shunt between the HT of the amp and ground, you'll only experience a limited amount of current.
Working routinely in tube amps with B+'s of 300,400,500 and more VDC's we forget that 120V is HIGH VOLTAGE and 1 A is HIGH current let alone 10 or 20 times that!
If I've got any of this wrong, I hope some one will correct me.
Ray
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Hey, I used an instructable to understand how to use the isolation transformer in an amp the first time. It's relatively simple, just need some fairly heavy gauge wire, a block of wood to mount the tranny on, and the triad transformer from Mouser. I'm pretty sure he even explains how to carefully work on a tube amp, install a fuse and holder, and a 3-conductor power cord along the way. You can find it here:
http://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/step4/Parts-and-tools/
Good luck, sounds like a fun project!