Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Cups on April 29, 2011, 12:26:15 pm
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I got a Lark amplifier in trade for some repair work and I finally traced it out as best I could. the preamp is simple enough but the power section gets weird for a couple of reasons:
- It runs straight off AC no power trans
- It has a no name 7 pin power tube and a 7 pin 35w4 rectifier
Actually the power tube has a couple of numbers. Mybe you guys can help me figure this out.
It seems to end with a C5?
Off to google to investigate further.
I apologise if this was covered before. I havn't seen much info on these.
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The only 7 pin output tube that comes immediately to mind is the 6AQ5. Super common. You're gonna want to add an isolation transformer to make that thang safe to use.
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It seems to end with a C5
maybe it's a 50C5 - common on the 1/2 wave direct line powered equipment.
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The 50c5 definately seems to share pin compatibility. Doesn't really look alike though. Hard to tell from an internet picture.
If I ever get around to drawing this on a computer I'll post the schem.
What is so dangerous about a power transformerless amp?? Is it more dangerous than any other amp of the time that had a 2 prong power cord?
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If you want to be cheap and don't want to buy a power transformer
you can use two cheap 120v / 12v back to back connected transformers
don't go on using it without a Power Transformer - may be you are lucky and nothing happen
but it can be really dangerous
Kagliostro
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OK, I understand it's dangerous, but why? Is it because you can't properly ground it? I'm just curious I'm not trying to be a smart ass or anything.
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Here's an older thread:
http://www.el34world.com/Forum/index.php?topic=11221.msg103538#msg103538
Here's my post from that thread:
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 the low current transformer overloading and the voltage dropping drastically close to zero, you're now relying on the fuse (if present) only, or the house circuit breaker. Voltage isn't going to drop at all 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 instantaneously for a few mS or longer if the oldie ain't got no fuse. Transformers are cool.
Stingray 65 wrote:
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!
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The numbers I used in my post are actually quite low. A normal healthy residential 20A circuit can hit 10,000 amps instantaneously before interrupting the circuit in a dead short condition. That 20A rating you see on the breaker handle is deceptive because that is the time delay current rating for a trip. Look a little closer at the AIC rating. Regardless, in a catastrophic failure within the amp itself, w/o a transformer is much worse than with a transformer.
Stingray touched on polarity and shock prevention. W/O an isolation transformer, you are relying on:
a. The receptacle you are plugged into to be wired correctly. I'm an electrical contractor. You'd be surprised how many receptacles are wired incorrectly.
b. The polarity of it's power cord to be correct. A lot of them old cords are not polarized.
c. Nothing but a single capacitor between you and high voltage. We all know that capacitors fail. It's not a question of if, but when.
W/O an isolation transformer, if you put a 3 wire cord on the amp and ground the chassis, then you end up plugging it into a reverse polarity receptacle, it's gonna arc.
A 1:1 isolation cost about $35.00 and is worth every cent. You wire it in front of the whole enchalada so you're not messing with the integrity of the vintage circuit. Safety MUST come first. Transformerless tube amps are not safe.
-Richard
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I actually understand that. Thanks.
Seriously, I learn so much from this forum. :icon_biggrin: