Power cord directly to rectifier tube?

[gayler]

I have a airline high fi amp that the power cord goes to the rectifier tube. This is uncommon right? I don't recall It even having a fuse! Also It has two 6F6G tubes, kind of coke bottle shaped. Are these similar to 6L6 or 6V6

[schmidlin]

Can't say for sure, but sometimes mfgrs used the unused rectifier socket pins as a terminal point. So it's just used for a soldering connection. Threw me when I first met one.

Sure about the 6FG6? My sources say it is an electron ray tube.

[Chris_M]

Not super uncommon lots of old tube equipment has the line voltage go straight to rectification. I bet if you added up all the first numbers of your tubes the filament voltage) it would add up to 110V -120V. Its just another way to cut costs, really cut them, transformers really contribute to the price of tube equipment.

The issue is your not isolated from the wall voltage. You need a 1:1 power transformer to isolate yourself. Otherwise it is very easy to get a shock.

6F6G seems similar to a 6K6, can only take approx 250V DC on its plates. It is a audio tube that was used in old radio receivers. Apparently it can put out about 11W in push pull. Might make for an interesting output section if you got a pair of them.

[Prairie Dawg]

Not super uncommon lots of old tube equipment has the line voltage go straight to rectification. I bet if you added up all the first numbers of your tubes the filament voltage) it would add up to 110V -120V. Its just another way to cut costs, really cut them, transformers really contribute to the price of tube equipment.

The issue is your not isolated from the wall voltage. You need a 1:1 power transformer to isolate yourself. Otherwise it is very easy to get a shock.

6F6G seems similar to a 6K6, can only take approx 250V DC on its plates. It is a audio tube that was used in old radio receivers. Apparently it can put out about 11W in push pull. Might make for an interesting output section if you got a pair of them.

You can build a cheap and effective isolation setup by getting two Radio Shack 120-12v doorbell transformers and wiring them back to back.

[gayler]

Can't say for sure, but sometimes mfgrs used the unused rectifier socket pins as a terminal point. So it's just used for a soldering connection. Threw me when I first met one.

Sure about the 6FG6? My sources say it is an electron ray tube.
6F6G not 6FG6

[Chris_M]

You can build a cheap and effective isolation setup by getting two Radio Shack 120-12v doorbell transformers and wiring them back to back.
Yep, I have a ton of non RS xformers that have dual 120V primaries and dual 12V secondaries.

The great thing is you get the isolation
120V - 12V - 12V -120V
And you also can tap of the 12V AC, rectify it and use it for your tube's heaters.

[schmidlin]

The 6F6G is similar to a 6V6, generally speaking.

Still can't say for sure, but we need verification on the PT status, if it exists.

If we were talking about tubes that started with a 35 or a 50, you guys are right on the money. But a 6? Not ususally used for series heater applications. Not very familiar with Airlines vintage hifis, just going by gut...

[ClinchFX]

You can build a cheap and effective isolation setup by getting two Radio Shack 120-12v doorbell transformers and wiring them back to back.

Just a friendly warning -

There are a number of small amp "designs" around the net that use back-to-back transformers as a cheap isolation method.

There is a potential problem in that the second transformer, the one connected backwards, may suffer from core saturation. If core saturation occurs, it will cause overheating in the transformer and possibly overload the first transformer. Many transformers have a thermal fuse in the primary. In the case of the transformer being used backwards, the primary is now the secondary and a thermal fuse won't prevent the temperature of the transformer from continuing to rise. The temperature can reach a point where it can cause combustion. This is what can happen if you don't choose the transformers carefully. You may get away with it, but, if it burns your house down, it's no bargain.:nono

Small signal transformers can usually be used in either direction, but power transformers are designed to work properly in one direction only.

If the mains is wired directly to the rectifier, you definitely need an isolation transformer to prevent electric shock and possible death. The power rating of the isolation transformer must be greater than the maximum mains power consumed by the amp.

Because tube amps are extremely inefficient, I'd guess the power consumption would be at least 4 times the output power of the amp. My Valve Junior (around 8W output?) is rated at about 35VA power consumption, and it has a solid state rectifier, far more efficient than a tube rectifier. I doubt that the doorbell transformers would be rated at more than a few VA.

Peter.

[mark norwine]

If the mains is wired directly to the rectifier, you definitely need an isolation transformer

Oh, come now....

If the amp uses 6F6 tubes, it is HIGHLY UNLIKLEY that this is a transformerless amp. Telling the OP that he needs back-to-back iso transformation only adds to the confusion of the initial question.

A rectifier tube uses only 4 of it's pins.....the other socket lugs go nowhere and are commonly used as terminal points. Connecting components....including the power cord.....to them is a very common thing to do.

[gayler]

The 6F6G is similar to a 6V6, generally speaking.

Still can't say for sure, but we need verification on the PT status, if it exists.

If we were talking about tubes that started with a 35 or a 50, you guys are right on the money. But a 6? Not ususally used for series heater applications. Not very familiar with Airlines vintage hifis, just going by gut...
Yes it does have a power transformer, two 6F6g's, a 5Y3G. I don't remember the others they start with a 6

[mark norwine]

5Y3 uses pins 2, 4, 6 & 8.

My bet is that your power cord is attached to 1,3,5 or 7 and is junctioning to other wires...like the PT primary.

The maker is simply using the un-needed socket lugs as terminal points. It's a smart way of utilizing real estate; it's still in practice today.

[schmidlin]

^^ Makes perfect sense.

[ClinchFX]

Oh, come now....

If the amp uses 6F6 tubes, it is HIGHLY UNLIKLEY that this is a transformerless amp. Telling the OP that he needs back-to-back iso transformation only adds to the confusion of the initial question.

A rectifier tube uses only 4 of it's pins.....the other socket lugs go nowhere and are commonly used as terminal points. Connecting components....including the power cord.....to them is a very common thing to do.

Seems I should have made myself clearer. When I said If the mains is wired directly to the rectifier, you definitely need an isolation transformer I meant If the mains is wired directly to the rectifier, not to spare pins on the socket. It was a general statement aimed at preventing electric shock, and it was not specifically aimed at this amp.

I generally agree that it's fine to make use of otherwise unused terminals. It was probably considered OK to connect mains to to these terminals when the amp was built, but modern mains wiring standards usually require that mains connections have a completely separate terminal strip or block.

Oh, and BTW, I recommended against using back-to-back transformers. That was really the main point of my post.

Peter.