Tube vocal preamp from old tube record player

[thomasks]

I have an old tube record player that I would like to convert to a tube vocal preamp. My question is, how do I feed the output from the converted preamp to my main amp? It would normally see a 4 ohm speaker load, so what would the load be from the input of the power amp? Anyone ever done anything like this before?

KT

[Enzo]

First question is WAAAYYY more important that how to connect it. Does this phonograph have a power transformer? If not, don;t do this project.

Or another way to ask is this - are the tubes numbers like 50C5 and 35W4, or 50L6 and 35Z5? We don;t want that on any microphone I would be touching to my lips.

[thomasks]

50B5, 35W4 are the tubes. Could I add a power transformer?

[Robert M. Martinelli]

Hi,
I think Enzo is ( righteously ) concerned about your health/safety.....

The tubes you mentioned have a high heater voltage, this was done to be able to connect the heaters in series and supply them without using a transformer ( something the current safety standards would never tolerate ).

I think that, to do it right ( safe ) you'd need to modify the thing extensively, changing the tubes' type, using a new transformer with a 6,3VAC secondary for the pentode and a separate 5 or 6,3 VAC secondary for the rectifier, plus another secondary giving whatever voltage you moght need to get the proper +B.....

I'm just wondering if the thing is worth the effort.....

Best regards

Bob

[thomasks]

Doesnt sound like it is worth the effort! I was just trying to find a good use for the circuit since I have it, and it still works....

[Enzo]

That is exactly my concern, the circuit conects directly to the AC mains. You could add an isolation transformer to it to make it safe. That way it still gets the 120VAC it wants, but now it is through a transformer.

[thomasks]

Ok, I'll keep it in mind. I see you are right here in Lansing !
I am just down the road in Belding!
Thanks again,

Kirk

[dscottguitars]

more on record player amps

Sir Envo...

Is there no way to make the amp safe with grounding? I'm not too experienced. I was about to hook up an old amp with the 50 series tubes to use as a preamp for an acoustic amp I'm building. I am glad I decided to see if anyone else has done this because now I'm scared.

I disconnected the power switch, moved the pots but kept the components connected. I also cut the output transformers. I used the direct line from the output of the tube to a pot and then to the main amp. (an old 3 channel motorola with preamp and controls) The other line, common, went to ground. I also have the chassis grounded to my main amp.

The reason for the small amp/preamp is to drive the reverb section.

Thanks much, Daniel

[JHow]

Do a search on "isolation transformer" on these forums. This comes up periodically. The concept is not hard. The device is not likely to be overly expensive, just like any transformer, depends on how much power you need.

[dscottguitars]

Thanks...

I did as you advised and I see the simple circuitry. I have another question: I am combining this little amp and the main amp in the same cabinet. Is there anywhere in the main amp that I can run the power line to that will be isolated?

I was planning on plugging this into the main amp's receptacle. It accepts a two prong plug. I was then planning on making the main amp a three prong connection to plug into the wall.

Thanks again, Daniel

[Steve Conner]

I think you're heading for at best, unusable levels of hum, and at worst, a small explosion. See that recent thread we had on the difference between neutral and ground. This distinction is important in modern electrical systems, but old transformerless sets like this can't observe it. "Ground" in them is always neutral, and that's assuming you put the plug in the right way round. But in your main amp, ground is actually ground, and this difference will cause you nothing but grief. (If not actual bodily harm.)

An isolation transformer is the only way to fix this. I don't see how there would be anything like an isolation transformer inside your main amp: it's a function that takes up space and costs money, so the manufacturer wouldn't include it.

If you're trying to amplify a low-impedance mic (the regular kind with a 3-pin XLR connection) then you'll probably find that a matching transformer would give better results than a preamp, anyway.

[dscottguitars]

Adding the isolation transformer would be easy, but my main issue with doing this is the full wave/half wave rectifier problem. Is the only solution a solid state rectifier? I am probably going to scrap the phono amp and look for a preamp or make one myself. Any suggestions on a simple schematic?

Thanks, Daniel

[J M Fahey]

[olddawg]

I was going to tell him to just buy an ART tube preamp for $30.... But I realized this post is 6 years old!