Transformerless Amp Safety

[atmars]

The tube layout

I believe this filmosound amp is L-2 from the 285 projector. It put out 10w. It's difficult for me to identify which tubes do what on this schematic, but the power tubes are 25L6s. There is a 6SL7 which appears to be a dual triode, so I am assuming this is the preamp tube ( phase inverter?). There is a additional 25L6. I'm not sure of it's function. The wiper from the volume knob runs to it's grid, so am assuming its also a preamp tube, but the volume knob is a dual pot. I don't understand what the function of the dual pot is. There is also a 5879 and another tube which I believe is the phototube. These seem to be part of a sub circuit and I understand that section of the circuit can be removed.

Any info or ideas would be greatly appreciated. Thanks.

[hasserl]

I personally know someone killed at a gig from the live chassis on guitar amp, touch microphone, death scenario.
It aint pretty, and totally avoidable with a trip switch feeding on stage power.
Transformerless amps are insanely dangerous.

My condolences for your loss, but I question whether this was a transformerless amp, as we're talking about here. With the old two wire power cords there were lot's of hot chassis amps back in the day. I've had my share of shocks between mic stand and guitar too, even with properly grounded amps. I've learned to test things before I go grabbing a mic stand.

But like I said, these transformerless amps put out about 1 watt or so of power and really aren't suited for live performance where you'll be needing your voice amplified thru a PA. These are little practice amps for home use.

[kg]

I believe this filmosound amp is L-2 from the 285 projector. It put out 10w.

But like I said, these transformerless amps put out about 1 watt or so of power

so which is it?

[Jag]

My guess would be 10w was power consumption.

Hasserl. In the '80's I did a fill in gig for a very well known band up here. The leader insisted that all members use the lowest power amplifier that could get the job done when mic'd through a PA. The guitarist in that band used a tiny line operated amp that sounded massive through the house speakers and the, very adequate, on-stage monitoring system. I used that rig when I filled in and it sounded awesome. At the time, I didn't give a thought to the potential grounding issue since the regular guitar player had been using that rig for several years. A month or so after I filled in, I heard that the regular guitar player had landed in the hospital after a severe shock on stage. Both of us were also singers.

I have two line operated amps that soudn really cool. I run them on an iso and sometimes use them to record guitar tracks while doing guide vocals.

IIRC. That's how Lee Harvey of Stone The Crows was killed.

It does happen.

[Enzo]

There is a large difference between the leakage voltage on the chassis of an old two-wire Fender, and the DIRECT wire connection to the mains of a transformerless chassis. One is the current flowing through a cap, the other is all the current the mains can provide.

[hasserl]

Good points Jag and Enzo, good points.

[slidincharlie (Carlo P)]

I don't want to hijack this interesting thread, but I have a closely related issue.
I have just bought a tiny Silvertone 1389 transformerless amp from ebay. The tube line-up is: 35W4, 12AU6 and 50C5 (btw: is the latter the power tube?).
Incidentally, I had heard about the potential danger in transformerless amps, but I was so stupid I didn't ask the seller about the tube compliment...

Now I live in Italy - where wall outlets are NOT polarized and are generally grounded - and I need a stepdown transformer to operate the amp.
I played this amp only once with a 50W stepdown autotransfromer at home and did not get electrocuted... yet.
Do isolation stepdown transformers exist?
Or should I use an isolation transformer AND a stepdown transformer?
This amp sounds very very cool for my style and taste, but I don't want to spend time and money for major mods (like adding a power or filament transformer).

[offhand35]

Hello! This is my first post. I found this forum when searching for info on the the Filmosound 07138 amplifier. I have been participating in a single ended and a push/pull amp forum. I have done some modifications and bias adjustments on my Epiphone Valve Jr. [sound familiar?], and felt ready to do some rebuilding and modification to something more interesting.....I saw the amp on ebay and won it, and it is on its way home. HOWEVER, even though I did find some encouraging references to the modification and use of Filmosound amps for guitar, I did not realize that this one was probably not one of them, being transformerless, and possibly full of AC hum.

Thanks to someone here who listed the link for the PDF containing most of the schematics for the Filmosound amps! That is when I discovered this amp has no main transformer! It was obvious to me that it could be dangerous to use as configured.

At the very least, it is a small chassis with 3 octal and 2 9-pin sockets, a questionable OT, and an interesting name plate.

To make this usable as is, it looks like it will minimally require the iso-PT, plus replacement of 4 can capacitors whose spec I cannot figure out from the schematic. These may be available at low cost from a surplus supplier, but I would need the specs. Some of the caps on the one coming in are very dark, and may not be legible. That sounds like a whole bunch of money to me, for an amp that might hum like a kazoo. However, it still remains a possibility to me, as it uses some unusual inexpensive tubes, and may be educational, but hopefully not TOO educational.

The alternative, would be as someone suggested a little earlier, to put in a PT and use different tubes along more currently conventional lines, yielding a nice little PP amp.

There. That's my hello/intro. I do have a couple questions so far that someone may help me with if you have looked at the schematic for the 07138 amp, page L2.

1. Which leg of the 117V input would be considered the hot lead, the one that is fused?

2. Is there any way to actually tell what the heater V's are for tubes 3,5,4, and 6 on the drawing? The taps are located between resistors of 20 and 75 ohms, on one of the incoming 117V leads.

3. The data sheet for the 6SL7GT states the heater V is 6.3, yet on the drawing it is not tapped at the 6V point used for the heater for the 5879, but is connected in series with the others near the 117V entry. Did they do this because of voltage drop the occurs during the series run?

Thanks!

I suspect that this thread may get pretty long and others may show up as people like me are looking try more in the amp mod field without getting into modifying an old PA amp.

[Albert Kreuzer]

Now I live in Italy - where wall outlets are NOT polarized and are generally grounded - and I need a stepdown transformer to operate the amp.
I played this amp only once with a 50W stepdown autotransfromer at home and did not get electrocuted... yet.

You were lucky.

Do isolation stepdown transformers exist?
Or should I use an isolation transformer AND a stepdown transformer?
This amp sounds very very cool for my style and taste, but I don't want to spend time and money for major mods (like adding a power or filament transformer).

You could use something like this:
http://at.rs-online.com/web/search/s...duct&R=2238241
2 x 55V secondary in series gives you 110V from 230V primary.
There are smaller ones if you don't need 300 Watts.

Cheers,
Albert

[Steve Conner]

IMO, the best safety precaution you can take with a transformerless amp is to throw it away, or rebuild it with a transformer. Using one in a 220V country with non-polarized outlets is like playing Russian roulette with three bullets

Yes, stepdown isolation transformers exist. Like I said before, I own two of them, so they must have come from somewhere! Albert's recommendation is good, though you probably won't get the 2x 0-55V transformer in a smaller VA rating. You may have to settle for two 2x 0-30V connected in series.

As a side note, people really shouldn't get electrocuted at gigs that way in the UK. We've had three prong outlets with polarization and grounding since the 1950s. I blame dicey imported amps.

[offhand35]

Oops, part of my previous post was actually meant as a response to a different forum.....that's why some of my questions may not make sense to anyone here...But just the same, I appreciate all that has been said here. Because of the potential risks AND the need to replace the cans with multiple caps, I have just about decided to regard this as a strip down and scratch build project. The whole thing was only $10 + shpg, which is a whole lot less than some pay for a bare chassis.

My initial impression of this amp on looking at the schematic only leads me to wonder how any of those nuns using this projector at schools around the world could have lived so long.

[Enzo]

Hi Offhand, welcome to the forum.

May I suggest starting a new thread for your amp instead of tacking on the end of a long existing thread. That way more people will see it and more likely respond.

If you look at the wall outlet, one slot is longer than the other. The short one is hot and the longer one is neutral - IF THE OUTLET IS WIRED CORRECTLY. before polarized two-wire plugs it was 50/50 which side was hot inside the amp - hence the ground polarity switches on older amps. Good practive has the fuse and switch in the hot side coming into the amp so if the fuse blows, it is the neutral that remains connected to the circuits.