WTF is this? Tru Tone Model 300A

[eccerr0r]

I got this from a hamfest from a pile of stuff that didn't sell and didn't want to take home... A cursory search revealed nothing, but I also wonder if it was "modded" to do something else due to the weird placement of some components/holes.

Anyway it seems like it possibly was simply a mono amplifier - but I don't know why it doesn't have an output transformer as tubes are not low impedance devices.

Tubes are:

35Z5 - diode (left)
12SJ7 - pentode (pre amp, middle)
50L6 - beam power tetrode (power amp, right)

Usually audio amplifiers do not have air or ferrite core inductors, which that thing in front of the 50L6 is. The hole is for a volume control pot that seems to go to the 12SJ7. The other side of the 50L6 has a ¼" phone plug which is the input to the pot.

The green wire on the left spiraling the power cord appears to connect to the 50L6 so I assume it's the output.

Might have to reverse engineer this to see how this really is connected, but this would be too poor to be a receiver from the get go, and this looks like a poor transmitter right now, and curious what it was originally...



As I don't know what it is, I don't know what the criteria is for it "working" ... so I don't know if it's broken or not

[petehall347]

Re: WTF is this? Tru Tone Model 300A

https://www.google.com/search?q=35Z5...RKXUH0s3_UYnyM

[eccerr0r]

Re: WTF is this? Tru Tone Model 300A

They appear to be very common tubes and google was how I initially concluded as as audio amplifier of sorts, but again, I did not see an OPT transformer which is needed for speakers.

The tubes are commonly used in receivers of the age and unfortunately I do not think this can be a receiver due to lack of stages (IF or whatnot) so I had to rule out receiver.

[petehall347]

Re: WTF is this? Tru Tone Model 300A

maybe someone borrowed the transformer

[eccerr0r]

Re: WTF is this? Tru Tone Model 300A

Possible, but where would have it been...not enough space inside (which I need to photograph at some point) and nowhere outside...

[petehall347]

Re: WTF is this? Tru Tone Model 300A

best i can find is phono amplifier
or it had an outboard output transformer like this one....https://www.youtube.com/watch?v=wllpeUzY2AE

[eccerr0r]

Re: WTF is this? Tru Tone Model 300A

Well, that seems really odd even as a phonograph amplifier, as if it were meant to power a speaker (50L6), it still needs a transformer -- though if it were line level, then the output of a 12SJ7 would be sufficient. Still very very odd...

[stj]

Re: WTF is this? Tru Tone Model 300A

for pa speakers with the transformer in them?

[eccerr0r]

Re: WTF is this? Tru Tone Model 300A

Hmm... That's a possibility... Wonder what impedance is needed for such, and was it common to do this, plus the fact that this amplifier is not isolated -- that doesn't seem very safe to have a "portable" device without isolation to the speaker...

[redwire]

Re: WTF is this? Tru Tone Model 300A

It looks like an AM transmitter with the RF inductors.
Careful because it has no power transformer and is a "hot chassis" device. The filaments are connected all in series 50+35+12 to work from 120VAC. Also with no audio O/P transformer, it does not drive a speaker.

I just sorted out a guitar amp that used the same notion - no power transformer... like the old 5-tube radios.

[eccerr0r]

Re: WTF is this? Tru Tone Model 300A

Yes it looks like it could well be an AM transmitter but it probably wouldn't pass FCC part 15 rules (too high power due to the 50L6, probably could do a part 15 AM transmitter with just using with the 12SJ7), unless it was a hack that someone did. True that this might be an unsanctioned device.

It is possible to run a device without a transformer and still have the chassis isolated, though unsure about noise filtering. The filaments are indeed in series.

[stj]

Re: WTF is this? Tru Tone Model 300A

how much current can it output?

[eccerr0r]

Re: WTF is this? Tru Tone Model 300A

who knows... The 50L6 is the output driver... usually these, like the 50C5 are good for at least 4W output

[petehall347]

Re: WTF is this? Tru Tone Model 300A

make your own pirate radio station .

[eccerr0r]

Re: WTF is this? Tru Tone Model 300A

I already have an amateur license, don't need to risk it doing that

[stj]

Re: WTF is this? Tru Tone Model 300A

an AM pirate station?
who the hell's gonna hear it other than shango066

[R_J]

Re: WTF is this? Tru Tone Model 300A

Any chance of a picture of what is inside? The 50L6 is for audio frequencies, not R.F.
The adjustable coil could be for hum reduction

[redwire]

Re: WTF is this? Tru Tone Model 300A

25/35/50L6 tube were used by hams in the 40m band, power to 7W but I think that is intermittent like Morse code. A few watts continuous. The tube is not that slow (i.e audio only).
Commercial low power AM transmitters I've only seen used in drive-through movie theaters.

You could probably make a small Tesla coil with it but mains power is risky? Apparently arcs/streamers between mains-powered TC and a person kills them- the arc is conductive so you are basically connected to mains if it touches you.

[stj]

Re: WTF is this? Tru Tone Model 300A

i still think it's for 100v line-speakers

[R_J]

Re: WTF is this? Tru Tone Model 300A

P.A. amps still have an output transformer to feed the 70v/100v line transformers mounted at the speakers.