AC/DC Radios

[boots]

I picked up a couple old radios at a flea market for dirt cheap. The loop antennas and variable tuning caps were worth the price for building crystal sets for kids.

Turns out both radios were AC/DC type. The tubes are oddball types designed for 35 VAC heater voltage. Pretty useless tubes for guitar amps. If anybody wants 'em, let me know. The AF amp in one was a 30C5 (I think - don't have it in front of me).

Can anybody give me a quick lesson on AC/DC power supplies? There is no PT apparently. How does that work? Sounds dangerous.

This must have been the era of the original consumer electronics crap. They seem to be very cheaply built. Most of them ended up in the trash, after stripping out a few of the parts.

[David Root]

They are potentially very dangerous. The tube string is in series and adds up to the wall voltage.

[shoggoth]

I wouldn't say they're useless for building guitar amps, just non-traditional, and you'll have to come up with a novel circuit to utilize the tubes.

I've got an amp I'm building using only radio tubes (described off in another thread), and it's an awesome learning exercise.

[Merlinb]

Can anybody give me a quick lesson on AC/DC power supplies? There is no PT apparently. How does that work? Sounds dangerous.

They normally consist of a half-wave rectifier circuit connected directly between live and neutral, so circuit 'ground' is actually connected to neutral. You're right- it's dangerous, and illegal today in a non-double insulated appliance.

The tubes are still useful though- you can easily buy a small transformer to power just the heaters. However, I would be wary of using one of those high heater voltage tubes in the first couple of stages of a preamp, due to hum (unless the heaters are DC).

[shoggoth]

You could run it all off the same isolation transformer. Bridge rectify it and drop it to 115v with RC filter(s). Then the full heater string runs off the same DC supply that feeds the plates.

[boots]

Ugh. Well, like I said, the tubes are up for grabs if anybody can use them!

[shoggoth]

Well, if you're giving them away, I'll build something with them. I'm almost done with my current radio-derived amp.

[boots]

Shoggoth, I'll send you a list of the ones I have. Also let me know if there are any in particular that you need. Are you in the US Lower 48?

[boots]

Shoggoth, I'll send you a list of the ones I have. Also let me know if there are any in particular that you need. Are you in the US Lower 48?

[shoggoth]

Massachusetts. Sent you a PM w/ my address.

It's more of a design challenge than a "need" - what can I do with an arbitrary grab bag of weirdo tubes. Since the heater chains are going to add up to 120v, it's easiest to work with full complements of tubes, but whatever you want to send I'll make use of - diodes, heptodes & remote cutoff pentodes will probably end up used for effects processing, sharp cutoff RF pentodes and triodes for gain stages.

[Prairie Dawg]

This may help a little.

http://judyboxamp.blogspot.com/2012/08/ ... fying.html

[boots]

I also have a bunch of Loctal tubes I will probably never use, if anybody wants to mess around with them. Things like 7W7, etc. All pulls, of course, I don't know if they are any good.