Unknown output impedance

[saltmiser]

Hello guys;

I have my Masco 375 tape deck, right. It's playback amp actually has a lot of power (5Y3, 2x6V6, 6SJ7 and 6SC7 tubes) so I ripped the tape deck off and now I'm using it as a keyboard amp. My problem is I have not even a hint as to what it's output impedance is. So far; I've been using an 8 ohm speaker. I guess it works; it sounds decent but it makes this smell after a few minutes. I have a multimeter; is it safe to run the speaker line through the multimeter/ohm meter to figure out the impedance? Is that even possible? Thanks.

[Tom Bavis]

8 Ohms will be safe, maybe not optimum. Most of this stuff used 4 Ohm speakers, though projector amps often used 16 Ohms since they had long speaker cables.

You could look up the Sams folder (117-7) and see... I'll see if I have it, but you might be able to get a copy from your library.

The smell is a bad sign... have you replaced the caps in it yet? Are you waiting for it to smoke first? Does it have a line fuse for when one of the transformers shorts? (one will short soon if you DON'T change the leaky caps).

Alan Douglas tells how how to test capacitors made before 1960:

Does it make a clunk when it hits the bottom of the garbage can? If it did, it was bad.

[saltmiser]

Just a side note, seeing as how I trash picked it and I don't use it as my main amp or anything (just a for-fun type thing), I wouldn't be too upset if it smoked/exploded/etc. I may use good parts from it one day to build my own amp though; specifically the plans I found at "Boozehound Labs."

I'm aware the cap(s) need changing because it already buzzes, and I swear this thing was made before fuses were invented so..

I have a 5 ohm speaker; I suppose I should be using that instead. It's a cabinet; so I'm sure if I ripped it apart maybe there'd be a (close to) 4 ohm in there. What is worse for an amp; having less ohms (2 ohms on a 4 ohm output) or more ohms (8 ohms on a 4 ohm output)?

I'll have to find this "Sams folder"; if you could find yours and check quick I'd appreciate it. Thanks.

[m6tt]

less ohms, but I wouldn't mismatch by more than 100%. Too many ohms increases the back emf from the transformer and will lead to anode-to-something arcs. 5 ohm on a 4 ohm tap is probably just fine or even great, it's 16 ohms or more on the 4 ohm tap that'll do you in.

[Tom Bavis]

I don't have Sams set 117 - but they're not hard to find - there are thousands of file cabinets of these still around...

[tubewade]

Originally posted by Tom Bavis

Alan Douglas tells how how to test capacitors made before 1960:

Does it make a clunk when it hits the bottom of the garbage can? If it did, it was bad.

I like this post. Although I wonder if he may have made these statements about ten years ago? I would say the same thing except I would say before 1970. It's not worth ruining an expensive set of output valves or transformers or, worse yet, starting a fire over some 30 + year old capacitors.

[m6tt]

About an hour after I read that, the electrolytics on a crappy set I have decided to go out. I guess I should start recapping… Of course they were 1961 and tubewade hadn't posted yet