Need help identifying old amp, specifically optimal ohm load

[veebis]

I got this thing a while back, just rediscovered that it works after all (after trying to reorient the on/off/vol switch). Now I'd love to hear it through an 8 ohm 12" Jensen I have, just want to be sure I'm not hurting anything by mismatching ohms...

The brand is "Dynamic" and the design looks 50s or 60s. Here are some pics of the innards, in case that helps. Tubes are marked 35W4, 50C5, and 12AU6.

Thanks!
-Vb

[Enzo]

Several thoughts.

First the speaker. I can;t think of any amplifier that would be harmed by an 8 ohm speaker. Little amps like this often used 4 ohm or "3.2 ohm" speakers, but an 8 ohm wold hardly threaten it in any way. And I can;t imagine the thing wanting a 16 ohm speaker, but if it did, again, 8 ohms wouldn;t hurt it.

If you wanted, you could disconnect that transformer from the speaker terminals, and measure the resistance between those terminals. The resistance your meter measures - referred to as the "DC resistance" - will usually be a little lower than the rated impedance. SO an 8 ohm speaker measures 6 ohms or so, and a 4 ohm speaker measures 3 ohms or so.

But what REALLY concerns me is the power supply. What we don;t see is a power transformer. (The small transformer on the speaker itself is the output transformer. And by the way, if you change speakers, that transformer has to stay or be replaced by something similar. It is part of the amp, not part of the speaker.) And your tube lineup with the 50v and 35v tubes verifies this is what we call a "hot chassis" amp. The 120v from the wall goes directly to the tube heaters, and is directly rectified and filtered to make high voltage for the tubes. This has the potential to be very dangerous. At the very least an "isolation transformer" should be added.

A very common symptom with this sort of amp is that sometimes it hums bad, so you turn the power plug over and it helps. You might also find that sometimes you get shocks touching it or touching your guitar, and again tirning over the power plug helps. That happens because there is voltage on this hot chassis.

Be careful.

[veebis]

Thank you.

Yikes; glad I didn't just tap out the speaker leads to a quarter inch jack before asking!

I came across this instructables page and am glad you've answered my question about that as well (i.e., "Do I need to do this?").

Does this look like a good fix? Any other references you know about? I will need to hire out this work but would like to know as much as I can beforehand.

[Enzo]

Oh there is the entire content matter of this forum, I suppose.

There are levels of work. The isolation transformer is for your own safety. In the days of that amp - and old record players and other things with hot chassis - it was acceptable to get shocks and such. No longer is that true. So the amp would funtion OK without it, but not in what we consider safe manner. Your linked article seems to cover it.

If it works as is and sounds OK, then it may be OK. But generally on something this old, we probably would replace the "electrolytic" capacitors - they dry out with age. That is the black cylinder off to the side. And many times the coupling capacitors need replacement as well. THose are the rounded black thing that says 600v and the similar brown things. They may be fine though. A competent tech would know about that.