The seller of my Dynacord Bass King 50W tube amp had been informing me slightly imprecisely and irresponsible sloppy, maybe he honestly forgot, and just told me it ran on 8 Ohm, although the label on the chassis of the amp still said 16 Ohm.
The fact are that there are two speaker outputs and that it from stock ran on 16 Ohm, but also from stock has pins on the output transformer so that you can set the two outputs to run at either 16, 8 or 4 Ohm.
Now the seller was a technician well versed in tube technology and had actually completely renovated the amp himself recently, and seemed to have done a really nice job at that as well, from what I could tell, so I trusted him.
And beside I was told correctly that putting in a cab of lower impedance up to half of the rated one should be fine, although it might cause in lower performance and higher rate of wear on the tubes, so either way I should be safe plugging in my 8 Ohm cab.
However when I plugged in my 8 Ohm 15" cab the amp got very hot very quickly where the power tubes are, to the point where you could scent the smell of burned electronics quite prominently and the few notes I got to play, before I decided it was probably best to turn off the amp again, sounded oddly thin and distorted and the speaker cone moved oddly as well as if overreacting to sound in an uneven way.
I decided to pull the amp out of it's chassis to get to the electronics to see if I could figure out, via reading the schematics, which the seller had kindly included, what pins the speaker outputs actually were connected to on the output transformer.
However my fairly limited knowledge about electronics and inexperience with reading schematics didn't help me much, until I noticed someone, the seller I presume, actually with a permanent marker had written the actual impedance above the two speaker outputs, which turned out to be respectively 8 and 4 Ohm.
Turned out I had plugged my 8 Ohm cab into the 4 Ohm speaker output.
So I mounted the chassis for the amp again and with a permanent marker wrote the correct impedance of the outputs above them before plugging my 8 Ohm cab into the correct speaker output, and this time it went much better.
No suspicious smell of burned electronics, other then the usual fume that normally is to be expected from tube amps, neither did the tubes get hot as a toaster within 2 seconds, but seemed to follow the normal heating pattern and no were close getting as warm as before when it had warmed up properly, also when I played the bass it sounded beautifully and warm without any unwanted distortion as well as the speaker didn't act strange and the cone movements seemed to follow the notes played nicely without any overreacting or sudden hiccups like when I first tried it out with what turned out to be a 4 Ohm speaker output.
The point of this wall of text I guess is, yes, I can indeed confirm from first hand experience that feeding your tube amp with higher impedance than it is rated for makes it appear dangerously strange, and indeed seems like it could damage it badly in the long term.
Luckly I acted relatively quickly and it has played beautifully ever since.
Also I can confirm that most tube amps seems to be of a very sturdy design, even if this amp in fact just was fully renovated, it is from 1963, and still seem to function flawlesly.
To further confirm this, I once owned an Ampeg B15S 60W tube head, build in 1969, and I used it for about 10 years, at one point playing in band over a five year period of time where I pushed it for all it could take 4 hours 2 times a week at rehearsals, plus occasional gigs, and it never one failed me, and never once did I have to have it looked after at an amp technician.
When I finally was stupid enough to sell it, it worked just as flawlessly as when I bought it.
Anyway, welcome to the world of tubes, those amp do really bring something special to the table that can't be achieved exactly the like otherwise.
In a way they seem more alive and dynamic the way they respond to your play, also have certain softness to them even with they are pushed into nasty and aggressive territory, hard to explain, but easy to feel and hear.
Personally at the moment I am bi-amping the 50W Dynacord tube head though with my 130W SS Trace Elliot combo of the SM series, and sort of get the best of both worlds.
Rock on
