Connecting 2 sets of speakers to a tube amp

I am wondering if connecting one set of 8 ohm speakers to the 8 ohm taps on a tube amplifier, and a second set of 4 ohm speakers to the 4 ohm taps utilizing the same neg. tap on the amplifier is possible and will it hurt the amp? Any other problems? Thanks for your help
I don't think that would be a good idea with most tube amps. That would be seen by the output tubes, on the primary side of the output transformer, as if a single 2 ohm load were connected to the 4 ohm tap. That would seem very likely to degrade sonics, with damage also being a significant possibility.

The stress on the amp would be lessened a bit if you connected both speakers to the 4 ohm taps, but in that situation the amp would be seeing a load of 2.67 ohms on that tap, which still seems very excessive.

And of course there are most likely some frequencies at which the impedances of the speakers will be significantly less than their rated 4 and 8 ohm nominal values.

-- Al
What I have done with my tube amp (Ayon Spirit II) is utilize the pre-out and send that signal to a second integrated amplifier through which I power a second pair of speakers. My tube amp powers the 4 ohm speakers (Montana EPS2's) and a McIntosh integrated powers my back speakers (8 ohm Vandersteen 2a's). This prevents entirely the problems that Almarg mentioned above. Further, with two integrated's I can taylor the front vs. back speakers beautifully. Running two channel 4 speakers makes the system sound DRAMATICALLY larger, more full, and realistic with vastly superior sound stage. The difference between the 2channl/4 speaker and the 2 channel/2 speaker is staggering (akin to tripling system cost or more)

So... that is what you should consider doing. Of course, your tube integrated needs to have a pre-out to do it...
Thank you for your help. I will use the pre-out to a second amp.
You could always wire the speakers in series instead of parallel.
Marty (Viridian), note that the speakers are different models, one being rated at 4 ohms and the other at 8 ohms. Series connection of different models will usually result in significant sonic degradation.

How the voltage and power being output by the amp at a given frequency at a given time will divide up between the two speakers depends on the relation between the impedances of the speakers at that frequency. Therefore the frequency response of each speaker will be affected in an arbitrary manner by how the impedance of the other speaker varies as a function of frequency.

Another factor that might also be an issue with a series connection is that the 8 ohm speaker will receive considerably more power overall than the 4 ohm speaker. Based on the oversimplified assumption that the impedances of the two speakers are purely resistive and correspond to their 4 and 8 ohm nominal ratings, the 8 ohm speaker would receive twice as much power as the 4 ohm speaker.

-- Al
Al, absolutely correct, and those are the issues that result in us generally not seeing series connected speakers, even of the same impedance, in high fidelity applications, but it's certainly worth a try as it is the only solution that will present a high enough load to the driving amp. There should be no danger in trying and it will only take a few minutes to find out the bad news, LOL!

let me know how you like the pre-out to a second preamp setup. I am sure you will love it. You can set the relative volume of the backfill speakers with the volume of the second amp... and none of the problems that were listed by other posters is an issue. Again... let me know how you like it.

My system at 2 channel/4 speakers for a total of 22K outperforms (or equals) ANY 2 channel/2 speaker system for 40K. No exaggeration. the 2 channel/4 speaker route is an unbeatable performance-to-cost upgrade. Enjoy.