If you're very comfortable waiting for a good deal and you're an expert online shopper
, some of the old analysers from Japan may be an option for you. More search terms cant hurt anyway.
I think the two stand out models are the Panasonic VP-7722A and the ShibaSoku 725 series (725B or 725C). The Panasonic is the more traditional analyser with a oscillator built in, whereas the ShibaSoku is a distortion analyser only, you'll need a separate oscillator (which ShibaSoku also makes).
The Panny VP-7722A performs much better than its sisters, even the later models like the VP-7723A, it is equal in residual distortion and noise to the flagship VP-7725A, which is basically a VP-7722A with an LCD display. It typically performs at 0.00023% (-112dB) THD+N, 1kHz, 30kHz BW. For THD only it will go down to 0.00004%(-128dB) or so at 1kHz, and the display limit for each harmonic is -140dB.
The VP-7722A is a fantastic analyser, it is as good as you'll realistically need for any tube amp, it can even display each harmonic up to the fifth at the touch of a button, no need to boot up a PC and fiddle with a sound card just to check the 2nd or 3rd when fiddling with biasing, too easy, it can even do IMD.
Next up is the ShibaSoku, I have to admit I've never even heard of them until recently and didn't give their analysers much thought because because their residual THD+N spec was something like -105dB, respectable for a 25 year old instrument but not remarkable in any way these days, even a sound card can reach that no problem.
But here's the secret to the ShibaSoku 725B/C, they absolutely DESTROY their rated spec. They typically do 0.00014% (-117dB) THD+N, 1kHz, 30kHz BW. That's AP 2700 series territory which is still 3.5k+ used if you're lucky!
Perform a few op-amp swaps and minor mods, and the 725 will go all the way down to -120dB THD+N without breaking a sweat, which is more than good enough for any DIY audio adventure. For reference the flagship Audio Precision APx555 goes down to around -123dB, but it starts at 28 grand...
I think I paid around $400 for the Panny 7722A, and $300 for the ShibaSoku 725B, they are big beasts though so shipping for each of them almost cost as much for the instrument itself. Unbeatable value IMO for the hobbyist. Unless you can get the older HP analysers for real cheap, these units are the only ones that hold up today.
I'll still echo the suggestions on getting a sound card to start with, just throw together a buffer so you don't accidentally blow it up with the full fury of a power amp
. You will also be able to use it to have a looksie at the monitor outs of an audio analyser if you ever get one, it's only good for looking at low level harmonics though and not at anything very close to the fundamental due to the notch.