Substitute 5V4G tube for 5AR4 tube

I won't go to the sonic differences, as these are subjective and I do not own your components. But I can address the electrical issue involved. First, all tubes with the same pin bases may be physically substituted, but this does not mean that they can be electrically substituted. The 5V4 is certainly not a substitute for the 5AR4 though it will function in some circuits without failure. First, the voltage drop is significantly greater with the 5V4, so your circuit will have to be stable at lower voltage, second it is more fragile than the 5AR4 and you really should not directly follow the tube with more than 35-40uF of capacitance. More capacitance will probably tax the tube. You can get away with quite a bit more with the 5AR4, though good engineering practice may put the cap values in a similar range as those acceptable with a 5V4. Capacitance after the choke really doesn't matter. The substitiution may work, it may not. It may sound good and it may not.

As far as 12AU7s go, electrically they pretty much suck, that is an opinion. If I have to use them, I go for the 5963 or 6680, which are both direct substitutes. My favorite is the 7058 which was designed for automotive use, of all things. Sadly, it is not compatible with all circuits and is not a direct sub for 12AU7 in all gear.

I should clarify the use of the 7058, it only works in circuts where the two filaments are wired in series, and will not work with them in parallel. Electrically is is pretty much a 12AX7, but I use it in certain 12AU7 circuits without issue. Of course it is a higher mu tube than the 12AU7. Your mileage may vary and this is one that should probably be avoided, except by the adventurous, and even then the 12AX7 circuit is probably your best bet.

Thank You very much Viridian. You have been very informative. I will probably look further into the 5963 and 6680 tubes.
The owners manual of the referenced components states that the 5V4G tube can be used in place of the 5AR4 tube but to expect sonic differences. If your interested you can review said owners manuals by going to www.upscaleaudio.com. You will find a special section on there web site for PrimaLuna components, including the manuals.
Thanks again for your help.
Brad

Brad, if the manufacturer says it compatible, then go for it! 5V4Gs are cheap. 5Y3s cheaper yet. You might also want to consider the Mullard 5AR4 which is the best example of a 5AR4.

The reason that I like the instrumentation or ruggedized grade of the 12AU7 is that they tend to be much less microphonic, which seems to be a great problem in this tube type. Many of the ruggedized versions are built to be much more stout for use in hostile environments or left on 24/7, the 5814 is another good example that is a direct sub for 12AU7. There are some more such as 6189, which I have not had good luck with, as well as 7730, 7318, and 6067, which I have no experience with. Sometimes these instrumentation grade 12AU7s sell for peanuts, so it is fun to try things out, as you will be doing with the 5V4. Good luck, and most of all, enjoy yourself.

While the 5V4 and 5AR4 can be interchanged in your circuit; you'll find the 5V4 less dynamic. A NOS Mullard(Blackburn Plant) 5AR4 will provide long life, dynamics, and a clean/smooth presentation(no sacrifices). ie: (http://cgi.ebay.com/MULLARD-GZ34-5AR4-CV1377-rectifier-tube-NOSNIB-/270598619300?cmd=ViewItem&pt=Vintage_Electronics_R2&hash=item3f00ef40a4) Many others on eBay as well.