This is from a post by support years ago of tubes that are usable in Fryette amps in a 12AX7 spot.
"12AX7
This is the original version of this tube. The earliest ones were all made in the USA by the usual manufacturers like RCA, Sylvania, GE and Tung Sol. Very early ones have rather large size plates. The "plain" version without the "A" suffix can only be used in parallel filament circuits, not series, unless the filament circuit is DC powered.
12AX7A
This later version has improved noise specs, mostly due to the improved filament design and can be used in series or parallel AC or DC filament circuits. Many manufacturers produced plain 12AX7's well into the 60's and 70's, but many of those could be 12AX7A's marked 12AX7. Current production noisy or untested tubes are often labeled 12AX7 while tested and selected ones are usually designated 12AX7A.
7025
This is the "hi fi" low noise version of the 12AX7. All other specifications are the same as 12AX7. Tubes marked with 12AX7 and 7025 both should be low noise, too.
ECC83
This is the European equivalent to 12AX7
ECC803, ECC803S E83CC
This is the "premium" version of the European 12AX7. The "S" on the ECC803S means "select" version. These are usually found on Telefunken brand tubes.
5751
This is the "milspec" version of the 12AX7, but is also slightly different. The 5751 has the same plate resistance as the 12AX7, but the gain factor is only 70 compared to 100 for the 12AX7. The 12AT7 also has a gain factor of 70, but since its plate resistance is lower, it draws more plate current than the 5751 and 12AX7, making it less suitable for use in low current high gain stages. The older "real" 5751's have extra thick mica spacers and extra support rods to minimize microphonics.
6681
Mobile version of the standard 12AX7. These are usually standard 12AX7's which have been specially tested to assure they will perform well in circuits with +/- 20% variation in filament voltage. Amperex Holland 6681s are a great example of this tube, and very well suited for use in guitar amps."
Here's a quote from Steve on the topic..
"As far as tube selection, it has always been and will always remain the case that our designs are based on generally accepted published tube data and design parameters for any given type. The word “type” is bold because unlike some manufacturers, we never have and never will optimize our designs for a particular brand. This is an important distinction. A tube type is defined by its intended function and operational parameters, 12AX7, EL34, 5U4, etc. A tube brand, Sovtek, Shuguang, Mullard, etc, is defined by its adherence to, or deviation from, generally accepted published tube data. These days, deviation from generally accepted published tube data gives rise to certain types which exhibit noticeably different and sometimes undesirable traits in a particular brand. Thus you find certain Russian 12AX7s that are problematic in cathode follower circuits and EL34s that exhibit excessive idle current behavior or peaky response characteristics (mid emphasis, spiky top end, etc).
Bottom line is that we encourage you guys to familiarize yourselves with various tube “personalities” and I want you to rest assured that any current manufacture or NOS type that adheres to generally accepted published tube data, is a candidate for use in our amplifiers. However, it is very important to be aware that some of the tube talk you see posted on various discussion forums can be flat out fabrication. Just because a 12AX7 can be swapped out in some amps where a particular circuit is somewhat forgiving of type-swapping, doesn’t necessarily mean that is true for any and all amp designs.
Our warranty is predicated on the assumption that the amp is built tested and guaranteed to perform as advertised using the correct types of tested, properly installed tubes. If you want to try different brands, you are certainly encouraged to do so, and I think you will find that while you make like the result at first, you may find that subtle changes in performance may affect your perception of overall performance down the road. So it’s a good idea to keep a stock set around in order to get reacquainted with stock performance. We often find our customers are very surprised to discover what they didn’t realize they were missing later on.
It is very time-consuming to explain repeatedly why swapping types is not a good idea and what the ramification might be since there are a myriad of possibilities there, so we just can’t spend the time rehashing that info. We have researched this stuff extensively, and I feel confident that whatever you may have read elsewhere, our expertise on the matter is probably pretty well founded, and we are not likely to concede our engineering decisions to the end user based on hearsay."
