Audiophile / Hi-Fi enthusiasts usually want to reproduce the purest, cleanest sound.
If you are a MUSICIAN, the chances are that you want to DELIBERATELY CHANGE the sound and characteristics of your amplifier. Your amplifier is an integral part of achieving your own personal sound, as is choice of pickup, string gauge, effects etc.
Changing V1 (The first valve after the input socket) will have the most dramatic and extreme effect. This is because any changes made here will be magnified by the gain of the rest of the signal path.
Only change ONE VALVE at a time! (Don't put a whole set of new valves in all at once). You then know exactly what each replacement has done and if a problem occurs, you know which replacement has caused it!
This applies to low level valves ONLY, in the pre-amp. With POWER OUTPUT valves change the WHOLE SET together as a matched set (Pair, quad, gang of 6, etc).
Table of valves and their characteristics/suitability.
Some valves available as NOS (New Old Stock) on offer were originally designed for use in early computers and are really unsuitable for audio. They have bad hum, microphony, and can even cause scraping and creaking sounds as the internal structure expands or contracts against tight fitting mica supports. Examples: 6414, 5963, 5965, E180CC. You may be lucky, but you have been warned.
Most modern production music amplifiers have exceptionally high gain compared to those from the 1960's and 1970's which gives easy strong distortion, but not everyone wants to play only heavy metal style. The most common and famous valve, the 12AX7/ECC83/7025A has a gain of 100, which makes comparison with alternatives very easy. The 12AY7, which was used in the earlier Fender Tweed series has a gain of 40 which I tend to prefer. This gives a cleaner sound for most volume settings, which I find easier to use, but it will still distort if you turn up the volume a bit further. The 12AU7 has the lowest gain and gives the cleanest sound, but lower gain means a less lively response. If you want to try just a slight reduction in gain to increase the headroom before distortion, then the 5751 is an excellent choice in the first V1 position. These are still in production although there appears to be only one supplier.
Anode current can be important for some valve functions e.g. driver for spring reverb units. These normally use a 12AT7/ECC81 with 10mA for each section, often with both sections strapped together to deliver 20mA of current to a very tiny output transformer. In this application a non-balanced double triode can be used with no problems. A 12AX7 will not provide enough current and will be likely to overload and fail if fitted in this position.
The Phase Inverter / Phase Splitter / Output Driver (The valve directly driving the output valves) in a push-pull output amplifier must be balanced, If it isn't, the sound is likely to be weak, feeble, tinny distorting, poor sound on all channels. Most double triodes are reasonably well balanced and I don't pay extra to have balance tested. If a problem occurs after changing the phase inverter valve, my solution is to swap the valve with another of the same type somewhere else in the circuit. Variation in actual gain is always going to occur to some extent ±10% perhaps, but in practice this hardly notices EXCEPT in the phase inverter. Many amplifiers use a 12AX7 for the phase inverter, but some, particularly the larger 200 Watt type use a 12AT7 which has 10 mA anode current to drive the larger output valves. Be careful not to replace this with a valve of lower current rating. A 12AU7 would handle the current but not have sufficient gain to drive the output valves to full power.
Some very low power amplifiers are now available giving only one, two or in one case five watts output from a single valve in push-pull mode. These use a 12AU7 with 10.5 mA anode current from each section to a push-pull output transformer. The five watt model uses a 12BH7A with 11.5 mA from each section. The important point about these is NOT to replace the output valve with one of lower anode current rating or it will overload and fail. The 12BH7A also takes DOUBLE the heater current of the 12AU7. I have replaced the 12AU7 with a 12BH7A in one of my amplifiers and the sound is different because the anode resistance / impedance is lower. This means that there is now extra strain on the mains transformer from the extra heater current, only 300 mA, so I will most likely get away with it. More than one 12BH7A and the mains transformer would be unlikely to cope with the extra heater current in the longer term. All of these very low power mini-amps that I have encountered so far have no negative feedback. They therefore reflect the sound of the valves and circuitry around them without 'Ironing out' the peaks, troughs, or any quirks in a particular circuit. If driven hard they distort heavily.
Most music amplifiers (Except VOX) do use negative feedback, but unlike Hi-Fi amplifiers (which are designed to have the flattest frequency response possible), the negative feedback is taken to the phase inverter stage, thereby allowing the earlier pre-amp and tone stack to have a wide range of control of the sound shape. This is true of all the major manufacturers including Fender, Marshall, Orange, also most 'Competitors' for powers of 30 Watts or more. At 15 Watts (A pair of push-pull EL84's or 6V6GT's) some do use negative feedback, while others don't.
The first valve (V1) handles the signal straight from the pickup (or pedal board) and provides the most gain before any tone stack, so this can be considered as having the most direct interaction with your instrument. It usually runs at only about 1 mA of current, so it's perfectly ok to substitute any of the valves listed above in the first table, as they all handle 1 mA or more.
Some modern current production 12AX7 / ECC83 have been given suffixes C, R, S, in addition to the long established A, W, WA, then B. This appears to be Chinese, Russian, Slovak. No definitive information on these suffixes has yet been discovered, but it appears to be from one particular reseller of valves, who supply valves produced from all three countries.
The 7025A was used by the Fender company for V1 position originally with 12AY7 or 12AX7 in following positions. It is thought by some that these were available from RCA in large quantities, while being slightly more expensive than 12AX7, more reliable and less susceptible to microphony. They are very slightly smaller physically and apparently more robust.
Valves all vary slightly, even from the same production line on the same day. You may find a good sounding 7025A that is decades old and still sounds as good as when first installed. You may have a new valve fail or seriously degrade after just a few hours. The table above lists the design parameters as a guide. If you have a rarer type or equivalent, or are offered an unfamiliar type number it is well worth checking the museum exhibits for special quality higher specification types. Some manufacturers apparently just wanted to give their products different numbers from everyone else. GEC in particular made some excellent versions and coded them B309 (12AT7/ECC81), B329 (12AU7/ECC82), B339 (12AX7/ECC83). There is a more extensive list of rarer equivalents elsewhere.
I have not mentioned changing the type of OUTPUT valves (Other than small double triodes), as these are best left as the same type as originally designed. The output transformer is designed specifically for a particular type. The supply voltage is also designed to power a particular type of valve.
Some Related articles
Audio Double Triodes ECC81, ECC82 & ECC83 and Pin Compatibles.
Mullard Circuits for Audio Amplifiers.
Some Notes on Phase-Splitters.
Double Triodes in Audio Amplifiers Advert.