It is common design practice overrunning output valves.
It is done with all types . There are some amps that use EL34’s and run them way over 500 volts. This is well above and beyond the maximum ratings of certain manufacturers. It is inviting trouble as the valves age, they have the possibility of flashing over and failing. We see similar effects with EL84’s.
This is not a new phenomenon. Amp companies have been doing this from the first days of amps !!
Let’s take a closer look and we use the 6V6 as an example.
A common valve in smaller Fender amps is the 6V6 type and you not only need to consider the sound, but can the valve type you have selected stand up to the volts in your amp for prolonged periods ?
Let us take an example of the Fender 68 custom Deluxe Reverb. Typical HT volts ( B+) are indicated to be around 415 volts and the volts on the screen grid is around 408 volts. All quite normal values found in these amps. The volts can be higher than this because the amp manufacturers reckon the mains voltage in the UK is 230 volts but it can be as high as 250 volts . This high 250 volts is still within the allowable mains voltage tolerance. The B+ and screen volts are therefore much higher
So the question is, “Are all 6V6 types able to stand up to these voltages ?” Are all 6V6 valves designed to meet the same specification? The flat answer is NO !
Here is a table comparing three common types of 6V6. The EH ( Electro Harmonix) JJ Electronic and Tung Sol. The numbers are taken from the manufacturers data sheets are published on their websites. At the end of the article I have put links to each of the data sheets from today, 27th December, 2019.
You will see that the JJ Electronic 6V6S has the highest voltage ratings of these three types. The Tung Sol 6V6GT is significantly lower on voltage ratings and on power dissipation.
If you use the Tung Sol 6V6GT in amp where the volts in the amp are much higher, then you are using them outside of their specification and so they are not covered by the warranty.
In our own 1970 Fender Deluxe Reverb, we have found that the EH 6V6GT has the best blend of sound and longevity. We were surprised that we started to hear pops and crackles from JJ’s on our lightly used amp after six months.
Our recommendation is to use either the JJ or Eh 6V6 versions. The JJ have thinner mod tones so great for 60’s sounds. The 6V6GT seems the best compromise of voltage rating and reliability.
This blog has not compared specifications for the Shuguang 6V6.I searched all over the internet and while there are warm words about this valve, I can find no hard data other than it is a 12 watt valve. If you have a copy please send and I can update this article
Here are links to the data sheets we referenced in this little overview
JJ Electronic 6V6S buy a matched pair here