can i sub a 6bs8 for a 12ax7? Amp Guru advice needed.
so i bought an old stereo amp and a couple of old tuners. the tuners contain a plethora of 6xxx tubes. From what i am reading most of them are dual triodes, the same as a 12ax7 or variant thereof. I looked up the pinout for a 12ax7 and for one of my radio tubes, the 6bs8. as far as i can tell the pinout is the same. the major difference that i see is that the heaters for 12axx tubes are designed to work with either 6 or 12 volts and that the 6xxx tubes are designed for 6 volt operation.
the question is, if i use these tubes in a guitar/bass amp that calls for 12ax7 style tubes what will happen. i know there are 4 basic possibilities. 1 the tube burns out either immediately or shortly thereafter. 2 the amp gets damaged in some way. 3 a combination of both A and B. or number D. the amp works perfectly and i end up with new and great undiscovered tonal bliss.
I am looking for answers from experts. answers like "hey, don't do it man because that's not what the manufacturer recommended" are not required. I know that this is not recommended by the manufacturer. i am also not really concerned if the tubes burn up rather quickly or even immediately, unless of course that causes the amp to malfunction in some way. i know there are more than a few amp gurus here on TB. I am hoping to hear from a few of them.
thanks in advance.
The 6BS8 has the same pinouts as the 12AX7, and they are both twin triodes. However, the 6BS8 has a maximum plate voltage of only 150 volts compared to 330 for the 12AX7. If your amp has more than 150 volts on the plate you would be exceeding the tube's maximum rating.
Also, the 12AX7 has an amplification factor of 100, compared to 36 for the 6BS8. In that respect the 6BS8 would be more like a 12AY7 (which has an amplification factor of 44).
The 6BS8 has different plate resistance, transconductance, and plate current than either the 12AY7 or 12AX7. I did not compare the load curves closely. Heater current is a tad higher with the 6BS8 but probably OK.
In my opinion, it may be possible to tweak a preamp circuit to use the 6BS8 provided the plate voltage isn't excessive (that could be a deal breaker right there). However, the plate resistor would be changed, and the cathode resistor also changed to bias the tube correctly for the different current draw. The 6BS8 was designed as a cascade amplifier for VHF.
I don't think you will achieve option D (the great and undiscovered tonal bliss), you may well drive the 6BS8 into cutoff. I'd stick to the 12AX7.
thanks nashvillebill for the reply. that is exactly the kind of answer i was looking for.
so if reducing gain is acceptable for the use that i have in mind, is there any 6xxx numbered tube with a pinout the same as a 12ax7 that would function without melting down and without modifying the amp. i have several different kinds of tubes and i have noticed that the 6xxx family of tubes are available very cheaply, probably related to the fact that they are no usually used in guitar amps.
Offhand, I can't think of any that wouldn't require rewiring the tube socket for a different pinout. The 6EU7 is a good sub for the 12AX7 but requires rewiring--and the 6EU7 is getting expensive as well.
Honestly...the 12A*7 series of tubes are not that terribly expensive, even for NOS ones, as long as you don't search for some of the elusive Bugle Boy's that are purportedly the best thing since sliced bread. (my ears couldn't hear a difference)
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