Do Ei silver plate 12AX7 tubes have very dim cathodes ?
I bought some, and at 6.3V on each of the heaters, the cathodes barely glow (You have to look very carefully to see that the cathode is glowing). If I increase the voltage up to around 14V (both heaters in parallel) it starts to glow a bright orange like the rest of the tubes.
I haven't miswired the socket, any other 12AX7 in that socket glows normally (bright orange) at 6.3V (AC, RMS), and it isn't a single tube, I tested 4 Ei tubes and they are all identically dim.
Is it normal for Ei silver plate 12AX7s to not glow brightly or is there something wrong with these tubes ?
I bought some, and at 6.3V on each of the heaters, the cathodes barely glow (You have to look very carefully to see that the cathode is glowing). If I increase the voltage up to around 14V (both heaters in parallel) it starts to glow a bright orange like the rest of the tubes.
I haven't miswired the socket, any other 12AX7 in that socket glows normally (bright orange) at 6.3V (AC, RMS), and it isn't a single tube, I tested 4 Ei tubes and they are all identically dim.
Is it normal for Ei silver plate 12AX7s to not glow brightly or is there something wrong with these tubes ?
It's normal, and running them at 14V will grossly overstress the filaments.
For 6.3V operation I assume you have tied pins 4 & 5 together and connected 6.3V to pin 9 on one side and 4 & 5 on the other.
For 6.3V operation I assume you have tied pins 4 & 5 together and connected 6.3V to pin 9 on one side and 4 & 5 on the other.
some 12ax7's i have seen glows like a lamp at turn on, then dims to normal a few seconds later....nothing to be concerned about if your filament voltages are within spec.
This type has particularly dim filaments and no flash upon application of power. The OP was complaining about the dimness of the filaments.
oh yes, i have seen those types too, looks as if the filaments are not lit, but measure voltage at socket and it is there, also the thing seem to work just fine...
I have quite a few of these in a closet somewhere and no they don't glow very brightly. If you look at the image you linked to, you will notice that there is a mica tab that rests on top of the cathode. That will tend to 'hide' the glowing heater that resides inside the cathode.
Hey that rhymed...well, kind of. 😀
About half of the dozen or so of the EI 12AX7's that I have, ring like a bell. The good ones are very good.
Hey that rhymed...well, kind of. 😀
About half of the dozen or so of the EI 12AX7's that I have, ring like a bell. The good ones are very good.
If it is normal for these tubes to glow dim, does that mean Ei developed a new cathode oxide coating that has sufficient emissions at a lower temperature ?
I feel that would be a major technological improvement and something that would be documented in books or patents or even advertisements? I can't find anything on google about low temperature cathodes.
While searching I found someone claims these tubes are low-noise. That should be a result of the lower cathode temperature ?
I feel that would be a major technological improvement and something that would be documented in books or patents or even advertisements? I can't find anything on google about low temperature cathodes.
While searching I found someone claims these tubes are low-noise. That should be a result of the lower cathode temperature ?
RCA had 'dark cathodes' (i.e. cool cathodes) towards the end of the valve era. Philips may have had something similar, and shared it with some of their outsourced producers such as Ei?
I seriously doubt it. There would be no reason for that.
They are not 'dim'. They look dark because the filament wires (the part that heats up) do not extend the entire length of the cathode. The part that's heated (glows) is hidden in the plate structure and as there aren't any holes for you to see into, it appears dark. The wires are folded like this: WWWW or spiraled, I'm not sure which for the silver' Ei's. They are low noise, if you find some that don't ring just by breathing on them!
The Ei's are reputedly built on the old Telefunken tooling. Maybe just the same template? 😉
Last edited:
The short plate tubes glow more brightly because the filament wires protrude or nearly protrude from the cathode. So not only does the coated part of the cathode glow, the not coated ends do also. 🙂 The Ei's have a long plate and a long cathode, so there's no need for the wires to extend that far. I have a few long plate 12AX7's that are not Ei's, that appear dim also.
If you heat a short piece of metal and a long piece with nearly the same amount of heat, the short piece will glow more along it's (or the entire) length than the long piece. Correct?
If you heat a short piece of metal and a long piece with nearly the same amount of heat, the short piece will glow more along it's (or the entire) length than the long piece. Correct?
Last edited:
I took some pics (I should have done this at the outset)
The dim cathode (12AX7 Ei):
A 6AK5 running on the same 6.3V supply:
The 12AX7 has pins 4 and 5 tied together and 6.3V across them and pin 9.
The dim cathode (12AX7 Ei):
A 6AK5 running on the same 6.3V supply:
The 12AX7 has pins 4 and 5 tied together and 6.3V across them and pin 9.
A 6AK5 is a higher current tube (not just heater current, but cathode current: perveance) than a lowly low current 12AX7.
Check out a 5965. It's a long plate also, but much higher current capability.
Check out a 5965. It's a long plate also, but much higher current capability.
Yes, that is not uncommon. That's where a part of the filament wire that doesn't have the insulating coating which conducts the heat away from the wire. In other words, that bare part heats up much faster than the rest of the filament. As it heats up, it's resistance goes up (the rest of the filament has low resistance) and it glows white hot for a few milliseconds until the rest of the filament 'catches up'. Hope that clarifies things a bit for ya. 😀
The dark heater was a filament with better heat conduction (from filament to cathode, through the alumina insulator) because of which the temperature of the filament did not have to be much higher than the temperature of the cathode.
In tubes which don't have this "dark heater", the filament is a lot hotter than the cathode due to the thermally insulating nature of alumina. (I got this information from the RCA handbook.)
In a tube with a dark heater, the cathode is not cooler.
I guess we're back the original question. How do these Ei tubes work with cathodes that aren't as hot as other tubes ? Is it a different cathode coating formula that has sufficient emission at a lower temperature ?
Filament wire
Just one question is that wire inside the tube or the filament wire to the tube socket? I tried yesterday 4 pairs of tubes 2 of them philips and ei are the same when turning the amp on one lamp light for a few msec and then are back to normal the other 2 pairs one china made and sovtek dont have that efect.
thx
regards
thx
regards
Just one question is that wire inside the tube or the filament wire to the tube socket? I tried yesterday 4 pairs of tubes 2 of them philips and ei are the same when turning the amp on one lamp light for a few msec and then are back to normal the other 2 pairs one china made and sovtek dont have that efect.
thx
regards
thx
regards
- Status
- Not open for further replies.