Twelve see five is a small six dubya six. When run at 110 volts they are almost the same. It’s supposedly the same guts crammed into the little bottle for little 1-2 watt table radios. George tortured a pair of the 50 volters, and supposedly survived. Don't know about the MTTF, though.
I built a 12b4a preamp, tried multiple iterations and landed on my current version using Ale's gyrator and I have no desire to build anything else.
I tried a lot other preamp circuits 6sn7 01a 4p1l 2p29l and two well respected commercial preamp.
I made a little test circuit on perf board with a pair of fifty see fives. It was going to be a small guitar amp, but it never got finished. Now that I am looking to make myself a new guitar amp, I got the old breadboard out of its nearly 3 year long sleep and fired it up. It still works and still makes 20 watts, but it hasn't seen much use. I remember going through a bunch of tubes to find two that worked, but all were old used tubes. The pair that's in the board are not even the same kind. One has a black plate and the other has a grey plate. I didn't remember that the 12AU6 driver is UNSET wired as well. Back when I built it, I let it run for about 16 hours straight with no observable issues and changes in operating conditions over the extended "burn in test." The UNSET configuration gets triode curves from a pentode. It also frees up the screen grid to be fed from a different supply so that TV sweep tubes can be run at high power levels in "triode" mode. Most Sweep tubes need 150 to 180 volts on G2. The secret to getting high power on any of the 6W6 variants is to run the screen grid at 125 to 150 volts MAX! Let the plate voltage go wherever you want as long as you keep the plate dissipation somewhere close to the spec. I ran the fifty see fives with 165 volts on the screen, 330 volts on the plates and about 35 volts on the cathode into a 6600 ohm load to get 20 watts at 3.76 THD.
The original build was seen here in post #39:
https://www.diyaudio.com/community/threads/unset-is-coming.340856/page-2
The original build was seen here in post #39:
https://www.diyaudio.com/community/threads/unset-is-coming.340856/page-2
Attachments
The 12b4, does look very appealing - plenty still about and specs are just what's needed.
https://www.diyaudio.com/community/threads/12b4-preamp.137771/
For anyone contemplating a 12b4 build this is a must read. Consensus seems to be -20V bias and 15mA. Big point is grid stoppers on both grids need to be at least 3.3K or even 4.7K in some cases. Active load seems to be preferred to a resistor.
For anyone contemplating a 12b4 build this is a must read. Consensus seems to be -20V bias and 15mA. Big point is grid stoppers on both grids need to be at least 3.3K or even 4.7K in some cases. Active load seems to be preferred to a resistor.
https://www.diyaudio.com/community/threads/12b4-preamp.137771/
I tried several bias, best bias is Elit Dutman schematic, but the best it's diy and get the best SQ you want. Good lucky.
I'm surprised that you aren't considering the 1626 since you've made so many positive comments about it in the past. Mu is 5 though Rp is not super low at 2.5k.
Based on your comments, I decided to try it when I was breadboarding various tubes (mostly indirectly heated) and that's what I ended up using. If my calculations were correct (???) I think the output impedance was ~1.4k. Regardless, it seems to drive SS and Class D amps just fine if that's a concern.
I posted a thread about it over on AK which includes many of your comments about the 1626. I even call it the Boogie Factor, in your honor.
Perhaps you're looking for a more DHT-like sound, though.
The DH tube that stood out during the breadboard phase for that build was the Russian 2P29L. As I recall, the Rp of the 2P29L is similar to the 1626 so the resulting output impedance should also be similar. I posted a thread here about the 2P29L.
As for the 12B4, I built a preamp with those quite a few years back and they sounded really nice. It was very full bodied with excellent bass. Definitely not a DHT-like sound signature.
Unfortunately, they were also quite microphonic. I ended up going through perhaps 20 or more tubes before I found a pair that were reasonable. I was surprised because indirectly heated tubes don't typically have a reputation for being microphonic.
It's the only tube I've ever used that would start howling in response to music being played. I don't even listen loud and the preamp was not sitting right in front of, or anywhere near, the speakers, which were on stands. It was on a terrazzo (stone) floor so there was zero vibration. But I did like the sound, though not as much as the 1626 or some of the DHTs I've heard since.
Ironically, I was avoiding DHTs at the time due to the stories I'd heard about microphonics but I've been using them for a while now and I've experienced almost no problems at all.
Based on your comments, I decided to try it when I was breadboarding various tubes (mostly indirectly heated) and that's what I ended up using. If my calculations were correct (???) I think the output impedance was ~1.4k. Regardless, it seems to drive SS and Class D amps just fine if that's a concern.
I posted a thread about it over on AK which includes many of your comments about the 1626. I even call it the Boogie Factor, in your honor.
Perhaps you're looking for a more DHT-like sound, though.
The DH tube that stood out during the breadboard phase for that build was the Russian 2P29L. As I recall, the Rp of the 2P29L is similar to the 1626 so the resulting output impedance should also be similar. I posted a thread here about the 2P29L.
As for the 12B4, I built a preamp with those quite a few years back and they sounded really nice. It was very full bodied with excellent bass. Definitely not a DHT-like sound signature.
Unfortunately, they were also quite microphonic. I ended up going through perhaps 20 or more tubes before I found a pair that were reasonable. I was surprised because indirectly heated tubes don't typically have a reputation for being microphonic.
It's the only tube I've ever used that would start howling in response to music being played. I don't even listen loud and the preamp was not sitting right in front of, or anywhere near, the speakers, which were on stands. It was on a terrazzo (stone) floor so there was zero vibration. But I did like the sound, though not as much as the 1626 or some of the DHTs I've heard since.
Ironically, I was avoiding DHTs at the time due to the stories I'd heard about microphonics but I've been using them for a while now and I've experienced almost no problems at all.
<< I'm surprised that you aren't considering the 1626 since you've made so many positive comments about it in the past.
I bought a selection of 1626 and 6AH4 tubes from the USA, and the 6AH4 were quite a lot better sounding. So I sold all the 1626. They went quite quickly - they clearly have fans. I can't say I've heard a better low Ri tube than the 6AH4 - it's seen off everything I've thrown at it (including 12b4), except DHTs. Downside is you don't find them used in the UK. The UK has a very different range of available tubes than the USA.
I bought a selection of 1626 and 6AH4 tubes from the USA, and the 6AH4 were quite a lot better sounding. So I sold all the 1626. They went quite quickly - they clearly have fans. I can't say I've heard a better low Ri tube than the 6AH4 - it's seen off everything I've thrown at it (including 12b4), except DHTs. Downside is you don't find them used in the UK. The UK has a very different range of available tubes than the USA.
I liked the 6AH4 a lot too, very nice tube. For what I listen to I preferred the 1626. Another that I really liked was the 6V7G which has a mu 8.3 but it has higher Rp. All my breadboarding was done with a tube amp so low Rp was not a primary consideration.
Lots of people use preamps with higher output impedance with SS and Class D so I'm not sure if there is a significant audible penalty though it's certainly not ideal from a technical perspective.
I do plan on trying the low mu section of some of the dissimilar triodes sometime.
Lots of people use preamps with higher output impedance with SS and Class D so I'm not sure if there is a significant audible penalty though it's certainly not ideal from a technical perspective.
I do plan on trying the low mu section of some of the dissimilar triodes sometime.
Several years ago I built 12B4a preamp for my friend.
As @FlaCharlie wrote, it was microphonic like hell (I have several Sylvania, but all of them ringing), so took it apart and rebuild it with 801. Gyrator loading resulted fairly low output impedance.
As @FlaCharlie wrote, it was microphonic like hell (I have several Sylvania, but all of them ringing), so took it apart and rebuild it with 801. Gyrator loading resulted fairly low output impedance.
