Welcome! Need support, you got it. Or share you ideas and experiences.
141.153.122.72
I recently changed the driver in my Paramours from a 12au7 (stock is 12at7) to a 955, a small, oddly shaped indirectly heated triode with an Rp of about 12K, mu about 25. These tubes are ofter referred to as "acorn" tubes which I think is a reasonable comparison. The socket is also odd and is a ceramic ring.
Physically, the switch turned out to be much easier than I thought it would since the mounting holes on the socket matched up to ones for the old 9 pin socket so no drilling is needed. I opted to mount the socket upside down so that the tube must be inserted from below. I did this so that the terminals would be harder to touch. The top of tube ends up about 1/2 inch above the top plate when it's installed. I see 2 problems with this set up:
- The tube can be pushed down and out by the curious or by accident and the hole is certainly big enough to get a finger down by the socket terminals (OUCH! as voltsecond would say). I plan to mount some sort of screen to prevent this.
- I think gravity/vibration may eventually push the tube out; I'm going to try to rig a "retaining wire" connected to the bolts securing the socket
Image from below socket: http://home.earthlink.net/~kawcvd/paramour_955_bot.jpg
The cathode resistor was changed to 1.2 K ohms as suggested by Paul Joppa (link below). The voltage across the cathode resistor was 5 VDC for both paramours and the plate voltage measured at 198.2 and 199.6 VDC (CCS upgrade is installed). The grid stopper is still the stock value but had to replaced anyway since the desoldering left the original too mangled to re-use.
I certainly think the 955 is a nice driver tube, and easily betters the 12au7 - I can't really compare to the 12at7 (the 12au7 might sound better if the current could be increased). The sound seems fuller and more musical. Visually, I find the tiny 955 is a nice contrast to the large 2A3. I've bought a number of acorn tubes and I have to say they're as cute as a vacuum tube can be.
Follow Ups:
Thanks for the post. I have wanted to try this for a long time, just too busy with other stuff to get to it. Glad to hear that someone tried it, and that it sounds good. The 955 has had a good reputation sonically, at least among the few who have tried it, and I've always been curious.Also of course, I'm pretty stoked to hear that my cathode resistance recommendation produced reasonably close to the target 200 volts! :^) These theoretical calculations don't always work out right ...
Isn't the 955 the same as the 7pin 902? or is the 902 the same as a single section of 12at7. I can't remember, but I was thinking of trying a 902 and now I am confused because I thought it was drop in after c4s and changed socket.
.
Paul, this may be a dumb question as I haven't even looked into it, but would it be possible to use the 955 as a driver tube in the Parabees? That is a cool looking tube!
Dave
Sure, ought to work fine. Being a single triode, it's not compatible with Tucker's "Parabee II" mod, but otherwise it should work well. Be prepared for reduced sensitivity due to the lower mu.
Nice, especially the below deck mount. I use 955 as the driver in a diminutive 5902 P-P amp and in a variety of pre-amp circuits, good tube. Seeing your implementation makes me itchy to do the same with my Paramours.
Given the state of the wire leads you might as well keep going. The DHT 957 and 958A will plug right on in, with the nod going to the 958A. Lift the filament wires and run a 9000maH NiCad D-Cell for the filaments, should be about 2 weeks or more of heaving listening per battery, about 90 hours. You can raise a filament on/off toggle most any where. Then its all DHT. 6F4 and 6L4 will also plug in but require a bit more lead work and a bit of design. The grids have two hook-ups as do the plates. If you're deft with the soldering iron you can attach wires directly into the grid pins, at the ends of course. And you may have to pinch down plate current to keep inline with the PS transformer. Nice pinstripes as well looks like a great amp.
Matt
I've wanted to try those as well, but both are rare, especially the 958a. I am interested in going all DHT as you suggest. I've also considered the 958a in a battery powered pre-amp for a remote cabin, but the preamp, like the cabin, has yet to be built.Are you aware of any sources for either tube?
By all means give it a try, but be aware that small signal DHTs like this can be super-microphonic. PJ did a really cool battery powered preamp that unfortunately sounded like a door bell. Too bad, as the audio output when it wasn't ringing was very nice.
I was able to control the bell ringing by dropping the filament voltage a bit. Steve Bench got me onto that when I built his 512 6088 battery phono preamp. Those little tubes will howl with the slightest filament over voltage--drop it down a bit and pay atention to physical vibration isolation made a big difference. Though 958As have not been as bad as others in my experience. Probably because their filaments are shorter.
Matt
Cool, thanks! I'll relay the lower heater V idea to PJ if he doesn't pick it up here.
Chris,
Tubes are listed at the following sellers, but they could be out of stock. Ebay is a good source as well. Good luck it’s a nice tube.
Matthttp://www.vacuumtubes.net lists the 958A at $6
http://home.att.net/~esrc/esrcs5.html 958 $7
http://www.vacuumtubesinc.com/new4.html 958A $7
http://www.radiotubesupply.com/Tubecatalog/catalog5.html 958 $7
http://www.elexs.com/6Indlrcvg.htm 958 $7
http://www.users.qwest.net/~tubes/page3.htm 958 $5
no text
This post is made possible by the generous support of people like you and our sponsors: