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In Reply to: RE: George Wright WLA10 help posted by frperdurabo on April 26, 2015 at 11:15:34
The production version of this preamp used two 12SN7 twin triodes for amplification, one 6EM7/6EA7 as a series-pass regulator, and a 6X5 rectifier. Starting from the end near the transformer, I believe the order is 6X5/6EM7/12SN7/12SN7. There were no shields on these octal sockets.
If you can post a photo of the unit, I or others can be of more help.
Follow Ups:
...but it is different. Not octal sockets, to begin with.The 7-pin socket is for a rectifier tube, almost certainly a 6X4. The two shielded 9-pin sockets are the signal tubes. I would guess they take 12BH7 or similar tubes, but it would be best to measure the AC voltage between pins 4 and 5 ( clockwise viewed from the bottom, 4th and 5th from the gap in the pins near the front of the unit.) This requires measuring the voltage with the unit plugged in and turned on, but could be done from the top with the bottom cover closed, if you can insert voltmeter probes into the socket holes on top. (Remember to count the pins counterclockwise from that perspective. Take the unit to a technician if you don't feel comfortable doing this.) If the voltage is 12.6V, 12BH7's should work, if 6.3V, maybe 6CG7's are needed.
The regulator tube is hardest to guess. It could be something like a 6DR7, maybe a 6DE7 or 6EW7. I think 6DR7 is more likely as it fits the pinout and is closer to the octal 6EM7 electrically.
Edits: 04/26/15 04/26/15 04/26/15
Yes - it's not the same as the WLA12, which used more common audio tubes. Not sure how comfortable I am with live AC.
Thanks so much!
Here are some pics - let me know if you need anything close up/more details.
Looks like it only lets me post one pic per message?
And finally, the guts.
George often built a fair number of variants within a given model number.
You will have to get a digital voltmeter and start taking measurements. You will also very likely need to trace the circuit out to really figure out what is going to work in there.
I agree, but we don't know if fperdurabo is up to the job of tracing the schematic. Also, while the AC filament and (with caution) plate winding voltages can be measured without a working set of tubes, DC voltages will require an actual tube set installed. I am pretty confident the rectifier was a 6X4, and that 12BH7 or 6CG7/6GU7 (depending on the filament voltage for the signal tubes, as I posted above) should work for the signal tubes, whether optimally or not. The regulator tube is hard to tell, but I think it has to be a dissimilar dual triode like the 6DR7. It isn't a 6BM8, and any other triode/pentode, regardless of pinout, would need all 9 pins in use (including to triode-strap the screen grid) while in this unit pin 2 on that socket is unused.
I looked up the tubes you mention above. The 6x4 and 6DR7 "look" like they might have been in it (it's been 12 years+ since I last used it). But the driver tubes were taller and skinnier. I THINK I still have one, but it has absolutely no markings on it at all. I'll take a photo tonight and post.
Thanks for your help! I liked the sound of this unit very much, albeit it has too much gain for anything but a flea power amp - I wound up using 15db attenuater plugs with it, IIRC. It didn't sound "tubey" - it was clean and extended in both directions.
I did a search and somebody said that winter 1999 issue of Listener Magazine
had a review of that preamp. I can't help you find that issue but I hope that helps.
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