DIY Audio Projects Forum

I put together the design for this for around $150 in parts, all brand new. The key is the tube, in lots they can be found <$1 a piece. (plus shipping).

The preamp will be a common cathode with humbucking. It HAS to be a common cathode because the tube has a single cathode.

All holes were cut tonight, phew...now the fun stuff can start tomorrow.

I gots no fire on my tubes!!

I measure voltage at the heaters but when the tubes are in they do not glow. I wired the heater circuit identically to the 6T9 heaters, reverse phase with a pair of 330ohm going to ground for cathode bias.

Secondly, when I power down the B+ bleeds off rather quick...this seems odd too...man I wish I didn't rush this preamp.

I don't have any pics but I don't think they would do any good because it is so cramped.

Hmmm!

First thing I would do,remove all the tubes from the unit

Connect a 25Watts light bulb in series with the unit. Connect to ac120V and see if the light bulb just make a flash (fine) or stays lit (bad)
If it stays lit, there is a primary, secondary wiring error! Correct this before going any further!

OK, suppose all is fine, measure directly at the tube sockets pins if you have proper B+ and heater voltages.
I make the mistake (quite too often that I like to admit ) to inverse tube pns from looking at the socket from the rear or the top view point..
Until you have this sorted out, do not go any further. The power supply issues are the very first thing to get going properly. Any other miswiring error will not damage anything, but you have to make double sure that the proper voltages go to the proper pins!

Luc

Well crap Scooby Doo, the mystery is solved.

I wired everything backwards.

What is the best way to recover? Pull the sockets, clean up and start over? Man this is gonna be a PITA. I did not realize all the pinout drawings were from the bottom so I reversed it...shouldve looked at the 6t9 for reference.

What a maroon!

I drew one in Excel, forgive me for not remembering resistor values.

I will update.

EDIT (5 Feb 2009): see 6J6 Tube Preamp Schematic for the most recent schematic.

A bit sketchy but it will do, you should try Express PCB, it is a free schematic drawing program that actually works!

I have a few 6J6's, but had no idea they could be used in a preamp, keep us posted

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My Mini 12AU7 Tube Preamp, Pioneer SX-D7000, JVC XL-V221BK, JBL L80T, DCM TF700

• Project Page - Mini 12AU7 Tube Preamplifier
• Support/Questions - Mini 12AU7 Tube Preamp thread

I dont know if they can either but Electra-print uses them as drivers on their SE schematics so one could reason that it should work.

The foibles continue...

I rewired last night and proceeded to swap the grid and plate leads....This pre is making a boob of me, I hope it sounds good.

I am going to add my coil to the PSU tonight and rewire the 4 pins.

Another question though: does it matter which # triode you wire first as long as you stick with it?

The 6j6 is laid out backwards to me so I was going to use triode #2 as the #1

Here is a schematic with the right pin numbers.

It's alive!!!!!

It is wild seeing the heaters exposed...it actually freaked me out enough that I traced the pins on a spare just to prove to myself that it was ok.

One thing that is odd, one heater is mucho brighter than the other...I need to meter it and see whats up, it could be that the valve is just that way...switching out would be a good way to test that.

I am a little gunshy about running it because I may have hurt the valves in my various mis-wirings. I have 8 westinghouse 6j6 on the way though so I will put it through its paces then.

The caps still drain relatively fast after shutdown. I cannot figure out what that could be a function of? I used different diodes in this pre vs. the 1n4007 of "standard" fare.

The data sheet is here, its a BA-159...was this a bad choice?
http://www.vishay.com/docs/88536/ba157.pdf

Anyways, the obligatory pics and first impressions:

Impressions, I only have a couple songs on it so I cannot give too much and the caps and whatnot should burn in also but first impressions are positive. This is no Aikido or Shrimp but it is no slouch either...for under $200 in parts I bet it could be mass-produced and sold for $500ish. Sound is smooth with good detail. Decent soundstage but not extremely deep. Good overall tonal balance that I first thought was thin on the bottom but it is more like very accurate. Warm mids, nice smooth highs...probably a little laid back.