Wiring second mains transformer

[MapleSingleCut]

Hi, folks. I’m UK-based and have built a few things from scratch and done a couple of conversions with some success. Found an interesting project in a portable record player that has a one-valve design (silicon rectifier, N108 valve). I pulled the amplifier section and got it working, but only barely audible at full vol (I think it was designed for a crystal cartridge, so guitar is pretty wimpy in comparison). Would love to add a preamp valve, but the N108 is a seven-pin with 40v heaters and I can’t find an appropriate triode in that configuration. Decided I could drop a 12A_7 in, if I could get ~ 6v for the heaters. I had just had a 6v wall wart fall apart on me  (the plastic case broke), so I thought I would take off the rectifier bit and try the transformer (which tested good). Figured out a way to mount it and attached the primary leads to the tag board that feeds mains voltage to the existing PT. On startup, the current limiter lit up super bright. I am obviously missing something. Any ideas?

[acheld]

Sounds like something I might try!

Many (? most ?) wall warts of the past decade are switch mode designs.  If I've got it right, the transformer used in these devices follows the rectifier and switching modules, and hence is not designed to handle your full 240VAC mains.   

Only a guess . . . 

There are plenty of wall warts that could easily supply heater current at 6VAC (or DC) to a single 12Ax7.  When I have tried this in the past, I've ended up with a lot of hum (I suspect due to improper grounding).

Another option is to use a Hammond 229E12 (or similar); these work great.

[Latole]

12AX7 heater may also work with 12 volts

[cfortner]

But different wiring!

[Latole]

But different wiring!

Yes. pins 4 and 5 are for 12 volts

[pdf64]

How are the 40V heaters powered, eg from a 40V winding on the stock mains transformer?

The description of the N108 is a bit concerning

The N108 is an output pentode designed for universal AC/DC receivers that operate at mains line voltage without a mains transformer. Small radios used a 100 mA heater chain

[MapleSingleCut]

Thanks for the comments, everyone.

acheld, the wall wart I used was not a switching model - I made sure it was direct-to-mains and desoldered the leads from the plug itself. Still don't know why it's pulling so much current, but I've probably just wired something incorrectly.

Latole, great shout on 12v - I probably have something like that lying around. Will give it a go.

pdf64, I hear you - I think that these valves were often used with the more dangerous serial heater circuits, but this one has a dedicated 40v winding on the PT. It occurred to me that I might do some sort of voltage divider off that winding, but I'd have to learn the maths involved...

I attach the schematic for reference, if anyone is interested. Should have done that first, but I was on my phone...

[MapleSingleCut]

Found a second-hand 6.3v mains transformer for a few quid online. Mounted it and attached it to the same contacts as the previous one and viola - no high current draw. The wall wart unit must have been faulty. Wired in 1/2 of a 12AX7 as a preamp using tweed Deluxe values for cathode/anode components. Works a treat. Drives the N108 very nicely with great crunch tones at bedroom volume levels.

[kagliostro]

Now that all works fine you can try to demage it using also the second triode of the 12ax7 tube

Matchless DC30 12ax7 input


Fender 5F2A



or use the unused 12ax7 triode as oscillator for a Vibrato

Gibson GA 5T Skylark



Or, better, use the triode as a Sluckey's TremOnator


https://sluckeyamps.com/trem/trem.htm

Franco