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Old 12th February 2013, 11:15 PM   #1
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Default Truly Silly Transformer Question

Hello all,

My power transformer only supplies 1.2 amps to my 2x 12AX7 and 2x 12AU7 tubes (ei. just barely enough). This makes it impossible for me to swap in some 12BH7s in place of the 12AU7s because the 12BH7s draw .6 amps each, which would push my current draw up to 1.8 amps.

I've just purchased a 120v primary to 6.3v @ 3 amp secondary filament transformer and will need some tips on installing it. Here's what I think I need to do.

1) Extend the leads on the new (to me ) transformer (they are fairly short)

2) Find a suitable mounting position in the chassis

3) Disconnect the existing 6.3v leads which power the 12**7 tubes, roll them up and isolate them (maybe use marettes, tape?)

3) Roll up and isolate the centre tap on the new transformer

4) Twist the new 6.3v leads and route them along the side of the chassis (shortest path) - solder where old 6.3v supply was connected

5) Solder primary leads to back of mains plug, parallel to existing leads.

Please correct any mistakes I have made in my thinking or provide any other tips which you think might be helpful. I am very new to this.
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Old 13th February 2013, 12:18 AM   #2
paulb is offline paulb  Canada
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That sounds fine. Is there anybody you could ask to help you? You're working with AC line voltage and high voltage DC, it would be nice to have someone with experience beside you.
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Old 13th February 2013, 12:31 AM   #3
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I have a friend who is an industrial electrician, but he is used to working on megawatt infrastructure, generators, UPS, PMMs, etc. Not electronics, per se.

I have worked a lot with mains AC, wiring panels, plugs, fixtures, etc. Enough to know what 120 feels like I suppose I could ask a friend to be there even if just to be a second set of eyes and/or pull me off the AC feed if I do something stupid. But I really wouldn't plug the thing in and start tinkering with it.

I assume, perhaps incorrectly, that as long as the caps are discharged (which they should do on their own) and AC is not plugged in, then I can safely work on the unit.

Transformers don't retain current, do they?

What are the less obvious risks that I might be missing?
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Old 13th February 2013, 12:37 AM   #4
paulb is offline paulb  Canada
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You should be fine. Google around a bit for articles on safety with tubes. Keeping one hand "in your pocket", things like that.
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Old 13th February 2013, 12:40 AM   #5
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OK. Will do. Have time to burn while I wait for the transformer to arrive.

Hard to solder one-handed, but I know what you mean.

Thank you!

Last edited by cogitech; 13th February 2013 at 12:42 AM.
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Old 13th February 2013, 02:41 AM   #6
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Please don't assume that caps will discharge on their own. Measure, then measure again. 400 Volts is just right to kill ya.

All good fortune,
Chris
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Old 13th February 2013, 02:49 AM   #7
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Thank you, Chris.

Indeed I will test with the voltmeter and discharge with a resistor if necessary.

It will be interesting to see if things play out according to this video Discharging.wmv - YouTube
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Old 13th February 2013, 03:01 AM   #8
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You could also simply remove the connection where the 6.3V branches off to drive the old 12AU7s, leaving the original connections to the 12AX7s.
This will "lighten" the load on the originals.
Your new filament TX will also see a lighter load & probably will run "cold".

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Old 13th February 2013, 03:16 AM   #9
paulb is offline paulb  Canada
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That's a good idea.
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Old 13th February 2013, 03:49 AM   #10
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Richard,

Thanks for your response. A couple of things to consider are:

a) the 4x 12A*7 tubes are on a PCB, which is fed by a dedicated 1.2 amp 6.3v lead from the existing TX, and

b) the existing TX is also powering 4x EL34 tubes (via 2 more 6.3v leads at 3 amps each).

The PCB makes removing the branching connection between the 12AU7s and 12AX7s quite difficult (not something I would attempt, I think).

Also, putting all 4 12A*7 tubes on the new, supplementary, 3 amp TX will divert a minimum of 1.2 amps (1.8 amps with 12BH7s) away from the main TX (making it run cooler) which I think achieves something similar to what you suggest. The supplementary 3 amp TX will only ever be tasked with 1.8 amps (1.2 amps if I roll back to 12AU7s) so it shouldn't run hot either. Currently, the main TX does run fairly hot, so the 1.2 amps of relief will be good for it.

This way, I also realize some of the benefits of isolating my power tubes from my preamp tubes with respect to power draw, noise, etc... ?

Am I correct?

Last edited by cogitech; 13th February 2013 at 03:56 AM.
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