Truly Silly Transformer Question

[cogitech]

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.

 

[paulb]

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.

 

[cogitech]

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?

 

[paulb]

You should be fine. Google around a bit for articles on safety with tubes. Keeping one hand "in your pocket", things like that.

 

[cogitech]

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!

 

[Chris Hornbeck]

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

 

[cogitech]

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

 

[Richard Ellis]

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".

_______________________________________________________Rick........

 

[paulb]

That's a good idea.

 

[cogitech]

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?

 

[Tesla88]

How much power is the originale TX , is it running hot? and how thick are the 6,3 V leads for the 12xx7 tubes?
What i mean is 1,2A @ 6,3 is about 8W , 1,8A is about 12W , if leads are enough to take 1,8A i don't think original TX would suffer from 4W more.

Anyway
1) Extend the leads on the new (to me ) transformer (they are fairly short)
I'll suggest to solder longer leads to the short and insulate with heatshrink tube (at least double strate for each juncion ) , or you can use terminal blocks

2) Find a suitable mounting position in the chassis
A couple of photo would help !anyway , not to close preamp tubes

3) Disconnect the existing 6.3v leads which power the 12**7 tubes, roll them up and isolate them (maybe use marettes, tape?)
I will use them to power a pilot light if any...otherwise insulate them with heatshrink

3) Roll up and isolate the centre tap on the new transformer
Better if grounded for HUM cancellation

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

Ok , is easy

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

Pay attention to the insulating due to voltage ! I suggest to insert a fuse for the new xformer !

 

[cogitech]

How much power is the originale TX , is it running hot? and how thick are the 6,3 V leads for the 12xx7 tubes?
What i mean is 1,2A @ 6,3 is about 8W , 1,8A is about 12W , if leads are enough to take 1,8A i don't think original TX would suffer from 4W more.

I have heard reports from someone else that 1.8 amps is just too much for the 1.2 amp tap on the TX. It runs quite hot as it is.

Anyway
1) Extend the leads on the new (to me ) transformer (they are fairly short)
I'll suggest to solder longer leads to the short and insulate with heatshrink tube (at least double strate for each juncion ) , or you can use terminal blocks
[/quote]

Yes, I plan to pre-tin each lead, solder them, then insulate with heatshrink.

2) Find a suitable mounting position in the chassis
A couple of photo would help !anyway , not to close preamp tubes

I think I can manage this, but I will certainly ask advice once I get the transformer if I am having problems.

3) Disconnect the existing 6.3v leads which power the 12**7 tubes, roll them up and isolate them (maybe use marettes, tape?)
I will use them to power a pilot light if any...otherwise insulate them with heatshrink

Pilot light?

3) Roll up and isolate the centre tap on the new transformer
Better if grounded for HUM cancellation

I will try this, for sure. Thanks!.

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

Ok , is easy

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

Pay attention to the insulating due to voltage ! I suggest to insert a fuse for the new xformer !

I suppose I will need to purchase a fuse holder and fuse. Can anyone tell me exactly what parts I need for that?

 

[Tesla88]

Pilot light?

What i mean is , now you get a spare 6.3V from the main xfromer , you can just insulate this secondary or use it for something else , like a pilot light or a Led to indicate the power on state ...
http://acimg.auctivacommerce.com/imgdata/0/1/2/8/8/9/webimg/1459963.jpg

I suppose I will need to purchase a fuse holder and fuse. Can anyone tell me exactly what parts I need for that?

Fuse rating depend on the new trasformer power , if the transformer is around 20-30VA , i will go for a 315mA slo-blow fuse.

For the fuse holder , try to find the same kind mounted on the amplifier , or there's not fuse at all now?
2 different kind of fuse holders on same chassis looks ugly

 

[cogitech]

Thank you very much, Telsa88!

 

[cogitech]

The transformer arrived today. It was waiting for me when I got home from work.

About 2 hours later, had it installed and running. Luckily it just barely fit inside the chassis.

While I was in there, I also took the opportunity to redo a few solder joints that looked suspicious. In fact, I found one joint where the wire was basically just resting on the socket pin. Scary.

Anyway, I now have an independent, 3 amp, 6.3v supply for my pre-amp tubes!

Thanks to all who contributed to this thread!

 

[cogitech]

BTW, am I hearing things? I swear the amp sounds better now that the front tubes are running off the supplemental supply.

 

[cogitech]

1955 GE 12BH7s arrived yesterday. Popped them in and let the amp heat up about 10 minutes.

They sound fantastic. The most significant tube roll I have done thus far. I don't know how, but the amp offers considerably more detail now. Elements in the music are more separated and defined. Of course, this helps with sound stage and imaging quite a bit. Bandwidth and tonal balance might be a bit better, too.

The little supplementary filament transformer handled the extra current draw without missing a beat. It was running a little warmer than with 12AU7s, but that was to be expected.

Overall, this modification was easily worth the $12 I spent on the filament transformer and the price of some nice old tubes. The fun and experience of installing the transformer is priceless.

Thanks again to all who contributed to this thread!

Oh, one other update: I ended up mounting the supplementary transformer on the outside of the chassis, as it was overheating and causing hum when installed inside. It is ugly, but it runs at a reasonable temperature now and the hum is gone.