Power Transformer Headache

[silverfox]

This probably doesn't happen often but- I'm trying to determine if a power transformer from an old amp is going to work in a new design. Can't find any info on the tranny. Here are the specs I have:

I have the schematic for the amp: http://i.minus.com/iWhvfOrQC6w7u.png

sec:  Tap a- 40 volts CT, Tap b- 6.3 volts CT, Tap c- 350- no CT

Previously used in a 35 Watt audio amp based on two 7686 power tubes. I want to drive two EL34's. Filament current for the 7686 is .800 ma, EL34's are 1.3 amps.

How can I determine if there will be sufficient filament current to drive the EL34's and 5 12AX7 filaments.

If I dead short the filament tap and measure the current will that fry the windings? I don't think so.

Another question relates to the bias circuit. Being a current based circuit for the bias, It shouldn't matter if I use the 40 volt tap for the bias circuit or even the 350 volt tap. I may want the 40 volt tap for something else.

Another question- I should be able to use the 350 volt tap for the plate voltage right? And how can I determine the output current available from the taps without creating an overload situation?

I'm building an amp based on the various circuits I've gotten from this site and would like to use this 35 watt PT and 35 watt OT but don't have any specs. Both worked fine previously.

Signed- Learning a lot about tubes

[DummyLoad]

This probably doesn't happen often but- I'm trying to determine if a power transformer from an old amp is going to work in a new design. Can't find any info on the tranny. Here are the specs I have:

I have the schematic for the amp: http://i.minus.com/iWhvfOrQC6w7u.png

sec:  Tap a- 40 volts CT, Tap b- 6.3 volts CT, Tap c- 350- no CT

Previously used in a 35 Watt audio amp based on two 7686 power tubes. I want to drive two EL34's. Filament current for the 7686 is .800 ma, EL34's are 1.3 amps.

How can I determine if there will be sufficient filament current to drive the EL34's and 5 12AX7 filaments.

If I dead short the filament tap and measure the current will that fry the windings? I don't think so.

Another question relates to the bias circuit. Being a current based circuit for the bias, It shouldn't matter if I use the 40 volt tap for the bias circuit or even the 350 volt tap. I may want the 40 volt tap for something else.

Another question- I should be able to use the 350 volt tap for the plate voltage right? And how can I determine the output current available from the taps without creating an overload situation?

I'm building an amp based on the various circuits I've gotten from this site and would like to use this 35 watt PT and 35 watt OT but don't have any specs. Both worked fine previously.

Signed- Learning a lot about tubes

If I dead short the filament tap and measure the current will that fry the windings? I don't think so.

it will leak magic smoke and you can't put it back. measure load carefully start with ohms law - 6.3V/1.6A = Rx, measure V with that load. you want 6.3V/2.4A = some Rx. strap that across the 6.3 sec. - if you measure at or less than 6.0V or so, you're heavy overloading. btw, those will need to be BIG (10W and more) watt resistors.

...35 watt PT and 35 watt OT but don't have any specs

what was this thing they worked in?  is it still in one piece? if not, did you dismantle? if you did dismantle, did you document the PS and output scheme?
you're asking for 1A more of filament - may or may not work for long. so build 7591 octal base amp. 7591 is same as 7868 - just different base.

--DL

[sluckey]

If I dead short the filament tap and measure the current will that fry the windings? I don't think so.

Absolutely! Think about it. Ohm's law... 6.3v across zero resistance equals infinite current. Varnish insulation will melt. Plastic insulation will melt.

DON'T DO IT!

[jazbo8]

Just went a similar situation >> http://www.el34world.com/Forum/index.php?topic=14970.msg143499#msg143499

[eleventeen]

Don't short the filament winding, you could cause rapid damage. Those leads are going to be heavy wires within the transformer itself, meaning they will carry a lot of current, and they will, and they could unsolder themselves internally and/or set the tranny on fire, or scorch it. Do not do.

The bias circuit consumes very, very small current, it's not a concern. Where you get it from (either a dedicated tap  like in Fenders or a completely separate winding like your pix or steal it off the HV of a tranny that has no sep bias winding nor tap) is also of little concern as long as it is referenced to ground or whatever the output tube cathodes think is ground.

You have a smallish to medium conundrum; either accept the notion that you are possibly overloading your power tranny by 6 or so watts (1 amp * 6.3 volts) leading to poss failure OR you have to throw in a separate filament transformer which you may not have planned for, chassis layout and random hum consideration wise. Hate when that happens.

My WAG is that you'll have no problem w/the extra load on the tranny; OTOH, if the thing ends up in a tight cabinet with poor ventilation, added to by tube heat, you could be inviting bad luck.

Fan?

Can you pull out a #47 incandescent light and make the pilot lamp an LED? I think those #47s are 150 mils. There's a watt or so fer ya.

[DummyLoad]

If I dead short the filament tap and measure the current will that fry the windings? I don't think so.

Absolutely! Think about it. Ohm's law... 6.3v across zero resistance equals infinite current. Varnish insulation will melt. Plastic insulation will melt.

DON'T DO IT!

Varnish insulation will melt. Plastic insulation will melt.

yes and yes, but will it make smoke? 

 

[silverfox]

Okay, so after the first post I figured I'd set up a rheostat of 5.4 ohms and monitor the voltage of 6.3 vac as I reduced the resistance. At about 1.4 ohms the voltage began to drop so I stopped. I think that is what was recommended.

The current then is: 4.5 amps.

I also checked the original circuit with the tubes in and the resistance was .6 ohms when measured across the filament leads that are still in the circuit.

Note that in the schematic there are no limiting resistor shown for the filaments.

Based on that the current should have been something like 10.5 amps.

There is something going on here that I don't understand. New to this. As I mentioned there are no limiting resistors.

4 6EU7 tubes and the two 7686.

As I recall I've never seen a limiting resistor in a filament circuit. There is a resistor in the circuit referenced above but that is on the center tap.

Okay finally, what limiting resistors do you put on a filament tap when you're building an amp?

[sluckey]

yes and yes, but will it make smoke?

Oh yes, very quickly too.

I also checked the original circuit with the tubes in and the resistance was .6 ohms when measured across the filament leads that are still in the circuit.

You're reading the low resistance of the PT filament winding that's parallel to the filament string. You must disconnect the PT leads to measure resistance of the tube filaments.

There is a resistor in the circuit referenced above but that is on the center tap.

Actually there are two resistors and a cap connected to the CT. The two resistors form a 10:1 voltage divider that's connected to point "A" which is probably a 380vdc B+ node. This elevates/references the filament string to some dc voltage for hum filament reduction.

Okay finally, what limiting resistors do you put on a filament tap when you're building an amp?

Nobody normally uses limiting resistors on parallel filament strings. The tube filaments are connected directly to the PT filament winding.

[silverfox]

I think I have it figured out. But first to make sure:

When I read .6 ohms resistance the filament transformer was out. I was reading the filaments. And I was reading the filaments of two 7686 and four 6eu7 at that time. I put all the tubes back in.

Ohh- I never could find the destination of "A"- Thanks that makes a difference. As far as loading the filament taps though I don't think so unless in an inductive sense. I do understand the hum reduction point and get that.

And this gets closer to the original question- Can I just connect the leads to the fillament tap. I realized I had already done this on another project and nothing burned out.

Here's why- The tube filament will only carry the amount of current listed in the tube data sheet. For an AX7 that is .3 amps. I may have this number wrong but the concept should be right.

Now if I connect too many filaments to the tap- Yes it will burn out the tap and possibly short the transformer.

What I did yesterday was load the tap until the voltage began to drop and came up with about 4.5 amps available for filaments. So if I add up all the filament draws that should give me an indication of suitability.

If this wasn't the case- The AX7 filament would draw 1.3 amps- based on a 4.3 ohm resistance

First project so correct me if I'm wrong. And all this is most informative and helpful, thanks!

[eleventeen]

When you did your rheostat test, you left things on for a few minutes and then stopped the test. In the real world, your brief test isn't germane to having the thing in a tight cabinet with poor ventilation, under lights, and you forgot to turn the amp on standby between sets so it has been on for 5 hours......

The tubes can run fine with slightly low heater volts. It's the excess heat the PT will have to endure/withstand/tolerate. Add in that the item and its internal insulation qualities are older....

That's the bottom line, as I see it. I myself believe you can almost certainly pull your extra amp of heater current. It will not cause sparks and flame. But it WILL cause a non-trivially stressed condition, that, combined with some other stressed condition, might cause an amp failure. And...that time might be five years of use from now. I have built stuff that ran a tad hotter than I wanted and it lasted for years; however, the failures were generally dramatic, LOL.