Fault in the power transformer of a tube amplifier

[rafaelctt]

hello!
Hope you can help me with the following problem:
The primary fuse (1.5A) breaks every time I connect, how could I know which winding is bad? I measure but they all have low resistance values. In the primary there is 15 ohms. In the output winding 300V 246 Ohm. The transformer is a Hammond 290PX
Thank you

[Latole]

Did you check the P.T. connected to the amp circuit  or all secondary disconnected ?

1- Connected to the amp circuit ;
    a- Wich amp ?
    b- Remove all tubes
        Did fuse blow or not ?


2-All secondary disconnected
  a - No need to know wich winding is bad, you need a new PT

[Latole]

Specs sheet and resistance values :

https://www.hammfg.com/files/parts/pdf/290PX.pdf

[rafaelctt]

Did you check the P.T. connected to the amp circuit  or all secondary disconnected ?

1- Connected to the amp circuit ;
    a- Wich amp ?
    b- Remove all tubes
        Did fuse blow or not ?

2-All secondary disconnected
  a - No need to know wich winding is bad, you need a new PT

I have done the tests with secondaries WITHOUT load. But I understand that if any of the secondaries were short-circuited, they would blow the primary fuse by asking for too much current consumption.

[SILVERGUN]

That is an easy transformer to wire incorrectly.
Show us how you have yours wired and what input voltage you are using.
Did it ever work?
Is this a new install?

[rafaelctt]

That is an easy transformer to wire incorrectly.
Show us how you have yours wired and what input voltage you are using.
Did it ever work?
Is this a new install?

Yes, it worked during installation and even after the amp was finished, it worked for 30 minutes until the primary fuse blew. I also have different measurements and once finished they were correct according to the diagram.

[rafaelctt]

[pdf64]

I think it should have managed an AC15; assuming you did, how did you use the EZ81 rectifier though?

[rafaelctt]

I think it should have managed an AC15; assuming you did, how did you use the EZ81 rectifier though?

Exactly as it comes in that schematic

[rafaelctt]

[SILVERGUN]

I'm still concerned about how you wired the primary side of that transformer.
You need to energize both of those 120V windings for proper operation. (even if you're not running 230V input - or are you? - that's still not clear to me)
None of those posted schematics addresses this.
Show us how YOU actually wired the primary side of the transformer.

[pdf64]

I think it should have managed an AC15; assuming you did, how did you use the EZ81 rectifier though?

Exactly as it comes in that schematic

That’s not really good practice though, puts very high stress on the rectifier’s heater cathode insulation. With modern valves especially, you may be pushing your luck.
If it shorts, there will be a hard short across the DC HT. Hopefully that would blow the mains fuse quickly, but perhaps not quickly enough to protect the PT from damage?

HT fusing would be a significant improvement, could be fast acting types if that nasty hot switching standby was eliminated (or rearranged to be non hot switching).

[rafaelctt]

I'm still concerned about how you wired the primary side of that transformer.
You need to energize both of those 120V windings for proper operation. (even if you're not running 230V input - or are you? - that's still not clear to me)
None of those posted schematics addresses this.
Show us how YOU actually wired the primary side of the transformer.

[rafaelctt]

I think it should have managed an AC15; assuming you did, how did you use the EZ81 rectifier though?

Exactly as it comes in that schematic

That’s not really good practice though, puts very high stress on the rectifier’s heater cathode insulation. With modern valves especially, you may be pushing your luck.
If it shorts, there will be a hard short across the DC HT. Hopefully that would blow the mains fuse quickly, but perhaps not quickly enough to protect the PT from damage?

HT fusing would be a significant improvement, could be fast acting types if that nasty hot switching standby was eliminated (or rearranged to be non hot switching).

I'm sorry, I do not understand. Could explain? Thank you

[pdf64]

Which point to explain?

[rafaelctt]

Which point to explain?

How else would I connect the EZ81? what is HT fusion? Are you referring to fast blow fuses?

[pdf64]

Which point to explain?

How else would I connect the EZ81?

A PT with a separate, dedicated 6.3 winding for it would be best.

what is HT fusion?

See under ‘secondary fusing’

http://www.valvewizard.co.uk/fuses.html

In case you were wondering why standby was nonsense, see
http://www.valvewizard.co.uk/standby.html

Are you referring to fast blow fuses?

Yes.
UL248 seems to describe fuse types as ‘fast acting’ or ‘time delay’, whereas IEC60127 uses the ‘quick acting’ or ‘time lag’ terminology.
https://dalmura.com.au/static/Valve%20amp%20fusing.pdf

[mresistor]

"A PT with a separate, dedicated 6.3 winding for it would be best."


Or since his PT has a 5V secondary he could change to a 5Y3GT rectifier.

[pdf64]

That transformer is now deceased 

[tubeswell]

If you’re using a Slo-blo Mains Fuse and the pair of primary windings are connected in series and in phase, and if the Mains Fuse still blows while all secondary windings are disconnected, you probably either have a bad PT, or your mains fuse rating can’t handle the current spike from self inductance of modern silicon grain oriented steel laminations at startup.

But if the mains fuse was initially holding when you first tested the PT without a load, but then it blew after you loaded up the secondary side, then it’s the way you wired up the secondaries. Or it’s the EZ81.

Even though EZ81 h-k insulation is supposedly rated to handle 500 or so volts, it’s not a great idea to put that to the test with modern production tubes, as pdf64 pointed out. It’s better to have a separate rectifier 6.3vac filament winding (thus avoiding stressing the EZ81 h-k insulation).

[rafaelctt]

Thank you for your answers, I ask you to forgive me for the expression since I do not speak English commonly.
I have ordered a new 290PX transformer from Tube-town. This transformer worked very well for me during all the tests, only I find the 6.3V output to be very tight. I finally had to put in a 4A slow fuse for tube heating. The 1A primary fuse blew when the amp was running for 30 minutes. The next one you install will be 1A fast. I also had problems with the circuit board as the turrets were very close together and I had a lack of insulation. Now I am going to build the board in two separate boards and with optimal insulation. Can you recommend a transformer just for heating the valves and that delivers about 5A? Thank you!

[acheld]

I've used the Hammond 166 series for filaments -- they work great, seem to be robust.   Their 266 series is similar, but can use 230VAC.

https://www.hammfg.com/electronics/transformers/power/166?referer=968

https://www.hammfg.com/electronics/transformers/power/266

These guys sell Hammond iron in the EU:

https://www.tube-town.net/ttstore/components-10662/transformer/power-transformer/filament/

In the US, https://www.hawkusa.com has the best prices and customer service (by far IMO).

Cheers!

[pdf64]

. Can you recommend a transformer just for heating the valves and that delivers about 5A? Thank you!

What’s the actual plan, why 5A?

[sluckey]

Can you recommend a transformer just for heating the valves and that delivers about 5A?

I think you missed the point of the recent comments. Use your 290 PT to power all filaments ***EXCEPT*** the EZ81. Use a small filament PT to power only the EZ81. The Hammond 266J6 should be fine. Don't connect the secondary to chassis ground.

[rafaelctt]

The 266J6 connection would be like the one in the image?
What is meant by NOT CONNECTING chassis to ground, can't I screw this transformer to the same chassis as the rest of the screwed elements (transformers, connectors, etc)?

[sluckey]

The 266J6 connection would be like the one in the image?

yes

What is meant by NOT CONNECTING chassis to ground, can't I screw this transformer to the same chassis as the rest of the screwed elements (transformers, connectors, etc)?

I did not say that. Read my reply again.

[rafaelctt]

The 266J6 connection would be like the one in the image?

yes

What is meant by NOT CONNECTING chassis to ground, can't I screw this transformer to the same chassis as the rest of the screwed elements (transformers, connectors, etc)?

I did not say that. Read my reply again.

and I was referring to this:
"Don't connect the secondary to chassis ground."
The secondary I understand is connected to the filament of the EZ81 only. Is that so?

[sluckey]

YES

[Williamblake]

The secondary I understand is connected to the filament of the EZ81 only. Is that so?
[/quote]
Isnt the second powertransfor installed so that you can apply the EZ81 heating To EZ81 cathode?

[pdf64]

The secondary I understand is connected to the filament of the EZ81 only. Is that so?

Isnt the second powertransfor installed so that you can apply the EZ81 heating To EZ81 cathode?
[/quote]
No, heat the heater / filament, which is electrically isolated from the cathode.

[Williamblake]

Well yes, heater winding for EZ81 goes to EZ81 pins 4 and 5 but pin 3 and 4 are connected to elevate the heaters to HT potential is what i thought was the goal of having a dedicated rectifier heater winding.

[sluckey]

Well yes, heater winding for EZ81 goes to EZ81 pins 4 and 5 but pin 3 and 4 are connected to elevate the heaters to HT potential is what i thought was the goal of having a dedicated rectifier heater winding.

I believe the reason for using a separate dedicated winding for the EZ81 is in case there is a short between heater and cathode, the B+ will not appear all the way down the other filament string. The other filament string is probably referenced to ground through a center tap or artificial center tap. Either the resistors or entire PT would be at risk in that scenario.

Connecting the EZ81 cathode to the filament should be OK ***IF*** the little filament transformer secondary has sufficient insulation. I really don't see any reason to do that though.

[mresistor]

deleted       

[Williamblake]

Thanks for pointing that out. All the EZ80 and EZ81 ciruits from the 50s i came across had their own heater winding and i thought this was for low heater to cathode voltage. I found the EZ80 being very capable of running with high heater to cathode voltage but totally missed out on the fusing i should apply when changing the arrangement.

[sluckey]

My experience has been just the opposite. All the small amps I've seen that use the EZ81 had it's filament powered by the one and only filament winding. This includes about 6 Hammond organ units and the amp in this thread, which is a 1960 Vox AC-15.

The only amp I've seen with a separate filament winding for the EZ81 are the popular Marshall 1974 18 Watt clones and the original did not have a separate winding for the EZ81.

I think the separate winding is a good idea but there are plenty of old quality amps that testify that it's OK to power the EZ81 from the same filament string as the other tubes.

[rafaelctt]

Should I do it like this?
Thank you

[pdf64]

Fuse protection for a heater circuit can be problematic. I’ve never bothered.
Heater fuses can cause issues, as generally standard cartridge fuse-fuse holder design is not intended for low voltage, high current applications. Low wetting voltage can tend towards excessive contact resistance between fuse and fuse holder, which generates heat and leads to malfunction / damage.

Automotive type blade fuses may be more appropriate.

[Williamblake]

So fusing the heaters to protect from HT failure may not work, since heaters draw much more current than HT and even more so at power on, you should better fuse HT than heater circuit? Or both?