Power Transformers

I have an old tube headphone amp that i think has some problems with it's PT but since we don't have a topic on PT's @Priidik suggested i make a dedicated thread for it.

Anyways this thing has some problems like a 50hz ish hum, definately lower then 60hz which is weird. There is some distortion going on, definitely noticeable with bass heavy music. And in retrospect i think it ate some nos tubes, they became very noisy quickly. So i have been reading and learning about possible issue's, i was thinking filter caps but the hum is too low for it to be the that. Then i read that too low heater voltage can cause damage to tubes and then a lightbulb went up.

So i measured the PT between CT and 6,3V and it reads 4,25v with no tubes inserted. There is a second 6,3v tap that goes to the rectifier and that one reads 5V with a tube inserted, so also too low.

Am i right in thinking this PT is toast? I even have theory on what damaged it, you see this thing is big and fully enclosed with vents on the side. It does get quite warm and i had a cat at the time and well you can see where i'm going with this. I put some cushion on top so the cat would not burn itself and i think i cooked the PT.

I took it back to the guy who built it a couple of times but he unfortunately had a stroke so he could not solve it anymore. He even gave me the wrong tubes 12AX7's which clearly have way to much gain for this amp, i didn't know much about tubes at the time but i doubt those damaged the PT if i look at how much current they draw compared to the 12AU7 which belong in there.

Here are some spec on the PT:
Primary winding 220V (225V from wall)
Secondary 270V (294V measured)
Heater winding 6,3V for 12au7's (4,25V)
Heater winding 6,3v for 6x4 (5v)
There is text on there that says 70 - 1,3 - 1,3 which i assume is the current for the High and two low voltage windings.

Some info on the amp:
It uses EZ80/6x4 Rectifier in the psu, 4 12au7's and two output transformers.

So what do you guys think? Is the PT toast?

 

Cooked could mean it has visible dmg. Pictures!
Do you have any sort of lcr meter? Is so, measure inductance of secondaries.
If not, some idea could be had from resistance.
Burned windings likely have shorted turns.
Also, if your dmm has megaohms, measure between primary and secondary if there's any serious leakage.
Ancient transformers could have serious leakage due to poor insulators used back then.

 

Yeah my mm can do that. If i did it correctly the 6,3v to the rectifier measures 119uF and the 6,3v to the other tubes measures 102uF. Not getting anything really on yhe 270v should i measure that between ct and the 270v out?
Resistance between primary winding and secondary heater windings are 1500M ohms and quickly rising to O.L. for both. The 270v one goes to O.L. right away.
Here is a pic of my mm i used the middle two and put the dial at 200uF.


And here are some pics of the PT:





Sorry for being a noob

Edit wait i think i might have confused inductance with capacitance lol

 

Just to clarify... CT is an abbreviation normally used for “current transformer”, whereas PT is “potential transformer”. When you use CT, are you referring to the point where the transformer is connected to the case? (Marked on the transformer as 0V).

Also, your readings are being taken right from the tapped points on the transformer itself, correct? I’ve seen bad readings if the common point(s) end up floating, due to a poor case or ground connection. From the pics, it looks like your leads are right on the transformer taps, which is the best location to take the readings for sure.

It’s entirely possible as @Priidik mentioned for the windings to short... in my personal experience, the windings will most likely burn open, if the transformer was subjected to a fault, and the fusing didn’t properly protect it. You’d also normally see heat damage as discolouration, or smell something like burnt varnish.

With the 225V applied to the 220V primary winding, and 294V measured on the 270V secondary, everything appears fine. The 6.3V taps most likely are a part of the 270V secondary winding, so I personally doubt the transformer is damaged...

 

Well i got the CT from watching youtube video's from Uncle Doug and he called it center tap and it is connected to ground here. There is also CT on the label. Maybe it's old school idk. When i measured the 6,3v there were 2 twisted wires, one connected to 0V and one to 6,3V. Those measured low but get this... i just measured again and now it's right where it should be. Both heater windings are giving 6,9v. Could this be a bad solder joint? The only place i pocked around was the CT joint.

Perhaps it's a good idea to just pop off the back panel and reflow every solder joint on the PT. Maybe check all the ground connections.

What do you think?

Edit: I just remembered measuring 150v on the B+ a while back. Didn't think or know much of it then. But now it measures 408v without tubes, the fact that this voltage and the heater voltage have doubled is odd.

 

Probing around a high voltage circuit when you don't know what you are doing is perhaps not the best idea.

CT is clearly center tap here, and PT is clearly power transformer.

Anyhow, lower than expected voltages could indicate a short. It could also indicate too much current draw from the amplifier which can drag the voltage down. Without a schematic and pictures, it is difficult to say any more.

 

From the pic of your multimeter it doesn't look right. The black probe should be plugged into COM.

 

Well like i admitted i am a noob but i know how to be safe, left hand behind my back when poking around. And when it is hard to reach i used zip ties to measure so i can keep my hands clear when it’s powered on. Still waiting on some clamps. I built a crack+sb and one of your T3’s with all the upgrades and i’m still alive

Funny thing is i didn’t do much i only touched the CT but now it measures like it should, 6,3v exactly for all tubes and the plate voltages get 220v which is also within spec. And it sounds allot better, no longer harsh and much lower distortion in the bass. I have a pic for you but i don’t have a schematic. Reckon reflowing a bunch of joints might keep it working now? Btw there is still some 50hz hum unaffected by volume pot.

As far as i can tell; input goes left through a 10k pot then it goes through the left most tube and the other one is in series before it splits of into those other tubes of which each goes to their own OPT.

Yeah i took the pic before i measured

 

Probably a loose connection. I didn’t think it was the transformer

AC doesn’t care whether the black meter lead is on the lowest potential point... it’s constantly changing, so the meter simply gives the voltage potential between two points.

As for hum, try checking for a DC voltage in your AC circuits. A small amount of DC can sometimes cause humming. This can sometimes be related to ground loops/other things which can take a while to chase down and isolate.

From my experience, reading “weird” voltages from transformers is usually a floating voltage issue. Supposing that the winding is tied to the chassis/ground, ensure the connection is solid and not corroded, or compromised in some way.

Shorts could pull the voltage down as well, but a true short should cause high current and a blown fuse.

Ahh, X0 is electricians nomenclature for the point of a secondary grounded winding. Thanks for the info @dsavitsk

 

I am going to reflow a bunch of joints tomorrow, check the ground connections and dc on the ac. But tonight I'm going to listen to it Sounds very good actually maybe a bit too warm but very engaging.

Thanks allot everyone!

 

Lots of topologies are possible with that tube compliment. Do you know if it is single ended or push pull? If you can put the values and voltages of some of those parts (particularly the electrolytic caps) it would be al ot easier to guess as to what's going on.

The 4 most likely sources of hum are grounding issues, transformers flux noise, B+ noise, and AC heaters. B+ noise seems likely in an amp with no choke and no regulator. I can't tell if the heaters are AC. If the noise is louder in one side than the other, that's likely a PT induced noise - try turning the power transformer 90 degrees. Ground issues are the most work to solve.

 

Ok i did some investigation and the 6,3v from the PT is going to what seems is a silicon bridge rectifier (JJ B40 C5000-3300) and then into a 25V 4700uF electrolytic cap from Jamicon(isn't that a pos brand?), a 25v 22000uF cap and a 5w 56ohm(?) resistor after that it goes to the heaters on the 12au7's. So that must be DC on the heaters then, the 6x4 in the psu gets AC from its own winding.

Other caps in the psu are 3x220uF 385v rubycon, a Philips/Vishay 385v 47uF cap and a Audyn mkp-qs 400v 4,7uF.

The electrolytics near the signal tubes are all 25v 220uF.

The hum is in both channels(pot has no effect) and if there is anything playing it is inaudible and i listen at low levels. But if I watch tv or a movie it's noticeable.
Couldn't tell you if it's push pull or single ended sorry. I did look under the pcb and it seems the left most tube takes in both channels and then goes into the middle tube and then it is split up and each goes to it's own tube and OPT.

For disclosure i put a voltage divider on both channels to get more travel on the pot using a 10k and 4,7k resistor but i'm pretty sure the hum was present before that.

I will look into joints and ground connections tomorrow

Edit: Been thinking, this amp is probably 20 years old time for a recap?

Edit2: So i reflowed all the joints on the PT and wiring that was easy enough to reach. This case is made up of 6 panels bolted together and there are filed down parts so the panels are connected to ground. Some parts were not very good so i improved them. Then i measured all the points connected to ground and the max i get is 0,1Ohm.

Then i checked for DC on the AC taps on the PT. 6,3v windings are fine but the 270v winding has 0,3v that goes up and down, i checked the wall the same thing happens there.

Now on to the result, the hum is still there unfortunately, seems a bit lower but i’m not 100% sure.

Reckon it’s noise on the B+ then?

Thanks for all the help btw, i’m very grateful

Edit3: Getting some blacker background now and i’ve noticed there are actually two different hums. One higher pitched and lower in volume(120hz i think) and a louder 50hz one. I’m just gonna replace all the electrolytics...

 

So i replaced all the caps in the PSU and the hum is still there
I did figure some things out though, it uses a full wave tube rectifier 6x4 so it has a slow start, there seemed to be a ground loop but lifting that didn't do anything. As soon as i turn on the amp i hear the hum loud, then it goes left and right while quieting down and stays the same on both channels.
Also the bass distortion is still there, tried it with two sources, it does not sound pretty on some Trentemoller tracks but on other music it's not so noticeable. The guy who built this was a big Grado fan(the old ones). Maybe this amp just doesn't like high impedance headphones, though i did listen to it with a hd600 10 years ago and i can't remember hum when it worked.
Then a tube died and the problems started, this is the 3rd time i got it out of storage in 10 years time trying to fix it.

Can anyone help me? Just spent money on caps and i don't want to toss it in storage again this time....

 

I have a theory about what happend, the caps after the bridge rectifier failed, that messed up the bridge rectifier and made it short out the PT thus causing it to give a lower voltage on the heater winding. And that is what is causing the noise, i mean this amp used to be quiet and nothing was changed grounding wise, tubes swapping didn't help so it has had to be a component right?

I'm just gonna order a new one, they're cheap so it's worth a shot...

Replaced the rectifier and it's definitely quieter now. But i was thinking the AC heater wire from the PT is next to the DC heater wire and also running across the B+ albeit at an 90 degree angle twice. Maybe i can try moving the ac heater wire more out of the way. It's no longer a loud hum but more of a slight buzz now.

Also found some more info on the topology. It is a split-load phase inverter aka cathodyne or kangeroo.

Gonna look into regulating the B+ i think.

@dsavitsk would you be so kind to give some more feedback? I really appreciate it

Edit: Bass distortion is completely gone after replacing the rectifier!!!