PT Failing under Load?

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Cygnus X1
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PT Failing under Load?

Post by Cygnus X1 »

Before I go and order another PT...
Question.
Built yet another Spitfire sorta clone and although the PT is delivering 300+VAC going into the rectifier I am only seeing 205VDC after the first filter cap, B+.

Not showing stress, not running hot, not blowing fuses or any other abnormal symptoms.

Should I:
Start taking out loads out down the power rail to see if something is dragging current down?
(Pain in the butt...who wants to start desoldering stuff?)

Or

Sub in another PT and let it fly to eliminate the PT as the source of the problem (Another PITB, lots of subbing there...)

Or...what should I probe for to find the culprit?
Could it be a bad ground?
Seems to me if the PT is running under so much stress that the B+ is low by 100V then something ought to be showing some kind of stress.
Cliff Schecht
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Re: PT Failing under Load?

Post by Cliff Schecht »

Are you overloading the PT? Is the bias showing up correctly? The power tubes could be drawing too much current and pulling the B+ way down.
Cliff Schecht - Circuit P.I.
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Phil_S
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Re: PT Failing under Load?

Post by Phil_S »

It could be an unintended ground.

I would check all resistors. Look for unintended substitutions, like 100K for 10K or 1K, that sort of thing. Look for a misconnect.

Take the schematic and a highlighter. Verify the build, marking each part as you confirm its value is correct and that it is placed correctly in the circuit. Do it twice. Once from right to left, and second from left to right. This will jar you out of seeing the same thing over an over again, allowing you to fool yourself into believing something is right when it isn't. IMO, something isn't right with the build and hiding in plain sight.
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Phil_S
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Re: PT Failing under Load?

Post by Phil_S »

Try disconnecting everything after the first filter cap and measure B+. That should be a one-wire lift. If VDC rises, you then know it isn't the rectifier/cap and that it is happening downstream. Almost 100V drop just doesn't look right.
tubesinside
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Re: PT Failing under Load?

Post by tubesinside »

Check the DC side of the rectifier with the meter set to AC. You might have a faulty/open capacitor and this will show up as significant AC voltage.
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Alexo
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Re: PT Failing under Load?

Post by Alexo »

What do you mean it is "delivering 300+ VAC?"

You mean it is a 300-0-300 center-tapped pt?
Life is a tale told by an idiot -- full of sound and fury, signifying nothing.

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Cygnus X1
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Re: PT Failing under Load?

Post by Cygnus X1 »

Yes Alexo.
300-0-300.
Sorry for the misleading post.
I will check the rest as recommended, thanks!
Cliff Schecht
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Re: PT Failing under Load?

Post by Cliff Schecht »

Did you Ohm out all of the windings? Some Google-fu will point you towards the results of other peoples PT winding resistance measurements which you can check against yours. The secondary is easy if it's center-tapped, each outer winding should measure about the same with respect to the CT (some imbalance is not uncommon because of interleaving and other winding techniques but it should only be a few Ohms at most).

Occasionally old power transformers will develop issues that only show under load. Arcing through thinner insulation and corroded wires may not cause problems at first glance but when put to the final test (i.e. working in an amp) they can go south. I've had a few PT's do this actually, one was really old and the other fairly new but it died seemingly randomly.

That article that someone posted about figuring out the current rating of unknown transformers had a good section on how to "bake" a transformer as well. IIRC you put the PT under a light load for an hour or so at it's rated primary voltage and see how hot it gets and how much it droops over the course of the hour.

Speaking of drooping, what do your secondary windings read when you put the amp under full load? If your 300-0-300 transformer is dropping down to 150-0-150 then there is obviously a problem somewhere..
Cliff Schecht - Circuit P.I.
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Cygnus X1
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Re: PT Failing under Load?

Post by Cygnus X1 »

Thank you Cliff, and all.
It's a budget Weber transformer intended for a 5E3.
In fact it worked fine when it was in my first 5E3 build.

I posted all of this too early...I'm in the thick of another amp right now but will go back and test as recommended.
10thTx
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Re: PT Failing under Load?

Post by 10thTx »

It's a budget Weber transformer intended for a 5E3.
Yes , it's a 300-0-300.
My understanding is the Weber PT's for the Deluxe are either
270-0-270 or 340-0-340.

Some of the Weber PT's also have an option for 120v or 125v.

Maybe that along with a wiring issues could be a factor.

With respect, 10thtx
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Cygnus X1
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Re: PT Failing under Load?

Post by Cygnus X1 »

FIXED!

Can't believe I sidelined this for 5 months when I really knew the answer.

It was that cooked transformer.
Tested OK with no load, and with diode rectification.
As soon as I put a tube rectifier on it, 100V low.

Replaced with a AO29 salvage...she's happy as a clam now.

Doink!
:oops:
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alvarezh
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Re: PT Failing under Load?

Post by alvarezh »

Congrats Cygnus, thanks for the follow up.

I had a weber PT on my Champ short by leaving the amp on overnight. The insulation on the leads are really bad quality. They are built in China, will never buy another one.

Bought one of their head cabs, not well built either, crappy woodwork and tolex unglues, and the amp has never left my house! I like some of their speakers though.

Cheers!
Horacio

Play in tune and B#!
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Colossal
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Re: PT Failing under Load?

Post by Colossal »

alvarezh wrote: weber PT...They are built in China
There's the root cause of the problem.

Glad you got it fixed Cygnus.
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M Fowler
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Re: PT Failing under Load?

Post by M Fowler »

Colossal wrote:
alvarezh wrote: weber PT...They are built in China
There's the root cause of the problem.

Glad you got it fixed Cygnus.
I would have agreed until I built my Fender Bassman 5b6 with Weber transformers and it sounds so good. If they blow, I'll post about it. :lol:

Mark
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alvarezh
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Re: PT Failing under Load?

Post by alvarezh »

Mark, you are way much more experienced than me in this trade. You well know that there is much more involved in a "good" sound from a rig than just the transformers. BTW mine didn't sound "bad" either, it just didn't last.

Glad you like yours! :wink:

All the best.
Horacio

Play in tune and B#!
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