Wow, that's one noisy transformer!

[Randall]

Just replaced the PT in a 66 Princeton Reverb with this one: https://www.cedist.com/products/tran...-0-325-v-70-ma

As soon as I powered it up the transformer made quite a racket, it startled me into thinking it was shorting out, but it wasn't. It was just making a loud buzzing that coupled with my bench top. I unplugged the amp and went about tightening the screws, three of them took almost a full turn. It quieted it down some, but it still makes more mechanical noise than I would like. I also find it quiets down even more if I suspend the chassis in the air by holding it off the bench by the sides. I have had noisy transformers before, but this one takes the cake. Surprised.

I don't know who makes these, but they are represented as exact Fender replacement for the 125P1B on the schematic and the 022772 stamped on the original. Anyone else coming across this?

[bob p]

that iron is made in Chicago by Marvel. it's a quality piece. I've used that exact model with no problems. nice and quiet.

the only time I've had really excessive PSU noise there was a problem with the amp dumping current and the transformer was growling because it was angry about that. i once had a Crown DC300-type amp that snarled so badly that it would walk across the bench at power-on. it turned out that someone else had driven a chassis screw into one of the big psu caps. fixing that problem made everything behave normally. you might want to check current and compare the transformers regulation specs to your in-circuit measurements. sometimes you can get hum when a tranny is underloaded or overloaded.

[stereomonostereo]

Is the noise all mechanical? I've run into that with some older transformers, but never a new one. I'd think it has a potting issue

[nsubulysses]

If you have the amp resting by sitting the transformers on the bench, the bench could be resonating and making the transformer seem louder. I've had that issue before where I could hear a noticeble buzz when turned on but it was only because the transformers were coupling the noise into the bench or the 4x12 the amp was sitting upside down on

Just a thought. Some transformers are just loud but perhaps you are getting tricked

[Randall]

It is not so bad now that it is installed in the cab, some of it was the recto tube I now see. It is making some chattering sound from inside the glass. I swapped it out and the new one doesn't do it, so I guess a replacement is in his future if it bugs him.

[nsubulysses]

Certain transformers are like that I guess maybe it's just crappy potting during production? Fender Hot Rod series comes to mind as typically buzzy transformers.Not buzzy through the speaker but buzzy to the ear when you can actually can hear the transformer

[Randall]

Well, this one is on me. I failed to check the 6V6's, realized my bad and measured them. Holy cow Batman, one was pulling over 50mA's! I guess we know why that PT needed replacing. Put in a fresh set of JJ's and noticed someone had added a 100K ohm from the bias cap neg to ground, so I was only getting -22v on pins 5. The fresh tubes were running at about 30mA @ 440v until I pulled that resistor. Now they are at 24 and 26mA. I know this guy is not going to want to pull that mod, because he told me specifically not to do anything to "change the sound" (New PT and all electrolytics). So I may have to send it out knowing it is way too hot.

At least the transformer and rectifier quieted down!

[The Dude]

I would have a conversation with the customer and explain to him that his amp is going to eat tubes in it's current configuration. He can make the decision as to what should be done, but it wouldn't hurt to educate him so that he can make that decision knowing the facts.

[Randall]

I just did that, and to my surprise he let me take out the mod. Funny thing is, They were probably the original RCA black plates, and he said he has played it that way on and off for the last 10 years! Way to torture a pair of choice tubes.

[bob p]

so what are the take home points form this experience?

1. always measure load when replacing a power transformer. transformers don't die for no good reason. always look for a problem in the circuit that caused the failure to occur, otherwise you're just hanging parts.

2. always check bias.

3. never trust anyone's home-cooked modifications. i hate to say it, but in this era where everyone watches youtube to learn about modding amps, you have to consider any modded amp to have been hacked by a novice. when i see mods the first thing that comes to mind is that somebody has buggered up the amp. Mods are hacks until proven otherwise. that goes double for bias circuit mods.

[nosaj]

bob p Could you elaborate on this statement maybe spell it out for me
What are you measuring voltage , resistance, etc I'm not clear.

1. always measure load when replacing a power transformer

Thanks so much,
nosaj

[bob p]

Measuring load is easy -- just measure the amount of current that your circuit is pulling across the transformer primary and secondary windings. You can do that with AC using a clamp-on meter or with DC by measuring voltage drop across the resistor on your input filter. The important thing is to make sure that your load is causing the transformer to operate within it's regulation limits, optimally at a center point of it's design specs.

Here's a very brief article that helps to clarify what I mean by transformer regulation. It's written for low voltage transformers but the principle is the same. Hope it helps.

The Basics of Transformer Voltage Regulation | Electrical Construction & Maintenance (EC&M) Magazine

this one uses a little math:

https://www.electrical4u.com/voltage...f-transformer/

[bob p]

and if you're a glutton for punishment:

[nevetslab]

I assume the P/T's internal Thermal Switch opened, leaving you with an open primary. I wish Fender would change to an automatic resetting thermal switch. That always pains me having to replace a transformer that would normally have survived the thermal overload. I haven't stopped to measure the temp rise on some of their power transformers, but, I do find a number of Fenders......Deluxe Reverb, Twin Reverb, Hot Rod Devilles, etc....letting them just idle for hours (Not S/B), after four or more hours, the power transformer is too hot to leave your hand on it.

The last one I had to replace (Hot Rod DeVille), I still have it sitting aside to open up and see if I can dig in a ways to see if the fusible link is anywhere accessible. I always used resettable thermal switches in the transformer designs I had spec'd out/approved over the years in manufacturing.

[Mick Bailey]

so what are the take home points form this experience?

1. always measure load when replacing a power transformer. transformers don't die for no good reason. always look for a problem in the circuit that caused the failure to occur, otherwise you're just hanging parts.

Another good reason is crap transformers that fail even though there's nothing wrong with the amp. Laney VC30, Chinese Vox AC4, AC30. Marshall AS50. All regular failures. And then there's the numerous Ashdowns where the toriodal PT cooks on the inner wraps. First it scorches, then it shorts.