Fender Bassman AB165 new power transformer buzz

[Duncan]

Hi there,

A little while ago, I received a Bassman AB165 that had been modified in the power section (different power transformer, some super sketchy wiring).

I pulled the old transformer out and rewired everything that was messed with, and I just dropped a new power transformer into it (Hammond 290EX), double checked, all of my wiring, and the rest of the circuit, and I can’t find anything that is wrong based on Fender’s layout diagram.

However, when I power up the amp, I am getting a very loud buzzing from the power transformer. Like, much louder than any other transformer I’ve heard before.

Any idea what this could be from? I’ve confirmed that none of the primaries are grounding out. Is this just a normal sound from this model of transformer?

Here’s a video:

eature=share

[Latole]

 " Is this just a normal sound from this model of transformer ? "
-Duncan

No

You may have mistake in wiring,
Folow Fender layout is not reliable, you need to follow a schematic.

Show us a schematic of your wiring as is

https://www.hammfg.com/part/290EX

[neskor]

I also had new buzzing Hammond 290EX power transformer.
3xx series are much better made

[Latole]

I also had new buzzing Hammond 290EX power transformer.
3xx series are much better made

I'd be surpri
sed if Hammond agreed to market noisy transformers.

[Esquirefreak]

+1 on a noisy 290EX, although a 290EEX (export model). There was a significant amount of mechanical buzzing that caused the chassis to vibrate.

It is the first (and so far, only) Hammond TR i ever got that had this problem. Tried it in 3 amps that were dead quiet with a different transformer (290DEX).

/Max

[sluckey]

There should be 8 nuts on that PT. Four to hold the PT together and four more to attach to the chassis. Remove the chassis nuts so you can lift the PT up enough to tighten the four nuts holding the PT together. Then bolt back to the chassis. Any better?

[dwinstonwood]

That sounds like metal vibrating to me, not electrical hum. I would try sluckey's recommendation first.

[Duncan]

There should be 8 nuts on that PT. Four to hold the PT together and four more to attach to the chassis. Remove the chassis nuts so you can lift the PT up enough to tighten the four nuts holding the PT together. Then bolt back to the chassis. Any better?

I’ll give that a spin and report back.

[Latole]

That sounds like metal vibrating to me, not electrical hum. I would try sluckey's recommendation first.

I agree.

[Duncan]

Removed the transformer, torqued the bolts. Hum is greatly reduced. Thank you for the suggestion, everyone.

Though when I took it off standby (with my lightbulb limiter), the bulb light up like a roman candle, so there's definitely something else wrong. I have a feeling the rectifier diodes need replacing. I was hoping I wouldn't have to pull more of the amp apart, but it looks like I'm going to have to go through it component by component, lead by lead.

Thanks again, guys.

[Latole]

"... the bulb light up like a roman candle, "

Low heater voltage at the light bulb ; solder ?
Nothing to do with rectifiers

[Duncan]

"... the bulb light up like a roman candle, "

Low heater voltage at the light bulb ; solder ?
Nothing to do with rectifiers

I’ll check the heater voltage after work. I used my multimeter to test the diode stack, and I was getting open lead for all of them. Admittedly, I was rushing out the door this morning, so I’ll try it again tonight.

[sluckey]

"... the bulb light up like a roman candle, "

Low heater voltage at the light bulb ; solder ?
Nothing to do with rectifiers

I’ll check the heater voltage after work. I used my multimeter to test the diode stack, and I was getting open lead for all of them. Admittedly, I was rushing out the door this morning, so I’ll try it again tonight.

You said the light bulb indicated a problem only when you took the amp out of standby mode. That eliminates the heater circuit as a suspect.

While in standby mode verify you have about -50V on pin 5 of both output tube sockets. Pull the output tubes to see if they are a factor. Keep chasing the B+ line.

 As for checking the diodes, this is a two part check. Connect the probes across the diode. Note the meter reading. Now swap the probes and you should get a different reading.  One reading will indicate OL and the other reading will indicate a lower resistance. If your meter has a diode check function it will read something like .5 one way and OL the other way. These two readings are for a good diode. Of course, if you read low or open both ways, the diode is bad.

Filter cap short is another possibility.

[Latole]

"  the bulb light up like a roman candle, "

I now understand you talk about the bulb limiter
If the bulb glow very bright , you may have a short, ( answer #1 )  that is what you call a roman candle 
Never hear that about a bright bulb.

I learn alway here
 

[TexasTone]

I just installed a brand-new 290EX and it sounds exactly like the one in the OP video.

I checked the nuts for tightness before installation and alternated tightening the mounting nuts until properly snugged.  All of the voltages are within spec.  I can pull the tranny and then recheck, but I checked before tightening. 

Duncan said that he "Removed the transformer, torqued the bolts. Hum is greatly reduced"  I wonder if it was eliminated.