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Pf50t rattling noise?

jlt5x

jlt5x

May 8, 2007
548
154
chicagoland
#1
My Pf50t sounds awesome. Bias lights are green, and flash red when I play.

But when the head is bumped or shook, there is a rattling like noise that actually comes through the speaker. Not super loud, but it is there. Happens if I tap on the head or even when I play a low e.

Haven’t tried to reseat the tubes yet - just figured I would see if anyone else has experienced this.

tia
 
#6
jlt5x said:
My Pf50t sounds awesome. Bias lights are green, and flash red when I play.

But when the head is bumped or shook, there is a rattling like noise that actually comes through the speaker. Not super loud, but it is there. Happens if I tap on the head or even when I play a low e.

Haven’t tried to reseat the tubes yet - just figured I would see if anyone else has experienced this.

tia
Click to expand...

Sounds like tube rattle. You can tap lightly on the tubes with a chopstick to track down which tube is the culprit.

It's basically the guts of the tube rattling around inside the glass.

The amp has two 12AX7s. Usually V1 will be higher gain so it's more sensitive to picking up this sort of noise. If you tap V1 and get the rattle swap the two 12AX7s and hopefully it will mitigate the problem.

FYI: If you decide to buy new small 12AX7 or 12AU7 tubes, long plate tubes are more prone to rattle than short plate tubes.

Found this image online:
ampeg-pf-50t-2975723.jpg
 
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#11
jlt5x said:
View attachment 4025362 So took off the cage and tapped the preamp tubes. Nothing. Tapped the power tubes - both made the same noise as when I tap the amp.
Click to expand...

Does the amp sound good and hold bias? If yes I would leave it alone for now. But I would monitor the bias on a regular basis. You could also buy a new set of tubes, and either keep the rattles as spares or test subjects, or throw them away.

I bought a Marshall JMP 2203 head a few years back. It came with a set GE 6550s which are claimed to be the grail of 6550s. The tubes rattled when bumped, but the amp sounded fine. After I owned the amp for a couple of years, I decided to check the bias. I noted the tubes were setup really cold and the current balance between tubes was really bad. I had the chassis out of the case upside down on the bench. I set the bias and put the amp back in the head case.

I usually run my guitar amps on a variac with a built in ammeter. When I set the bias, I note the total current on the variac. The idea is to have a reference so I know the normal current range when the amp is running on 120V. As long as it sounds good and the current has not shifted much, I assume the tubes do not need attention. If the current shifts significantly, I open the amp to see what's going on.

When I put the amp back in the case and turned in on, I noticed the total current had changed more than 15%. Turns out this variation in current was due to the metal structures shifting around inside the tubes. If I rotated the amp backwards the current would change, and you could see the guts in the tubes moving around. Actually I sort of amazed they were still working.

Turns out only two of the 6550s were totally thrashed. The other two were stable and did not rattle. I wound up swapping in a pair of KT88s that were current matched to the two 6550s within a few milliamps. Some people say me it's a bad idea to mix output tube types and I won't argue the point, but the amp sounds fine and is stable. Eventually I will buy new set of 6550s.

FYI, running the tubes too hot can superheat the metal parts of the tube. I believe this can cause the internal structures to deform, resulting in rattle. That's not to say you ran your tubes too hot...just a warning to avoid running the tubes too hot.
 
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#13
My pf50t makes a rattle sound as well, but it was much worse with the original power tubes. The original 12AU7 was also microphonic, so I swapped an EHX 12AU7 I had sitting around along with new matched Ruby 6L6s. Unfortunately, several months in the new Rubies rattle a little bit too. :(

Next time around I'm going to try a different brand.
 
#16
Jack Butler said:
Looking at this picture, I noticed something that I had noted on my PF-50t, one of the power tubes seems to be glowing significantly brighter than the other. Wonder if that's normal?
Click to expand...

It depends upon what is producing the glow. The most obvious source of the glow is the heaters. The placement of the various components inside the tube is not all that precise, so its not unusual for different tubes to produce different amounts of heater glow. This is especially noticeable when comparing various brands and models of tubes since the structures may be drastically different. The heaters come on and are the only source of glow when an amp is in standby. When a tube is operating there are other potential sources off glow.

The plates of the tubes may also glow if you run the tubes too hot. If the glow is at different levels it could indicate different levels of electron flow through the tubes...but you should not run your tubes this hard.

There is also the glow cause from the electrons passing through the gases in the tubes. This can vary with the quality of the vacuum in the tube and also the current passing through the tube.

If the amp is noise free, sounds good and is not humming, I suggest leaving it alone unless it's running excessively hot or read plating.

Most tube bass amps have push pull output sections. The tubes are organized into two circuits on opposite sides of the primary winding of the output transformer. The two circuits of tubes need to pull approximately the same amount of current, or the amp will produce a dull low frequency roar. Essentially when the current is balanced in both sides of the output transformer the noise cancels out because the tubes are connected out of phase.

Say you have an amp with four output tubes. We will call them AA and BB. Ideally all four tubes pull about the same current, but this is not necessary for hum cancellation. The hum will also cancel when the total current dissipated by AA equals the total current dissipated by BB.

Since the PF50T only has two output tubes, it's only required for the current of A = B. However it's fairly common for one side of the transformer to run a bit higher current than the other. This is due to the way transformers are wound. Theoretically both sides should be perfectly symmetrical and balanced, but usually they are not. Tube matching is generally not perfect either, so a potential solution is to put your hottest tube on the cold side of the transformer so the current matches better.
 
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#19
So I took out the 2 power tubes, tightened up the clips and reseated the tubes. They now have a little less wiggle room. Although the rattle sound is still there when I tap the tubes, it’s “decay” is a bit shorter. Playing through speaker, the shorter decay makes it less noticable. And now it happens on different notes, which is interesting.

played the head into a mixer - no cab vibrations, no rattle sounds coming through

so, pretty confident the power tubes are the culprit. Ordered a couple more, but probably not a huge hurry to make install. When I do, I will update

thanks
 
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