Ampeg PF-50t microphonic transformer/power tubes

[casual_platypus]

Hi all!

Just recently picked up a PF-50t from Zzounds that was labeled 'scratch and dent'. It was roughly 30% off, and they didn't provide any information about the actual condition, no photos or comments on the supposed scratches and dents. I decided to take the 'risk' anyway, but they have a seemingly good return policy.

It arrived yesterday with no original box and a grand total of 1 (one) faint scratch on the back panel. Zero other visible physical damage. Ended up playing with it for several hours, trying the DIs and micing it up, sounded great.

This afternoon I noticed something unfortunate: there is a certain buzzing that happens when playing specific notes; it is a sort of microphonic metallic ringing. It is not present on the preamp DI; it is however present on the output transformer DI, but only when a speaker is connected (head resting on cab, resonance).

Knocking on the amplifier at various places and tube cage yields a somewhat similar noise, but knocking on the output transformer yields the exact same ringing I mentioned earlier in full strength. Taking off the cage, none of the preamp tubes are even remotely microphonic, but both power tubes are slightly microphonic, one more than the other. The noise they produce when knocked on is slightly different to the resonance/buzzing I'm hearing through the DI and the cab. Increasing or decreasing preamp gain and master volume has no effect on the ringing heard when knocking on the transformer or on the power tubes; the strength of the buzz/ringing is only related to the strength of the tapping/resonance they experience. The output transformer is very securely seated; I could not find external loose screws or items; I have not yet tried reseating the power tubes.

To me, it seems like both the output transformer and the power tubes are all slightly microphonic. The ringing is noticeable at medium volumes when playing through a cab, but it is much more noticeable at the transformer DI (no loss of high end vs the cab, buzzing is a lot clearer).

Have others had similar experiences? Curious as to what may be the smartest path forwards or where to start trouble shooting. It seems worthwhile to just get another set of 6L6GCs to see if that fixes it, but given that this was a returned item and any prior damage was not documented, there could be other issues in the transformer that are not a quick fix. Returning and trying again could also be a good option. It does still have a full factory warranty which is yet another route.

tl;dr slightly microphonic output transformer & power tubes, curious if others have had similar issues

 

[yonkrz]

I bought one 4-5 years ago basically the same way you did, as a demo unit. It was microphonic right out of the box, I contacted the vendor and they sent me a new tube,which if I remember was a preamp 12ax7 and it took care of the problem. I have since sold it due to the (hum) not being busted, PUN intended. But it was very noisy with that bad tube, and the first time I ever encountered such a problem.

 

[casual_platypus]

Believe it or not, the hum on this unit was, in my opinion, very acceptable. It was completely unnoticeable when playing at volume. Some hum is definitely present, but it is fairly quiet. I have yet to open it up, but doing the mod was definitely on the docket assuming all else was in order.

Since selling yours, what amp did you end up with and why? So far I love the sound and feature set of this amp, but I'm definitely open to recommendations for similar amps. Just looking for some power tube breakup in my sound without breaking the bank or my windows.
Edit: or my back. 20 lbs is nice, and a total of 65 with the 115 cab is very manageable.

 

[Wasnex]

Any chance you can try a different pair up of 6L6GCs. The idea is to try and confirm where the problem is coming from. Not even certain if it's possible for the transformer to be the culprit. But I am certain it could be tubes.

If it actually is the transformer, I would send it back. If it's the tubes, then you have a decision to make. Try with the amp not sitting on the speaker.


The hum buster mod increases filtering on the bias supply. The bias supply is only sent to the output tubes. Normally when the tubes are matched, hum cancels in the output transformer. This is because in a push-pull output section the tubes are connected to opposite ends of the output transformer, which puts the signals out of phase.

The red and blue signals are exactly the same size and 180 degree out of phase, so they cancel out.

If the tubes are not well matched there will be more hum. The idea is one tube will produce a bigger signal than the other tube, so cancellation is not perfect.

 

[ahbass2500]

Could also be the preamp tube protectors. They are known to rattle. Easiest fix is removing them (push down and twist to remove if I remember correctly). I had this issue and others have also reported it. https://www.talkbass.com/threads/pf50t-rattling-noise.1486278/

Edit: this may not come through the DI so may not be your issue. More posting it here for reference on anyone chasing down PF50t noises.

 

[jthisdell]

I bought a PF50 a few years ago, was very disappointed when it came with a very microphonic tube. Was very disappointed when neither ZZounds or Ampeg would send me a replacement tube. They said it was not a user replaceable part. I replied that the manual explains how to replace them but no dice. I ended up returning it to ZZounds and still regret that decision, I should have just bought a tube but I was kinda pissed at the lack of support.

 

[jdjk7]

I bought a PF50T + 115HE from the Bass Club in Chicago in 2019. The only problem it had was that the spade connector to the speaker was loose and led to intermittent popping. But, several years in, I did indeed end up having to replace the power tubes due to microphonics.

 

[seamonkey]

Does it happen with the head in Stand-by?

 

[yonkrz]

Believe it or not, the hum on this unit was, in my opinion, very acceptable. It was completely unnoticeable when playing at volume. Some hum is definitely present, but it is fairly quiet. I have yet to open it up, but doing the mod was definitely on the docket assuming all else was in order.

Since selling yours, what amp did you end up with and why? So far I love the sound and feature set of this amp, but I'm definitely open to recommendations for similar amps. Just looking for some power tube breakup in my sound without breaking the bank or my windows.
Edit: or my back. 20 lbs is nice, and a total of 65 with the 115 cab is very manageable.

I bought a V4b and a couple of other class d heads, at the time they were sort of just talking about the hum buster mod.If it was today I probably would have kept it and had the mod done.

 

[Wasnex]

If the transformer housing is ringy, check to make sure the bolts mounting it to the chassis are snug.

If the windings are somehow ringy, I would expect it to hum and my guess is the windings would not last a lifetime: Vibrations in wire lead to metal fatigue and eventually the wires will break.

I believe tubes can ring from the glass envelope or the internal grid structure may ring.

There are silicone rings you can put on the tubes to dampen the glass.

 

[micguy]

It is possible for the tubes in the power section (which usually includes a preamp tube acting as the phase inverter as well) to be microphonic - microphonic meaning that, when vibrated, they pass a signal from that vibration to later stages in the chain.

I can believe that an output transformer would vibrate from input signals - if you're hearing it from the transformer itself. Lots of power transformers buzz or hum from the power going through them, and an output transformer is a very close cousin to a power transformer, especially with an instrument that can make signal above and below 60 Hz running through it.

However, an output transformer being microphonic in the usual sense of the word - meaning tapping on it, and then hearing a signal from it later in the chain (in this case from the loudspeaker) - that would be a new one for me, I doubt that highly. I suspect when you tap on the transformer housing, what you're hearing is the vibration going through the chassis, and shaking the microphone tube.

 

[JeezyMcNuggles]

Believe it or not, the hum on this unit was, in my opinion, very acceptable. It was completely unnoticeable when playing at volume. Some hum is definitely present, but it is fairly quiet. I have yet to open it up, but doing the mod was definitely on the docket assuming all else was in order.

Since selling yours, what amp did you end up with and why? So far I love the sound and feature set of this amp, but I'm definitely open to recommendations for similar amps. Just looking for some power tube breakup in my sound without breaking the bank or my windows.
Edit: or my back. 20 lbs is nice, and a total of 65 with the 115 cab is very manageable.

V4b bro. Get one, and never look back

 

[el murdoque]

Is the output transformer the thing on the (viewed from the front) right of the tube cage?

 

[el murdoque]

My PF 50 T did hum a bit from that thing, and IIRC, this part seemed to cause problems, too - I contacted the seller since it was brand new and they sent me new preamp tubes. The one in the third position was the culprit. It caused the microphonic behavior and swapping that also eliminated the hum and noise that came from the thing that sticks out to the right of the tube cage. It would still hum, but not audible on the DI signal.
I sold that amp because I was hoping for a better quality of the transformer coupled DI out.

 

[Rabidhamster]

The output transformer in every tube amp will make audible noise while the amp is cranked into a load, you just can’t hear it over the speaker in normal playing situations. (Sorry to those of you who never noticed before) But since you’re hearing similar noise in the signal path, it sounds like there is potentially something defective. If it’s microphonic tubes you can tap them with a chopstick and get them to act up, anything else and it would be a return for me on that one.

 

[GreyBeard]

Back in the day I had a blackface bassman doing the same thing. Cold solder joint on a cap.

 

[casual_platypus]

Update: did a lot more testing and found that, as you all suggested, it is in fact likely just the tubes that are microphonic. One of the power tubes seems to be the largest culprit, and it seems V2 and V3 preamp tubes are also slightly microphonic. My current guess with knocking on the transformer is as @micguy suggested: those vibrations went through the chassis to vibrate the tube(s), and the transformer is likely just fine.

Unmounting the head from the PF-115he cab and setting it next to the cab made any and all microphonics while playing unnoticeable. What's more, the specific ringing I was hearing on certain notes was a bad resonance in the cab which then got further amplified by the microphonic tubes. Time to trouble shoot that. No loose parts that I can see in the cab, so that might just be an outright return. In every other way, the 115 sounds great.

One thing I have noticed through all this testing is how different the transformer DI sounds when a cab is or isn't attached. With a cab attached, the recorded DI signal sounds fantastic: it's full and driven. Unplugging the speaker, the transformer DI loses a lot of low end and just seems anemic. I'm sure you could EQ it to sound better, but I would rather go straight in than use the DI without a cab. This is true both with the head mounted on the cab and with it resting on the side, so this is not another microphonic issue. I don't think I would ever use that DI to record without a cab hooked up. It's enough to make me seriously consider getting a dedicated load box, as the sound of using it without a speaker load just doesn't come close to the DI sound with a speaker load.

As of now, I already need a new set of tubes and I have yet to figure out the bad resonance in the cab when playing a middle D. The transformer DI was one of the most attractive things about this amp to me, and if I need a load box to get a useable sound out of the transformer DI, it almost makes more sense to just get a V-4b and be done with it; I'd need a load box for that head anyway and would always have a post-output transformer DI using the line-out on the load box. I love the majority of sounds out of this amp/cab, but the issues I'm having are leading me to think about a full return on both.

Thanks for all your thoughts thus far.

 

[Wasnex]

It's enough to make me seriously consider getting a dedicated load box, as the sound of using it without a speaker load just doesn't come close to the DI sound with a speaker load.

Not sure a load box will achieve the desired results. A speaker provides a reactive load. Many if not most load boxes are going to be primarily resistive. The amp already has built in load resistor.

Always best to try before you buy, however impractical that may be.

 

[casual_platypus]

Does it happen with the head in Stand-by?

No; no sound comes through the speakers on standby; I could be wrong but aside from the heaters, I don't think anything is actually 'on' when in standby.

Is the output transformer the thing on the (viewed from the front) right of the tube cage?

Output transformer is to the left, closest to the speaker jack. Power transformer is to the right, closest to the power input.

Could also be the preamp tube protectors. They are known to rattle. Easiest fix is removing them (push down and twist to remove if I remember correctly). I had this issue and others have also reported it. https://www.talkbass.com/threads/pf50t-rattling-noise.1486278/

Edit: this may not come through the DI so may not be your issue. More posting it here for reference on anyone chasing down PF50t noises.

These did rattle, but none of the rattle seemed to come through the DIs. Only real rattles/resonances seem to come from the cab, and the microphonic tube(s) picked that up.

I bought a PF50 a few years ago, was very disappointed when it came with a very microphonic tube. Was very disappointed when neither ZZounds or Ampeg would send me a replacement tube. They said it was not a user replaceable part. I replied that the manual explains how to replace them but no dice. I ended up returning it to ZZounds and still regret that decision, I should have just bought a tube but I was kinda pissed at the lack of support.

That's embarrassing on their part. They have two entire pages in the manual detailing how to change them.... Sad. That's a good heads up as to what to expect when dealing with them.

Not sure a load box will achieve the desired results. A speaker provides a reactive load. Many if not most load boxes are going to be primarily resistive. The amp already has built in load resistor.

Always best to try before you buy, however impractical that may be.

That is a good point; I wrongly assumed the load in the amp was reactive. The Two Notes Torpedo is rated for 100W RMS, so it seems there are some good solutions out there for higher(ish) power reactive loads.

 

[seamonkey]

No; no sound comes through the speakers on standby; I could be wrong but aside from the heaters, I don't think anything is actually 'on' when in standby.

I meant through the transformer DI. With the speaker plugged in, Put it in standby, and whack it while listening on the transformer DI