Ampeg PF-50T Humbuster

[Wasnex]

I wonder if it might have something to do with the "humbuster" mode that I applied to the amplifier because of the very loud noise it produced when the speaker was turned on?

The humbuster mod only increases filtering on the bias supply.

Top image original schematic. Bottom image Bertr mod with C1 and RT added

I can't imagine how these added components would possibly have any significant impact on the frequency response of the output section, other than reducing power supply ripple. If anything (AFAIK) the added resistance of RT (3.3K) would reduce any possible reactance.

One possibility you could consider. There has been discussion on whether the output impedance should be set to 4 ohms or 8 ohms while running the dummy load.

I am about 99% sure the dummy load is 8 ohms. So if you had the amp set to 4 ohms, try again with the impedance set to 8 ohms. Ref: Ampeg PF50T question


Another possibility is your dummy load is open or not connecting.

 

[_Obra_]

The humbuster mod only increases filtering on the bias supply.

Top image original schematic. Bottom image Bertr mod with C1 and RT added
View attachment 5168486

I can't imagine how these added components would possibly have any significant impact on the frequency response of the output section, other than reducing power supply ripple. If anything (AFAIK) the added resistance of RT (3.3K) would reduce and possible reactance.

One possibility you could consider. There has been discussion on whether the output impedance should be set to 4 ohms or 8 ohms while running the dummy load.

I am about 99% sure the dummy load is 8 ohms. So if you had the amp set to 4 ohms, try again with the impedance set to 8 ohms. Ref: Ampeg PF50T question


Another possibility is your dummy load is open or not connecting.

Thanks for Your answer. I set impedance to 4 ohms like my cab is. I will try to change it to 8 for sure .

 

[Wasnex]

Thanks for Your answer, I will try that for sure

Please let us know if it helps.

 

[_Obra_]

Please let us know if it helps.

Just come back from the studio after few hours of playing with outputs. Here are the results:

Yesterday when I was recording both mic on cabinet and Transformer line out at the same time, impedance was on 4 ohms like my cab is. Result was unusable Transformer line out channel with volume goes up and down in the scope of 10db, and there was some phase problems.

First thing to do today was to plug the line channel into Preamp out instead of transformer line out and keep the mic on speaker on, recording both channels again. The result was much better considering line channel. There was no drops or jumps in volume (only some phase problem which I will refer to later).

Test three: Speaker unpluged (no mic) only one channel at the time - first the transformed line out and then preamp out - both on 8 ohms (low volume, at 9 oclock and gain on 9 oclock). Result was fine, no jumps or drops in volume on both outs - transformer or preamp. On 4 ohms - the same result. No problem, and both outputs sound pretty much the same regarding output volume.

Test four: Also without the cab, just two output channels one at the time, first transformer then preamp out, but the volume and gain was set much higher- at 12 oclock both. Result is again no drops or jumps in volume, but preamp out was much louder then the transformer line out (maybe this has something to do with humbuster mode?). Also, transformer line out sounded much more compresed and ''produced'' then the preamp out.

I do not have an 8 ohms speaker, so couldn't tray micked speaker + transformer line out in 8 ohms mode.

In any case, these are my conclusions:
- (as You said) Impedance of the speaker must match the impedance set on the amp for transformer line out to work properly (at least it worked on 4 ohms).
- As soon as you unplugged the cabinet, both preamp out and transformer line out works without any problems, on 4 or 8 ohms.
- Even without cabinet plugged in, on higher volume, transformer line out sounds quieter, more compressed and more ''organic'' then the preamp out, although preamp out is not bad at all.
- When the cabinet is pluged in and recorded together with any one of the line outs, there is a little phase mismatch problem, which can be solved by moving a mic channel for 6.6ms in my case (depends on the position of the microphone). After that, this combination of two channels sounds the best.

I hope this helps anyone who has the same problem as me.

 

[Wasnex]

preamp out was much louder then the transformer line out (maybe this has something to do with humbuster mode?).

I believe this is normal and not related to the humbuster mod.

 

[casual_platypus]

Yesterday when I was recording both mic on cabinet and Transformer line out at the same time, impedance was on 4 ohms like my cab is. Result was unusable Transformer line out channel with volume goes up and down in the scope of 10db, and there was some phase problems.

- (as You said) Impedance of the speaker must match the impedance set on the amp for transformer line out to work properly (at least it worked on 4 ohms).

I'm very confused by this post - you claim "result was unusable transformer line out channel" and then conclude on the second day that the very same setup works with no issue.

On the first day, how were you and the engineer monitoring? Were you still summing both the mic'd signal and the transformer line out when you were having the volume issues? Or were you soloing just the transformer line out? If you soloed it, then there's a piece of the puzzle missing here: you tested that same setup on the second day and had no issues (4ohm cab, 4ohm amp, monitoring transformer signal only, no volume drops). However, if on the first day you weren't soloing the transformer line out signal and were still mixing both the out-of-phase mic'd signal with the transformer line out, then it sounds like the only real 'issue' was with your out-of-phase setup and mix, not the transformer line out.

Just trying to seek some clarity for strangers (like myself) who roll into this thread from a google search.

 

[_Obra_]

I'm very confused by this post - you claim "result was unusable transformer line out channel" and then conclude on the second day that the very same setup works with no issue.

On the first day, how were you and the engineer monitoring? Were you still summing both the mic'd signal and the transformer line out when you were having the volume issues? Or were you soloing just the transformer line out? If you soloed it, then there's a piece of the puzzle missing here: you tested that same setup on the second day and had no issues (4ohm cab, 4ohm amp, monitoring transformer signal only, no volume drops). However, if on the first day you weren't soloing the transformer line out signal and were still mixing both the out-of-phase mic'd signal with the transformer line out, then it sounds like the only real 'issue' was with your out-of-phase setup and mix, not the transformer line out.

Just trying to seek some clarity for strangers (like myself) who roll into this thread from a google search.

Hi
First day we recorded Transformer line put + Cabinet, and the impedance was set on 4ohms = volume problems on recorded transformer out channel.
At the secon day, we recorded preamp out + Cabinet aslo at 4ohms and there was no problems. You are right that at my last ''conclusion'' I did not clearly indicate that. What I meant toosay is that there is a phase difference when recording the speaker together with either output, but with preamp out sound is OK, and with transformer out sound is not ok. Transformer out goes crazy as soon as You plug in the cabinet jack.

 

[agedhorse]

Of course there will be a phase difference and maybe a polarity difference when comparing the 3 parallel sources. This is not surprising.

What do you mean by "crazy"? Perhaps there is an issue with a ground loop or missing ground reference that is causing an instability problem (just a speculation).

A good (knowledgeable, experienced) recording guy should know this.

 

[Wasnex]

Hi
First day we recorded Transformer line put + Cabinet, and the impedance was set on 4ohms = volume problems on recorded transformer out channel.
At the secon day, we recorded preamp out + Cabinet aslo at 4ohms and there was no problems. You are right that at my last ''conclusion'' I did not clearly indicate that. What I meant toosay is that there is a phase difference when recording the speaker together with either output, but with preamp out sound is OK, and with transformer out sound is not ok. Transformer out goes crazy as soon as You plug in the cabinet jack.

My understanding is there will be an interaction between the reactance of the transformer and the reactance of the cab. The impedance curve of a speaker swings quite a bit...and I would expect this to impact the Transformer Balanced Out.

With a sealed cab you get one big peak. This is the Eminence B810 in a large sealed box.

I believe the peak corresponds to driver resonance in the box.

This graph shows the peak at 52hz, which AFAIK is the driver resonance in free air.

With a ported design you get two peaks. AFAIK one is the driver resonance in the cab, and the other is below the port tuning frequency (Fb).

B810 is large ported box.

Fb is 55.73hz, which corresponds to the dip in the impedance curve.

B810 in small ported box with Fb = 55hz.

The look pretty similar but look closer. Notice the driver resonance has been shifted higher and the dip at Fb is not quite as deep as the previous design. The peak below Fb has also been shifted a bit.

As you see, the tuning of the box effects the shape of the impedance curve...and the shape of the impedance curve most likely effects the Transformer Balanced Out.

 

[Engle]

This is interesting to me. I multitrack rehearsals using the transformer line out with a 210AV cab (usually) connected.

I do get some pretty strong peaks overdriving the audio interface input, but I was assuming it was technique and/or the bass. It's maybe worth doing some experimenting.

 

[agedhorse]

This is interesting to me. I multitrack rehearsals using the transformer line out with a 210AV cab (usually) connected.

I do get some pretty strong peaks overdriving the audio interface input, but I was assuming it was technique and/or the bass. It's maybe worth doing some experimenting.

It may be, what's going on at the input metering of the interface? You could also be over-driving something in the amp?

 

[Engle]

It may be, what's going on at the input metering of the interface? You could also be over-driving something in the amp?

I should pay more attention, but I tend to set it up and then just focus on playing as it's challenging material.
My Ric copy has very hot pickups, so I have assumed it is too much gain when I dig in.
Maybe I'll try the other line out.

 

[_Obra_]

I'll put some new data after going to the studio again. Probably record how the input meter behave when on transformers output with and without the cabinet conected.

 

[_Obra_]

Of course there will be a phase difference and maybe a polarity difference when comparing the 3 parallel sources. This is not surprising.

What do you mean by "crazy"? Perhaps there is an issue with a ground loop or missing ground reference that is causing an instability problem (just a speculation).

A good (knowledgeable, experienced) recording guy should know this.

By ''crazy'' i think signal is jumping to much up and down (+ - 10db) while playing, and I can relate it to the notes I play, not the dynamic. In the preamp out it's flat and ''notmal''. It's OK also at the transformers output but without the cab connected.
Also, there was no ground loop problems (confirmed by knowledgeable, experienced recording guy )

 

[Hmmmmm]

I received a new PF50-T today. After reading through all of the threads, I was pretty confident that the hum problems would have been sorted out by now. Unfortunately, the hum is still present on the new models, and in addition, on my new amp there is a loud pop that occurs every 8-10 seconds.

The serial number indicates it was made in 2023, but there doesn't appear to be any resistor or capacitor modifications that I can see.

Needless to say, I'm a bit disappointed and will be sending it back.

View attachment 5143301 View attachment 5143302 View attachment 5143303

I also just purchased a PF50-T. It had the hum. Bummer. The vendor quickly sent a replacement. The replacement arrived damaged, like it had been dropped by the factory prior to packaging (the ampeg box and the shipper box were in perfect condition). Double bummer. The vendor quickly sent another replacement, only this time they agreed to have it be checked prior to shipping. "functions according to factory specs.." according to them. This one hums just as bad as the 1st one.

I wish I had stumbled on this thread prior to buying. I just want a smallish bass rig for recording. Thought about going vintage but figured a modern take would be nice, reliable, and quiet. It really does sound good. I want to keep it but I feel like I'm getting hosed. I'm torn between sending everything back for a refund vs just keeping the amp and doing the mod myself. I'd have no problem doing the mod but I'd hate to void the warrantee on a brand new amp. Any advice?

 

[agedhorse]

I also just purchased a PF50-T. It had the hum. Bummer. The vendor quickly sent a replacement. The replacement arrived damaged, like it had been dropped by the factory prior to packaging (the ampeg box and the shipper box were in perfect condition). Double bummer. The vendor quickly sent another replacement, only this time they agreed to have it be checked prior to shipping. "functions according to factory specs.." according to them. This one hums just as bad as the 1st one.

I wish I had stumbled on this thread prior to buying. I just want a smallish bass rig for recording. Thought about going vintage but figured a modern take would be nice, reliable, and quiet. It really does sound good. I want to keep it but I feel like I'm getting hosed. I'm torn between sending everything back for a refund vs just keeping the amp and doing the mod myself. I'd have no problem doing the mod but I'd hate to void the warrantee on a brand new amp. Any advice?

Rather than void the warranty, contact Ampeg and ask them how to proceed with getting it fixed.

 

[J-Bass RMR]

I took delivery of my PF-50T in May.
It had a low level ~60 cycle hum when powering up. Nothing noticeable when playing.
I thought I could live with it, but, over time, it started to bug me, and my other tube amps are all quiet (B-15, V4-B, Traynor YBA-100).

I called Ampeg last month.
They confirmed the new ones do not have the mod, but it would be covered by an authorized repair center.

Unfortunately, they also clarified the Authorized Ampeg re-seller closest to me (Full Compass) is not an authorized repair center.
I likely will just take it there anyway and pay to have it done, considering by the time I drive or ship it, it won't make much difference, and I know the guys there and can talk to the tech.
The Ampeg rep I spoke to indicated this wouldn't be a problem in the event there was another warranty issue down the road, since the repair is pretty basic for a qualified tech.

 

[Hoochie Coochie Man]

Rather than void the warranty, contact Ampeg and ask them how to proceed with getting it fixed.

It would be interesting to see if Yamaha/Ampeg can approach the level of customer support that Mesa provides.

I am not bashing Ampeg, (I do own more than twenty Ampeg rigs).

 

[Hmmmmm]

Thanks for the quick replies. My vendor also happens to be an authorized ampeg repair center. When I talked to their service department (after amp#1) they said they didn't see any reports of hum issues with the PF-50T (he was looking things up on a computer). I then pointed him to this thread and the main humbuster webpage, which he acknowledged.

After I received the damaged amp (amp#2) I talked with another support person. I explained what I went through with the 1st 2 amps and she was very helpful to offer that their service department verify no hum or physical damage prior to shipment. I thought this might have been enough for my amp to get the royal treatment by their authorized ampeg repair center if in fact it did not have the mod, but alas it was not. I will contact ampeg directly and let everyone know how it goes.

***I say "vendor" so as to not sully the name of a retailer that I have had nothing but good experience with in the past***

 

[onestone]

It would be interesting to see if Yamaha/Ampeg can approach the level of customer support that Mesa provides.

I am not bashing Ampeg, (I do own more than twenty Ampeg rigs).

My humming 50-T was fixed at no cost and no questions asked. It was repaired by the company that imports Ampeg to my country and i assume they're also an authorized repair center.