I'm ok with a bit of breakup, not ok with two cabs. I'd like to switch from combo to head + 1 cab for flexibility but I can't afford to buy and carry two cabs at the moment.
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Ampeg PF-50T Humbuster
- Thread starter mnats
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- ampeg ampeg pf-50t pf-50t
There's the Ampeg PF-210 which is really good value, I went for 2 PF-112 cabs for scalability.
Yes, I find it strange that most 1x15 combos are below 200W. I suppose mose people prefer 2x10s nowadays.
11/4/23 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.
Update; Just got mine back. Finally took it to Full Compass.
Contrary to what I was told by Ampeg, Full Compass is an authorized Ampeg service center.
Picked the amp up tonight - no charge - no questions asked.
The service department there has treated me well over the years.
I highly recommend having the mod done if you haven't already.
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That's cool. It's a pretty easy mod.
Enjoy your quiet amp!
I just set mine up for re-amping. It's a lot of fun being able to adjust gain and tone while my digital self is playing.
I did a version of the humbuster mod last year. The hum was much improved, but not totally gone. I suspected the output tube matching was a bit too loose.
Last week I ordered output tubes for a couple of amps, including a pair of JJ 6L6GC for the PF50T. I installed bias probes and the old tubes measured 40mA and 45mA, with is an 11% difference.
The JJs balance at 40mA and the amount of hum is noticeably reduced to an almost imperceptible level. Hopefully the balance holds.
I guess I need to compare the PF50T to my 30W Ashdowns again. Previously the LB30s seemed to have slightly more clean headroom and the Little Stubby had about the same amount of headroom.
Last week I ordered output tubes for a couple of amps, including a pair of JJ 6L6GC for the PF50T. I installed bias probes and the old tubes measured 40mA and 45mA, with is an 11% difference.
The JJs balance at 40mA and the amount of hum is noticeably reduced to an almost imperceptible level. Hopefully the balance holds.
I guess I need to compare the PF50T to my 30W Ashdowns again. Previously the LB30s seemed to have slightly more clean headroom and the Little Stubby had about the same amount of headroom.
I just rolled a JJ 12AU7 in V1 in mine after I got the humbuster mod done, and I basically have clean headroom all the way to the top now. It sounds great!
Still happy with the matched 7581A tubes in my PF-50t, with the humbuster mod hum is barely perceptible even in a small quiet room.
Yes output tube matching is the critical factor. The original bias circuit was essentially identical to the Fender AB763 back in the pre-CBS era when each new amp had matched tubes. When CBS switched to cheap random B.S. tubes they implemented that "bias/balance" pot to adjust out the hum, figuring the increased distortion from the imbalance would be of no issue. If the output tubes are PERFECTLY identical, it doesn't matter how much ripple is on the bias supply, it is being applied common mode and cancels out. Unmatched tubes magnify the ripple on the bias and the ripple on the B+ as well.
Interesting.
I got a new open box PF50-T from Guitar Center online delivered yesterday and it sounds fantastic through my twin AV-210's but has the pronounced hum. Amazing that these are still going out the door like this after, what, 8 years???
Anyway I contacted the local Ampeg authorized service center about a warranty repair, but I'm also curious about ordering a matched pair of power tubes - is there any advantage to this versus the mod, or advantage/disadvantage of doing both? I'm a MechE so speak slowly...
I'm a retired MechE so I no longer have to shout over banks of running chiller compressors. Tube balance is #1. Increasing the bias filtering is still advisable as the original circuit is a copy of a circuit from an era when amps came from production with matched tubes and a little ripple didn't matter.
I haven't read through all the pages, but with your mod, if the control wiper fails, you lose all bias. With the original circuit, if the wiper fails, the bias increases protecting the output tubes from burning up.
A better method would be to make a voltage divider from fixed resistors and put the control in parallel with one of the fixed resistors.
A better method would be to make a voltage divider from fixed resistors and put the control in parallel with one of the fixed resistors.
IMHO it would be beneficial to do both. I believe Ampeg will do the humbuster mod free of charge. I don't believe they will replace the tubes unless they are actually bad.
Tubes are generally considered matched when the difference in bias current is within 15%. The tubes I took out were still matched to 11% (current was 40mA and 45mA). The resulting hum was audible, but not as bad as some of my other tube amps.
The tubes I just installed appear to be almost perfectly matched. However, as tubes age they drift, and they don't always drift in the same direction. It's up to you to decide when the hum is bothersome enough to spend $60 on tubes.
I probably would not have replaced the tubes in my PF50T if it was just for the residual hum. I replaced them partly out of curiosity, but mostly because one of the tubes is intermittently noisy. It makes a staticky sound randomly while it is warming. The noise is not just a quick crackle, it tends to go on for awhile.
Matched pair of JJ 6L6's installed, no perceivable difference. Stock tubes were also JJ's.
Update: took the amp to the (only) authorized service center in San Diego; they looked up the problem & found the Humbuster mod in their system. Great. However, they are 10-12 weeks backlogged. $50 to cut it in half, $100 to get it done in 7-10 days.
This is a real pain in the a$$ - I can do the work myself but would void the warranty on a brand new unit. Still can't believe Ampeg's churning these things out almost a decade after the problem was identified and a fix made known. I sucked it up & paid the $50 in hopes of getting the unit back in time for some late summer gigs. So now I'm in for ~$115 plus the $950 purchase price, and I don't have an amp for another month. Hopefully when I get it back it'll be perfect. Come on, Ampeg.
This is a real pain in the a$$ - I can do the work myself but would void the warranty on a brand new unit. Still can't believe Ampeg's churning these things out almost a decade after the problem was identified and a fix made known. I sucked it up & paid the $50 in hopes of getting the unit back in time for some late summer gigs. So now I'm in for ~$115 plus the $950 purchase price, and I don't have an amp for another month. Hopefully when I get it back it'll be perfect. Come on, Ampeg.
I own over twenty Ampeg rigs going back to 1960, (including a PF50-T) so please don't think that I am an Ampeg hater. I agree with junglebike when he says that Ampeg/Yamaha should not be foisting off new amplifiers with a known design flaw on customers. I also own Mesa gear, and I bet that if there was a similar situation with a Mess Boogie amplifier the level of customer care from them would be far different than what Ampeg has demonstrated.
