Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dpm309 on July 10, 2019, 11:43:49 am
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My 5E3 which I built over 10 years ago has developed a bad hum. The first 2 OTs I installed (the first 2 from AE failed after a couple of years) did not have a heater CT so I used 2-100 ohm resistors for an artificial tap. The third one I installed several years did have a center tap but I did not use it since I had the artificial tap already in place. I have the chassis pulled and am going to check out the various components that might be causing the hum. My question is should I remove the artificial tap and use the heater CT? Is one method quieter than the other?
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Either method is fine. Neither should cause bad hum.
Hum should be either 60Hz or 120Hz (assuming 60 cycle el supply). 60 is likely heater supply. 120 indicates B+ filter, or ground, issue.
Always suspect tubes first.
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Checked all tubes, input jacks, resistors, caps, re-floated solder joints and still had the hum. It is definitely 60Hz. Removed the 2 100 ohm resistors and connected the heater CT to ground and the hum disappeared! Checked the 100 ohm resistors after I removed them and they read 100 ohm. I have had the artificial tap in this amp since I first built back in 2007 with no issues with hum up to now. I am glad the hum is gone but baffled on why it suddenly occurred.
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Checked all tubes, input jacks, resistors, caps, re-floated solder joints and still had the hum. It is definitely 60Hz. Removed the 2 100 ohm resistors and connected the heater CT to ground and the hum disappeared! Checked the 100 ohm resistors after I removed them and they read 100 ohm. I have had the artificial tap in this amp since I first built back in 2007 with no issues with hum up to now. I am glad the hum is gone but baffled on why it suddenly occurred.
Just be aware that a virtual center tap is actually safer long run than using the CT. The 100 ohm resistors can burn up for you if something wrong is happening on the heater rail. The CT won't and can burn up other more expensive things.
The fact that it resolved the issue means maybe one of the resistors was starting to go bad or drifting in resistance creating an imbalance in the virtual center tap. That or a ground loop was developing through it somehow. I've used them on many, many of my builds, with no negative hum. It's your call, but I'd suggest restoring the virtual one with new resistors and see if the hum's still there.
~Phil
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Thanks for the advice. I'll give it a try with a couple of new 100 ohm resistors. All my other builds used the CT with no problems, yet, but I can see you point on damaging more expensive components.
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The CT maybe developed a poor ground connection; or lost continuity with one or both windings.
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The hum is back. Checked all ground connections, re-wired the pots, cleaned tube sockets and jacks with no luck. Tried reinstalling the artificial tap with new resistors and get the same amount of hum whether using the OT CT or artificial tap. The hum is definitely after the pre-amp stage as the volume does not change the level of the hum. Looking at my voltages, I am getting no reading on pin 5 on either power tubes. Pulled the preamp and power tubes and am getting high voltages on pins 1 and 6 on the preamp tubes and pin 3 and 4 on the power tubes and no reading on pin 5 of the power tubes. The power tubes get extremely hot after running for only a few minutes. Could this be a bias problem?
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My 5E3 which I built over 10 years ago has developed a bad hum.
The first 2 OTs I installed (the first 2 from AE failed after a couple of years)
Hi,
can you please elaborate a bit on why did the 2 first Output Transformers failed?
It's not normal in just a short 10 years period to have 2 failed output transformers and being on the third.
People are still using Fender and Marshall amps with 50 years old with the original output transformer
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Could this be a bias problem
Yep
I ALWAYS suspect tube stuff 1st, then PS stuff 2nd when hum just shows up outta the blue. fixed bias IS a serious thing, lots of expensive pieces can burn up fast.
fwiw, when I take in an amp for repair, before I even think about their problems. I verify bias without PA tubes, IF good, PS rail VDC with tubes, quick.
all good, THEN, I can start on the issues noted
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5E3 is cathode biased. There shouldn't be any voltage on Pin5 @ idle. Que Passa?
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:think1:
good catch, I knew pp, assumed fixed :BangHead:
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My 5E3 which I built over 10 years ago has developed a bad hum.
The first 2 OTs I installed (the first 2 from AE failed after a couple of years)
Hi,
can you please elaborate a bit on why did the 2 first Output Transformers failed?
It's nome normal in just a short 10 years period to have 2 failed output transformers and being on the third.
People are still using Fender and Marshall amps with 50 years old with the original output transformer
Something's amiss here. dpm309 did say OT. it appears he meant PT, though this is not clear.
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Sorry, they were power transformers not OTs. The first 2 were Fender PTs that just crapped out. The third PT was from Weber and have been using this one for 8 years. I have also replaced a couple of Fender PTs on a couple of customer's 5E3s with the Weber PT with no issues to date. Don't know if there was bad batch made several years ago. Have been playing this amp hard for the last 12 years or so and except for the 1st 2 blown PTs, I had zero issues up to now. I need to get a matched pair of 6V6s to rule out bad power tubes as these are the originals I only had 1 6V6 when I tried swapping out tubes.
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...Just be aware that a virtual center tap is actually safer long run than using the CT. The 100 ohm resistors can burn up for you if something wrong is happening on the heater rail. The CT won't and can burn up other more expensive things...
I take the opposing view; a failure that results in fault current via the heater 0V reference (ie due to HT current) is rather likely to pull the heater circuit up near the full HT Vdc if that reference is lost, thereby likely damaging the heater to cathode insulation of every tube in the amp.
OK it's likely that in that scenario a power tube is kaput anyway, but unless the remainder of the tubes are of the super cheap variety, their value might easily be significant.
Hence in the absence of (or disinclination to use) a heater winding CT, I use 5W 100ohm resistors to balance the heater circuit.
And, of course, HT fusing (why wouldn't you?).
It seems perverse to consider sacrificing expensive components for the sake of fitting a fuse? In the case of a 5E3 and it's tube rectifier, a quick blow 250mA, either in the HT CT to 0V return, or in the rectifier output, could offer a significant degree of protection.
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It's unusual for a tranny to blow due to its own fault. Usually some other circuit condition causes a tranny to fail. Hence, replacing a failed tranny, withiout first correctng the circuit, will fry the new tranny.
What are the specfic failures of all these 5E3 PT's? HT, filament? Is there an explanation how you have so many blown PT's?
Is this thread about hum, PT''s; or are these issues somehow related?
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I don't know why the 1st 2 PTs failed (or the couple of others I changed out on customer's amps). Both times my PTs went, I used my light limiter when powering up after installing the new PT each time with no apparent shorts. The Weber PT I have in there now has been performing flawlessly for the last 10 years or so. I know this thread started out as CTs, but I am now trying to get rid of the annoying hum. My next step is to check out the PT for any possible shorts and putting in another set of 6V6s. I am still using the same set of power tubes from when I first built it. I have only one on hand and when I tried swapping out all of the pre and power amp tubes, including the rectifier, the hum level did not change. I will also check out the filter caps (sprauge atoms) that I have in there since day one. Is there anything else I might be missing?
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Is strange, may be you have a pair of Power Tubes that ocassionnally shorts ?
Franco
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UPDATE! Turns out the ground prong came out of the plug. Found it sticking out of my power strip so the amp had no earth to ground. Replaced the plug and the amp has no hum and is working perfectly. Has anyone had this happen to them? Don't understand how I got this much hum even though the polarity was correct on the 2 remaining prongs. Wish I would have discovered this before I started tearing apart the amp.