Hum Busting a Single Ended Tube Amp
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- Опубликовано: 24 авг 2024
- I rebuilt a noisy Champ clone and made it worse!
Eliminating the hum, step by step, taught me some valuable lessons.
Join me in the Colonel's shed and enjoy the cinema-verite hand held camera work and technical banter.
I love the 6n2p. I've used them in a couple builds. You can wure the amp in such a way that you can wire a switch and choose whether to use a 12ax7 or the 6n2p.
Choke is the main one obviously, but I was perfectly happy just putting an extra 22uf on the power stage filter.
I built a 5F2A Princeton in the standard Fender chassis. Hum is nearly inaudible. I added a 5H choke after the first power supply cap and made the preamp caps all 20uF. For grounding I isolated all the jacks and control pots from the chassis, and ran every ground in the circuit to one 18awg bus wire grounded at a single point on the chassis, at a ground lug under a transformer bolt. Center taps for B+ and 6.3v line are also grounded there.
Fender's layout is a total mess. I ignored it.
The filaments can be supplied with a DC voltage using a voltage regulator which would eliminate filament hum.
That vintage .1 mustard cathode bypass cap just doubled the value of that amp !! Hahah
You seem like a really nice guy.the longer the leads of elec. Component.the better they are as antenna s for unwanted noise
Don't use a transformer mounting bolt as a ground point. Earth ground from the power cord should get its own dedicated chassis connection (this is part of the electrical code for most countries); and separating the audio signal ground from earth ground by several inches tends to lower hum. Also, transformer mounting hardware tends to loosen over time from speaker vibration and transport shock.
I've been using the transformer bolt "star-grounding" method for 30 years... never once had a problem, and I don't have hum issues. I see people repeating this "wisdom" all the time on RUclips, Insta, etc, and then they show their amp with the safety ground either soldered to the chassis directly, or bolted to the chassis on it's own. Both of those methods can also come loose from drops and vibrations. So I don't get it. I was taught by old timers who told me they used the transformer bolt, and also "solder isn't glue".
Recently came across your RUclips channel and subscribed to it, really enjoying watching your videos, I have a mid 70's (I'm guessing) non-reverb Fender Super Twin all valve guitar amp that a friend sold to me for only Au$300.00, brand new I think it would easily have costed ten times as much as that brand-new, it's housed in a wooden Fender head cab, it works really well as a bass amp too, it uses six STR 6L6 power valves, a 12AT7, a 12AU7, two 12AX7A, and a 7025, puts out 180W RMS, or a whopping 395W peak, very loud clean amp.
Sounds brutal!😄 Too much power for the gigs I do, but yeah, nice for bass.
Hard to tell from the vid but it appears you have some inductive coupling going on between the windings of the P.T. and O.P. transformers. The bobbins need to be at 90 degrees to each other and not with the same orientation as you appear to have them.
Even up to 10 Mohm on the grid .........indeed rarely seen today.
Does it hum when the chassis is out of the amplifier
Hi, great video, my small champ 600 reissue, makes ugly hum, and after improve componentes, move place transformer and put a cable direct to heaters, it's still there... It is a way to kill it? Or diminish...
Thanks, sorry for my English, greatings from Spain!
They hum because they don't know the words! 😂 That thing is a killer harp amp! I've had to troubleshoot a lot of home builds over the years and a fair number of pro builds too. On the fender style amps ive always found that duping their grounding scheme with locations, brass ground bus is just the easiest way. Not the best way, but definitely good enough for rock n roll and its pretty amazing that the fenders are as low noise as they are. A few addl ideas that might help someone: Some Champ copies use the same "one wire" filament that original tweeds had. Not a good way. I would always mod those with the artificial ctr tap like you have there. Another way is the hum balance control (later fender) that connects the wiper to ground and the outer lugs to each filament lead. That can help. And yet another consideration might be your use of the 6v preamp tube: the 12ax7 is wired normally as 6v humbucking. Ive seen minor hum issues in preamps using single 6v triodes. I'm curious as to whether your minor hum is audible with the preamp tube pulled.
One other common issue ive seen is when people copy the early marshall way of grounding the pots AND running a bus wire on all the backs which terminates on the chassis. Cheers!
Thanks for that additional info. Really useful. I will try pulling V1 to see if the remaining hum disappears or stays.
Good point on the humbucking qualities of a 12AX7 wired to run off of 6 volts; I've read that 12AX7's also had a pair of heaters that were spiral wound in opposing directions, for further hum canceling.
I have a fender 007 bassbreaker amp, that has the low tone hum.
I read that it's a single ended amp.
I actually bought two head versions and both hum.
I'd like to reduce the hum.
I've seen some very messy amps that were dead quiet and then some pretty neat ones that noisy as hell on a Saturday night! You should be able to get that one pretty quiet with some more tinkering. It's funny tho cause it's not always what you think it is or what makes sense ya know! Maybe instead of the isolation technique just ground to the chassis? I know it's against the "rules" but plenty of others have achieved good results. Also, as a harp amp I'd be careful about over filtering that little jewel. Sometimes you can kill the nice soft sort of "bounce" these little amps have. But, that's just a suggestion.
Thanks. All good tips. Each change I made DID give an improvement, but you are right, what seems correct in theory doesn't always lead to a good result.
Could the hum just be the result of where the transformers are positioned?
Definitely can contribute...move the power and output transformers apart and also ensure they don't couple due to complementary allignment of the windings...i've changed this on a champ 600 and it immediately helped as a sole modification.
Good sequence of tips. I've had similar adventures chasing hum. Here's another good video on the importance of grounding the first stage cathode DIRECTLY back to the input ground without sharing the ground wire: ruclips.net/video/GuN35X0Wd5c/видео.html
Thanks for that. It's a pretty long-winded explanation, but makes sense.
@@TheZedvox Yeah, the main thing is that in at least some scenarios it works like a hot damn to bust hum.
@stormriderkaos every amp I've ever built has 1 ground bus shared by everything and terminates at 1 point on the chassis under a transformer bolt ground lug. No hum.