Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxacthirtee on January 31, 2016, 11:49:40 am
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Hi folks,
had an old handle on here i can't seem to remember/recover so this is a first post for this handle.
I've built a bunch of amps over the years, but i'm having problems with a new build.
Built a Blonde Tremolux using new caps/resistors/pots/board, but with vintage transformers.
Its a Fender style fibreboard.
I fire it up and it works great, just with a loud "buzz". Not 60hz hum.
I get a "buzzy" sound in the transformer at power up, with NO rectifier installed.
Since i've had mechanical transformer buzz translate to actual amp buzz before, and since i can't find the buzz elsewhere,
i swap out the transformer for another vintage power transformer. Same thing...buzz
So now i'm thinking the likelyhood of 2 bad power transformers is not great, but i can't troubleshoot it and i'm less than happy.
Don't have a scope, but i can't trace any audio AC signal causing this. The new coupling caps don't leak DC.
I have all but no signal(Mv) at the gridstoppers on the 6L6 at idle. ANd the buzz is there.
Electrolytics are a new can, its not popping any fuse, and actually works and plays, including the Tremolo, just with a loudish buzz.
It bias' up fine, all voltages are in line with what they should, tried different rectifier tubes, power tubes, preamp tubes.
Pulling the preamp tubes doesn't stop it(driver still in)-pulling the driver does stop it, but it would have to....
Any way to test for a "bad" transformer when its actually powering everything up fine?
I haven't replaced the filter cap can yet, as that would be a huge pain, but its my last though at the moment,
other than going over every connection and solder joint....again...
thanks
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So just to clarify, are you saying you get buzz (you call it) with no rectifier (tube) installed and thus the two high voltage leads go to unused tube socket terminals which is nowhere?
With two separate transformers. (Used but assume salvageable)
Same for the 5 volt winding.
Also re: the 5 V winding, is there any CT goofiness, is the HV drawn from a CT on that winding (versus pin 2 or pin 8 of the 5V rect tube) and somehow you have half that winding shorted?
Two ideas that should not matter: Flip phase of 5V winding. Change whether you take your B+ from pin 2 or pin 8 of the rect tube.
Got 6.3 volt CT real or syn?
Other than that....stumped.
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Hi Eleventeen, thanks for the thoughts.
Sorry, to clarify. Yes i seem to get some mechanical buzz out of two different used but (seemingly) salvageable transformers.
While connected to empty tube sockets.
Not the worst mechanical buzz i've ever heard out of a transformer but i am second guessing as it COULD be 2 separate bad transformers.
i was told by a couple people/mentors i trust that you can get mechanical buzz when windings dry out(wax drips out/etc) or if the laminations get rusty/loose.
I've experienced it before, so i know it happens. Sometimes it manifests itself as noise/buzz in the amp, sometimes not(?)
Either way, in this case i get buzzing transformer AND buzzing from the audio, using 2 different transformers.
Anyways, on the first transformer, it had no center tap for 5v or 6.3v. For the 6.3v was running a faux tap using a 100ohm resistor per side to ground.
Using the 5v as it was. On the second transformer, it has a center tap for the 6.3v so i disconnected the 100ohm resistors. No center tap for 5v.
So its straight up yellow-yellow to pins 2 and 8 - pulling HT off of pin 8 both times. I'm getting a little over 5v while a rect. is in there and a little over 6.3v when the tubes are in, both Within limits, and nothings popping a fuse or getting hot or smelling burnt. So i'm not thinking i have anything obvously shorted.
can try to switch "polarity" on the heaters. I am pulling the bias supply off one side of the AC connections to the rectifier tube. Never had a problem doing that in the past. I know this transformer has the juice for it and more.
i'll try some more later tonight.
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ok, so after taking a day or two to walk away from the frustration, i fired it up (and it still buzzed of course)
so i put in the preamp tubes and disconnected the "in" wire from the pre to the driver and the buzz mostly stopped.
So i have a problem in the preamp somewhere(dammit)
I guess i have to go through the Pre and the Trem and see what i can dig up.
If one is signal tracing hum/noise and you have your Meter on 200Mv AC and you get .1mv after a coupling cap is it leaking or just a mediocre meter?
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I would consider .1mV as zero leakage. But if you are specifically testing for a leaky cap your meter needs to be set to measure DC volts.
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I would consider .1mV as zero leakage. But if you are specifically testing for a leaky cap your meter needs to be set to measure DC volts.
Thanks, thats what i meant.
although the question also rises. Whats the lowest amount of Signal voltage that would usually translate as a perceptible noise/buzz?
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That depends on where you are measuring. 10mV or 20mV on the input jack would make quite a loud hum with the volume turned up. But that same 20mV at the grid of the output tube would be nothing.
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That depends on where you are measuring. 10mV or 20mV on the input jack would make quite a loud hum with the volume turned up. But that same 20mV at the grid of the output tube would be nothing.
Thanks, i should have been more specific.
As mentioned above i'm chasing down a buzz in the preamp and not getting a lot of obvious signal ac anywhere.
None of the caps in the pre pass DC
Built the amp and it functions and passes signal, playing (relatively) normally.
It just also passes the buzz at moderate level (louder than low, softer than loud) that does not change with volume or tone control changes. There isn't any "Hum", per se, and turning the volume up with no guitar/cable hooked up gets me only a modest boost of hiss/noise at three quarters volume and above.
Disconnecting the wire from the second gain stage to the input of the Driver stage makes the buzz go away.
It is technically a single channel Tremolux (6G9-b) so other than the Driver tube i just have the one signal tube, and the Tremolo tube.
Just fought it for a while. Changed a couple of signal wire runs in the pre section to shielded wire, no change.