Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: repoman on September 03, 2019, 08:37:25 am
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Hello
I recently completed a Trinity TC15 and I am having difficulty eliminating a very loud hum coming from the amp.
I am very inexperienced in building amps, and would like to learn some ways of locating and solving issues in them.
Schematic
http://www.trinityamps.com/docs/TC15Schematic12.pdf
Layout
http://www.trinityamps.com/docs/TC15Layout11.pdf
Couple pictures:
https://i.imgur.com/kAUs3OA.jpg
https://i.imgur.com/VRYEDaz.jpg
https://i.imgur.com/zzAAFmH.jpg
Video of the buzz/hum
I completed the amp, built the cab and head, and had experienced this hum immediately on start up and testing. The hum persisted as I did things like check ground connections, continuity, taking voltage readings from the various points of listed appropriate voltages on the layout, swapped tubes (I have purchased all new power and preamp tubes for this amp but changed them out with other tubes to double check).
As the day went on, I noticed the buzz had dissipated a considerable amount after a couple hours to what I would consider somewhat normal levels of amp noise. The most prevalent annoying noise coming from the amp was a faint, fast ticking sound but it was not terribly noticeable. The next day I turned the amp on and the hum had returned louder than ever. I've gone about rechecking voltages and ground points and have not located anything odd except perhaps one point which is low, this is the turret at the very end of the board which has a 330k resistor on it and is connected to pin 6 of the EF86 as well- this reads around 84v when the schematic indicates it should read 131v.
During changing tubes, I did swap the 5AR4 with a 5Y3, I suspect this was around the time I noticed the hum being reduced but I am not totally sure. However, the hum is present now with both 5Y3 and 5AR4, both of these tubes are somewhat old and this is next on the list to replace with a new part to see if that reduces the hum but aside from that I am at a dead end on where to go if that doesn't fix the issue aside from completely rebuilding everything.
Are there some procedures I should take that can maybe give me some more clues on locating the source of the hum and eliminating it besides the new rectifier tube? The hum is volume dependent and seems to be 120hz, so I have been instructed to look into the power section of the amp but I don't know what that specifically entails. I have taken ohm readings from the filter capacitors and they are high, this is the only method of checking them I am aware of.
I have chopsticked around on the inside a bit and several of the small capacitors produce audible noises through the speakers (thumping from the tapping of the chopstick), I don't know if this means anything. I have reflowed solder on most of the points of the turret board and ground points.
Any help would be much appriciated!
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here's a good start place;
http://el34world.com/Forum/index.php?topic=23286.0
Hum is ambiguous so ya gotta break it down
both channels?
60hz/120hz
with no guitar plugged in?
effected by volume or tone pots?
......
your 84V vs 130V indicates a heavy current draw, a mis-read R value.
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Hi
I have a video posted of the hum, I am not sure if it is 60hz or 120hz due to inexperience, I am guessing it sounds like 120hz though?
The amp is relatively quiet when nothing it plugged in, the hum occurs when a guitar is plugged in, it does not seem to matter if it is a humbucker or single coil (I can hear the normal 60hz hum from single coils when plugged in, that is normal and not the loud hum I am trying to address).
Both channels exhibit hum when guitars are plugged in.
Hum is volume dependent and some what high pitched.
I have rechecked the resistors using a meter and all seem normal except the I am seeing 33k on the 2 resistors coming off pin 2 on the 12ax7 (V2) and 33k on pin 9 of the EF86 when they should be 68k...? They are 68k resistors from the color code.
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Hum is volume dependent
Both channels exhibit hum
so you're starting left of volume pots and common to both channels
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Any fluorescent lights nearby?
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No, I have removed all electronics from the room I am working in to see if something in there was causing the hum. If I move around the room with the guitar the hum will decrease but never to a point that is normal levels of amp noise. Specifically if I move to the side of the speaker cabinet I have built the hum lessens the most.
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this reads around 84v
does pin 3 read 1.9v?
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Hi,
Pin 3 reads 2v, although I am seeing 60v on pin 1 of the EF86.
When the amp is in standby, I can hear some scratchiness coming through the speaker when I adjust the volume control for the 12AX7 channel.
Do the 68k grid resistors on the V2 and V1 reading 33k indicate a problem or are the values affected by them being tied to another 68k (I would think that would show double value, not half)?
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here's a great file to file, well read 1st pg 7 :icon_biggrin:
your PS volts mighta dropped when you changed Rec tubes, nothing on the schematic shows what each Tap should be
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Have you tried posting over on the Trinity forums? Very helpful bunch there too who will be very familiar with that amp.
EDIT: I just popped-over there and saw you are getting help from Mitch M... you are in good hands :icon_biggrin: Between the folks here and Mitch you'll get to the bottom of it, but Mitch knows all those Trinity builds inside-out. I built a TC15 (my first build actually.) Great amp.
I would suggest you check everything in the power supply section again, reflow all the solder joints and make sure your connections to ground are nice and tight... you scraped-away any paint at the ground screws, right?
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Do the 68k grid resistors on the V2 and V1 reading 33k indicate a problem or are the values affected by them being tied to another 68k (I would think that would show double value, not half)?
That's normal. They're in parallel.