Tube Amps / Music Electronics
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|10/23/2002 11:00 PM|
||Intermittent noise in Tweedy Deluxe|
I'm really excited about finishing my amplifier today. It really screams! I have a couple of questions that require answering.
1) When I plug into the number one inputs (both channels), I get no sound. The number two inputs work great. Is this normal? Should I take another look at my layout, perhaps?
2) The amp sounds great, lots of gain, and pretty good clean sound at a good volume, too. BUT...there's this noise, like gentle surf, with an occasional crackle and pop. It kind of comes and goes, louder, quieter. Any suggestions as to the cause and cure of this? Thanks, Mike
|10/24/2002 6:54 PM|
I can't help you with the surf music, but the dead inputs are not normal. You should double check your wiring. These are a little tricky, take a look at Bruce's site for detailed pictures of the wiring.
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|10/24/2002 10:41 PM|
Thanks Rich, I found the input problem and fixed it, but the white noise is still there. More constant than intermittent now. Not noticable when you play, but whenever you stop...ssshhhh! Mike
|10/25/2002 1:48 PM|
Are you using carbon comp resistors? If so, that may be the source.
If this is a standard Bruce kit, I would look for a cold solder joint or a bad connection, possibly on the preamp side.
I am sure Bruce can give you a better response when he checks in.
|10/25/2002 4:40 PM|
I'm getting exactly the same sounds from mine. They weren't there initially and have grown worse over time. Here's to hoping for Bruce's reply soon.
|10/25/2002 9:58 PM|
I get a little bit of 'blow' occasionally in mine too, but ususally just when I run my EL84/Yellowjacket setup. I chalked it up to the tubes.
However, it IS intermittent, and when it does happen it seems to only occur for a few seconds. just after I flip the standby switch (after the amp has warmed up in "standby" for a couple minutes).
Another bump to the top....
|10/25/2002 10:02 PM|
|Bruce /Mission Amps
I don't know off hand.
I have a few kit amps that have been running for many many months here in the Denver area and I've not heard this complaint yet.
First thing I'd try is a new 12AY7 followed by the 12AX7.
Also, the 5E3 power tubes are running pretty hot and you might have one that is getting noisy and or drawing way too much current at idle.
So, check the idle current of the two power tubes.
With the amp off for a while, measure the DC resistance of the 6V6 biasing resistor and then with the amp on and both volumes to ZERO, measure the DC voltage across the cathode biasing resistor.
Then divide that voltage by the DC resistance value of the biasing resistor.
If you have 20vdc / 270 ohms, that would be around 74ma for both tubes.
If the tubes are still good and matched, that's about 37ma each.
Now measure the B+ on one of the 6V6 tubes from plate to cathode and multiply the ma times the plate to cathode voltage to see where the tube is sitting in idle power.
Plate voltage: lug 3 to lug 8 is 340vdc
Cathode voltage: lug 8 to chassis ground is 23v
20v/270 = .074a
.074a/2 tubes = .037a
.037a x 340vdc = 12.6 watts
Thats a bit hot but I've seen real ones much hotter!
A decent pair should be around 10-13 watts.
A very hot idling power tube can sound pretty hissy.
Parasitic oscillations can cause the tubes to appear to be idling at very high currents to, when infact, you are measuring the DC idle current plus the AC current needed to supply energy to sustain the oscillation.
Things can get tricky then.
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