martin manning wrote: ↑Sun Jan 24, 2021 2:03 pm
You can do a quick check measuring the voltage drop across each side of the OT primary, and use the resistances to calculate current.
I will try that procedure in a little while here.
One thing I noticed which I am not sure if it is relevant:
When checking DC voltages on the electrolytic caps, I found that it made an audible popping sound when I checked voltages on the 2 large Sprague Atom 20uF 500v caps. The snap/crackle/pop artifact was higher/lower in volume with master volume up or down, but never muted (just like the hum). There I did not hear any of that type of noise checking any of the other 'lytics. Both of those caps also seem to have a strong ground connection.
Is this condition indicative of anything awry?
-Matt
It may very well be that the sole purpose of your life is only to serve as a warning to others.
No it is not, those B+ connections go directly to the anodes of your power tubes, or other tubes therefore you're injecting a noise directly into the signal path.
It's a good proof that stage of the amp is working.
Okay, so I had the amp on my bench, powered off, caps drained, unplugged from the wall as well as the speaker.
I measured resistance on my blue side, it took a while to settle down and stabilized at 13.8ohms. I measured the resistance on the brown side (which also took a while to stabilize) at 12.8ohms.
Plugged back into the wall and the speaker, all knobs at 0.
I measured my plate voltage at 448 on both sides. Using this I came up with .0368A, rounding to .037A, giving me .074 per side.
So on the blue side I calculated .074 * 13.8 = 1.021
On the brown side I calculated .074 * 12.8 = 0.947
I adjusted the bias pot until I got 0.950v on the brown side.
When I checked that on the blue side I read .940v. It seems that I cannot have both sides biased "ideally" but I am not sure if the difference is within an acceptable tolerance.
I also noticed that when I had the multimeter connected, if I had the COM (black) side connected to the red OT lead (standby switch), and the other multimeter lead clipped to the tube socket pin 3 soldering lug I would read negative voltage. When I reversed them I got positive voltage. I am not sure if this is relevant, but I expected the opposite.
-Matt
It may very well be that the sole purpose of your life is only to serve as a warning to others.
professormudd wrote: ↑Sun Jan 24, 2021 8:56 pmWhen I checked that on the blue side I read .940v. It seems that I cannot have both sides biased "ideally" but I am not sure if the difference is within an acceptable tolerance.
I also noticed that when I had the multimeter connected, if I had the COM (black) side connected to the red OT lead (standby switch), and the other multimeter lead clipped to the tube socket pin 3 soldering lug I would read negative voltage. When I reversed them I got positive voltage. I am not sure if this is relevant, but I expected the opposite.
Bias is usually about compromise. You're still using four power tubes, right? So there's no way to tell whether the individual tubes are well matched. You'll get a better look when you install the 1R resistors.
About the voltage drop, the DC voltage will be highest at the CT because that's connected to B+, and lower at the tube plates, thus the voltage drop is negative WRT the CT.
I build and repair tube amps. http://amps.monkeymatic.com
There should be a voltage drop yes. Think about it, the windings of the transformer are like a resistor, so the voltage at entry is B+ of say 350, but at the other end, it drops per the resistance of the winding on that half. That is the voltage difference you're measuring to get the current. (You know the winding resistance.)
Another way to look at it, and technically measure it, is to put the common lead at chassis, then measure the entry (standby switch), then measure the voltage at the anodes and they'll be slightly lower, as the voltage has dropped between those two points. maybe 349 or whatever. you'd get the same voltage difference by subtracting the b+ from the anode measurement.
xtian wrote: ↑Sun Jan 24, 2021 10:48 pmBias is usually about compromise. You're still using four power tubes, right? So there's no way to tell whether the individual tubes are well matched. You'll get a better look when you install the 1R resistors.
About the voltage drop, the DC voltage will be highest at the CT because that's connected to B+, and lower at the tube plates, thus the voltage drop is negative WRT the CT.
Thanks for the info.
I don't know if it is related or not, but I found the overdrive channel to be pretty raspy. There is plenty of volume, and not so much noise, but the clipping sounds a bit sizzle-y.
-Matt
It may very well be that the sole purpose of your life is only to serve as a warning to others.
professormudd wrote: ↑Sun Dec 27, 2020 2:58 pm
Finally got some tube sockets. I had previously ordered an Amplified Nation chassis, but decided to do that build with eyelet boards. For this build I'm going to use a Ceriatone chassis with some custom faceplates (editable artwork is shared in the Dumble files section). I still have to order quite a bit, but I'm trying to take my time.
Was wondering how you made the faceplates, did you print them yourself or get it done somewhere? Looks great...
professormudd wrote: ↑Sun Dec 27, 2020 2:58 pm
Finally got some tube sockets. I had previously ordered an Amplified Nation chassis, but decided to do that build with eyelet boards. For this build I'm going to use a Ceriatone chassis with some custom faceplates (editable artwork is shared in the Dumble files section). I still have to order quite a bit, but I'm trying to take my time.
Was wondering how you made the faceplates, did you print them yourself or get it done somewhere? Looks great...
I had a friend re-draw them from the ceriatone plates I had (the editable graphic files are in the Dumble Files section here), and I had a local company called American Data Plates print the aluminum. I also had the same company do my SSS faceplates with aluminum.
-Matt
It may very well be that the sole purpose of your life is only to serve as a warning to others.
Still waiting on those 1R resistors... In the meantime I wanted to go ahead and experiment.
Continued probing with chopsticks, verified and re-flowed ground connections, and clipped filter caps of equal value in parallel one at a time. No change.
I have been searching and it looks like the tubes are the most likely culprit. Will the 1R resistors allow me to glean enough to confirm/rule-out the tubes as the issue?
If not, I might go ahead an order another set of tubes. I am open to recommendations on brands. I have only ever bought JJs and this set of TADs.
-Matt
It may very well be that the sole purpose of your life is only to serve as a warning to others.
xtian wrote: ↑Sun Jan 24, 2021 12:37 am
Another simple solution is to pull out one pair of power tubes, leaving the others in far L and R sockets and see how they're balanced. Repeat with other set, then mix and match--should be able to see if one is way out.
xtian wrote: ↑Sun Jan 24, 2021 12:37 am
Another simple solution is to pull out one pair of power tubes, leaving the others in far L and R sockets and see how they're balanced. Repeat with other set, then mix and match--should be able to see if one is way out.
Did you do this?
I swapped the tubes out in pairs and still heard hum with any combination I tried, but I did not re-do the voltage measurement procedure each time. I only get to go out in the workshop for limited periods of time.
-Matt
It may very well be that the sole purpose of your life is only to serve as a warning to others.
norburybrook wrote: ↑Tue Jan 26, 2021 5:30 pm
have you got a dedicated ground for your speaker jack or are you relying on the chassis/socket?
I've had a hum before on an anodised chassis because of this so I now always put a ground wire from the speaker jack.
M
I am using my speaker jack with lock washer as a ground point there, and I had considered what you are mentioning, so I tested running a ground to the preamp ground point and it did not make a difference.
I am going to install those 1ohm resistors today and since I am going to be in there I decided to remove the half-power switch since I will most likely never use it. This will also make it more tidy back there. I also replaced the 5k1 resistors on each socket because I believe I originally set them in such a way that led to a lot of over-manipulation and possible loss of integrity.
My suspicion/hope is that somewhere in my cleanup I resolved the issue. I will report back after UPS stops teasing me and finally delivers those resistors.
-Matt
It may very well be that the sole purpose of your life is only to serve as a warning to others.
You're still using four power tubes, right? So there's no way to tell whether the individual tubes are well matched. You'll get a better look when you install the 1R resistors.
