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In Reply to: RE: "Ping" Fender Lover posted by FenderLover on May 12, 2014 at 23:26:39
1) Funny you should mention checking the outlet voltage. When I replaced 4 trim pots and checked bias at 2:00 am, the reading was steady. I replaced 4 more pots during the day time and the reading was waivering. I have a UPS and will give it a whirl. Otherwise that meter you sent me a link of would sure come in handy.
2) Yes. When all but the output tubes are pulled, all stable. As mentioned before, even if I pull 1 channel only, the "empty" channel is fine. the other unstable.
3) Have no idea how global and feedback work... :( But I found a link to the MKII D70 schematics. Same as the ones Mike S. had sent me. Mine is a MKI D70. Apparently minor change, different signal tubes. Hmmm... :)
"I know just enough to get into trouble. But not enough to get out of it."
Follow Ups:
If you're running new output tubes,they also can vary in bias because they are changing somewhat thru the first 400 hours..It isn't major but neither is your bias issue but how many hours are on your tubes.They may stabilize with break in.
Honest amplification is better than excessive 2nd order distortion anytime.
Edits: 05/13/14
New output tubes now. But had same symptom using the old tubes. Just in the past couple of months of course. Otherwise they seemed good.
I'll try putting the old ones back and see what happens.
Many good tips to try.
Have a long weekend coming up if I don't get any chances during the week.
Thanks Mike!
"I know just enough to get into trouble. But not enough to get out of it."
Does the amp have the same idle bias readings with the phase inverter pulled?
Anyhow, good luck with your amp!
Oh geeze. I don't even know where to check the plate voltage.
So far all I have been able to do over the years is check resistors, capacitors, and replace things that bulge, leak or look burnt!
Or I just follow the advice someone gives me on the forum.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
`Check the plate voltage at pin number 3 of each of the 6550 tubes.Also check the negative voltage at pin 5 and see if it changes.
Honest amplification is better than excessive 2nd order distortion anytime.
Thanks Mike! Will add that to my list of things to try.
"I know just enough to get into trouble. But not enough to get out of it."
Maybe able to find a pattern.
Somethings been nagging at me. When I used to check my output bias and DC balance every 6 months or so, I thought I always had to change from DC mA to DC volt settings along with switching one of the test leads. But at some point I recall getting zero reading set at DC mA for output so thought I was mistaken and both tests were to be done on the same DC volt setting, because only then would I get any readings.
But reading through past posts I think it should be DC mA for output and DC volt for balance. I know not to touch A.C. balance as I need a scope.
Either I've gone crazy or my DVM has. Hopefully the latter.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
When you measure the voltage (milli-DC), you are really measuring the voltage drop across a known set resistor tied from the cathode-to-ground. The idle current passed can be related to this voltage drop, by Ohm's Law.To measure the actual current passed, you need to break the connection between ground and the cathode of each tube. Then, insert your DVM probe to complete the circuit.
Edits: 05/15/14
The test points are set up such that 65 65mV equates to 65mA.
Going to get down to some testing this weekend.
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
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