So I was installing a NFB switch on my sons amp and after I installed it a new problem has surfaced. Now the amp does not get loud at all and starts to oscillate if the MV is anywhere past 9 O'clock. It is driving me crazy. I don't know what I could have done wrong. Any ideas?
I am going to fire up my scope and try to find the prob.
FYI, when I turned the amp on, it was fine for about 15 seconds and then the volume dropped and the oscillation started. I have replaced all the tubes and still the same problem occurs.
Maybe you have a phase problem and you've got positive f/b instead of negative. Try reversing phase. Either swap the OT primary leads or swap the feed from the PI, which ever is easier to do.
.022 coupling cap died, It was one of the NOS mustards I had in there. I also changed the .1 mustard, but kept the .1 Prescence mustard. It also took out my 1 month old JJ 6CA7's
I have a pair of old Ruby EL34's that test fairly good in there for now.
Quick question for the sake of others working on amps. (this is not personal!)
Did you bring the amp up on a variac or bulb limiter after you you worked on it?
I can't help but feel the tubes could have been saved with good start up habits.
The faulty caps should have shown a high amperage draw on an amp meter while bringing the AC voltage up from 0-120 (117). The amp meter should have spiked around 35volts giving plenty of time to power down before any damage was done to the circuit or parts. Pointing out a problem in the circuit.
selloutrr wrote:Congratulations on getting the amp fixed!
Quick question for the sake of others working on amps. (this is not personal!)
Did you bring the amp up on a variac or bulb limiter after you you worked on it?
I can't help but feel the tubes could have been saved with good start up habits.
The faulty caps should have shown a high amperage draw on an amp meter while bringing the AC voltage up from 0-120 (117). The amp meter should have spiked around 35volts giving plenty of time to power down before any damage was done to the circuit or parts. Pointing out a problem in the circuit.
No, unfortunately I don't have a working variac at the moment. I have one that needs the output plug replaced, but I have been lazy in replacing it. This is a good kick in the behind to get that going. Thanks!
I can't stress enough the importance of a variac for trouble shooting.
I use an Eico 1078.
It has a Voltage meter and Amperage meter that is switchable from 2.5A to 7.5A
The meters are fully useable but a tad bit small so I tapped on to the lines and added two large 6"+ Western Electrics VU meters. - ebay for $10. Used my fluke and a screw driver to calibrate the meters
I found the variac on ebay in early 2000 it was under $100 shipped. It's easily saved me $thousands in stupid mistakes and troubleshooting.
A bulb limiter is very simple to build. All it is, is a box with a mains line to an outlet with a 20 to 40-watt lightbulb on the high side. The bulb does the current limiting. If a component is shorted the bulb is full on, if operation is close to normal the bulb barely lights. Used in combination with a variac, it's very effective but the limiter alone will save your output tubes or your power transformer.......
Back when I was cute but dumb, I put filtre caps into a Heathkit WM-5 monoblock and fired her up hot off the mains. A shorted grid-stop cap took out a power pentode (6L6 or EL 34, I can't remember now) and the plate winding of that beautiful Peerless output transformer.......
I like to use a 25 watt light bulb for the first power up.
This limits the current to less than 250ma.
Then you can go with larger wattage like 60w or 75w for more current if you like.
