Very low output from Knight KN928 amp
I have a Knight KN928 integrated amp that I have used from time to time over the years. About ten years ago it was in use in my kitchen, when I noticed the output of one channel was getting
lower than the other. It did accumulate a lot of grease on the tubes in that environment, which I have since cleaned. It has
not been used much since then.
Now the output from both channels is VERY low. This unit uses 4 El84s in the output, 4 12ax7a in the preamp, and a 6CA4 rectifier.
I have swapped out all of these tubes with ones that I know to be good (well, they work in other units; I don't have a tube tester). The output level remains VERY low. This unit should put out at least 12 watts per channel, I'm sure it's nowhere near that now. When it was in my kitchen it drove smaller Advents easily.
I have little experience with working on tube amps, and have not woked on curcuits much since getting my EE 10 years ago.
However I have a lot of vintage tube gear and this one seems to be a relatively easy one to start with. I've read the FAQs on the dangers of working on tube gear.
Can anybody suggest where to start to look to troubleshoot?
Also I have been unable to find a schematic for this, anybody
have one they can share?
I can provide pics if that helps.
I've heard that schematics for old Knight amps can be tough to track down. I do not have one for the model you own. I do have half a schematic for a KN734, and a uselessly small photograph of the schematic for a KA55. Both of these amps are somewhat similar to the KN928 in that the use the same complement of output tubes (four 6BQ5). Perhaps there is enough similarity in the circuits for these to be of some use to you. Good luck.
Since you've done the first obvious thing which is changing tubes, the next thing would be to open it up and check voltages around the power supply. You will need a voltmeter or multimeter of some kind. Begin by looking for dark or discolored resistors. Especially any that are connected to the filter capacitors or elsewere.
Voltages coming off the 6CA4 and the first filter cap should be over 300 volts, and progress slightly downward as you get farther away from there. If you find a serious voltage letdown somewhere, you will then need to determine why. Is it a bad resistor that has increased in value because of age, or because of something else like a leaky capacitor.
If you don't have a multimeter, you'll need to buy or borrow one. This is something you should have anyway at your disposal. So for now, voltage checks are the next logical step in trouble shooting. Since both channels are affected, the PS is the most likely place to start
Thanks for the schematic. It does look similar to the KN928.
I do have a Fluke DMM (without capacitance checker), a variac, and an old B&K single trace scope. Looking at buying a capacitor checker; is the one in a DMM good enough? Or for large filter caps used in old tube stuff do I need something else?
I will check the voltages (carefully)
You don't need a capacitor tester to trouble shoot this amp. Your scope will tell you if the filter capacitors are ok by looking at the ripple on the B+. Use a 10X probe with the scope input set to AC to keep the trace from shifting up off screen. If you connect a small test speaker to the amp, your ears will tell you much too. A small amount of ripple right out of the rectifier is normal and should smooth out to almost nothing at the next node.
Those small cheap capacitance meters are not very good for the most part because they only measure by the low voltage DC charging or time constant method. But they will tell you if a cap is probably ok or bad. They have no means of testing for leakage which requires higher voltage across the cap.
Replace that 6CA4 tube with a known good one. I have an amp where the 6CA4 tube went bad (not shorted) and volume diminished drastically. Replacing it brought it back to full power.
I used the same 6CA4 in a Pilot 680 tuner and it worked fine
(good volume, etc) so I think it's OK.
Is that a sufficient test?
I tested the voltage coming off the secondary, and it's
544v, off the first filter cap it's 305, off the second 303v.
This is all new to me, but from what I've been reading for the 12AX7 and EL84, it looks like 250-300 volts is typical for the plate voltages but so far I haven't checked those, I assume that's the next thing to check? To be honest I'm a little spooked by the FAQs about guys getting fried working on these things, but I'm using one hand, clipping one lead, etc....I'm just going to have to get used to it.
I have the following tube gear
Pilot SA260 amp
Pilot SP210 Pre
Pilot SP216 Pre
Pilot 680 Tuner
Pilot FM200 Multiplexer
Knght KN928 intergrated
(2) Altec Lansing 1569a monoblocks
Altec Lansing 1566a mic amp
This was all given to me over the past 30 years as transistors replaced all the 'old junk' and I was the only one who was interested in taking it.
ALL of this stuff needs some kind of work, and I'd like to be able to do some of it myself. The Pilot amp is the only piece that has been recapped (not by me).
As Victor suggested I checked the voltages.
Here's what I got (correct me if I have the terminology wrong):
voltage coming off the secondary is 544v
Off the first filter cap it's 305
off the second filter cap 303v
On 1st preamp 12ax7 tube:
Pin1 (looks like plate from data sheet) 240v
Pin 6 (the other plate) 140
Pin1 ~118 < ---these voltages jump around
Pin6 ~128 < ---these voltages jump around
Pin1 111 <----is this low for a 12ax7?
pin 6 102
On the 1st el34 :
pin 7 (looks like plate from the data sheet) 318 v
pin 9 (grid 2) 292v
pin 7 325v
pin 9 292v
pin 7 323
pin 9 299
pin 7 318
pin 9 300
can we conclude anything from this? The voltages on the
third 12ax7 jump around, might this indicate a bad
element (resistor or capacitor?)
I suppose it would be easier to take it to the shop but it
looks like a simple enough unit that I (maybe) can fix with
some help from this forum. Also it's not worth a zillion dollars if I screw it up....
Again I don't have a schematic. I do have the one for the KN734
that Ty provided, but the resistance values for the KN734 are different than for the 928 that I have, and it uses a different rectifier tube.
Thanks for any advice you can offer.
Wandering voltages might indicate you simply didn't get a good connection with your meter's probe. I wouldn't conclude anything at this point.
Your output tubes are EL84, not EL34.
It's OK to run 100 volts on the plate of a 12AX7.
Old resistors tend to drift upwards in value, especially the old carbon composition types. This can put an amp out of the "ideal" operating area, but usually isn't fatal. Some resistors can overheat, burn, or open completely. These should be easy to spot.
Old capacitors tend to "leak". A healthy capacitor shouldn't have less than (?) 100k ohms DC resistance across it. If the amplifier is powered off and the big caps in the power supply have been drained of any high voltage, you can stick an ohmmeter across the cap. Analog ohmmeters are easier to use here. The cap will initially show a low DC resistance, then it should quickly move towards a very high DC resistance as the cap charges up.
Leaking caps in the power supply cause humming. Leaking coupling caps (like C26, C27, and C28 in the KN734 schematic) allow DC from one stage to pass to the next. This can cause a tube to be driven into saturation.
You've checked the plate (anode) voltages on all the tubes, and the screen voltage on the EL84. Now it would be useful to check the grid and cathode voltages too. Cathode voltages must be much lower than anode voltages. Grid voltages must be lower than cathode voltages (sometimes even negative). If you have a leaking coupling cap (described above) you'll probably see a "bad" voltage on somebody's grid.
7 = anode (plate)
9 = screen
2 = grid
3 = cathode
1 = 1st anode
2 = 1st grid
3 = 1st cathode
6 = 2nd anode
7 = 2nd grid
8 = 2nd cathode
I am really grateful you've taken the time to look at this...
Yes of course, these are EL84 tubes in this amp (the Pilot has EL34s).
So I poked around and got the other readings:
pin7 0 (makes some buzzing noise, not through the speakers,
when I take this reading...did I fry something?)
pin8 1 v
pin7 .002v (same buzz as above)
pin7 -0.06v (no buzz)
pin2 0.7 v
pin7 -0.02v (no buzz)
pin 2 .065v
(BTW this pin is not connected to anything on this one tube, but it is on the other three EL84)
pin 3 10.4 v
pin3 9.42 v
pin2 varies between -.001 and +.002v
pin3 9.54 v
I made up a discharger resistor (these filter caps hold >20v
for several hours) and drained the caps.
the first filter cap has three 20uf elements and one 10 uf.
The three 20uf caps measure >13M ohms.
The 10uf (which is connected to ground via a 100 ohm resistor)
has DC resistance of ...100 ohms. according to the can the 10uf
cap is only rated for 50v, the others rated to 350v.
The second filter cap has 2 10uf elements and they
show DC resistance of over 13Mohms. I do not hear any hum when using the amp.
Do these reading sound right?
On a whim I checked the volume control, and it looks OK....
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