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Re: One step at a time.

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In Reply to: One step at a time. posted by PeterP on March 27, 2006 at 16:16:09:

Hi Peter:
I've made some progress using this approach.
I disconnected the coupling caps (both the 4 .1uF caps from the bias pcbs and the 2 .47uF caps to ground, correct?). MOST of the noise dissappeared, however I still have some low hum, prob. 60hz, present.
Reconnected coupling caps. Noise present, loudest in left channel. However, it will occasionally drop out, and things are relatively quiet. I suspect I have a loose connection in that channel somewhere, I just haven't been able to track it down yet.
I did some further tests with the volume control. Starting from lowest position, I can raise the volume pot quite a ways without any increase in noise. Then I reach a point where the noise decreases dramatically to where it is almost inaudible. With any additional turn of the pot, the noise increases rapidly and is very, very loud.
In conclusion, I think it's clear that I am looking for 2 sources of noise. The first is in the power stage, the second is ?? A final question: When shorting the signal inputs (step 2, previous post), should I short them to ground, or connect them to one another?
dv

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Re: One step at a time. - PeterP 17:51:44 04/08/06 (0)
IIRC the .47uF caps are part of the phase splitter. All you need to do is to remove the (4) 0.1uF caps. That is as low as you should expect the hum to be. This hum could be due to a couple of things such as: the output tubes, the tube socket connections, ripple on the B+, heater voltage connections, etc. But from what I'm hearing from you, it seems like the hum in the power stage is acceptable. Am I right?
BTW
When you are "all done", if you haven't already done so, you should look at adding the power supply choke option. I think Ella really benefits from the choke and more capacitance on the B+.
Have you checked and then double checked your feedback connections? What happens if you disconnect the feedback? A lot of people like Ella in triode mode with no feedback. (I happen to like it for what it is. A nice push-pull UL amp.) It's an easy test to disconnect the feedback and play some music. Then, without changing anything else (volume control levels, source, music), reconnect the feedback. Did the volume of the music go down? If so, then the feed back is correct and you can move on from there. If not, you need to find out why not.
Have fun.
Pete.
Re: One step at a time. - Victor_A 16:05:19 04/07/06 (9)
You have to short inputs to the ground.
- Re: One step at a time. - PeterP 17:21:33 04/08/06 (8)
  .....and when you do short them to ground you should:
  1. Remove the wires from the volume control (at the PWB)
  2. Solder in a jumper to short the inputs right at the PWB
  3. Do you still have hum?
  And now for just a couple of general questions.
  Have you tried with the feedback disconnected?
  Do you have the optional power supply choke installed?
  In the ones that I've played with, I feel that Ella needs the choke and could use more capacitance. But it should still be quite acceptable stock.
  Have fun.
  Pete.
  - Re: One step at a time. - tokyofist 18:26:49 04/10/06 (7)
    Thanks for all the help, Peter. I shorted the inputs and the hum was still present, if not a bit louder. When I disconnected the coupling caps, I do still have hum, but it's barely audible and only in one channel. I noticed that it follows one of the output tubes if i switch in from left to right. Possibly a bad tube? Not a huge concern as far as hum, but if it could have other repurcussions WRT the functioning of the amp, I'll of course replace it.
    Now on to look closely at the amp stage. Any pointers at this step?
    Also, is it normal for B+ to drop from about 430V to ~390 or so once tubes heat up? I also notice that the B+ drops really quick when power is cut, down to ~10V within a few minutes. I seem to remember it holding a steady high voltage for much, much longer than that previously. Victor got me thinking that a tube might be shorting out internally when hot.
    - Re: One step at a time. - PeterP 05:11:35 04/11/06 (6)
      Seems to me that your power tubes are fine and I don't remember what the B+ dropped down to (seems a little low but not too much it's less than 10%). As long as the current through the power tubes remains stable after things warm-up you should be just fine. What is the voltage at the test points after 30 minutes or so? Is it the same after an hour? Have you disconnected the feedback yet? You need to keep the inputs shorted and disconnect the feedback. You should have more hum than just the power section alone, but let's see if it is less than with the feedback connected.
      The other area to look at is the heater voltage. Have you checked, double, and triple checked your heater center-tap connection? As well as the connection to chassis? Did you remove the paint at that connection? Is there any DC or AC voltage the the heater center-tap? Verify that there is a dead short between the power safety ground and the circuit ground. Do a visual inspection of the heater wiring on the PWB and make sure that the heater wires (bare wire) and solder connections are not in contact with anything else.
      Lots of things to look at. And...who knows, it may turn out to be a tube after all.
      Keep at it. You'll find it.
      Pete.
      - Re: One step at a time. - Victor_A 19:12:58 04/14/06 (5)
        
        In Reply to: Re: One step at a time. posted by PeterP on April 11, 2006 at 05:11:35:
        
        Maybe one of the recifier diodes is dead (shortcut). That may explain the constant hum and fast hight voltage drop.
        
        Re: One step at a time. - tokyofist 16:44:29 04/15/06 (4)
        
        In Reply to: Re: One step at a time. posted by Victor_A on April 14, 2006 at 19:12:58:
        
        I desoldered the 5 diodes on the PS board. they all tested fine.
        I spent several hours going over every connection in the amp section, rewiring the filament wires, tube connections, ground connections, etc. the amp still hums - left channel is 2.3mV, right is 1.8mV.
        Peter - bear with me :) - i'm familiar with what feedback does, but i'm can't figure out which connections are part of the feedback loop just by tracing the circuit. could you be specificy which wires need to be disconnected? also, what is the heater center-tap?
        I had previously removed paint at the circuit ground and safety ground. There is continuity between the ground bus and chassis. Resistance between ground bus and safety ground pin of the IEC socket floats between 0-0.3 ohm.
        also, i'm confused by your reference to heater wiring on the PWB. are you referring to the filament wires?
        btw - in response to your questions from a while back: this is the P-P ella and with choke.
        
        Re: One step at a time. - PeterP 10:00:11 04/17/06 (1)
        
        In Reply to: Re: One step at a time. posted by tokyofist on April 15, 2006 at 16:44:29:
        
        I haven't abandoned you. My home computer has a virus so I'm using a laptop on a different network.
        The feedback that I'm referring to is the small gauge wires connected to the output transformer or output terminals. IIRC there is an RC across the output terminals and feedback is the wire that goes from the output to the input stage. (Since I haven't seen one, in the P-P Ella, it could be either from the output transformer or the output terminals.)
        As for biasing up the heater voltage, don't worry about that for now. Let's get it working stock first.
        I'll be back. ;^)
        Take care.
        Pete.
        
        Re: One step at a time. - tokyofist 18:50:38 04/18/06 (0)
        
        In Reply to: Re: One step at a time. posted by PeterP on April 17, 2006 at 10:00:11:
        
        With inputs shorted and feedback disconnected, the hum is audibly louder. In fact, with just the inputs shorted, hum is also louder than with inputs connected.
        
        Re: One step at a time. - Victor_A 02:55:00 04/16/06 (1)
        
        In Reply to: Re: One step at a time. posted by tokyofist on April 15, 2006 at 16:44:29:
        
        Are the Power Supply filter capacitors OK ? I am almost sure, that the problem is in the power supply. (B+)
        
        Re: One step at a time. - tokyofist 06:32:08 04/16/06 (0)
        
        In Reply to: Re: One step at a time. posted by Victor_A on April 16, 2006 at 02:55:00:
        
        Unfortunately, I don't have equipment to properly test them. Upon visual inspection, none are bulging and I don't see any signs of leakage. On the PS PCB, the smallest caps (on the far righthand side) measure ~12Kohm across the leads. The larger two pairs of caps measure in the megaohms.

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