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It is currently 16 Dec 2018, 13:58

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PostPosted: 06 Dec 2018, 12:20 
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Joined: 29 Jun 2016, 15:59
Posts: 60
Bruce & Team,
The latest findings. Sorry to say that:
1) grounding to the signal grounds the metal cases of the coupling caps did not make a difference,
2) placing a short between the grid of the lower triode section of the driver stage and ground, did not make a difference also. The noise is still there and at the same level.

Danny


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PostPosted: 06 Dec 2018, 21:26 
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Joined: 04 Jun 2008, 20:59
Posts: 3860
Location: Arizona, USA
Hi, Hummm. What I would like you to do is describe exactly (as possible ) what is still going on as I have lost track of all the things we have tried and we should start to trouble shoot from the basics. Clearly we are missing something. Additionally please tell us about the set up / configuration and everything that is connected to it. I don't know about anyone else, but I have lost track of what has been tried and what happened. I appologize for that, but with four fourms, 5 email accounts, a business web site and bunches of individual inquiries I sometimes get lost. BTW, where do you live? There may be a forum member that is nearby and can assist.

Good listening
Bruce

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PostPosted: 07 Dec 2018, 03:30 
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Joined: 24 Oct 2010, 07:05
Posts: 283
Maybe contradicting with the previous message:
- taking out the driver tube eliminates the noise;
- switching off the block (working just on the capacitors charge) eliminates the noise;
- blocking input signal (grounding input RCA or the lower tube grid) does not help;
- grounding FT-3 capacitors does not cure the problem;
- changing ground layout does not help also.
What may I suggest to try: remove the LR8 and replace it with a resistor (the value of the resistor can be estimated from the previous versions of the power secion of the driver tube or just measuring the voltage between in and out of LR8 and count).


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PostPosted: 13 Dec 2018, 11:57 
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Joined: 29 Jun 2016, 15:59
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What may I suggest to try: remove the LR8 and replace it with a resistor (the value of the resistor can be estimated from the previous versions of the power section of the driver tube or just measuring the voltage between in and out of LR8 and count).

Poty,

OK. Thanks.
See the before and after figures. I want to make sure I get it right. Is this what you are suggesting to do?
What are you trying to determine? If the LR8 is introducing oscillations?
What do you mean by "count" in "measuring the voltage between in and out of LR8 and count".

Bruce,

I live in NJ.

Thanks,

Danny


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PostPosted: 13 Dec 2018, 19:11 
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Joined: 24 Nov 2010, 14:39
Posts: 27
Location: California
I've been following this thread and the troubleshooting steps. The one thing that stood out for me was that the noise disappeared immediately when Danny switched off the AC power. The audio continued, noise-free, until the capacitors discharged. Switching off the AC removed whatever ripple content that remains in the rectified and filtered DC. Is it possible that the 120 hz ripple could be exciting some weird oscillation in the LR8? Maybe just a freak thing with just the right amount of capacitance around the LR8? I don't know if that is even possible, but....
The test that Poty suggested, removing the LR8 from the circuit will be interesting....


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PostPosted: 14 Dec 2018, 02:19 
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Joined: 24 Oct 2010, 07:05
Posts: 283
Danny wrote:
See the before and after figures. I want to make sure I get it right. Is this what you are suggesting to do?
Yes, but if you have a capacitor on hand (voltage - more than 350V, capacitance 40-100uF) please put it across the 0.22uF. I also doubt you'll have to remove 910R/220k divider
Danny wrote:
What are you trying to determine? If the LR8 is introducing oscillations?
Any weird thing with LR8. As Jim_O already mentioned - all the experiments points to the power supply circuit.
Danny wrote:
What do you mean by "count" in "measuring the voltage between in and out of LR8 and count".
Sorry, my English is far from perfect. There is a voltage which is dropped on LR8 (let's call it Vio). There is a current for driver part (Id = 1mA for SRPP + 1.3mA for 910/220k divider + 1mA for heaters' lifting). You can use the Ohm law to count the resistor with the same voltage drop: R=Vio/Id. Alternatively you can determine Id just by measuring the voltage across the 33k resistor (Vr): Id=Vr/33k.


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