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PostPosted: 26 Oct 2018, 09:10 
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Joined: 24 Oct 2010, 07:05
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Danny wrote:
1) Not clear what you mean by the negative feedback setting being correct and not backwards. The 8 ohm speaker output of the transformer is currently connected to the negative feedback SPST switch. Which is normally in the open position (negative feedback off). What do you mean by backwards?
If the switch is open there is no negative feedback, so the question is out of scope. If the switch is closed there can be a mistake in connecting the primaries' wires which make not negative, but positive feedback.
Danny wrote:
2) By shorting the inputs you mean just connect a cable having RCA connectors between the two inputs, correct?
No, just short the RCA jack (the central plug to the shell) of each input.


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PostPosted: 27 Oct 2018, 14:19 
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Joined: 09 Dec 2013, 01:11
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Location: Canada
Danny wrote:
Team,
I wonder if you can give me some suggestions on how to try to fix this.
Few months ago I built 2 OddBlocks (using KT88s & 5751s) with subwoofer outputs.


The subwoofer outputs are one possible input for the noise, if not isolated properly. Try to physically disconnect the subwoofer.


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PostPosted: 27 Oct 2018, 17:05 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The outputs could cause issues, but not all that likely. Just disconnect them when looking for the noise. When you have a system with noise (or hum) issues it is key to figure out which components are doing it. The only way that typically makes sense is to start at the power amps disconnecting all input sources and work forward to preamps and input sources. When you find a combination that causes the problem then you need to determine why.

Good listening
Bruce

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PostPosted: 28 Oct 2018, 12:46 
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Joined: 29 Jun 2016, 15:59
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Well, sorry that say that shorting the inputs (connecting positive to ground of the plug) did not make a difference. So it’s not the source.
Also, same for disconnecting the subwoofer cables.
Negative feedback is off.
Again, it is not a buzz or oscillating or hum noise. It is white noise.
I am not an expert at this. Thus, if anyone can describe the procedure for how to isolate the problem, I would appreciate.
Could it be the choice of power tubes? I am using JJ blue KT88.

Thanks again,

Danny


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PostPosted: 28 Oct 2018, 14:07 
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Hi, In these amps it is safe to remove the 12SL7 driver tube (when the power if off) and then turn on the amp and see if the noise is in the driver or output section. Please verify first that the idle currents are correct as a failed LM317 can cause noise and other problems (like tube over heating). If you have more than one amp (I recall you said you had a pair) try swapping output tubes from one to the other and see if there is a combination that is quiet.

Check also to verify that the dc voltage on the driver side of the coupling capacitor is about 1/2 the B+ applied to the upper triode. This can vary by as much as 25% and be OK, but if it is way out of wack it usually indicates either a wiring error that could include a component out of tolerance or a failed tube. Also verify that the heater circuit is not some how connected to the cathodes of the 12SL7s. Easiest way to do that is to measure the DC to signal ground on the heaters and on the cathodes.

Hiss in general is quite abnormal in these amps. What speakers are you using? Any with sensitivities of 96 db/watt or lower should be very quiet. Those unusual ones with sensitivities much higher than that might have audible hiss. In that case I would reset the amps (regardless of tubes type) to the lower power setting of 60-62 ma. It ought to be quieter.

Good listening
Bruce

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PostPosted: 28 Oct 2018, 21:36 
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Joined: 07 Jan 2018, 07:13
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Hi Bruce,

Would you be able to explain the difference in the heater reference in the Oddblock schematic vs the Forewatt? I notice the reference connects to the positive side in the Forewatt and the negative side in the Oddblock.

Thanks


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PostPosted: 30 Oct 2018, 04:54 
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Joined: 24 Oct 2010, 07:05
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I think the question has been answered here: viewtopic.php?f=9&t=3545&p=53960&hilit=heat%2A+lift%2A#p53960


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PostPosted: 30 Oct 2018, 12:27 
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Joined: 29 Jun 2016, 15:59
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Bruce & Team,

Hi. In my investigation to eliminate the background white noise that I am experiencing, I have come to the suspicion that the LM317HVs may be faulty. Can you please let me know how to check them? What voltage levels should I see between the I, A, O points and ground?
I am using KT88s and the voltages across the test point are 92mV for one amp and 93.5mV for the other.
But I have notice that, most of the times, when I power up one of the amps the voltages across the test points will hover around 240mV for up to 4 minutes and then will go down to 93.5mV. It varies. Sometimes it will settle quicker.
The other amps goes directly to 92mV. But, both amps seem to work fine.
The voltages between the tab (V out) and ground is about 1.5V. The heat shield is mounted to the metal plate, thus grounded.
Thanks,

Danny


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PostPosted: 30 Oct 2018, 14:59 
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Hi, The currents are right, but it seems that one tube is heating up much faster than the other. The max value should not exceed about 180-190ma though when both are added together. 240 is high. The voltage from the tab to ground should be between 1 and 2 volts so that is correct. It should be the current (sum of both tubes or about 0.18 amps) times the sense resistor value in ohms. So typically .18 time 6.8 = 1.2 volts. The LM317 will either work all the time or be cooked. I have never found any other mode. Since it does control the current when both tubes are conducting (roughly 92 plus 92 ma) then it is functional. They typically fail in a shorted through mode and the currents will be way high...essentially as much as the power supply can deliver.

What happened when you started up the amp without the 12SL7 inserted? Did the noise go away or was it still there?

Good listening
Bruce

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PostPosted: 30 Oct 2018, 18:57 
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Bruce,

I don’t use then12SL7 driver tube. When I remove the 5751 the white noise goes away. Thus, I guess this means that the problem is in the driver section. Where should I look next? A while ago I used a 12AX7, but that did not make a difference.

Related to the issue of one of the amps hovering about 200-240 mV (on both test points, thus both power tubes) upon power-up and then settling to 93.5 mW after 1-4 minutes, I swapped the power tubes from the second amp. But, that did not make a difference. Thus, the problem is with the LM317 I guess. Should I replace it? Is it worth it since the amp seems to be working? Is this going to damage the power tubes eventually?

Thanks,

Danny


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