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PostPosted: 31 Oct 2018, 17:20 
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Hi, A very strange behavior. Next time you start it up measure the dc voltage between any of the test points and the signal ground. The negative speaker terminal works fine for this. Let us know what you find. The 5751 is virtually identical to the 12SL7 in performance and sound. The early amps used the 5751 and later ones 12SL7. I just liked the looks of an all octal tube set up. Both sound and perform the same. What is the date on the schematic you built the amps from? Then I can tell you what else to check.

Good listening
Bruce

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PostPosted: 01 Nov 2018, 04:34 
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I'd measure the voltages/currents in the driver side and all hints related the heaters lifting.
There could be a faulty capacitor (driver-to-output connection).


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PostPosted: 01 Nov 2018, 11:47 
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Bruce & Poty,
Thanks. Just to be clear, there are 2 issues which are independent of one another.
1) The background/white noise, which is affecting BOTH amps exactly the same.
My main speakers are 96 db/watt. I tried 90 db/watt speakers and the noise went down significantly. The schematic that I am using is the Oddblock Octal dated Febr. 2, 2012. For the power supply I used the May 6, 2015 Unified Schematic for Oddblock Amplifier. I put an LED at the base of the 5751s (for show). Could this create interference in the driver tube? Poty you are suggesting that I could have a faulty capacitor (driver-to-output connection). I use 0.22 uF Russian FT-3 teflon for the coupling caps. But, what are the chances that both of them are faulty? My DC Heaters voltage is rather low (on both amps) - 11.6V. Could this be the problem?
2) Only ONE of the amps is 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'll do more checking/measurements this weekend and report.
Thanks,
Danny


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PostPosted: 01 Nov 2018, 14:23 
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Danny,
I've never come to a faulty FT-3 even when they worked with max allowed voltages across them, but I think it's not impossible. Not both, certainly, but who knows... Maybe the whole batch is bad... You can easily check them taking all tubes out, putting a jumper on anode-cathode of the upper driver tube and measuring voltage across the active power tube grid resistor after powering the block on.
Where is the LED connected to?
While I do not think that heaters voltage is problematic - could you measure ripples here? Could you measure the voltage between a heaters power line and the signal ground (heaters lifting)?
About the second problem: do you have B+ delay?


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PostPosted: 01 Nov 2018, 17:39 
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Hi, I doubt the capacitors are at fault for the unusual current on start up. Since one tube has its grid at ground potential and is not affected by faults with other one. If the other one went high current it would need to go low if everything is functioning as it is supposed to. What I suspect is that something is causing the joint cathode voltage to go quite high and in turn causing something like an avalanche mode in the LM317. Normally this is fatal for the LM317. I have never seen one recover from this event. The really odd thing is that it settles down and works OK after the incident. I wonder if there is a thermally related issue. Something is not behaving when cold, but settles down when warmed up. The only component that would seem likely is the sense resistor in the LM317 output terminal to ground. I believe I would replace it first, before condemning the LM317.

The hiss is quite unusual. It can be tube brand related. I have experienced that some brands and types are quite noisy. Another possibility is the NFB is not functioning properly. Something I may have already mentioned is to measure the DC voltage on the heater string to a signal ground. It should be about 1/3 the B+ on the top triodes anode. The mid point between the sections (where the coupling cap is attached) should be about 1/2 the B+. Anything much different (say 25%) is a cause for investigation. I have found in preamps that sometimes attaching a 47-50uf electrolytic capacitor from the signal ground to the heater string (you have to try both sides as it varies which will be best) can reduce hiss a few dbs. About 1/3 the time it makes no difference and with power amps it might not matter at all as the gain is much less.

Good listening
Bruce

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PostPosted: 02 Nov 2018, 12:09 
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Hi,
I tried attaching a 47uf capacitor from signal ground to the heater strings and it did not help.

If I want to remove the negative feedback circuitry (4.7K resistor and 470pf capacitor right after the neg feedback switch) can I just disconnect them from the rest of the circuit? I.e., at the junction of the 2 resistors and cap of the bottom driver’s cathode. This to determine if the problem is in the negative feedback circuitry. Plus, the switch is always open (neg feedback off).
In the meantime I’ll be taking all sorts of measurements this weekend.
Thanks,
Danny


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PostPosted: 02 Nov 2018, 17:02 
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Hi, Why remove them, just turn the switch off. If wired correctly the volume should increase slightly when the NFB is off.

Good listening
Bruce

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PostPosted: 04 Nov 2018, 17:52 
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my stereo is coming!!!


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PostPosted: 05 Nov 2018, 11:51 
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Bruce & Poty,

Hi. First the good news. I was able to resolve the issue with 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 mV after 1-4 minutes. It had to do with a terminal block that I was using to connect the LM317 to the mother board (housing the power supply, the LM317's sense resistors and tube selector switch). Once removed, the joint cathode voltage has a rush of ~140 mV upon power-up for each power tube, but it quickly settles to to 93.5 mV in 2-5 seconds.

Still having issues with the background/white noise. I replaced the FT-3 coupling cap, but that did not solve the problem I am attaching various measurements that I took for both amps. But, they look good to me. Let me know otherwise. The only one that I am not sure is the lower drive cathode to GND voltage. It is ~1.5 V for both amps. Is this right?

Thanks,

Danny


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PostPosted: 05 Nov 2018, 14:52 
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Hi, All the voltages are fine. Since everything seems OK I would suspect that the driver tubes are noisy. See if you can find another one of a different brand or at least different manufactured batch. Since the amp makes the noise with nothing conencted to it and pulling the driver tubes makes it quiet that puts the problem in that stage. One thing I would try though is putting a 5-10uf capacitor at the upper anode of the driver (where the appx 300 volts is) if you are using the LR8 modification be sure the amp is powered down first as the sudden change on the output of the LR8 will almost always kill it.

A long shot on the noise.....what is the way the grounding is done on the driver stage. Do the input resistors and cathode resistors and capacitors connect in such a way as any part of the power grounding to the output stage has to feed through it to return to the power supply. The order of things does matter. You want to minimize the chance of any power flowing through the same path as a signal return. It will introduce noise. It is true there will always be a little as the cathode current of the driver has the same path. But if the current from the output tubes uses that path it will act like a signal and cause noise.

Good listening
Bruce

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