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PostPosted: 10 Oct 2018, 06:27 
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Joined: 07 Jan 2018, 07:13
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Will do! Thanks a lot for your input. I might even leave the 680R alone and put an additional resistor going to each LR8 to drop to 235V.

I think I’m going to temporarily install a pot before the LR8 and dial it to 235V going into the regulator then replace it with the appropriate resistor.

Also, I’m probably going to replace the the LR8N with the LR8K because the K version has about 3 times the power dissipation. I also ordered the appropriate heat sink for the K version in the 252 package. I believe those specs are with a suitable heat sink installed. I figure it can’t hurt. Otherwise, I think they are the same.

Thanks


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PostPosted: 10 Oct 2018, 15:51 
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Joined: 24 Oct 2010, 07:05
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Please allow some additional voltage to cope with the mains deviations.
Also, taking into account that the problem repeats I'd think of a deeper understanding of the voltage "before" LR8. You measured RMS voltage and it is not the same as minimal voltage at the point (you may have big ripples for example due to a bad capacitor in the raw (input) filter).


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PostPosted: 10 Oct 2018, 16:46 
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Location: Arizona, USA
Hi, For one thing the 160 volt rated cap is not enough. It needs to handle the output voltage of the LR8 minus about 1.25 volts. A 250 volt or higher one is needed. An LR8 will not by itself motorboat. My experience is that when they think or are actually running too hot they just shut down and will not restart until the power is removed. I have never seen one cycle. To motorboat one of two conditions come to mind. Feedback through the power supply or a self resonating portion of the circuitry. I believe I would first replace the 160 volt caps as they may be failing and then if needed check the filter caps on that channel. A meter on the B+ while the unit is operating may point to the fault area as well. You can increase the 0.1uf capacitor by a large margin. I have not discovered any issues with up to 10uf there. 1 uf ones could be used there if you choose. I often do in my builds. They do form a time constant with the resistors. Bigger capacitors may improve filtering a bit but at the expense of fast transient response.

Good listening
Bruce

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PostPosted: 10 Oct 2018, 17:22 
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Thanks guys, I really appreciate the input.

I will replace the caps with 250V units. I didn’t realize they needed to take the output voltage. Each of the three times this happened it happened slowly( after about 15+ hours of use) and would get progressively worse. The failing cap makes sense now because the problem went away when I replaced all of the parts in that section, unfortunately I used the same caps. I don’t think I have bad caps upstream because only one channel is doing it.

As for the input voltage to the LR8’s, should I drop the input voltage to 240V or so with a dropping resistor?

Last question, would using the “K” variant be ok in this case? I prefer surface mount. I’m not sure if it wouldn’t work as good in this case though.

Thanks again


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PostPosted: 10 Oct 2018, 18:01 
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Hi, The K4 is designed for surface mounting. There is another version that is not though. They don't seen to be all that common. I think it was called a "G" something. I like the K4 version as it can be soldered to a "surf board" easily. It won't handle the max power that way, but it is at least twice what the N3 will handle. With the K4 You can easilyt go for 50 volts across it at 10 ma or so. I would juggle the B+ feeding it to between 25 and 50. It is not critical.

Good listening
Bruce

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PostPosted: 10 Oct 2018, 18:11 
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Ok sounds good,

I’ll go with the K4 then on a surfboard... FYI Mouser carries some tin plated aluminum heatsinks that are meant to solder right to the heat tab of any TO-252 package. If you never need max dissipation.


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PostPosted: 10 Oct 2018, 19:53 
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Joined: 03 Feb 2018, 00:47
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Location: Brisbane, Australia
The Master has arrived. Sorry if I sent you off on a wrong track.

Bruce, I want to build a Forewatt but I have access to a transformer that has 150-0-150 winding as well as heaters. 300V (full wave bridge) will be way too much, but 150 will be too low. I could waste the extra voltage in resistors before the regulator I guess. Am I wasting my time trying to use it?
The right transformer is ridiculous postage over here from Edcor.

Thanks,
Glenn.


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PostPosted: 10 Oct 2018, 20:28 
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Hi, Gotcha. OK. Two possibilities. One would be to use a choke input filter. It will hold the B+ down to around 350ish. Then use suitable resistors to get it down to the 260 range. Let the LR8K4 deal with the rest to get it to 215. Alternatively you can use cathode resistors in the Forewatt of 680 ohms and use 275 B+ on it. Then you could juggle the B+ and LR8 to have at least 20 volts but better with 25-50 drop across it. There would be a very slight increase in noise (1-2 db measurable, but not audible since the design is for -90 to -95), and an increase in output (as is it can easily do 20volts RMS and why would you need more?). The heater reference voltage would have to go up as well.

Good listening
Bruce

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PostPosted: 12 Oct 2018, 19:55 
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Hello all,

So in an attempt to fix this motorboat issue again, I replaced the LR8N with a new LR8K and I replaced the .1 160V cap with a .1 250V. I also added dropping resistors going to each LR8K to drop to 250V. The motor boating is still there on the right channel.

The only other components it could be are the diode, 470k resistor, or the 4.7cap. When I swap the left and right channel plate wires coming from the 4.7 cap and going to the plates with the 1uF cap, that tells me that the problem couldn’t be after the 4.7cap because the problem follows the right channel regulator portion.

Could the diode, 470k resistor, or 4.7 cap cause this, or am I missing something else? Also, would it be ok to use a 2.2 cap in place of the 4.7 since I have those in stock?

I’ve fixed this twice before by replacing all the components in the regulator portion but I’m not exactly sure which one fixed it.

I’m wondering if I should just get rid of the regulator portions all together and just use some dropping resistors to get to 215V like the earlier schematic to simplify things?

Thanks


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PostPosted: 12 Oct 2018, 20:34 
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Hi, A most irritating situation. First I would not get rid of the regulators. They enhance the B+ filtering. The loss in S/N is not great, but a few dbs, but still the circuit works extremely well and is out there in hundreds of amps and preamps. Do you have access to an oscilloscope?

So yes any of those components could cause it. I would replace the diode first. It could be breaking down and not withstanding the B+. Not common, but possible. The capacitor could be causing it. Try the 2.2uf ones. My sense however is that it relates to either grounding or possibly physical placements of components. Possibly the ground circuit of the capacitor or on the LR8. A bad ground or weak one could pulse the B+ to the tube. Another possibility is if your power amp has input blocking capacitors and the 1meg resistor after the coupling capacitor is not well grounded or connected. The combination could act like a low frequency oscillator. BTW, other circuitry that has a capacitor that is in parallel to a resistor can form a "relaxation" oscillator. I have seen power supply filters do odd things like this. Usually it can be traced to a grounding or high resistance connection problem. The actual configuration of the preamplifier itself has rather low gain and as such nearly impossible to get to oscillate on its own. Did you use silver or lead free solder by chance? I'm all for the environment, but those types tend to make connections that look fine but are not good on occasion. The silver and lead free solders require more heat for a longer time to work correctly.

Good listening
Bruce

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