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PostPosted: 20 Aug 2018, 19:29 
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Joined: 03 Feb 2018, 00:47
Posts: 52
Location: Brisbane, Australia
Very sad this morning. Turned on my Stereo Oddblock EL34 to warm up the workshop a bit and after about 10 seconds, started to produce sound, then crackle, pop and the 100 ohm B+ feed pair to one channel destroyed themselves. :(
Voltages measured last time I power her up were:
main B+ 485V.
driver regulated B+ 285/289 (L/R)
Heater lift is 64 volts.
No delayed B+ at present.

My primary filter caps cop about 520 volts for 10-15 seconds until the tubes conduct, which doesn't seem to worry them. If they had a problem, I reason that it would die immediately power was applied cold.
I suspect I have an EL34 failure.

Q1. Do any of those voltages raise any concerns?
Q2. How do I safely figure out which tube is faulty?

I have a variac to slowly wind things up. for testing. She has only done maybe 4-6 hours running on my bench.

Edit: Definitely taken out the 317.. dead short. Currents were 62mA/63mA within a mA each pair.
Thanks,
Glenn.


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PostPosted: 22 Aug 2018, 22:56 
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Joined: 03 Feb 2018, 00:47
Posts: 52
Location: Brisbane, Australia
Nobody?

It occurred to me while thinking about this, that while nothing is drawing current, not only am I putting excess voltage on the caps, but I am also exceeding the LR8 maximum input voltage whilst in this state. I think I need to get my delay up and running.


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PostPosted: 23 Aug 2018, 07:38 
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Joined: 07 Jan 2018, 07:13
Posts: 46
Agreed,

With a delay those initial high voltages will be pulled down immediately when the HT turns on.

What wattage 100R resistors are you using?


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PostPosted: 23 Aug 2018, 15:39 
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Joined: 03 Feb 2018, 00:47
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Location: Brisbane, Australia
Thanks for responding. 2 x 100R 1 watt metal oxide as per the recommendation. They have been fine up until now, no sign of overheating in normal use.


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PostPosted: 23 Aug 2018, 17:28 
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Joined: 02 Oct 2014, 20:36
Posts: 28
can i use a pot bias and lm 317 to each kt 88? separate bias for each tube... thanks!


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PostPosted: 24 Aug 2018, 16:12 
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Joined: 24 Oct 2010, 07:05
Posts: 296
gonzalo guerrero wrote:
can i use a pot bias and lm 317 to each kt 88? separate bias for each tube...
No, you can't. In the device the common cathodes' LM317 works as "balancer" transfering the signal to the passive tube.


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PostPosted: 25 Aug 2018, 15:08 
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Joined: 04 Jun 2008, 20:59
Posts: 4052
Location: Arizona, USA
Hi, I have been away for a few days. 2 one resistors are not enough. You need at the very minimum a pair of 2 watt ones. The design calls for 2 five watt ones. The dissipation is nearly 2 watts when operating. The logic is as follows. With normal 5 watt resistors the tolerance is about +/-10% so while the are supposed to be the same they very seldom are. If there is a fault (usually very large current, up to what ever the power supply can deliver) one resistor will fail before the other. But both will fail in sequence. A single ten watt one might not. This arrangement is for catastrophic protection of the power supply. The amps will often still make some music if the CCS fails. I have encountered one such case. It didn't sound right, but still functioned a bit. It took a short while for the resistors to fail but the amp was not damaged. (BTW, the failure was caused by using the amp in the rain!) Depending on the failure mode, the fuse will go first.

You definitely should use a delay to protect the caps.

Good listening
Bruce

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PostPosted: 25 Aug 2018, 19:22 
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Location: Brisbane, Australia
Thanks Bruce. I think I can get 3 watt in either carbon or metal film easily, 5 watt are wire wound.
If the resistors simply failed though, would the 317 not be still intact? It is dead short, and just losing the feed it should not have died?
Thanks,
Glenn.


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PostPosted: 26 Aug 2018, 08:59 
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Hi, Typically if the resistors blow, then the current has to exceed 300ma for a significant time, 500 or more cuts the time to less than a minute usually. To get that sort of current, the LM317 has failed together with something else that caused either an open circuit in the adjust terminal, excessive over voltage (across the LM317) or if the LM317 heat sink is not large enough to handle the usual load. When a LM317 fails it will normally fail in a shorted through mode. This allows the tubes to conduct everything the power supply can deliver. The tubes are still often fine as they can handle the excess better than the resistors (that was why they are used). When the resistors fail almost always the power supply and tubes are protected and still fine. A quick check is to power it up and measure the voltage across the LM317. Any of the test points to the ground will do fine. If it is not over about 25 volts (usually it is in the 30-40 range) the LM317 has failed. Sometimes when it fails it measures about 4 volts, most of the time near 0-1. Based on the normal causes of failure (heating, over voltage, ground connection and once in a while tube internal short) you need to figure out which one caused the problem. Start by verifying that both output tube's grids show about the same resistance to ground. If one is not grounded properly, it will float and upset the circuit. This can lead to one tube hogging the current and causing it to fail. If the one hogging the current is the driven one the amp will still produce sound until something fails. It oddly acts like a single ended amp and not push-pull when such anomalies happen.

Good listening
Bruce

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PostPosted: 26 Aug 2018, 18:04 
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Location: Brisbane, Australia
Thanks Bruce.
It blew as it was warming up, had just started to produce sound so 317 overheat doesn't make sense. It has been working fine, so wiring error is out (no bad joints that I can see and I am very careful with every joint I solder). Resistance measurements match the good channel, so biasing should be fine. LM317 is definitely short circuit, so either LM317 has failed randomly or one of the tubes has failed and taken out the 317 is my current theory. I was sitting next to it at the time and heard a crack, so my thought goes to the EL34's.
To move forward I guess I replace the 317, replace the resistors with 3 or 5 watt (is wirewound ok?) check everything cold again, power up and see what happens. If it fails again, then replace the EL34 pair. Not sure how you decide which of the pair might be crook without sacrificing 317's and resistors???

FYI, The EL34 heaters are AC, driver heaters are DC 7806 regulated, and the whole lot are lifted to 63 VDC by a resistor chain off one of the regulated driver B+ supplies. I assume that is all in order?
Thanks again,
Glenn.


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