This seems wrong?

WildOne · 24th May 2011, 01:14 PM

I was looking around the bay and found a ST-70, This guy shows a pic of it lite up and it looks like it is going to blow up! The tubes are glowing very bright?

Is this right! Or something is causing these tubes to really glow. None of the 18 I have do this?

Dynakit Stereo 70 Tube Amplifier | eBay

WO

SY · 24th May 2011, 01:16 PM

Combination of photographic exposure and some red plating of the output tubes. I'd only take that amp if I could get a lowball price.

quikie22 · 24th May 2011, 01:19 PM

seems dodgy, plus the fact that it is sold as is is extremely suspect. All he needs to do is connect a source and a pair of cheap speakers to test the thing.
Also the pictures show that he has powered up the amp with no load attached to the secondaries.. one of the best ways to blow the OPT if there is any spurious signals...

WildOne · 24th May 2011, 01:31 PM

Thanks

I thought it looked funny. If mine did that I would look for a problem.

WO

bigwill · 25th May 2011, 11:51 AM

The guy said he just plugged in the tubes and turned it on, the bias is probably way off, hence the red plates...

sreten · 25th May 2011, 12:39 PM

Quote:

Originally Posted by quikie22

Also the pictures show that he has powered up the amp with no load attached to the
secondaries..one of the best ways to blow the OPT if there is any spurious signals...

Hi, this is a complete myth ... what alleged mechanism operates ? , rgds, sreten.

Doz · 25th May 2011, 12:40 PM

... or it's a 120V amp on 220v

Doz · 25th May 2011, 12:42 PM

Quote:

Originally Posted by sreten

Hi, this is a complete myth ... what alleged mechanism operates ? , rgds, sreten.

Output tubes push current into the OPT, which is now just an inductor in effect... the reflected voltage is large enough to damage the opt insulation. Whilst I've never done it myself, I have certainly seen a few output tubes destroyed because if it.

quikie22 · 25th May 2011, 12:51 PM

@sreten: I have not tried it but was advised never to do it. Feel free to try/do it anytime and let us know. Anyway to quote what someone once kindly explained to me...

"Unloaded means basically reflecting an infinite impedance to the output stage. This will make the output signal much higher than normal, potentially causing output tube and socket arcing, and worse, perhaps arcing over of the wires inside the OT secondary, causing OT failure. "

Loren42 · 25th May 2011, 01:08 PM

Quote:

Originally Posted by SY

Combination of photographic exposure and some red plating of the output tubes. I'd only take that amp if I could get a lowball price.

+1

24th May 2011, 01:14 PM	#1
WildOne diyAudio Member Join Date: May 2011 Location: Great NorthWest	This seems wrong? I was looking around the bay and found a ST-70, This guy shows a pic of it lite up and it looks like it is going to blow up! The tubes are glowing very bright? Is this right! Or something is causing these tubes to really glow. None of the 18 I have do this? Dynakit Stereo 70 Tube Amplifier \| eBay WO

24th May 2011, 01:16 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Combination of photographic exposure and some red plating of the output tubes. I'd only take that amp if I could get a lowball price. __________________ If there's a sucker born every minute, where do the rest of them come from?

25th May 2011, 12:40 PM	#7
Doz diyAudio Member Join Date: Jul 2010 Location: Sat Down	... or it's a 120V amp on 220v __________________ "To err is human.. to make a real balls-up requires a computer"

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24th May 2011, 01:19 PM	#3
quikie22 diyAudio Member Join Date: Sep 2009	seems dodgy, plus the fact that it is sold as is is extremely suspect. All he needs to do is connect a source and a pair of cheap speakers to test the thing. Also the pictures show that he has powered up the amp with no load attached to the secondaries.. one of the best ways to blow the OPT if there is any spurious signals...

24th May 2011, 01:31 PM	#4
WildOne diyAudio Member Join Date: May 2011 Location: Great NorthWest	Thanks I thought it looked funny. If mine did that I would look for a problem. WO

25th May 2011, 11:51 AM	#5
bigwill General Nuisance diyAudio Member Join Date: Dec 2004 Location: UK	The guy said he just plugged in the tubes and turned it on, the bias is probably way off, hence the red plates...

25th May 2011, 12:51 PM	#9
quikie22 diyAudio Member Join Date: Sep 2009	@sreten: I have not tried it but was advised never to do it. Feel free to try/do it anytime and let us know. Anyway to quote what someone once kindly explained to me... "Unloaded means basically reflecting an infinite impedance to the output stage. This will make the output signal much higher than normal, potentially causing output tube and socket arcing, and worse, perhaps arcing over of the wires inside the OT secondary, causing OT failure. "

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