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Is my 274b/n defective
Is my 274b/n defective
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Old 22nd June 2016, 01:54 PM   #1
cowneko is offline cowneko  France
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Default Is my 274b/n defective

Hi everyone,

I'm facing a problem with a new rectifier tube in my 300B SET amp.

The amp is a Raphaelite CSK30 kit I completed some times ago that works wonders with the actual tubes (5Z3P rectifier, WY2p regulators, 6SJ7gt GE drivers and Shuguang 300B/98 Power tubes).

I ordered a Full Music 274b/n mesh plate rectifier and the beast is beautiful with it's visible filaments.
I put the 274b tube in (it's listed as directly compatible with the amp) and turned on the amp:

-at first, no problem, the filaments started glowing normally, as well as the other tubes, no vibrations or strange noises. Playing some music, no problem, great sound......then.....
-after nearly 1min, one of the filaments (not the other) began to glow brighter and brighter I switched the amp off and waited for it to cool down.
-I checked everything, from solder joints to unloaded voltage from the transformer and all was normal.
-I tried a second time, this time the tube seemed to behave normally, but after more than 4 minutes, the same filament began to go brighter again, I tried to take a picture of it before it became white, but the fuse blown before I got the time.
So the delay before the filament goes wild is inconsistent.
-I had the time to notice a white dot on the filament.
-The tube is not dead, but I don't want to risk trying again.

I think it's a defective tube, but I may be wrong,

What do you think of it?

Here's a picture of the tube before it begins to go nuts (it's the rear filament, lined with pin 2 and 8)
Click the image to open in full size.

Here's the shematic of the amp
Click the image to open in full size.
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Old 22nd June 2016, 02:06 PM   #2
petertub is online now petertub  Sweden
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Is my 274b/n defective
Replace with 2 1n4007 diodes and be happy!
The 274 seems broken. In the meantime you can reinstall the 5z3p
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Old 22nd June 2016, 02:07 PM   #3
Mooly is offline Mooly  United Kingdom
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Is my 274b/n defective
Interesting (and a beautiful valve as you say).

It would appear to be a problem with the valve... and just a thought... could the dim (normal) side actually be the faulty part and the overheating side the one that is passing all the current ?

If it all works OK with the original parts then it does seem odds on favourite the new valve is faulty. I assume its being used within its ratings (current) ?
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Old 22nd June 2016, 02:41 PM   #4
cowneko is offline cowneko  France
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Quote:
Originally Posted by Mooly View Post
Interesting (and a beautiful valve as you say).

It would appear to be a problem with the valve... and just a thought... could the dim (normal) side actually be the faulty part and the overheating side the one that is passing all the current ?
Clever that may be exactly what's going on...

Quote:
Originally Posted by Mooly View Post
If it all works OK with the original parts then it does seem odds on favourite the new valve is faulty. I assume its being used within its ratings (current) ?
The 300B are biased at 65mA, but I don't know how much current is drained from the rectifier, I suppose it's within tolerances as the 274B is explicitly given as drop off replacement for the 5Z3P, and the amp worked no problem till then.
How can I check the current drained from the rectifier? looking at the voltage drop through the 30R 10W resistance maybe?
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Old 22nd June 2016, 02:54 PM   #5
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Do you have an oscilloscope?

If this is the case, measure the waveform at the cathode of the rectifier, where it joins to the resistor. There, you must see a sinusoidal rectified of equal amplitude. If not, it may be that the switch in the transformer's secondary be defective, overloading one side of the double diode.

Also, try removing that switch, it appears to be useless.
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Old 22nd June 2016, 02:55 PM   #6
azazello is offline azazello  Bulgaria
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Put res. 1 ohm after chock and measure U on it. Current I = U/1ohm.
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Old 22nd June 2016, 03:01 PM   #7
cowneko is offline cowneko  France
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Do you have an oscilloscope?

If this is the case, measure the waveform at the cathode of the rectifier, where it joins to the resistor. There, you must see a sinusoidal rectified of equal amplitude. If not, it may be that the switch in the transformer's secondary be defective, overloading one side of the double diode.

Also, try removing that switch, it appears to be useless.
I have a scope, so I can check that
That's right that I don't need the switch since I don't plan on using 2A3 tubes in a forthcoming future.

Quote:
Originally Posted by azazello View Post
Put res. 1 ohm after chock and measure U on it. Current I = U/1ohm.
I'll try the shunt resistor also

thanks a lot for those advises, I'll let you know how it turns out.
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Old 22nd June 2016, 03:05 PM   #8
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Good luck. Keep us posted, and if you can, paste a photo of the 'scope screen, so we can help better.
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Old 22nd June 2016, 03:08 PM   #9
cowneko is offline cowneko  France
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Good luck. Keep us posted, and if you can, paste a photo of the 'scope screen, so we can help better.
I will, thank you so much
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Old 22nd June 2016, 05:22 PM   #10
Mooly is offline Mooly  United Kingdom
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Is my 274b/n defective
Quote:
Originally Posted by cowneko View Post
The 300B are biased at 65mA, but I don't know how much current is drained from the rectifier, I suppose it's within tolerances as the 274B is explicitly given as drop off replacement for the 5Z3P .............
That is well within the ratings of the 274B.

If you add two identical resistors putting one in series with each anode then you should be able to see very similar volt drops across each. If one is doing all the work then that shows a problem with the other half of the device.

I would think two 10 ohm 1 or 2 watt carbons would be fine as a test.
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