Red spots appeared on 300B anode

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IDM
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#1 Red spots appeared on 300B anode

Post by IDM »

Hi,

Quick question. I was listeningto my 300B SET's last night and noticed a red spot on one of the 300B's mesh anodes. I have used the amps for about 2 years with no sign of this in the past, and when built did check the HT was correct. Is the most likely reason going to be that the valve needs replacing or can HT lift up for other reasons? The amps are LD2+ with GZ34 rectifiers.

I assume the best way to check this out is to swap the 300B's round and see if the fault follows the valve and if so replace and if not pull the amp apart and check the HT voltage?

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Ian
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Nick
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#2

Post by Nick »

Yep, I would also check the voltage on the cathode (assuming cathode bias) and work out the dissapation from that.

Also worth checking if there is any DC across the grid to ground, there may be some grid current. Not knowing the circuit its hard to say beyond that.
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andrew Ivimey
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#3

Post by andrew Ivimey »

If I remember the LadyDay had a driver valve or various shapes cap coupled into 300B, so you shouldn't get any nasties if all the components are fine and no dry joints and HT is what it ought to be.

Measuring dissapation would be reassuring anyway, only if you compare the current across the cathode resistor in both channels. But it could just be the valve.

fingers' crossed.
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Mike H
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#4

Post by Mike H »

Meshes are a bit lower Pa are they? A plate type one is 25W or something max., IIRC


 
 
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#5

Post by Nick »

Standard 300b spec is 40w max dissapation.
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Mike H
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#6

Post by Mike H »

OK, so it would seem - had to look it up! Although Western Electric datasheet says 36W
 
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#7

Post by Clive »

TJ Mesh are typically setup in the LD amps at 21W. They can't take anything like the full 300B power for long.
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#8

Post by IDM »

I checked the voltages on both monoblocks and found the following:

Both amps had 491V of HT, the instructions say 470V so about 4.3% high which I thought would be ok.

One amp has a measured cathode resistor of 1.03K with a measured voltage of 81V (the instructions say 75V) so I calculate the current as 78.6mA.

The other amp has a measured cathode resistor of 1.01K with a voltage of 77.8V so a current of 77mA.

I am not confident of calculating dissipation, so do these numbers sound OK?
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Ian
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Dave the bass
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#9

Post by Dave the bass »

I work it out to be 80mA and 77mA Ian.

Can you measure the voltage 'across the valve' from anode to cathode? If yes, we can work it out. Put the +ve meter terminal on the anode and the -ve on the cathode and measure the voltage.

Supposing we have say.... 350V Anode to cathode. We multiply the voltage across the valve by the current running through it, in this case approx 80mA. So, 350(V) X 0.08(A) = 28W disp.

Howsat?

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#10

Post by IDM »

Hi Dave, just relaised I should have measured on the anode not the HT!! I will have to pull the amps apart again.

Howwever the output transformer is 3K so could I calculate the voltage drop from 3000X0.077= 231V so 490-231=259V. Or is it not so simple?
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#11

Post by Dave the bass »

The '3K' of the OP TX is probably the Impedance not the DC resistance so no it's not that simple this time Ian.

If you really want to be 'tweaky' you could measure the DC voltage drop across the primary of the OP TX and work out the Resistance of it as we already know the current is 78 to 80mA or so on either amp.

If you could measure the volts across the valve (anode to cathode) you'll be able to work out the power disp yourself I reckon.

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#12

Post by IDM »

Thanks Dave, I will pull the amps apart again tomorrow morning.
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#13

Post by pre65 »

Heres a bit about working out plate dissipation.

http://www.world-designs.co.uk/forum/sh ... .php?t=153
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#14

Post by Mike H »

Given the above numbers, I make it around 32 Watts, but the above numbers could be the wrong ones :D

 
 
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#15

Post by Greg »

If it's 32W, it's far too high for a mesh plate and I'd expect hot spot plate glow. It needs to get down to 21/22W dissipation to ensure valve longevity. This is not my personal research but rather the recommendations of the quality suppliers of the mesh plate valves. They know 'cos they've had to deal with loads of complaints/returns etc. A mesh plate 300B cannot cope with the original 40W dissipation spec'd by General Electric on their original solid plate 300B valve.
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