Can anyone help me identify any problems with this TT DHT - looking at the silver grid, it seems 'blackened' (maybe heat/current) . Is that normal?
Tube is a Hytron 210
Tube is a Hytron 210
Grid to filament short circuits can happen if the valve has been mechanically damaged.
Try bringing it up on a low anode voltage, (80 -100V) with a low value of grid leak resistor (as low as 1k even).
Measure voltage across the 1k to look for current in the grid.
Inspect the grid to filament gap on both sides, with and without heating and then anode voltage
If you shake the valve (cold) is there any powder loose in it?
Try bringing it up on a low anode voltage, (80 -100V) with a low value of grid leak resistor (as low as 1k even).
Measure voltage across the 1k to look for current in the grid.
Inspect the grid to filament gap on both sides, with and without heating and then anode voltage
If you shake the valve (cold) is there any powder loose in it?
It appears to me that the vacuum has been removed (missing the glass envelope does that).
Is the glass envelope still there?
If not, of course it is no longer possible to test the tube.
A grid that has a localized blackened section, but nowhere else along the grid is not normal.
1. If the working tube had a red plate localized hot spot, it might blacken the grid that is near the plate local hot spot.
2. If the grid is held to a very low DC resistance to ground (like a low DCR interstage secondary to ground), then if the filament also has a low DC resistance to ground, and if the filament touched the grid, that might be the cause of the blackened section of the grid.
No other ideas from me.
A complete and accurate schematic might be helpful in determining other possible causes of the blackened section.
Is the glass envelope still there?
If not, of course it is no longer possible to test the tube.
A grid that has a localized blackened section, but nowhere else along the grid is not normal.
1. If the working tube had a red plate localized hot spot, it might blacken the grid that is near the plate local hot spot.
2. If the grid is held to a very low DC resistance to ground (like a low DCR interstage secondary to ground), then if the filament also has a low DC resistance to ground, and if the filament touched the grid, that might be the cause of the blackened section of the grid.
No other ideas from me.
A complete and accurate schematic might be helpful in determining other possible causes of the blackened section.
Here are more images and description. The seller claims:
"No cracks in base. very clean pins. Base to bulb seal very tight.
This tube has been fully tested using a Hickok 539C tube tester recently calibrated. Testing for Transconductance (Gm uMhos) and Plate current (Emission Ma.). Tested as follows:
1650/790 at 10 ma plate current"
https://www.ebay.com.au/itm/126626620988
"No cracks in base. very clean pins. Base to bulb seal very tight.
This tube has been fully tested using a Hickok 539C tube tester recently calibrated. Testing for Transconductance (Gm uMhos) and Plate current (Emission Ma.). Tested as follows:
1650/790 at 10 ma plate current"
https://www.ebay.com.au/itm/126626620988
The grid can also be heated if it is driven positive in some cases. If there is no grid to filament short, that could the other reason. The best option may be to test for emission and then get the tube hot enough to activate the getter.
If there was a short between filament and grid, then would the seller be able to get the reading of Transconductance (Gm uMhos) and Plate current (Emission Ma.). (Tested as follows) :
1650/790 at 10 ma plate current
1650/790 at 10 ma plate current
The short can be position dependent. Might short if on its side and the filament can sag against the grid. Upright no short may exist.
I’ve seen this in many tubes, including Pentodes and beam tetrodes.
It’s due to heating of the grid wires, leads to a coloring of the metal. It just looks black, normally it’s dark brown or blue.
From many heat cycles. It just means that the tube is used.
It’s due to heating of the grid wires, leads to a coloring of the metal. It just looks black, normally it’s dark brown or blue.
From many heat cycles. It just means that the tube is used.
Thanks.
The seller provided additional photos, and the coloring seems is uniform in relation to the filament.
Attachments
Not seen a datasheet for this version so perhaps not such a strong reading. Would need to find a stash of boxed NOS to test for comparison. The heater requirements may be different too. Bit like an early VT25A.
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The white tape appearance makes it oxide. TT is metalic metal. AFAIK there were two types of oxide, azide being the earlier.
I've got an amp that has four Hytron 841s (same tube family) and these tubes see regular A2 duty. The grid wires are plain silver and don't exhibit this coloration.
Nice. I love the look of the silver (colored) grid wires.
I guess I'll see how this 210 sounds, as an alternative to my 801a.
I so want to get and try an 841 😍
I guess I'll see how this 210 sounds, as an alternative to my 801a.
I so want to get and try an 841 😍
In DHTs the grid discoloration can be a symptom of excessive grid current, or just some hours of normal operation. I would be cautious if you do not know the seller and the tubes are not NIB. The fact that he is disclosing this sort of detail is actually rather encouraging. I guess it depends on whether you are willing to take a risk of getting an occasional dud in a batch and whether the asking price reflects what you think is a good value.
Make sure he packs them extraordinarily well, I have had a few rather sad moments when much anticipated DHTs arrived and were completely trashed.
Good luck, I hope the transaction ends up to your satisfaction.
Make sure he packs them extraordinarily well, I have had a few rather sad moments when much anticipated DHTs arrived and were completely trashed.
Good luck, I hope the transaction ends up to your satisfaction.