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Old 14th September 2008, 10:36 PM   #1
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Default Used freebies precipitate questions

I picked up a couple of hands full of freebies today, no white getters but as I examined them I came up with a few questions.

I am sure that they are all used. There were

2 RCA 6SN7GTB
2 6CB6 (Westinghouse and Motorola)
1 Hitachi 6AU6
1 Motorola 6AH6
1 Motorola 6U8
1 Motorola 12AX4GT
1 Westinghouse 6W6GT
1 Unknown 7pin stuck in a shield

At first I thought the 6SN7s were shot as I didn't see any getter. Then I noticed a little bit of getter flash visible just above the black plastic base. I did some searching and it appears that it might be something called a bottom getter version (first I had heard of it). The visible getter only comes up 1/8" to at most 1/4" above the base and is rather uneven. It looks a lot like the picture attached. Also note the plates on these are black. Does this condition sound like a normal used tube that might still be good? How high up is the getter on this type of tube generally in new condition?

On several of the small tubes I noticed blackened spots (looks like heat) on the getters (top getters) in either smallish spots or in one case a broad line nearly all the way across the tube top. Does this give any meaningful indication of condition (other than that it is obviously used)?

On the tube that I can't identify there is a metal ribbed shield (marked "standard shield) that is press fit over the tube. By applying moderate pressure to the tube from either end I can not get it to move. I am afraid to push too hard for fear of damaging the tube. Any tips on how to get it off without hurting the tube or removing any markings on it?

Appreciate your input
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File Type: jpg bottomgetter.jpg (38.1 KB, 228 views)
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Old 15th September 2008, 04:22 PM   #2
kevinkr is offline kevinkr  United States
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Can you photograph the 6SN7 tubes in question? That might provide a clue as to what they really.

I have seen side and bottom getter versions of the 6SN7, mostly Russian IIRC, and had to say who made them here. I tend to favor Sylvanias, (all mine are top getter) but I do also have RCA/GE/Tungsol amongst others - most, but not all are top getter types.

In terms of being able to determine whether these are good or not either you need a tube tester or you can build a simple jig and do some measurements. Visual inspection is not going to tell you much.

Sorry I could not be of more help. Try to find/build a tube tester, either DIY or a cheap score on eBay or hamfest/flea-market/yard sale to answer these questions. Get one that measures mutual conductance or transconductance. Some of the older Sylvania 200 series testers are a good compromise, and cheap.

The metal shield is probably one of the cheap things seen in tvs and relatively low end am/fm tuners, usually it just one piece of overlapping metal - find the overlap and gently pry it just slightly further apart with your fingers and then the shield is likely to come off easily.
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Old 15th September 2008, 10:39 PM   #3
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Thanks for the tips. I plan to put together some kind of chassis that I can easily rewire (barrier strips) to do static mutual conductance tests. The tricky part will be putting together the variable power supplies needed. Bias supply could be easily done with a trio of 9V batteries. Heaters... 6 & 12 V no problem 17, 38, 22 etc... a little harder. The B+ Trickier. No spare high V trannies on hand. Got one that provides 300V by using doubler that is slated for a later project so could use it in the mean time. For my purposes regulation would seem to be unnecessary as I will be able to measure and adjust each supply before each measurement. Just need a high power rheostat or something like that to voltage divide the B+.

I should be able to get a camera late tonight to take some pics.
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Old 16th September 2008, 02:16 AM   #4
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It looks like I won't have the camera until tomorrow afternoon. I did get the shield off however and it was another Westinghouse 6CB6. From the data sheet it looks like a nice little pentode with good gm and rather high Ip capability.

Unfortunately, unlike the data sheets for the 6AH6 and 6AU6 there is no data on triode connetion. The other two have pretty good gm in triode mode too and it would be interesting to know how the 6CB6 compares in triode mode.
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Old 16th September 2008, 12:34 PM   #5
ilimzn is offline ilimzn  Croatia
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The ones in the photo look like 6N8C, soviet version of the 6SN7 and yes, they do have a bottom getter, and appear to be perfectly fine.
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Old 17th September 2008, 12:06 AM   #6
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Default Here are the pics

Here are some pictures. The first one is the nicer looking of the two 6SN7s.
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File Type: jpg better6sn7sm.jpg (99.6 KB, 67 views)
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Old 17th September 2008, 12:07 AM   #7
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A closeup of the getter on the better 6SN7.
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Old 17th September 2008, 12:08 AM   #8
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The other 6SN7 doesn't have as much getter showing.
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Old 17th September 2008, 12:13 AM   #9
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This shows the darkening on the top of one of the 6SN7s. It doesn't look bad but is a telltale sign that it is used.
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File Type: jpg 6sn7darkeningsm.jpg (72.8 KB, 58 views)
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Old 17th September 2008, 12:15 AM   #10
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This shows much more severe darkening on one of the 7 pin pentodes. Notice that it is very dark all the way across the dome.
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File Type: jpg littleguydarkeningsm.jpg (83.8 KB, 52 views)
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