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Old 24th March 2012, 12:43 PM   #1
johnvvc is offline johnvvc  United Kingdom
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Default Wrong base ???


My first posting so hopefully will not get it 'too' wrong... My question is not directly related to audio - well not to hifi audio anyway

I'm restoring an old marine radio receiver and the audio output valve is an N37. The radio stopped working a few days ago and I'm doing some checks. I suspect the fault is in the audio stages. I can hear lots of stations just like when the radio was working previously but the volume is drastically reduced and it is distorted.

The above should not present too many problems I hear you say - check voltages etc...

When I removed the N37 I see the valve in there is a GEC 7025 - a double triode. The N37 which should be in there is a pentode. Now I presumed someone had rewired the valve base to accomodate the 7025 which annoyed me a bit as I'd prefer the radio to be as original as possible.

However - and here's the puzzle - when I downloaded data sheets for both these valves I see they have different bases.

The N37 is a B7G base whilst the 7025 is a B9A base. The base in the radio does not appear to have been changed - and is a B7G.

So I have a 7025 with a B7G base - which the data sheets tell me should have a B9A base.

I'm confused as I'm obviously getting something wrong - but what???

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Old 24th March 2012, 02:47 PM   #2
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Michael Koster's Avatar
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Location: Eureka, CA
You don't have a 7025 dual triode. The tube may be marked 7025 somewhere but it's not the tube on the 7025 data sheet. Your socket is probably original and you should try the exact replacement tube or a proper substitute. If you post a photo of the tube maybe someone will be able to provide more info.
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Old 24th March 2012, 06:14 PM   #3
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is this your valve ? except for the marking ?
N37 @ The National Valve Museum
Judge: This court appreciates that you invented physics, Mr.Newton, but unfortunately you can't have a patent on it.
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Old 24th March 2012, 08:03 PM   #4
DF96 is offline DF96  England
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Is 7025 a date code? Made in week 25 in 1970?
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Old 24th March 2012, 08:40 PM   #5
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hidnplayr's Avatar
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As for the distorted audio, likely it's the coupling capacitor wich is leaky.
Real tubes have top-caps
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Old 25th March 2012, 11:06 AM   #6
johnvvc is offline johnvvc  United Kingdom
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Default N37

Thanks for the feedback guys.

Michael I think you have it in one.

The valve obviously 'is' an N37 or an equivalent. The radio worked pretty well before it developed this low very low volume distorted audio so the valve was OK.

payloadde - yes mine looks pretty similar to that in the museum...the internal construction looks similar and the size is about the same - the 7025 could be a date code.

DF96 - you've probably hit the nail right on the head.

I'll try and get some photographs of this valve but I'm trying to handle it as little as possible to stop any more of the lettering coming off.

What's printed on it is:-

The GEC badge
What might be AR_83
Underneath it 7025 followed by the letter Z

In other words:-

7025 Z

The AR_83 is a bit smudged but I think I have it correctly.

The above in a printed box.

I've looked for some equivalents on the 'net but no luck.

Thanks again for your input, time maybe to do some fault finding. Coupling caps high on the list. The valve 'appeared' to be drawing excessive anode current pulling the HT rail down a lot. In this radio the rail is ~110 volts (designed to work off ship's power) so not much to play with...
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Old 25th March 2012, 01:24 PM   #7
DF96 is offline DF96  England
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Sounds like a military version of the valve. I believe Z is the military code for GEC manufacture (but that is from memory, could be wrong). The valve may have a CV number on it. I understand that A3283 is a GEC code for an extended life variant of the N37 - ypu could be seeing a smudged version of this. GEC valve names often begin with A.
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Old 25th March 2012, 01:50 PM   #8
johnvvc is offline johnvvc  United Kingdom
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Default N37

DF96 - thanks for that.

Interesting as the "_" I'm seeing could be the bottom bit of the figure 2 in which case it all makes sense.

I was surfing earlier and read someplace that the letter Z on GEC valves indicate a pentode - and that again would figure as of course the N37 is a pentode.

If we can assume that 7025 is a date code that just about wraps it up. Now all I've got to do is to try and breathe some life back into this old radio!

Thanks everyone for your help.
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Old 25th March 2012, 03:53 PM   #9
johnvvc is offline johnvvc  United Kingdom
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Just as a follow up the N37 debate, see the pictures at this site:-

新管 GEC A3283 - 金春音响 - 港澳台深圳珠海地区及国外 - 胆艺轩音响材料网 音响|交易|电子管|胆机|胆机网站|胆机论坛|胆机材料| 涑雠变压器|电源牛|电阻|电容

(the link looks a bit weird but it does work...)

Interestingly, and on a totally different note, look at the "8" on these valves - seems as if it's printed upside down...

Are these Chinese copies ???
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Old 25th March 2012, 04:02 PM   #10
DF96 is offline DF96  England
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Z in a valve name can indicate a GEC small signal pentode (e.g. Z759). The N in N37 indicates a power pentode or tetrode, so in this case the Z shows the manufacturer. In most cases UK military valves would have something like KB/Z to indicate GEC, KB/D for Mullard etc. GEC special quality valves often start with A (e.g. A2900).

Early military valves used a letter code for dates; later ones used the numeric code you have.
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