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Old 3rd March 2007, 12:33 AM   #1
phn is offline phn  Sweden
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Default 7-pin miniature tubes for audio?

This just hit me. I knew of their existence. I just never cared about them. I don't really care about them now, either. But I thought it could be fun to ask. I googled and searched here for "7-pin." I didn't come up with any relevant hits. (From Google I learned that lots of dealers carry 7-pin sockets.) I'm sure this has been here. But since I don't know which, if any, 7-pin tubes are popular, or even used, I don't know what to search for.

I looked into these tiny tubes and they seem to have specs similar to those of "space-age" novals. They look pretty impressive. Some are designed for VHF, some for audio. They also have relatively low maximum voltage rating. I don't know if the latter is good or bad.

Anyway, has anyone here had any experience with these?
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Old 3rd March 2007, 01:01 AM   #2
wfmali is offline wfmali  Europe
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EL95 - fine tube, runs pretty hot, though. AB with 10k Raa in UL43% should give you about 7W. They are on my radar screen as well.
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Old 3rd March 2007, 01:12 AM   #3
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Well, taking a look at an old RCA tube manual will give some hints.
The 6AV6 is the exact equivalent of 1/2 a 12AX7, and can be used this way, in spite of it having 2 tiny diode plates (which can safely be ignored). These diode plates were used in the 12AV6 version of "all-american 5" radios as the detector & AVC.

Other than that, I've seen circuits using the 6AQ5A (pentode power tube), 6AU6A,
6C4 (half of a 12AU7), the 6CB6A in some voltage regulators,
6J6A,
the 6X4 (power supplies), etc.
In a lot of old Scott hi-fi equipment you'll see tubes like the 6AU6, 6X4, and 6BQ7A (the latter is actually a 9 pin).
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Old 3rd March 2007, 01:15 AM   #4
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I'll check in. I've just started looking at them, and the progress so far is:

I have a power amp that uses a singly-driven diff-amp to drive a pair of (various) outputs in PP. Some guys here are aware of it:

http://www.audiotropic.net/Projects/machine1.html

Until recently, I used the 5965 / 7062 / E180CC for the input tube(s), because it has the best gain with the lowest RP I could find, making it suitable for this service almost uniquely. There are complications. It's a computer tube, and so not fabulously linear (varies by brand, some are not so bad), and since it's only really specced at full on saturation and cutoff, they tend to be somewhat wayward in between. Especially in regard to the two devices in the single bottle. Since I'm using them as a diff-amp, I need pretty close matching between sections, a requirement that is currently putting about 60% of my tubes in the circular file.

A few months ago, I went looking for a replacement, and one of the criteria was that it had to be a single triode, to get around the sections-in-an-envelope predicament. I came upon a bunch of the 7-pin minis you have been looking at (I bet), all called 'VHF amplifiers'. I went researching with the vendors, and came upon a poster girl.

The 6GK5. There are half a dozen near equivalents, but they all have the same chars; two thirds the rp of the 5965, and more than twice the gm, and so a mu of ~65, instead of ~45. Plus, of course, they completely sidestep the section matching problem. The kicker? They're widely available in bulk nests (look on eBay) for $250/hundred (Mullard) or $150/hundred (Hitachi). The vendors will kiss you where the sun don't shine, you take a C of these things off their hands, I have several hundred; thus I divulge.

I now have a breadboard 6V6 amp up with these, and it sounds noticeably better than the 5965 version. I'm gonna keep my 5965s for past client servicing, and these little darlin's are going in ALL the future amps.

Was that an okay report?

Aloha,

Poinz
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Old 3rd March 2007, 01:27 AM   #5
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Actually, you can design a good amp, using the 6X4 (full wave rectifier) for the power supply, a few pairs of 6AV6's (or others), and 6AQ5A's for the output stage. The 6AQ5's are actually touted as having the same characteristics as the 6V6's, providing you keep and eye on the max voltage (etc) specs and don't push them too hard.
So an all 7-pin amp should be fairly easy to set up, and very close, component wise to combo 9 pin/octal designs, at least in the 10-12 watt or so push-pull designs. You'll just need a few more tubes, as most are not dual-element.
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Old 3rd March 2007, 02:51 AM   #6
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6AU6 (or 6AU6A) is quite often used in audio, either as a pentode voltage amplifier or as a CCS. It can also perform very well strapped as a triode.
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Old 3rd March 2007, 03:13 AM   #7
rdf is offline rdf  Canada
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9002 triode, the 7-pin equivalent of a 955 acorn. Dirt cheap, nicely linear CCS loaded.
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Old 3rd March 2007, 03:45 AM   #8
mach1 is offline mach1  Australia
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I'll second the 9002 and add the 8532.

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Old 3rd March 2007, 05:30 AM   #9
phn is offline phn  Sweden
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Yeah, it was the low Rp, high mu tubes, like the EC900, that caught my attention. They aren't quite the Western Electric 417A and 437A. But an Rp of 7-8k and mu of 70-80 isn't too shabby. Tube matching could be a problem, I guess. Though not as severe as the 437A.

Edit: Error correction.
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Old 3rd March 2007, 06:51 AM   #10
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The 6AV6 (1/2 a 'X7) has already been mentioned. The 6AB4 is 1/2 a 'T7.
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