[tomb]

CA2 is the cathode bias capacitor on the tube. It shunts the circuit from the backside of the tube and is not even in the circuit unless the music wave exceeds the bias differential relative to the supply voltage. As such, it sees very little voltage - much less than 10V, for instance. When you see some caps selected at 25V, it's because that's the most readily available cap in that brand and at the capacity needed (1000uf). As with any capacitor, there's no penalty for selecting a higher voltage rating - except a higher price and a larger cap (larger may not fit).

CA7 is directly in the circuit - every bit of the signal passes through that cap. The two CA7 caps split the supply voltage and sit at 13.5VDC. However, good practice says we size the caps to take the full potential voltage of the circuit in case of failure. If one of the CA7 caps fail, the other CA7 will see the full supply voltage on the other side - 27V. Hence, we rate those capacitors at 35V (25V is too small).

 

[aphexii]

So I snagged a killer deal on three NOS 12FM6 tubes on eBay the other day, and they just arrived. I figured it wouldn't be bad to have some backups, and at $5 shipped for all three, I just couldn't pass it up.

How would I go about matching the tubes up? (or seeing if they match at all). Not exactly sure what I'm looking for I suppose.

Any help would be greatly appreciated!

 

[Beefy]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif I just bought up this blokes last 6 12FK6 tubes. Normally I wouldn't be bothered, but sitting at home writing grants is *boring*......

I finally managed to pick these tubes up from the post office today.

I've never seen photos of top getters on 12FK6 tubes before, only 12AE6? But here are six of them......

The three in the first pic are all Philips MiniWatt (USA), the second pic are Mullard and AVW and claim to be made in Australia. The date on one of the Mullard boxes is 15/10/64...... always amazes me that these things are over a decade older than I am

 

[tomb]

Quote:

Originally Posted by aphexii /img/forum/go_quote.gif So I snagged a killer deal on three NOS 12FM6 tubes on eBay the other day, and they just arrived. I figured it wouldn't be bad to have some backups, and at $5 shipped for all three, I just couldn't pass it up. How would I go about matching the tubes up? (or seeing if they match at all). Not exactly sure what I'm looking for I suppose. Any help would be greatly appreciated!

A tube tester.

Short of that, you can try to match the construction at least. There's no guarantee that they're actually Tung-Sol, but usually the tubes of the manufacturers that made them were theirs, but not always. Balancing the tubes is easy with the bias, but if they are mis-matched on output, you'll notice that one ear in your headphones seems more dynamic than the other. You can still try to match them by ear that way.

EDIT: I've listened to these so long now, my ears have begun to easily tell the difference in mismatched tubes. It's seldom that the mistmatch is great, though - the differences are most often very subtle. Every once in awhile, though, the differences are so great that mismatch is easily heard.

 

[tomb]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif I finally managed to pick these tubes up from the post office today. I've never seen photos of top getters on 12FK6 tubes before, only 12AE6? But here are six of them...... The three in the first pic are all Philips MiniWatt (USA), the second pic are Mullard and AVW and claim to be made in Australia. The date on one of the Mullard boxes is 15/10/64...... always amazes me that these things are over a decade older than I am

Wow - you've made a real find with the ones on the right. I've never seen getters and top spacers such as the ones you have on the right. I'd guess that they may really be made in Australia, perhaps. Another mfr to add to the big four, perhaps.

The ones on the left are GE's with top halo getters and "J" plates.

The white etching and dots on the glass are tell-tale. Also, no one else made "J" plates like that - flat with a lipped edge all around. Top halo getters are also rare with 12FK6's - only GE's did that, AFAIK.

 

[Beefy]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Wow - you've made a real find with the ones on the right. I've never seen getters and top spacers such as the ones you have on the right. I'd guess that they may really be made in Australia, perhaps. Another mfr to add to the big four, perhaps.

FWIW, all three of these have the 12FK6 in a 'flattened octagon' like the webpage says is common for RCA tubes.

 

[tomb]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif FWIW, all three of these have the 12FK6 in a 'flattened octagon' like the webpage says is common for RCA tubes.

That's true - RCA's always are marked with an etched, squished octagon, but it doesn't quite look like an RCA octagon. Theirs is thinner than that, but I'd have to look and compare them up close to tell for sure.

One thing's for certain - RCA never used top spacers like that. I've never seen any like that on any Millett tubes - or the getters. Those getters are giant rings - never seen any like that anywhere near that size.

The plates, the grid housing, and the glass look typical, though, but those weren't limited to one mfr. Maybe RCA sold many of the components, but assembly was done by someone else in Australia.

 

[Beefy]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Maybe RCA sold many of the components, but assembly was done by someone else in Australia.

In any case, nothing blew up on any of them in initial testing

 

[bperboy]

Okay, long story short, my minimax got yanked off my desk.. tube broken.

I've replaced the 12AE6As with a pair of 12FK6s, but I'm having some voltage reading issues. Here's the rundown.

PSU: now reads about 14v, down from 27v

TubeL: -2V at minimum bias. (always stays negative)
TubeR: 0V at minimum bias (biases up correctly)

BufferL: about -2V (turned off immediately, but maybe transistors are fried?)
BufferR: 90mV (biases relatively normally)

Looks like my left channel is the problem. Anything specific I should be testing for?

 

[tomb]

Quote:

Originally Posted by bperboy /img/forum/go_quote.gif Okay, long story short, my minimax got yanked off my desk.. tube broken. I've replaced the 12AE6As with a pair of 12FK6s, but I'm having some voltage reading issues. Here's the rundown. PSU: now reads about 14v, down from 27v TubeL: -2V at minimum bias. (always stays negative) TubeR: 0V at minimum bias (biases up correctly) BufferL: about -2V (turned off immediately, but maybe transistors are fried?) BufferR: 90mV (biases relatively normally) Looks like my left channel is the problem. Anything specific I should be testing for?

Sorry to hear about it. Obviously, something else got broken. I'd go over the entire board and collection of parts very carefully. Also look for traces on the bottom of the board that may have separated from their pads.

 

[bperboy]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Sorry to hear about it. Obviously, something else got broken. I'd go over the entire board and collection of parts very carefully. Also look for traces on the bottom of the board that may have separated from their pads.

Remind me not to do anything requiring intelligence in the morning. The ground test point is in the middle. Not the right. D'oh.

 

[pinkfloyd4ever]

Quote:

Originally Posted by Beefy /img/forum/go_quote.gif I've never seen photos of top getters on 12FK6 tubes before, only 12AE6? But here are six of them......

FWIW I've actually been listening to my Max with a pair of top getter 12FK6's for pretty much all of the year or so since I finished my max...guess I got lucky. They're RCA branded but made by GE. They came in a fairly new looking box, with their newer modern RCA logo as opposed to the older circle logo. The tabs of the box tops have a "(4-69)" printed on them, so I assume that's the manufacture date. I got them in from vacuumtubes.net in October '07 if anyone cares (not that I have any clue if they have any more like em)

Of course, I've never seen any like those on the bottom (hell I've only really seen the 20 or so Millett tubes in my own stash in person.) If even tom has never seen any like em, they MUST be rare. He's been collecting Millett tubes since before yours in the Mullard boxes were even made!

 

[adn6244]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif CA2 is the cathode bias capacitor on the tube. It shunts the circuit from the backside of the tube and is not even in the circuit unless the music wave exceeds the bias differential relative to the supply voltage. As such, it sees very little voltage - much less than 10V, for instance. When you see some caps selected at 25V, it's because that's the most readily available cap in that brand and at the capacity needed (1000uf). As with any capacitor, there's no penalty for selecting a higher voltage rating - except a higher price and a larger cap (larger may not fit). CA7 is directly in the circuit - every bit of the signal passes through that cap. The two CA7 caps split the supply voltage and sit at 13.5VDC. However, good practice says we size the caps to take the full potential voltage of the circuit in case of failure. If one of the CA7 caps fail, the other CA7 will see the full supply voltage on the other side - 27V. Hence, we rate those capacitors at 35V (25V is too small).

Thanks tomb. I made a mistake the silmic is only 25v so I guess I need to place another order with beezar really soon!

 

[tomb]

Quote:

Originally Posted by adn6244 /img/forum/go_quote.gif Thanks tomb. I made a mistake the silmic is only 25v so I guess I need to place another order with beezar really soon!

OK - I was afraid of that, because I've looked at Silmic's before to try them on the MiniMAX, but they just don't make anything that will fit.

There are a couple of guys on e-bay who sell the Cerafines at 470uf 35V. Those are 25mm tall if you're interested - kind of expensive if you include shipping ($3-$5 per cap w/o shipping if memory serves), but still less than Black Gates.


EDIT: Well, shoot - he must've sold them all. There's some 50V on ebay, but those are way too big. Oh well - guess they had to run out of them at some point.

 

[n_maher]

Test fit of panels and initial setup complete. Other than 1 RTFM moment during construction things went without a hitch. Great job on the board and case guys!