6148 vs 12AT7 - anyone have experience?

[nashvillebill]

Let me add one more clarification on heater voltages: We often say "6 volts" or "12 volts" when actually we mean 6.3 volts and 12.6 volts respectively. (Note that this happens to coincide with the voltages available from a lead-acid battery)

Now there is a tolerance of roughly 5% , so we can run a nominal 6.3 volt tube on something like 5.9 to 6.6 volts on the heater without seriously affecting tube life. Get further away and tube life suffers, too far away and the tube won't work. The tube handbooks have a nifty little graph showing tube life versus voltage.

One advantage of indirectly heated cathodes is that tube characteristics wont' change too appreciably with a slight deviation in heater voltage, so if you read 6.25 volts on the heater there's no reason to get too obsessed...unless you want to get obsessed....
And note this is the actual heater voltage, not the transformer readings unloaded without tubes--the transformer may read closer to 7 volts, unloaded.

 

[datafone]

Well, there's several confusing posts here. My books don't list a 6415. It might be in Frank's Tube Pages though.

So perhaps it may seem redundant to state this, but for anybody who may stumble across this thread in the future, here goes: A 12 AT7 or 12AX7 or 12AU7 may be run with a heater voltage of 12 volts by applying the heater supply to pins 4 and 5. One heater wire goes to 4, the other wire to 5. Pin 9 is not connected to anything. The same tube can be run on 6 volts: wire one heater wire to pin 9. The other heater wire must be wired to both Pins 4 and 5 (merely bridge them together).

Not all tubes have this dual-voltage capability. A 12FQ8 for example does not have the middle tap (got any Wurlitzer organs? Plenty of 12FQ8's) so it must always be run on 12 volts. Same with the 6$$* tubes where the $ would be a letter and the * a number, like the 6SN7, they must always be run on 6 volts.

The pure number tubes like the 7027and 7247 and others don't follow any scheme as far as heater numbers... They are whatever number was open next. The 7027 is a 6 volt big bottle power tube, the 7247 is a 9 pin miniature dual triode either 6 or 12 volts

So when somebody has a tube XYZ and they want to sub it for a ABC tube, they need to look at the number of pins, first. Obviously a 9 pin miniature tube won't fit a Compactron socket. If the number of pins are the same, look at each individual pin to see if it's the same pin out. Are pins 4,5 and 9 the heater pins? Same with plates and cathodes and grids. If they are the same, no rewiring necessary, if they aren't the same, the socket will need rewiring to even attempt to use the different tube.

So let's say the wiring is the same, or you can make it the same. Now...is the heater voltage the same? How about heater current? Not so critical in miniature tubes, but critical in power tubes. Ok, now how about plate voltage? Does the replacement tube have limits on plate voltage? Then comes the rest of the electrical characteristics. Similar gain (transconductance)? Then look at the tube charts and ideally draw load lines to see if they will be biased similarly without changing anything. It's possible the new tube will be biased into cutoff, or clip too easily if the new tube will be at a far different bias point.

I may have left something out, that was the speedy version, so if I've fat-fingered a response, don't shoot me. I'm only the piano player...oh wait, I don't play piano.

And if all this is old-hat to anybody here, I'm not trying to sound condescending either, but there are some new folks who may think "hey this new tube has 9 pins and it's a 12 something, this tube in my amp also has 9 pins and it's a 12 something also. Let's see if this new tube works!!"

All I was trying to point out is it isn't as simple as it may seem. And I was a little off on the 6 meaning 6v and 12 meaning 12v, as you have pointed out that this doesn't always apply.

That's a much better post on it, thanks mate

 

[cbox83]

I'm pulling them out today. They started popping yesterday. Today, one channel was nearly out when I turned it on. Not sure whats happening yet, but will say as of right now they are not fit for Fisher gear. Replacing them with the 12AT7's eliminated the problem. Will research further in due time.

 

[mjw21a]

A shame. They work well in the Yaqin MC10T

 

[datafone]

Lots or reasons why they can work, and why they may not, and it comes down to the application. How the heaters are wired up is one thing.

I won't use any valve that is not a direct replacement, but that's just me.

 

[GordonW]

My guess, is that since half the tube is used in a cathode follower, that maybe the 6414 didn't like that much heater-to-cathode voltage differential (you exceeded the max heater-to-cathode voltage for that tube). I can't find the spec for that voltage in the online references- it would be interesting, if that COULD be found, to see what that rating is, for the 6414.

If the Yaqin is not using a cathode follower and doesn't have the tube inside of a feedback loop, I'd guess it would work better there?

Oh man, I just noticed something else. Don't know why I didn't notice this immediately! The max plate voltage spec'd for the 6414 is 200v. The 12AT7 can handle 300v. Perhaps the Fisher was exceeding this voltage (especially on startup), by enough to cause an issue...

Regards,
Gordon.

 

[datafone]

Nice pick up on plate voltage (one reason I wouldn't have used this in the Yaqin without checking)

And the other points!

So many possibilities to why this tube may often not be suitable.

Edit: Actually Gordon, I thought someone would have noticed the plate voltage sooner, maybe you were the only one that looked at the pdf.

 

[nashvillebill]

I didn't quite figure out if it was a 6414 or 6415 or ??? so I did not call up the tube data sheet.

For anyone who does not have this link, Franks Tube Pages are here http://frank.pocnet.net/sheets11.html

This is the most thorough site on the 'net.

 

[mjw21a]

I'm more worried about my having accidentally turned on my amp for 30 minutes without speakers hooked up. Seems to have adversely affected sound. Stupid of me

 

[datafone]

I'm more worried about my having accidentally turned on my amp for 30 minutes without speakers hooked up. Seems to have adversely affected sound. Stupid of me

Half hour is not good, if music was played while there was no load, it can be very bad news for the OPT's as far as I understand it.

I have gotten away with if for brief moments with no signal present (no music passing), but only for those reasons.

 

[datafone]

I didn't quite figure out if it was a 6414 or 6415 or ??? so I did not call up the tube data sheet.

For anyone who does not have this link, Franks Tube Pages are here http://frank.pocnet.net/sheets11.html

This is the most thorough site on the 'net.

indeed a good reference, I bookmarked it long ago, but so many bookmarks I tend to google it instead.

I need to clean up my bookmarks......too many :scratch2:

 

[mjw21a]

Yes, it's definitely had a negative impact on sound. Introduced a slight buzz on the leading edge of some notes. Annoying

 

[datafone]

If the OPTs are damaged they don't get better. But I have no idea what has happened as a result of the mistake.

To know exactly what has happened, you would need to get a good tube/valve tech' to check it out.

 

[mjw21a]

Damn.... Once fixed I think I'll remove the speaker switch and the Denon amp so it can't happen again

 

[datafone]

Ah, a speaker switch, now I see how the mistake was easily made.

Even though I rarely disconnect my speakers at either end, I have a habit of scanning the connections on my amp and speakers before each power on. It's a ritual, check connections volume at zero, pre turned on first and then the amp, and switching off volume at zero and amp off first then pre off.