NewYorkDave 2-bottle killing 12av7s ??? help

Hi,
Ive had my NewYorkDave two bottle for about 3 years now and it's been great but recently it developed a big hum . I tracked it down to the 12av7 so I bought 2 NOS 12av7s and swapped out the bad tube. One of the new tubes sounded great, quiet with no hum, but the other new tube had a huge hum like before. What could be going on? Can I just expect that it's a roll of the dice for tubes?

Two bottle? as in one bottle + the pentode wired as triode as the first gain stage?

I built this same thing in the past, a really ugly layout but decent performance nevertheless. oh and brilliant sound! Oddly enough, it started eating 12AV7 tubes as well. I would swap the tube, and after about a month or two of heavy use the tube would develop nasty 50hz hum related harmonics. Swap the tube, and everything was fine again. And then the tube would develop hum after a while. ad nauseum. I measured everything in it. All numbers (voltages, current, audio) were fine, and never managed to track the reason for this behaviour. It was my first tube project and I was eventually so unhappy with its longevity I ended up recycling its parts for other projects.

I find it very odd you seem to have a similar problem. Maybe just bad luck with old unpredictable tubes?

Take my two channel MILA-1 for example, I have never had to change even one of its four 12AV7´s. It has measured exactly the same from the first time I turned it on.

My only advice is to buy a big load of 12AV7 while they are still cheap and keep rolling the dice.

How are the heaters wired? Are they referenced to ground through a CR or resistors, or floated?

PRR said:

How are the heaters wired? Are they referenced to ground through a CR or resistors, or floated?

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They are not floated...If I measure from one of the heater wires to my chassis ground I get 0.3 ohms. Is that what you mean?

Sounds like you have grounded center tap heaters; you measure same from either end of filament to ground (4-9, 5-9)?  I'm doubting it's the tubes, but at the same time it sounds like what some tubes do when dying.    I wonder about intermittent socket contact on the filament center tap, or in wiring itself.  Parallel or series filament?  12V/6V? 

for what it's worth..when I built my 2 NYD one-bottle's, I ordered 4 12av7 from the states, since I couldnt find any localy. . one started huming for no apparent reason after only a few hours. ..swaping around, showed it was the tube, replaced the tube, everything was fine for a few hours, then hum..?...tried with DC heaters(in a different amp), no hum. back to AC heaters, referenced to ground..hum....with no local source for 12av7 and not wanting to fry another one before I know I didnt mess up my wiring, I replaced with 12au7. no hum. didn't notice any real difference in gain or tone, so left the 12au7 in..still working without a hitch two years later

the 12av7 I got are all NOS Sylvania, and 2 (maby three, one is still in its box)out of 4 where bad...not used to that on nos tubes...I can still use them with DC heaters thou..

I am planing on building two more one-bottle, but this time try ecc85...

j

Since the tube was primarily used for non-audio purposes, it may have filament layout or support issues from some suppliers.  Always a possibility.    Sounds like it's maybe time for DC, and I'm almost certain the 'faulty' tubes will still work fine with DC for years to come. 

Has anyone searched for hum commentary trends on the hi-fi and guitar amp sites?

emrr said:

Sounds like you have grounded center tap heaters; you measure same from either end of filament to ground (4-9, 5-9)?

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yes, it measures the same from either end of filament to ground and I'm using 6.3 V AC

Here's my PSU scheme
http://www.box.net/shared/x527fx9qa0

Sounds like you are using series wiring, which is meant for 12.6V rather than 6.3.  I don't have any experience with this tube, so can't comment on it's behavior when under-heated. 

Looks like your ground reference is through half of the bridge rectifier.  Maybe okay, but kinda creeps me out.  If you look at the JLM PSU, you'll see a pair of 1000 mfd/50V caps in series with the AC path to the 2nd parallel bridge rectifier.  My guts says you want to do that here (between the filament wiring and the bridge), and use either a pair of resistors or a hum adjust pot to create an artificial center tap on the transformer side of the iso caps. 

thanks for the insight Doug,
why did you think I'm using series wiring? just curious....It is actually wired parallel, that is pins 4&5 and pin 9.
I'm going to try DC for the heaters though

edit....ooops, now I see

you measure same from either end of filament to ground (4-9, 5-9)?

Click to expand...

sorry I misread your question

gotcha.   I'm just thinking half of a diode bridge isn't really a proper center tap, and there may be other problems with the approach.  Maybe it is proper, but my gut says no.  I think you wish to be isolated from that, and it may clear some of the 'tube eating'.  

Also, 12VAC wiring with 9 as CT may be an improvement. 

What is the 5VDC used for?  Can you disconnect the bridge and the 5VDC from the 6.3VAC?

What you might want to try is to wire the 6.3VAC to a raised DC voltage above ground this would require a extra supply for the 5VDC.

lots of options to try ....thanks guys!
Gus, the 5V is for relays switching phantom, pad & polarity.

wire the 6.3VAC to a raised DC voltage above ground

Click to expand...

How do you do that?