Russian "Mullard" 12Aat7 tube failures after less than 12 months

[Guest Simonon]

I replaced both of the 12AT7 tubes about 11 months ago on my tube amp with one developing a rather audible hum at 6 months and the other channel failing yesterday with the same issue. A check on my AVO mark 5 tube checker reveals a substantial heater cathode leak on both tubes increasing when warm. Hence the hum.
My conclusion here is that new old stock tubes made in the valve era are the choice.
I also had some Sovtek EL34 tubes for a short time but found they were microphonic and hard to bias after a short time. Hope this info is useful for future purchases of tubes.
 
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[LogicprObe]

Could be too much voltage on the heaters causing premature failure.....................among other things.

[Guest Simonon]

Heater volts are all good and the amps are now running the original 12AT7 tubes from the 1960s again. One thing for sure is that these Russia Mullards bear no resemblance internally to NOS Mullard 12AT7 tubes. I reckon the matetial coating the heaters has broken down and caused a heater cathode leak as indicated by my tube checker.
Will be interested by the experience of others. Note my amps are separate monoblocks

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[unclekanus]

1 hour ago, LogicprObe said:

Could be too much voltage on the heaters causing premature failure.....................among other things.

 

If everything is ok with the amps then you shouldn't be having a problem I would have thought so might be worth a thorough check over? That said, current production tubes don't seem to last like the old stuff so it may be a quality issue..

 

1 hour ago, Simonon said:

My conclusion here is that new old stock tubes made in the valve era are the choice.

 

If the amps are fine then maybe try tracking down some Siemens e81cc or ecc801s. I have used both and they sound great and are 10,000 hour rated tubes. I run an e182cc in my pre (also 10k hour rated) at high current and have been flogging it hard for 3 years. I tested it on an AVO recently and a new tube should measure 36ma per section and my used one measured 38/40ma respectively. High quality and great sounding tubes . If I'd been using an ecc99 of current production I would have chewed through at least one by now.

[Guest Simonon]

I shall look out for some Siemens tubes I think and also give the amps a birthday as they are still running original 4uf filter caps in the power supplies dating from 1963. Some of the original resistors on these Centre Industies type 3 amps have also drifted in value so reckon I have an excuse for a rebuild. I still have the original EL34 tubes which measure okay on the AVO. One of the original mullard 12at7s eventually went gassey which made me put in the Russian Mullards. Same batch so maybe they had an issue.
Time to order some quality parts for the next 50 years of flogging [emoji3]

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[Guest Simonon]

I shall look out for some Siemens tubes I think and also give the amps a birthday as they are still running original 4uf filter caps in the power supplies dating from 1963. Some of the original resistors on these Centre Industies type 3 amps have also drifted in value so reckon I have an excuse for a rebuild. I still have the original EL34 tubes which measure okay on the AVO. One of the original mullard 12at7s eventually went gassey which made me put in the Russian Mullards. Same batch so maybe they had an issue.
Time to order some quality parts for the next 50 years of flogging [emoji3]

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[Guest Simonon]

I shall look out for some Siemens tubes I think and also give the amps a birthday as they are still running original 4uf filter caps in the power supplies dating from 1963. Some of the original resistors on these Centre Industies type 3 amps have also drifted in value so reckon I have an excuse for a rebuild. I still have the original EL34 tubes which measure okay on the AVO. One of the original mullard 12at7s eventually went gassey which made me put in the Russian Mullards. Same batch so maybe they had an issue.
Time to order some quality parts for the next 50 years of flogging [emoji3]

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[Bronal]

10 hours ago, Simonon said:

I shall look out for some Siemens tubes I think and also give the amps a birthday as they are still running original 4uf filter caps in the power supplies dating from 1963. Some of the original resistors on these Centre Industies type 3 amps have also drifted in value so reckon I have an excuse for a rebuild. I still have the original EL34 tubes which measure okay on the AVO. One of the original mullard 12at7s eventually went gassey which made me put in the Russian Mullards. Same batch so maybe they had an issue.
Time to order some quality parts for the next 50 years of flogging

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 I have a matched quad of NOS Siemens 12AT7s if you're interested.

[unclekanus]

Simon,

 

There is some good info on the Trimax amps here in the forum - 

In that thread there are some links to circuit diagrams which will be useful I'm sure.

Do you have the tube rectifier version? Great amps and definitely worthy of a a rebuild. I'm sure a lot of those old carbon comps will have drifted over the years and leaky caps can be a problem too. I'd love to see some photo's if you can find the time 

[VanArn]

The 12AT7 has a low heater to cathode peak voltage rating; 90 max volts. Given that  the Trimax directly couples the 6AU6 valve  first stage to the cathode coupled phase splitter, then there can be the danger of a heater to cathode breakdown if the cct dc  voltages drift. I think that is what LogicprObe is alluding to in his post. Perhaps another valve type and some resistor changes may be a better proposition in  the Trimax amplifier.

[Guest Simonon]

Agreed and I just replaced some out of spec resistors around the 12at7 and found one of the paper oil filter caps had spewed. These 12at7 tubes also lay horizontally so I reckon the heater formers had sagged due to the way they are constructed. Looking inside the tubes they look flimsy compared to original Mullards . Found some Nos Siemens tubes that measure good and have just rebuilt the amp. Can somebody explain why I should use paper oil caps for HT filtering as I can fit a higher rated electrolytic and get better specs.

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[Guest Simonon]

If anybody has 4 Ducon 2S40 4uf 400v paper oil caps that are good I am interested to return the amps to original. I have stuffed some 10uf 600v electros in to get the amps going again. To be honest Ht rail regulation is much better.

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[Guest Simonon]

Maybe I will forget about the ducons and Russian KBG oil filled types. Research suggests they contain PCBS. My old capacitors have therefore been wrapped and disposed of in our pcbs bin at work as they cannot go on landfill. Safer to stick with electrolytics and glad I wore gloves when I removed them.

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[VanArn]

The 12AT7 has a low heater to cathode peak voltage rating; 90 max volts. Given that  the Trimax directly couples the 6AU6 valve  first stage to the cathode coupled phase splitter, then there can be the danger of a heater to cathode breakdown if the cct dc  voltages drift. I think that is what LogicprObe is alluding to in his post. Perhaps another valve type and some resistor changes may be a better proposition in  the Trimax amplifier.

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 Problems with the site have caused my previous message to be repeated. Another headache for Marc to solve.

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On with the issue of old  oil filled capacitors. Not so long ago PCB* fluids were banned for use in capacitors and transformers and disposal required special treatment. The world has moved forward and I would suggest that modern rectangular cased  high crystalline segmented polypropylene capacitors  ( HCPP )  available from Element 14 and Mouser would be the solution to your filtering problem and they would avoid the need to try and find high voltage electrolytic caps to suit.

                                                                                                         * PolyChlorinatedBiphenyl 

[unclekanus]

Obbligato make a modern 630v 4.7uf polyprop/aluminium film in oil cap which I have used before in power supplies with good results. Well made and not too pricey but I don't think anyone in Oz sells them so you'd have to order in.

[LogicprObe]

7 hours ago, unclekanus said:

Simon,

 

There is some good info on the Trimax amps here in the forum - 

In that thread there are some links to circuit diagrams which will be useful I'm sure.

Do you have the tube rectifier version? Great amps and definitely worthy of a a rebuild. I'm sure a lot of those old carbon comps will have drifted over the years and leaky caps can be a problem too. I'd love to see some photo's if you can find the time 

 

See!?

 

Mustud is useful!

[Guest Simonon]

All back to sounding very nice and minus the dripping pcb oil [emoji2]

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