All You Tube Guru's

[iccaros]

I don't keep messages several years old but you can always email a manufacturer and ask them. Maybe this will answer some of your questions:

http://www.newsensor.com/pdf/electro-harmonix/12ax7eh.pdf

I'll be elevating heaters when I have CF. Period.

I understand now.. I am a little slow..
Your saying that if you do not elivate the heaters, some new 12ax7's have issues as they have changed the speck from 200 or 180 in the case of JJ to 100. So your not saying a 12AX7 makes a bad CF just you must elevate the heater voltage to keep it from dying early 


I do have a Carvin with out raised heaters that has has a JJ 12ax7 for over  year as the cathode follower, and a sovtek 12AX7 before that, which did not die, I was tube rolling as I had a microphic tube, but that turned out the be a NOS 5751

in any case my RCA tube manual dated 1959  says dc component of H-K should not exceed 100v, So I do not believe there was really a change, just Russian manufactures may be building to the Specification and not to what the 12Ax7 of the Past could do.  JJ still list 200 as of 2005,  along with sovtek but I am sure if the fine print was there they would list the DC max at 100 as the RCA manual does.

I also see that EH has the same specification fro 12au7 http://www.drtube.com/datasheets/12au7eh.pdf

In the end I see  a lot of 1960's datasheet saying the same 100v limit H-K. I will research more, and will  keep in mind to elevate my heaters with cathode followers and because even 12au7's  list the same specification in some datasheet.
better safe than sorry.
Thanks for the information. 

[MetalGuy]

Dude, chilax, no one's calling you a liar!

I just don't get why are getting stubborn on the subject. Anyway no bad feelings.

So your not saying a 12AX7 makes a bad CF just you must elevate the heater voltage to keep it from dying early 

Exactly.

[iccaros]

Dude, chilax, no one's calling you a liar!

I just don't get why are getting stubborn on the subject. Anyway no bad feelings.

I think the issue is that we have not seen the issue and your first post just stated that 12ax7's should not be used as CF. We all use them, and have not seen the issues. But then I have a Metric but tone of NOS tubes.. and have only bought a few newer tubes in the last year.

[Cliff Schecht]

So who is making a modern AX7 that can handle higher K-H voltages? I too have the crap-ton of old tubes (NOS and pulls) to source from but I don't like to put these in amps I'm building or repairing for others. I saw someone mention Chinese tubes, are they referring to Sino?

Also how much are you guys elevating your heaters by? I've got a few amp builds coming up that have quite a few cathode followers and...well...better safe than sorry!

[merlinb]

I just don't get why are getting stubborn on the subject. Anyway no bad feelings.

Because it's all too easy for misinformation to spread via forums and become received truth. As a conscientiousness engineer, I don't like myths and legends. While people report incidents honestly on forums, they often then draw false conclusions. Sure valves fail, but WHY they fail needs more then speculation and heresay.

[MetalGuy]

So who is making a modern AX7 that can handle higher K-H voltages?

I know about chinese tubes for sure but which ones exactly I can't tell. They are usually marked 12AX7A or B. Mesa is using rebranded chinese tubes so this should give you an idea. Marshall also started using elevated heaters in some later models (2203KK and JVM410 come to mind). Whether it's a coincidence I don't know.
Since that issue came up I don't feel like playing lottery but if someone is willing to conduct such experiments you're most welcome. I would like to see the results as well.
NOS tubes should be able to handle high h/k voltages the question is how high. Sometimes in CFs h/k voltages is around 150-180V in amps like mesa DR it goes above 200V so I assume if you have h/k specs of 200V for a tube it should be OK in those circuits. If you're going for Mesa DR type of preamp maybe you would need to give it a thought.
I'm elevating at 80-90V regardless of whether heaters are AC or DC.

[craigmillard]

Hey guys! Great info Sounds like elevating is the safest bet no matter what with new tubes and normally i would!

With the current power layout it isnt possible to raise the heaters, so, just for testing should i be able to run the tubes with a higher K-H voltage or is it likely to kill the tubes straight away! Im using the 6n20-ev military russian tubes by the way??

Im thinking of geting a heater transformer to supply both heaters and smps at the moment just need one small enough to fit in 1u:)

[MetalGuy]

If you mean 6N2P-EV (6Н2П-ЕВ in russian) they have 100V h/k rating. According to some russian DIY-ers that they don't handle 250V+ voltages very well and start sounding crappy but that's of course subjective matter. Also don't forget the different pinout.

[craigmillard]

right im thinking of getting this transformer:
http://www.rapidonline.com/Electrical-Power/Toroidal-Transformer-80va-0-12v-0-12v-88-3805

should provide enough for both heaters and smps, can anyone see an issue with this choice before i buy:)

Im assuming i can also create a fake center tap to allow me to elevate the heaters...?

MetalGuy: Yup thats right, mistype by me:) 6n2p-ev. Wanted to give them a go as they are dirt cheap and plentiful as well. I also know banica who is on this board built a couple of preamps with these tubes, mkii and soldano slo, and was happy with them.

[MetalGuy]

MetalGuy: Yup thats right, mistype by me:) 6n2p-ev. Wanted to give them a go as they are dirt cheap and plentiful as well. I also know banica who is on this board built a couple of preamps with these tubes, mkii and soldano slo, and was happy with them.

I also have a bunch but never used them that's why I said "according to some...". Give them a go and decide for yourself 

[iccaros]

MetalGuy: Yup thats right, mistype by me:) 6n2p-ev. Wanted to give them a go as they are dirt cheap and plentiful as well. I also know banica who is on this board built a couple of preamps with these tubes, mkii and soldano slo, and was happy with them.

I also have a bunch but never used them that's why I said "according to some...". Give them a go and decide for yourself 

I have used them, and have them in one amp as a CF that is putting over 100v on the cathode. Still not dead, but its datasheet does say 100v. watch the pin outs.. Heaters are different, you need 6.3 across 4 and 5 with pin 9 should be connected to ground for the shield.
but I have created adapters for placing 6.3 across 4 and 5 leaving 9 unconnected and they work.

[craigmillard]

Cheers guys, i saw on the datasheet that the heaters a different, was thinking of putting some sort of switching inplace to allow different tubes:)

Right im going to give it a go without the elevated heaters for now, but have ordered the traffo above for the future! Should sort the issue and allow more flex!

Just got to fini building the thing now:) what do you guys use for tube socket holes? tried a stepped drill but doesn't work reliably. Considering these:
http://www.bluebellaudio.com/toolspunches.htm

Anyone know what the difference is between a standard punch and thrust race punch?

[merlinb]

Anyone know what the difference is between a standard punch and thrust race punch?

The standard ones have a hex/allen key hole. I think the thrust race ones have a different connection. Unless you have a thrust race, you want the standard ones.
I use these for socket holes:
http://www.rapidonline.com/Tools-Equipment/Circular-sheet-metal-punches-34566/?sid=13286fbc-fc7b-4544-b43e-9ff886d09f1d

[craigmillard]

ow man thats annoying! I have just placed an order with rapid last night and they have already shipped

Never even thought to check if they sell punches!! they are pretty cheap too!

cheers
Merlin!

[craigmillard]

Hey merlin, do those socket punches use any 8mm allen key? or do you have to use the q-max ones?

[merlinb]

Hey merlin, do those socket punches use any 8mm allen key? or do you have to use the q-max ones?

Any allen key will do. I certainly don't have any Qmax ones.

[craigmillard]

Cool cheers merlin!

[brett]

Hi all
RE: over-voltage
A lot of amps run valves hard. In general, rating are soft and the popular types of valves are rugged (if they weren't, they wouldn't be popular). My JTM60 runs EL34s with a B+ at 505V and about 35 mA of bias. That's 18W of dissipation at idle  - for a tube rated at 25W maximum power (!).

RE: Chinese tubes.
The 12AX7 ones with the red star are fantastic. They sound as good as a Sovteks and JJs, but seem much more rugged. I had a couple in my somewhat poorly designed JTM60 - other brands would go microphonic in a year or two, but those Chinese ones lasted 5+ years. The EL84 equivalents were also good (not great) in an 18 watter.
cheers

[Cliff Schecht]

I've heard good things about Sino tubes and also especially about the Shanguang Golden Treasure series if you don't mind spending a little more scratch on your tubes. The hifi guys seem to really like these.

[DougH]

I have heard the stories about Tung-Sol reissues failing when used as a CF. I have a Mullard reissue that died recently. One section was used for a CF. I don't know if that's why it died or not. I did replace it with an old tube I pulled from a Lowery organ. No big loss with respect to the Mullard RI, it was not a particularly good sounding tube to begin with. I'm kind of surprised I blew a preamp tube though, in any case. This was in a modified Windsor which uses DC for the preamp heaters. Haven't investigated elevating the heaters and won't for just this issue, as long as I have a sturdy tube in that position that sounds good.