How much voltage is too much for 12ax7 plate?

[frank57]

There was a mod(it looks like a factory mod) that was done on one of those Korean Hiwatts online.
They raised the cathodes to 15k on v2b and v2a.
So a trainwatt express I guess.
I did a quick test and now the plate voltage tends to go to a whopping 311(from 200)
on v2b with a 100k plate resistor.
Is that safe?
Can that cathode resistor cold bias trick be used to generate more gain
say in a lower gain setup?2 triodes in  preamp instead of 3?
Some schematics of the amp here:
http://www.el34world.com/Forum/index.php?topic=11524.msg106674#msg106674

[sluckey]

Is that safe?

yes

Can that cathode resistor cold bias trick be used to generate more gain
say in a lower gain setup?2 triodes in  preamp instead of 3?

no

[stratele52]

On the RCA tubes manual , they said 330 volts maximum rating

[frank57]

Well I'm a little confused.
I see the sovtek lps data sheet says 330v also, although the jj sheet says 300.
General electric 12ax7 just says design center value:300.
Eh says 300v too.

[sluckey]

Fender routinely put 380-390v on the plate of their tremolo driver tube. Here's one that even put 440v on that tube.
http://www.el34world.com/charts/Schematics/files/fender/twin_reverb_sf_100_schem.pdf

And take a look at the Marshall CF tone stack driver.

[frank57]

I see what you mean.Marshall has 296v on this schematic of a jcm with a similar cathode resistor thing.
I'm sure that would go over on some of them.
Might not be a bad idea to use 10k too.
I think on some of those odd Hiwatts like the lead 30 or DC40 they're doing a similar thing but using 4.7k there.

What is the advantage of doing it that way? The distortion is from the tube being cut off but less noise as opposed to having a 1.5k resistor let's say?What's the worst that can happen to the tube?Burn out sooner?

In general will lower voltages will get you more breakup and higher voltages will get you
more headroom just like a power tube stage?

[firemedic]

Lower plate voltage= earlier breakup.
I have seen a lot of sources say a 12AX7 is rated for 350v, a lot of 12AX7 driver circuits see well over 400.
That draws more current thru the tube, technically wearing it out sooner? 
Raising the cathode resistor value brings the heater-to-cathode voltage limit into play also. This too is routinely exceeded.
So I wouldn't worry about it if I was you, if you somehow fry a tube, get a new one. I haven't had to replace any "worn out" tubes yet.

[frank57]

How would I measure the heater to cathode voltage?
I get 3.3 dc on the cathode and 311 on the plate.
On the grid I get .001.
Would this type of cathode setup increase hum?
One other thing I notice is the speaker wires are not twisted on this amp.Make any difference noise wise?

[firemedic]

No, you have to raise the cathode resistor a lot, i.e. in a cathode follower circuit, for cathode-to-heater volts to ever be an issue. IIRC a 12AX7's limit is something like 180v, and you have, for all intents & purposes, 3.3 volts cathode-to-heater. You're good.
 
I don't know if yer setup will increase hum. But hum is usually induced from the heater wires or anything anywhere near the PT, or tiny/bad filter caps. You should get several yards of Doug's shielded wire & use it in any signal-carrying wire more than a couple inches. It may be overkill but it's worked well for me so far. My builds always start out real pretty but after all the tweaking they degenerate into rat's nests. That's when shielded signal wires help a lot. Ground the shield at only one end. Doesn't matter which end.

I've never twisted my speaker leads- maybe I should but I've never had issues.
 

[RicharD]

Don't confuse B+ for plate voltage.  In that old Fender circuit where B+ = 440V, about 160V is dropped across the plate resistor leaving the plate voltage at about 280V.  Sure when the tube starts swinging it'll exceed its listed rating but preamp rubes are tough.

[frank57]

You might be right. It's a little confusing the way they drew it up.
They do have 100k on the cathode there on v5b too.
So is it 440 on the plate or less?

[sluckey]

According to the schematic, it's 440 on the plate and 13 on the cathode.

[frank57]

Well I gave two 10k cathode resistors a try on this amp and what I get is
about 312 on the plate and 3.24 on the cathode of each section of v2b.

On the one hand it does reduce the noise way down(but I am bypassing the efx loop which adds noise and have a 100kk on the plate of v2b),but you also lose a ton of volume.
So you have to put the master way way up(practically on 10) and the gain on say 4 to get a decent clean.Distortion is ok although perhaps a little clippy.
Now I can see v2b at 10k or 15k which was working,but both?
Not so sure you can get away with this setup as a fix.Not sure what to do there.

A big part of the original problem is way too much gain in the preamp I think plus the bad layout and so on.
You might be better off bypassing v2b altogether(almost eliminates the problem and keeps the volume but tough to play on because the gain way way up the dial ) like I did before and try to get more gain going somehow(any ideas?) or reduce r8 to 47k (like the Hiwatt overdrive lead)in the original amp setup and see what happens.The problem there is the bass disappears to some extent.
I might try a quick battery on the preamp tubes and see what happens to the hum test.