What happens if ...

[krash]

What happens if I operate a 12AX7 into a smaller-than-recommended plate resistor for long periods of time? I can't seem to divine this info from the data sheet I presume it'll just wear out faster. ??

[MarkB]

I've seen data sheets with sets of recommended component values that list 47K plate resistors, so I don't see any problem. You'd want to change the cathode resistor to rebias the tube. As long and the plate dissipation is not exceeded, there's no harm to the tube. Max. plate dissipation per triode is 1W. There may also be a max. cathode current - I'm not sure about that.

[krash]

yeah i'm talkin' like 28K plate resistor.

so 62.5K + 28K + cathode res. is about 91K ... 250V plate V, 2.7mA plate current, that's about .7W plate diss... under the limit!

[rfgordon]

Seems to me that I read somewhere that plate resistors at 100K or less may, depending on other circuit circumstances that my middle-aged brain cannot recall, stimulate some grid emission and, if it happens in the first gain stage, put DC on the guitar's volume pot. This results in scratchy pot syndrome. That may be why I've seen some designs with a cap between the input jack and the first grid. I'm no theoretician, so I may be way off on this...

But I reckon you can try those values. The only "wrong" values in an amp are the ones that either sound bad or blow stuff up! Conventional wisdom is that moving from 100k to 220k increases chime. I'd be very interested in what you find, tone-wise, from lower values.

[krash]

Here's what I get from the 28K ...

it's in the third gain stage in this amp:

http://revelationamps.com/media/72-tele-blues.mp3
http://revelationamps.com/media/72-leads.mp3

Sounds good to me but I just want to be sure I'm not going to have amps out there that go thru a 12AX7 a week.

[Allynmey]

Hi Josh, IIRC the range for plate resistance for the 12ax7 or any preamp is 47K-330K with 100K being the best middle ground. I very rarely see less than 82K on plates.

Allyn

Maybe your on to something...the clips sound cool! Hopefully the tube won't fail after delivery.

[krash]

Well I have been running one like this in the prototype amp for about two months without any problem but I am considering putting a 12WD7 in that spot since really the tone is not all that different AX/AU ... I don't need the gain there. But I think for the most part I prefer the AX so if I don't need to change it, I don't want to ...

So as long as I'm under a watt of plate dissipation I should be pretty safe, right?

I know we rarely see <47K for a plate resistor on a 12AX7 but I think mostly that's because the 12AX7 is usually used as a voltage amplifier and the gain is low enough with sub-47K that it would normally indicate a different tube choice. So if you need less gain, then rather than stress out a 12AX7 use a 12AY7 with a 47K ... but in my case, I want it to sound stressed out

[Normster]

Sounds very cool, Josh. Of course it may just be your chops that are swaying me here.

[krash]

haha ... thanks Norm.

you know I would have done a lot more takes if I thought anyone was paying attention to the actual PLAYING... That was quick and dirty and on a day filled with frustration since I really haven't "bonded" with the monitors in the studio ... and I thought everything was insanely bright all day only to discover I get home and it sounds great.