PI grid leaks

[iknowjohnny]

Well, this was my mistake. It was a 2.2k, not a 220k ! Got home from work thinking i was going to figure out the volume issue and have the tone i've been after for so long. Thats when i took the plate R off to try a 180k and see if any of the volume came back. Then i noticed not red red yellow, but red red red ! ARG ! Well, i wasn't too disappointed because the tone with fresh ears wasn't as great as i thought at all. In fact it wasn't very good aside from the fatness.

So i'm back to playing with it. But i have a gig tomorrow and would like to get it the best i can in the short time i have. The voltage i got when i tried that 200k on the CF cathode went from around 140 to 170 or thereabouts. i didn't think that was a huge increase, but it is huge compared to a regular gain stage. But like i said, i don't get cathode followers at all so i figured if 140 is normal on a CF cathode 170 can't be catastrophic. I'd like to use that tweak if i could because it did sound good....or at least i thought it did yesterday. I gotta try it again just to see. i'll look at the specs, but heater to cathode is 155 with 200k and a little less with 100k. How high can you go?

EDIT:I found the spec which is 100v, but they also said thats often exceeded by quite a bit in cathode follower circuits. So just how far can i take it? it DOES sound good, as i just tried it again and i'd like to use it like this or add a bit less resistance.

[Jana]

See, I made you do some digging and research!

Yes, the spec is 100 volts. Marshalls typically exceed that by about 40 volts or so. Sometimes a little more. Most times it isn't a problem at those voltages EXCEPT for the new tung sol reissue 12AX7--they won't handle it. I would be concerned though if you are pushing it and reaching 170 to 200--that is going a bit far. One way to get around this is to elevate the heaters. If you have the heaters elevated to +50 volts, and the cathode voltage is 150, well, 150 - 50 = 100. The heater to cathode voltage.

Guitar amps are known to exceed maximum ratings but it is still something to be aware of and not push things too far. Unless you want to earn the nickname "Sparky!"

[David Root]

Marshall used the 100K CF cathode resistor at about 170V or so on the cathode IIRC back in the day. These were probably Mullards at the time so beware new production in that spot.

I have run cathode follower cathodes on NOS 6SL7s and 12AX7s above 200VDC (6SL7s were Marshall style at 68K plate V2A and 68K cathode V2B) but NOT for extended periods. In my experience the old stuff will do it but how long it will last is an open question. My guess is below 150V with NOS (or UOS) 12AX7s you should be OK longterm. Above that you are likely pushing your luck.

[Andy Le Blanc]

You might as well play it safe, float a positive voltage on the filament supply.
It's one thing to exceed a static rating, but when you start adding signal,
you'll end up eating tubes.

[iknowjohnny]

thanks guys....oh, and gal. I appriciate not ribbing me for being so ignorant. But i need to investigate the larger cathode resistor idea a bit more because i'm not sure it is as good as i 1st thought either. But Jana, your idea to tweak that last stage B4 the CF was just what i needed because to this point i always felt those component values were somehow supposed to be unchanged because of the CF that follows it. That thought combinded with the fact that the 100k plate was tightly soldered to the pins and in a way that was hard to get out, i never messed with it. And while i HAVE tried different cathode resistors, never with the right intention at the right time. So tonite i realize the 220k plate wasn't really helpful and that the cathode R just may be at this point because of how i have the preamp set. Sure enough a 2.2k softened up the top end and allowed me to up the value of the grid leaks on the first stages a bit. I always felt 68k was low for that because it's right before the cutoff point where the tone just goes down the tubes. But i needed them to get the gain down to where i wanted it. But with the 2.2k on the 3rd stage cathode i could bring those up to 100k and it also softened up the top and made the amp a bit richer.

Anyways, i have one of the first gigs i've done in a long time tomorrow so i'm anxious to hear it. I stopped gigging after 25 years several years ago, but every now and then i'll get one. And so far i have used this amp a few times and i've always been thrilled with the tone and feel and the way it cut thru. I think right now it's better than ever so tomorrow nite i believe i will have a good time. Thanks again, and thanks Jana for giving me some things to think about with that edited schematic.

[breakfastbuddy]

cf catode resistor max 100k ?H iwatt have a 220k

[Jana]

"cf catode resistor max 100k ?H iwatt have a 220k"

Yes, some do. And they also have a 100K to drop the B+ voltage.

[iknowjohnny]

Well, further experimentation tells me the CF cathode doesn't do what i thought, so both tweaks are now back to thier original specs. But i have another idea that i am going to try before i go to the gig tonite (took the day off:))that i only thought to try because i am now considering that 3rd stage for the first time thanks to you Jana. What i realized is it's that stage that has been the place which pretty much every issue i have had is occurring. So what i will try today is to replace the 68k grid leaks with larger values and to counter the extra gain i'll cold bias the heck out of the 3rd stage. Maybe a 4.7k at first, then if more if needed. I know marshall uses a 10k, tho thats not on this stage but the one before. maybe i'm blinding trying something foolish due to my lack of knowledge, but what the heck. I'm just looking at it from the point of view that till now i haven't been aware of the fact (or at least i THINK it's a fact) that the 3rd stage has been the one taking the beating all along and that i need to balance things out with the first 2. Gain staging if you will.