unortodox design....will this work

[6267]

I've ended up having to "productionize" or "human proof" a couple of my designs as well (amps for other musicians or for sale). When there is too much gain, I've seen a couple things happen that make for bad sounds, often right after the best sounds.
1) Grid current
2) Oscillation
3) Transformer distortion

They all kind of sound similar, in that they occur "night & day", they sound like crap, and they're kind of hard to get around. Personally I'd just put series resistors on the gain pot so that it can't go there, and call it a day. An oscillation at ultrasonic frequencies just steals sound from everything else. Transformer distortion just sounds nasty, kinda like transistor clipping but worse. Grid current does some crazy stuff when there are caps involved too...try paralleling a smaller cap with the larger coupling caps, or there's some trick with a diode I forget. Good luck, it's the little bit of polish that makes a good amp great!

[ETR]

Putting a voltage divider before or after the master volume in a JCM800 type circuit i would say is unorthodox since i have never seen it done. Trying to drop gain at the output of the CF is also something i never saw done. Anyways, those a moot points because everyone who's answered so far has offered up suggestions i can use, and thats what the title was meant to evoke. So it worked didn't it?!

Ahhh, the ol bait and switch.

[daz]

Personally I'd just put series resistors on the gain pot so that it can't go there

Well, that was my game plan at first, tho i was thinking more like a voltage divider after the pot or that sort of thing. Same thing tho really. I just gotta get the amp back so i can install a post PI master again just for testing purposes. (don't wanna have to see what works by trying it over and over at full volume !)

[chipaudette]

+1 on putting the resistor after the MV.

Chip

[daz]

Ok, i have tried a 100k grid stopper right at the pin and tried a 1 meg pot coming out of the master which should be the same as BEFORE the master because the master is dimed and i'm using a post PI master too, which allows me to test this at less than ear splitting levels. in any case, neither had any effect. Next i will try different values of the 1meg PI resistors and the cathode/tails and see what happens there. I'll report back.

EDIT: well, on the PI, 100k resistors in place of the 1 meg grid leaks and a 1.2k at the cathode and no change at all. Turning the pre PI master to 1/2 stops it, but then the volume isn't as loud as it should be. But as a last resort i'll put a voltage divider after the master using a second pot to determine the values to use. But it is happening at the PI because with the pre PI master down to 1/2 it stops, which tells me it is NOT before the master. And with the POST PI master down and the pre up it's doing it so we know the power tube grids aren't getting slammed. So it's happening between the 2 masters which is of course nothing but the PI.

[daz]

I just realized something....if i turn the post PI master all the way up or remove it completely and test it by turning the pre PI master up, I don't get the nastiness ! so i guess the problem is part and parcel of the POST PI master ! I could only do this for a second because of the extreme loudness. In fact, i only put the post PI master in so i could test the issue at low volumes. I first noticed the issue when trying a post PI master thinking it may be a better sounding master. But it was horrid and i thought the amp had a design issue because of the nasty tone i got with the post PI master way down. But why would it cause this? I see crossline masters in many amps, and i even at one point tried one of those dual pot masters and the same issue happened. What could it be that having a master at that point causes this thin nasty sound? I suppose it doesn't matter now that i realized it's ok w/o the post PI master, but i would like to understand.

[ETR]

Where did you ground the PPIMV and how did you run the wires?

[daz]

I was using a crossline, so it's just a variable resistor from one EL34 grid to the other, but on the OTHER side of the 5.6k grid stoppers, not at the grid themselves. just like any crossline. The dual pot one i tried i don't recall, but trust me it was right.

[mooreamps]

Am I missing something? What's unorthodox here?

It's not. unorthodox is more typical of some of my work...