Mullard 12AX7 RI issue

[daz]

Not the GT version, but the russian one. I can use any 12AX7 in this amp in V1 with no problem. Yet if i use a grid stopper on the v1a input grid thats say 10k, the tube goes into cutoff till i turn the gain WAY down. If i use a 68k stopper it works fine. The amp is a homebrew single channel MV marshall style amp and the 1st stage is very typical with a 100k plate and 1.5k cathode and .68uf bypass. signal comes from V1a thru a .0047uf coupler then a 470k/500pf filter into a 1m gain pot. About 125v at the plate and a 40uf filter at that node. Nothing out of the ordinary at all. I have 2 of these mullards and they both do this till the grid stopper is raised high enough. If it's any kind of hint, they have a low hum too thats pretty loud with the gain up high at all, and all other tubes are dead quiet. They are quiet in V2 tho !

EDIT: well, i'd have deleted the post but theres no delete option. But i found the reason so maybe someone can theorize why this happened. Oddly, i've struggled with this a long time and i post about it and 5 minutes later i try something and it works ! What i did was remove a 2.2k grid stopper i had on the second stage. I don't have the knowledge/theory to understand it, but somehow looking at it i began to suspect it. Removed it and sure enough the mullard now works fine and in fact it doesn't seem to be humming like it was.

[Chuck H]

I have no theory on the effect of removing a grid stopper from V2 resolving the specific problem with the Mullard tubes used in V1. I am skeptical of the level of scientific approach that may have been employed.

It seems to me that a larger grid stopper on the input may have been squelching a parasitic oscillation. If the Mullards are a little higher in gain or have more HF than your other tubes this would be the reason a higher grid resistor needed to be employed with those particular tubes.

I suppose it's possible that the lead dress may have been altered when the grid stopper on V2 was removed. This may have helped resolve the issue. But as far as I know there is nothing strictly electronic about the use of a grid stopper on V2 that should cause any problem with V1. Unless...

If the grid stopper for V2 was very near a V1 lead of the same phase you might have had a positive feedback loop via inductance. Within the preamp itself this seems unlikely though.

[daz]

If the grid stopper for V2 was very near a V1 lead of the same phase you might have had a positive feedback loop via inductance. Within the preamp itself this seems unlikely though.

Hmmm....maybe. They both had stoppers that were connected to a 2 terminal strip (or whatever it's called) to make them solid. With the shielded cable moved to the socket pin it's considerably further from the V1 lead/grid stopper. I suppose i should connect the resistor up again but not on the terminal and move it as far from the V1 lead/resistor as it is now and see if it does it. In any case, whatever it was it had to be bad for tone if it caused that, regardless of what tube i used. So i'm glad i changed it.

[Chuck H]

I actually miss out on a lot of "happy accidents" now that I have more experience with layouts, grounding schemes and operating parameters. By happy accidents I mean when things are not ideal but it works in favor of the overall tone. In my early days as a tweak an amp guy at a local shop told me that whenever I made an amp that I liked I should just install it in a cab as it was. Be it on a breadboard or otherwise. His reasoning was that layout can have so much to do with how high gain circuits interact and distort. Another case in point would be an amp I built as an "intermediate" designer (which I still am. But closer to the experienced end). The OT position was causing some oscillation problems. But when I moved it the amp lost all it's mojo. The distortion character lost all it's hair. Whatever was causing the problems was also the reason the amp sounded good!!! That amp is gutted now. But if I had it to do over again I would have experimented more with twisting leads or other stabilizing ideas before grounding the project. Lots of guys attest to that old saying about an amp sounding it's best right before it blows up! An amp on the edge of instability can give you that without the smoke. I guess what I'm saying is...

If that particular amp sounds good with a certain tube roster, put it back exactly the way it works best. Obviously it's at the edge of stability. And I know (from previous postings) that you've worked hard on this project. Trying to idealize it now may only take it from great, but unstable with the Mullard preamp tubes, to mediocre with any tubes. Get what I'm saying??? Lots of revered vintage designs enjoy their reputation because the original designs were never intended to distort but do specific things when they do. Not all of them ideal. But it works for good tone. This amp game is as much an art as a science. And in art, if it's good, it's good. No reason to pick it apart on a technical level.

[daz]

I get what you are saying, and I do consider that in my experimentations. At this point it's not longer a matter of trying to get the amp to where i want it. It's long been more than acceptable to me and better in most ways than the marshalls i used for many years. But now it's like gardening to a retired person....it's something i do to relax and enjoy. Looking for that tweak that makes you feel like you found blackbeard's treasure, ya know?
Lead dress is the most mysterious thing to me because i have yet to hear a change when purposely altering it in search of better tone or troubleshooting. So i have no experience with it as far as noting what it's capable of causing/fixing.
As for happy accidents, I at times lament my extreme lack of knowledge, but on the other hand i at times feel it's what makes the search more likely to score unexpected big points and therefore makes it more fun. If you were to play the lottery and be mostly in the dark about how impossible the odds are , you are going to enjoy it more and have more hope. If you are acutely aware of it, you probably won't even play, and if you do you'll likely have much less hope and get little enjoyment from it. So i feel sort of in a good place where i understand amp circuits enough not to kill myself and enough to understand for the most part how various parts/values/design aspects affect tone, but not so much that i lose the feeling i could find that treasure any minute. That makes it fun and exciting. But i do only do this from time to time because it's a pain to keep the solder joints clean and keep the solder renewed, plus at my age it gets old after a certain amount of time spent. So i tend to screw with it for days or weeks, then bolt it back in and just play it for months or even up to a year before i get the itch again. Something to do at a time in my life when things to quell the boredom become harder and harder to find.

[Alf]

The OT position was causing some oscillation problems. But when I moved it the amp lost all it's mojo. The distortion character lost all it's hair. Whatever was causing the problems was also the reason the amp sounded good!!!

This what I read about the secret of Ken Fisher amps. He could put an amp on the verge of oscillation and get a terrific tone.

This is what I read in articles about Trainwreck amps , I don't have any experience with it myself but my instinct tells me this could be part of the secret about getting a really good sound. And so imho the parts you use and especially the way you wire an amp (leaddress) are very important.

I intend to experiment with this when I have more time ( next year when I quit working).

Alf