grid bias resistor value 6V6

[12AX7]

Just wondering. My amp from cathode follower to output are all the same values  as a JCM 800 pretty much. I have 220k bias resistors at the 6V6 grids and i was looking at a 6V6 plexi and noted they used 100k there. Can anyone tell me what the difference is as to why and what difference you'd see if any given the same bias with each?

[12AX7]

Don't everyone answer at once, it'll be too confusing !  Seriously, i tried 100k and i was pleasantly surprised to find this actually improved the tone. I did some searching on the subject and from what i found i saw nothing to suggest it would improve tone with the possible exception of tightening the lows, which is not what i heard. What i DID hear is just additional harmonic complexity which is something i always strive for. So definitely a very a worthy tweak. Another improvement was the master is no longer a 1-5= full volume affair with 5 to 10 doing little if anything. I read that 6V6's are supposed to use a max of 100k, but that many including fender have ignored that spec using 220k. But this tells me that spec might be worth adhering to. Sure helped mine. Tried this on a el34 amp once and it wasn't the same result at all.

Oh, just noticed i put 6L6 in the title when i meant 6V6. Luckily this forum lets you modify the title, but those who earlier saw "6L6" take note.

[sluckey]

Sometimes you just gotta do stuff yourself.   

[12AX7]

True, but the way things are set up here it's a major pain to pull the chassis and try it. But i will have to pull it yet again because with all that extra complexity came some serious very high content that i need to tame. Never quite understood the need for snubbers because they took too much off the top. This may be the first time i have a sound that i think will benefit from them w/o dulling the tone. I'll try the plate to plate snubber and tubeit's (sp?) "enhance' cap.

[12AX7]

I tried tubenit's enhance cap in the past but now i know why it never worked well for me. It works when you have too much very high end content that manifests itself as what i think tubenit calls "hash". I call it a staticky high end where it *seems* to extend beyond typical guitar tone into the ultra high region and gets nasty. Lots of good harmonic content but the fundamental is what i thing gets tamed while the swirl of the harmonic content is not interrupted. But in any case it never worked in the past for me because i never achieved this tone before and it was mostly fundamental without enough high harmonics so that it cut out the only thing that contributed to chime and cut.  Thats the best i can describe what i'm hearing. I think with changing the grid resistors to 100k and adding that cap i just achieved my best tone to date. By the way, i tried 150k before i tried the cap thinking about 1/2 way between the 100k and 220k grid resistors might give me just enough of that added complexity w/o the hash. But nope. The 100k;s and the 250pf enhance cap did the trick.  And in the past if i used 250pf there it would just totally destroy the top end !

[PRR]

Did you RTFM?

Self-bias maximum resistor is 500K.

Fix-bias maximum resistor is 100K.

Yes, Leo and Jim often violated the factory specs. 99% of the tubes in a crate will not be at the grid-current reject-limit; especially in the 1960s golden age of mature tube production.

> additional harmonic complexity

You are making the driver work harder. Distortion rises. This is not what most audio amplifier users want. It may be what some guitarists want.

[HotBluePlates]

... I did some searching on the subject and from what i found i saw nothing to suggest it would improve tone with the possible exception of tightening the lows, which is not what i heard. ...

Amp circuits are about striking a balance of competing tradeoffs.

The "bias-feed" resistors influence low-frequency shaping going into the output tubes, voltage gain (and possibly headroom) of the phase inverter, and possibly the susceptibility of the output section to blocking distortion.

A change of the bias-feed resistor value in one direction to "improve" one of those effects could make the amp sound worse due to one of the other effects.  Whether a change is worthwhile depends on other issues within the amp.

I doubt you'll find a universal "best value" or even "best direction to change value" across many different types of amps.

[12AX7]

Amp circuits are about striking a balance of competing tradeoffs.

I'm just a player, not a tech. But thats one truth about designing/tweaking i know well. In fact, the game of tweaking circuits is rarely one where you find a change that makes the amp great. It's usually a scenario where you hear potential but it also causes other areas to need change to work well with the new tweak. Thats what i found with this change, as the top end was a bit much hence tunenit's enhance cap which tamed it. But that said, in the end i went back to 220k and dropped the cap because while i was able to crank it all the way up with no ill effects, later on that was NOT the case. And what i found was the lead dress had to be exact or it would start oscillating when turned up. So back to the 220's and ll is well. Must have disturbed the dress a bit between the initial try and when it started oscillating.