6L6 to 6V6 opinions

[jjman]

I reduced the valued of the grid resistors (not grid stoppers) from 220k to 100k on my 6v6 amp at one point to reduce the drive they were receiving. I didn't want to be able to push them into crossover distortion and this worked. I now use a dual 100k pot there instead (as a post PIMV.)

Reducing the potential strength of the signal anywhere will not increase headroom anywhere, BTW. You can always turn the volume down on the guitar or the amp to reduce clipping, but that's not increasing "headroom." Increasing headroom means increasing the clean volume potential.

Some people want to be able to turn the amps volume knob(s) further before clipping starts, which is fine. But that is not automatically an increase in "headroom." Although I guess one could call it "knob headroom."

[Ian444]

jjman, that's what I was thinking. Once the PI delivers a voltage swing that equals the bias voltage of the output tubes, thats max clean power and there's nothing can be done to increase clean power. If the PI gets driven even harder, it goes square wave trying to drive the grid current of the output tubes, which seems to sound a lot louder, but doesn't make much more power.

[megawhat]

This is makes sense, and it the "knob headroom" description is probably what I'm after. The Pre PIMV to be a little less sensitive. It's also true that you really don't need to turn the gain or MV knob up much at the same time but everything is going square wave. I think the amp reaches max output pretty quick and then just clips more.

The benefit is you can use less preamp gain and run the master up a bit and get 6V6 breakup pretty early on in terms of where the MV is positioned (10 o'clock), or you can have the MV next to 0 and get the preamp distortion more than the 6V6 breakup while the amp is still pretty darn loud!

I just want to be able to turn up the MV a bit before the output reaches max and just starts clipping. The preamp is clipping the signal with the gain past 9 o'clock, before it even gets to the MV.

Someone suggested putting a resistor (100K) across the pins 1 & 3 on the MV to reduce it's value and reduce the level into the PI. I fail to see how this works to reduce voltage though.

[megawhat]

ok!

I put two 220K resistors in parallel with my grid leaks on the 6V6's which gets me down to 110K.

That does make it quite a bit better. The Volume increase when using the pre-pimv now stops once you get just over a half turn on the MV (1Meg log) pot.

I tried a 120K across the pot in addition to this (as someone suggested to me) but that didn't seem to do much, and at the full turn it was making something weird happen. So i took it off.

I then tried a potential divider before the MV made up of a 330K at the top and 120K at the bottom (what I had to hand). That way the MV seems to have a useful sweep through 3/4 of the turn.

I'm just wondering if that is the most elegant solution.

[Ian444]

I just want to be able to turn up the MV a bit before the output reaches max and just starts clipping.

You could use a 500K resistor to feed a 500K pot, or something like 680K feeding a 220K pot. This will give much greater control and less voltage output to the PI.

[megawhat]

I have a lack of pots currently... eek!

I just put two fixed 100K's in as the grid leak/bias feeds.

I did try a 1M in line with the MV input and it does seem like a more easy going MV control.

[megawhat]

Just wanted to thank you all for the help. This is sorted out now. cheers

[dynaman]

Yeah, I also run a pair of 100Ks. What did you finally decide on? Did you wind up raising preamp voltages too? On average, you're did seem about 50V lower than standard SLO.

[megawhat]

I got the b+ up in the preamp to start at around 350v by swapping the 10k dropping res after the screen supply for 1k.

I went with the 100k bias feeds, and a 1m resistor in line with the master volume input.

right now it also has a 5751 in v1 and a 12au7 in the PI position. I'll probably change the au7 for other types depending on what I'm doing with it.


one day I want to try the LTP scaling circuits from merlins book