AC4 biasing the ef86

[shzmm]

hello
just finished up building an ac4 amp-- ef86 into an el84, minus the vibrato. i used the schematic below. my question relates to the voltages on the ef86.
my B+ is around 250V, anode is at 187V with 220K, screen grid at 23V with 5.6M and cathode at .45V with 1.5K.
obviously these voltages are way off the schematic values. the amp sounds ok-- just seems to be lacking gain and the highs seem to be somewhat muted. i tried lowering the screen grid resistor down to 1M but that just threw everything drastically out of whack.
any suggestions on how to get closer to the schematic values? does it matter?
anyone with experience with this? good modifications?
i don't have any troubles with hum, microphony or the like on the ef86.
the el84 is pretty much within 5% of the schematic.
thanks for any replies.

[jbrew73]

probably has something to do with r8(100k) on the trem circuit. not sure what to recommend other than using values from the ac15 or ac30/4 circuit and see how it sounds.

[HBamps]

Did you ever find the problem? I have a EF86 build that is lacking gain and the high end is muted as well. I have been searching high and low, what did you end up doing?

Thanks

[unparalleled]

probably has something to do with r8(100k) on the trem circuit. not sure what to recommend other than using values from the ac15 or ac30/4 circuit and see how it sounds.

R8 is feeding the LF modulation into the cathode of the EF86, If you remove it you will get no Trem but the amp should function. I have built an AC4 with the trem and have added a trimpot across the 1k5/ 25uF with the wiper to the 100K to adjust modulation depth.
The only other mods I did were a 470k to ground at the input grid of the EL84 and 4 series IN4007s clamping the plate.
Try another EF86 or raise the screen resistor to 6M8 Ohms.

[black_labb]

the screen voltage seems a bit low to me. wouldnt a voltage of 60-100v be more normal? that should give it a bit more gain as well. i'm not that familiar with ef86's but thats not much voltage on the screens.

[shzmm]

Hi
I know now that ef86s are a bit tricky. Sometimes they work great with the stock AC4 setup sometimes better with the AC15, it seems to depend on the individual tube. But I think the trick is mostly to play with the screen resistor-- that has a lot of affect of gain and compression.
Good luck.