Tube Town EF86 AC18 Tweaks

[jjschessmaster]

Hi All!  I bought a tube town AC18 kit to try out ef86 tubes.  I followed the schematic linked below with mods to toss in a spitfire preamp into the phase inverter. 

For the group, the ef86 channel (as designed) is a little dark and I'd like to get some sparkle out of the cleans.  I've seen suggestions for bright caps, changing tone switch to Matchless 6 way wiring, and snipping the cap on the phase inverter plates. 

Bright caps I'm real familiar with adding (big fan on 5f1s).  What would I gain with the 6 way matchless tone switch?  Does snipping the cap across phase inverter plates have an actual audible increase in high end or just noise dogs hear?  Just trying to focus efforts before heating up the iron.  Thanks all!

https://media.tubetown.net/cms/userfiles/downloads/ttac18-schem.pdf

[tubeswell]

If you’re satisfied that you ruled out other factors such as deteriorating guitar cable, then Lower frequencies can be ‘tamed’ by any or all of the following;
1) reducing capacitance of the screen bypass cap (the cap between g2 and ground)
2) reducing capacitance of the cathode bypass cap
3) installing a stepped mid-high frequency boost in parallel with the cathode resistor
4) reducing capacitance of the coupling cap

These will all have an audible effect of reducing overall volume somewhat.

Happy experimenting

Adding a bright cap to the vol pot will only help when pot rotation tends towards the ‘cut’ end.

Also, Having the ‘wrong kind’ of tone stack directly following the EF86 will ‘darken’ it due to the relatively high output impedance of a standard EF86 gain stage. (If the tone stack is a high-loss type - like a standard FMV TMB tone stack, it loads the EF86 output too much, resulting in sucky toan.) One workaround for this is to add an impedance buffer (e.g., cathode follower or source follower) between the EF86 and the tone stack. In your case, the bandwidth selector switch/tone control is ‘simple’ as far as lossiness goes. So maybe go for smaller cap values.

[jjschessmaster]

Appreciate the input!  Seems like lots of options to try!  Did a little a/b comparison of the matchless dc30 schematic and on the tube town.  The ac18 schematic has a 100nf cap off the plate before the tone switch whereas the dc30 schematic doesn't have any cap.  Is it worth trying to do an alligator clip around that cap as a bypass or is that the cap that blocks the ac voltage?

[tubeswell]

Appreciate the input!  Seems like lots of options to try!  Did a little a/b comparison of the matchless dc30 schematic and on the tube town.  The ac18 schematic has a 100nf cap off the plate before the tone switch whereas the dc30 schematic doesn't have any cap.  Is it worth trying to do an alligator clip around that cap as a bypass or is that the cap that blocks the ac voltage?

That is the coupling cap and it’s purpose is several fold, namely:
1) Blocks DC from the plate so only pure AC gets to the next stage’s grid
2) Is part of a filter that (in combination with the load impedance of whatever follows) sets a frequency dB roll-off point (going to the next stage’s grid). Lower capacitance = more bass cut.
3) Protects any lower-voltage rated parts from excessive DC from the plate.

Bypassing the coupling cap with a jumper is effectively shorting the cap (so any components in the tone control will need to be rated for sufficient voltage to deal with whatever is coming off the plate). And because caps in series add up to lower capacitance, effectively taking the cap out of that circuit will let more bass through for each of the freq toggle positions in the tone selector (i.e., assuming these caps each have the same voltage rating as the coupling cap)

[jjschessmaster]

Gotcha, appreciate the additional info.  Was playing around with the bright cap and ended up putting a 3 way on off on with a 220pf and 680pf.  220 pf is near perfect and 680 is a little much but may find a use.   I'm going to play it for awhile then see if I'm happy with the bass cutting that happens through the tone switch.  If I find I need to cut more I'll start messing with the bigger caps.  Thanks!