Express build finished with PPIMV - cut control and NFB advice needed!

[pedro]

Hi folks
I just finished up an express build which I am very happy with - no oscillation, quiet, and sounds like an Express should do, when comparing it to the soundclips I have heard. I have clean to mean to scream on the guitar volume.

I just used it for some recording work and I love it.

I initially built it with a MV and put together an Airbrake using the info in the files section here.
Heresy I know, but I actually prefer the MV to using the attenuator.

One question though - if I wire in a vox style Cut control, and remove the Presence, do I need to hardwire any NFB still or just remove it completely ?

Cheers
PeteC

[martin manning]

I'm assuming you want to use the panel space from the Presence for the Cut? I would keep the GNFB, and maybe replace the pot with a fixed presence setting. I find the Cut control to be very useful at low MV settings.

[pedro]

Thanks Martin,

Yes I intend to use the front panel presence hole for the cut control
At the moment I only have 250k pot and 0.002ufs on hand ( no 0.0047s ). Would this be worth trying ?

I tend to keep my presence settings very low or around 9 o’clock. What fixed value would be worth trying - 100k ?

Thx
Pete

[martin manning]

Since the Presence is a 5k linear pot, I'd try a 3k7 and a 1k, with the 0.1 uF cap across the 1k to approximate your 9:00 setting. For the Cut, I used a 250k and an 0.002 uF.

[pedro]

Thats great Martin - many thanks for your help its much appreciated

I love this amp and need to build a rocket or liverpool next this Autumn

I guess i have been lucky with my lead dress and layout judging by many other build posts

PeteC

[pedro]

Happy to report thats its all now wired in and working fine. I mught experiment with different value caps to see whether the cut frequencies land

[pedro]

After trying a few different values I have settled on paralleling a 0.002uf and a 0.0033uf cap, and using a 250kA pot. I like the extra cut this gives. I also have switchable “bright” caps of 100pf or 500pf (or none)

There is now a huge range of tones available using the combination of the bright cap switch, the treble Eq pot and the cut control. And using the guitar volume control it’s possible to access so many good tones.

It’s great to be able to boost the treble early in the circuit and then smooth it out later - some very creamy lead tones indeed.

Of course I guess the GNFB and cut control effectiveness all depend on the nature of the individual output tx , speakers and tube choices etc

[martin manning]

After trying a few different values I have settled on paralleling a 0.002uf and a 0.0033uf cap, and using a 250kA pot. I like the extra cut this gives. I also have switchable “bright” caps of 100pf or 500pf (or none)

Great! I also have 100p/none/500p bright switch on mine.

[pedro]

The 100/ 0 / 500pf switch is really handy. When I find a setting that’s just a bit too much on 500pf the 100pf option is great
The 500pf does seem to boost the gain at higher volumes quite a bit though.

I haven’t yet tried a long cable as Glen K does to tame the top end - that looks like a good option too

I think the only other “mod” I would like to try is a fast/gradual switch with a split plate load to see whether I like that. I do have a couple of 12AY7 from a fender pro-amp build I did about 15 years ago which has been cannibalised for parts but I think swapping preamp tubes out will become a bit tedious just to have some more clean headroom from time to time.

It’s primarily going to be a recording amp, but I will try gig with it once I have got the hang of using the guitar volume pot to move between a clean, crunch and lead sound. We always mic up the guitar cabs live so I have to get that right so I don’t screw up the PA mix

[ViperDoc]

I have tested 12AY7 in V1 of my Express and I found it reduced the detail too much when playing cleaner at lower volumes. I found the Sovtek 12AX7WB in V1 offers a nice bump in the low end to offer a more balanced tone; the Express otherwise has such a truckload of high end, it is challenging to render down properly without squashing the magic!

[pedro]

Thanks for the info viperdoc.
I am going to stick with the existing tubes and try adding a split plate load switch to V2 using the komet approach

[Roe]

for taming the fizz with a cut control consider decreasing the 4.7nf cap. Aiken uses 470pf and a 1m pot. Larry uses 2.2nf and a 250k pot

[pedro]

Thanks Roe,

At the moment I have a combination of parallel caps to give me 0.0055uf with a 250k pot - probably cutting more highs for a slightly thicker tone. My tinnitus tends to push me towards rolling off a few more highs these days! I love being able to boost the highs early on ( at the guitar and tonestack) then being able to cut them later - this seems to give some really smooth sustained tones. The closest I have come to this kind of tone before was with an overdriven Kitchen-Marshall 100 PA amp back in the late 70s which I think had an ef86 in the first stage.

Will probably tinker more with the cap values no doubt.

Cheers
Pete

[Mark]

Is the PPIMV really worth implementing, I thought the joy in these amps was pushing the output valves, though I imagine a fair component of the overdriven/distortion is from the PI stage.

[pdf64]

Is the PPIMV really worth implementing, I thought the joy in these amps was pushing the output valves, though I imagine a fair component of the overdriven/distortion is from the PI stage.

I recall that katopan showed that a significant part of the overdrive character was the output valves sagging the HT supply to the LTP. And another significant part was the output valves clipping the output from the LTP.
Both of these mechanisms would be absent with a type 1-3 master volume.