Ampeg VH140C

[cniers13]

Did anyone figure this out?

[Blackboarcult]

I've been meaning to breadboard the whole thing and see what could have gone wrong, as I was not able to find the fault(s) in my build, but the past three months have been hell for me and I haven't had the time to do it.
However as soon as I find the time to test that, I'll report

[cniers13]

Good luck! I've been wanting to build this so hopefully we can figure it out

[Blackboarcult]

Well I breadboarded the whole circuit according to both the handdrawn and original schematics (omitting the J112 switching on the latter one.
 It works, so the fault may be in the layout (or our builds). The only thing that I could spot at quick glance is that the caps on the feedback loops are reversed,but I did that on my breadboard and the sound does not change that much, maybe more sputtery.

Anyway I love the dirt this circuit pulls out, but didn't like the tonestack, as the bass acts more as a depth/volume pot, so I'm gonna swap it and try other ones, maybe the HM2 or the GK250ML which is a damn versatile active 4-band tonestack.
If I come up with something cool I'll draw a layout, in the meantime I'll also try to check the layout and my old build to see where the issue is.

[Blackboarcult]

UPDATE:

Replaced the original tonestack with the Gallien-Krueger 2000CPL one on my breadboard, and to my eyes it has improved a hundred times.
Cranking Bass, Treble and Lo-Mids and lowering the Hi-Mids to almost zero (or the other way around on those two, my pots aren't marked )  yields a wonderful swedish Death Metal chainsaw tone, and the circuit takes TS-ish overdrives in front surprisingly well.

I only tested it with my homemade Class D amp and a 2x12 Celestion V30 open cab, I'd like to test it through the FXret of my Engl E650, but it'll have to wait...

I'll tinker and tweak a bit more to improve the noise and volume and try to do a vero anytime soon if anyone is interested...

[cniers13]

I'm interested. Glad you got it to work. Would love to see the Vero of the working pedal

[Hex_97]

Any updates on the circuit? I'm looking to build one because I can't get my hands on an SNK VHD

[mnemonic]

OK i think I figured this one out. Back when I built it, it was around christmas time 2018 and once I got back to work I kinda forgot about it.  Clearing out my cupboard, and I found my half-working pedal.

issues were, not much gain (light crunch at best), extremely woofy with tons of bass.  Sounds like a bad doom amp.  

I built mine based off Anders' layout on the previous page, copied here for reference:
https://i.ibb.co/bd36BPP/vh140c-anders-layout.png

I remember previously tracing this layout based on the hand-drawn schematic posted previously, copied here for reference:
https://i.ibb.co/mTGnfDt/vh140c-hand-drawn-schematic-errors.jpg

I previously found no errors (except one I recently found, more below).

This is the Ampeg Vh140c schematic, again posted on the previous page:
https://i.ibb.co/XSXZhMH/vh140c-offical-schematic.jpg


there are several omissions on the hand-drawn schematic that may or may not make a difference.  
1. at the input, 1N754 diodes (D1 and D2 on schematic) back to back, right after the 22k are omitted.  I added them, no change.

2. C3 on the schematic, electrolytic 2.2uf, between the 1k5 resistor and ground.  I added it, no change.

3. C23 (3.3uf) and C24 (0.22uf) are missing.  I added C24 and it greatly improved the bass response, no more excessive low end.  still not enough gain.  

4. R38 is the wrong value in the layout, it is 100R in the ampeg schematic and the hand-drawn schematic, but 100k in Anders' layout.  I changed this at the same time as adding the 3.3uf cap, and the gain level is greatly improved, about what I expected for a VH140c preamp.  Death metal heaven.  

Note that the 3.3uf in the ampeg schematic doesn't have polarity marked so not sure if its supposed to be polarised, the other caps 1uf and larger that arent electrolytic are specifically marked 'N.P.' on the schematic, so i'm not sure.  I put in an electrolytic since it's what I have on hand.  I put the positive end to ground, like the other electrolytics in the schematic.  

5. the 22uf caps are backwards on Anders' layout (correct in both schematics).  I flipped them but noticed no change.

6. C39 is missing, 0.047uf between IC5 and IC6.  I haven't added this one yet.

7. Gain pot is wrong, Ampeg schematic states 100K audio taper but hand-drawn schematic shows 10k audio taper.  hand-drawn schematic also omits 10k resistor (R48) after the gain pot.  I added them but I don't think there was a change (I had a break between adding and testing so I can't say for sure).  



Its late now so tomorrow I will give the pedal a go through a good power amp.  Currently its late and I'm testing it half-open through a cheap class-d power amp board into a speaker cab.  It oscillates a lot over 3/4ths gain, but the pedal is also open and my leads to the gain pot could be shorter, and shielded.  

[fx-fidden]

Thanks for your report.
I didn't build the veroboard layout by Anders, but today I had fun emulating the schematic.
I confirm all you said, some corrections make minor change, something else limit the gain in the low end.
The 10k resistor after the gain pot should tame a bit the gain, but the diference could be not enough to hear it.
As well the 47nF that should be a low pass filter. The 47nF cut too little to make any difference I guess, but I'm not sure if it's better to add it in the signal or not.

[cniers13]

Glad you got it to work! I'll have to build it via Anders layout and try your changes

[mnemonic]

I boxed up the pedal, pretty crammed-in, still oscillates above 3/4 gain if I have the guitar plugged straight in, but if I put a buffered pedal before it, no oscillation even all the way up.

I think I still have some gremlins to track down, it was working fine, then the bass control stopped working. Opened it up and nothing seemed amiss, closed it up and it’s working again. Probably a loose connection somewhere.

Also one error from what I posted above, I think in the original schematic the gain pot is 100k reverse audio, not 100k audio. I have 100k audio in there now and the taper seems quite natural though.  This wouldn’t change anything except knob position required to get a specific gain level, right?  

I played it though a KT88 tube power amp but the pedal has a lot of bass on tap and the additional resonance from the tube power amp was a bit much. I think this preamp sounds best into a solidstate power amp.

[fx-fidden]

Could this Ampeg VH140C schematic sounds like this?

https://www.youtube.com/watch?v=ducGePqFHYE

[mnemonic]

fx-fidden wrote

Could this Ampeg VH140C schematic sounds like this?

https://www.youtube.com/watch?v=ducGePqFHYE

it looks like he is running a boost into it, then direct into an impulse.  I don't know what that boost is like but i tried boosting mine with a modded tubescreamer and running it direct into some impulses into headphones so I could A/B between his recording and mine.  Sounds like the general character of the distortion is about right, though that clip is brighter and more scooped than the same settings on my clone pedal.  

it may be that there are still some issues with mine and highs are getting pulled down somewhere (I do think its kinda dark sounding) or maybe he's just using a really bright and scooped impulse.  I got the tone about right when I added a big high shelf boost (+5db at 3800hz) then a wide mid cut at 800hz (-3.5db, Q of 0.34).  

[Sensei Tim]

i too have found this pedal to have too much bass to the point where it sounds muddy to the point where im wondering if the component values in the bass opamp feedack loop are correct (C44, R62, R63)

[Sensei Tim]

After messing around, i found that a 250k Audio pot is MUCH better suited than the linear taper pot.

[cniers13]

Anyone finalize this?