[BUILd] PM660 / PM670 SCAmp Booster build and tips help thread

[ChuckD]

Ok sorry I do not know this old way of marking the electrolytics and I have my + and - switched in my instructions !!!

It should read:


1) On the Scamp Board C103 and C203 correspond to C109 on the FC670 schematic.
2) R17 on the Poorman Board Corresponds to R137 on the FC670 schematic.
3) Leave C* connectors empty.
4) Build the network in the FC670 Schematic by hand using a 2pole 6position switch connect that to R* connector.
5) The poles of the switch connect to the RED side of the wiring diagram while the shorted side of the network connects to the Brown Side.
6) If using electrolytics then the Negative terminal of the cap should go towards the resistor and Positive connects to the Brown shorted side of your hand built switch network

 

[jyrki3101]

HI, could this TC network board w/ 2p6pos rot.switch work in analag's and fairchild mode. Here is the pic. ( if I succeded in attaching
an image )
  - jyrki3101-

 

[lolo-m]

HI, could this TC network board w/ 2p6pos rot.switch work in analag's and fairchild mode. Here is the pic. ( if I succeded in attaching
an image )
  - jyrki3101-

I looked at it fastly and I didn't notice any error... It should work properly. Nice work  !!!

 

[Nele]

maybe add some extra pads for the caps so larger types will fit as well ( 7.5, 10, 12.5 and 15mm)

 

[jyrki3101]

Thanks, I just want to check one more thing :
1. 50-63V is enough for the voltage rating of caps in the TC network(scab mode), because the voltages in SCamp are not more than 17V ! 
please correct me if I'm wrong
    -jyrki3101-

 

[lolo-m]

You're perfectly right. PM670 doesn't use huge CV voltage swing anyway even in Analag's mode... 63V is already overkill. we don't deal with the real Fairchild's CV voltages.

 

[TheGuitarist]

I'm just sorting out how to wire up my TC switch.


Just checking that this sounds right to everyone (desoldering this may be a pain so i'd prefer to get it right the first time)


I'll be using a 6 pos 2 deck switch (or only 6 of a 12 etc)

for one deck it will be either resistors in series or just 2k 4k 6k 8k 10k

For the second deck, which will be the fairchild mode, it will be like from the 6th position. capacitor from the input to output tabs (hope that makes sense.. where you would normall solder resistors.) then use a diode to link the input of 6 to the input of 5. Then repeat that process.

I think that should work and paralell the caps. What sort of voltage/current is going through the switch. Just need to work out what diode to use (unless someone wants to point me in the right direction )


EDIT:

After looking at jyrki3101's board... i'm wondering.. is it even possible to do this without a seperate board? I think i'm in over my head electronic wise. Just getting veeryyy confused as to how to implement the FC TC. Not too worried about going custom.

 

[mrclunk]

Hi guys,
I have a question for anyone who's experienced the small amounts of distortion (under compression) in the normal 'analag' TC mode.

After you added the SCAB sidechain amp does this still happen or does the added balls alleviate the problem?

Just finished tweaking my standard PM670 and noticed a little distortion on a piano solo in a test track, the distortion only occurs on quite hard hit parts. (But its not a signal overload problem)
Just thought perhaps the added power of the SCAB mod would help the SC 'keep up' with the faster transients of the piano. (programed midi piano, with high velocities)
I do have a SCAB pcb waiting to go in btw so when added i'll see for myself, just wondered if it helped?

 

[Moby]

After you added the SCAB sidechain amp does this still happen or does the added balls alleviate the problem?

Adding SCAB board makes PM totally different regarding TC network, so it of course sounds different  .

Just finished tweaking my standard PM670 and noticed a little distortion on a piano solo in a test track, the distortion only occurs on quite hard hit parts. (But its not a signal overload problem)

From my experience distortion is caused mostly bu "Varimu" tubes, then by transformers . First U must match them and when you do that there is still  a problem because it's not really Vmu tube. It will never work as  6BA6 or 6386 ! Adding SCAB helped a bit to catch faster so "headroom" improved a bit.
Anyway, that's my 0.2 cent's.

 

[mrclunk]

Thanks Moby,
I'll try some different tube combo's and take it from there.

 

[Moby]

Thanks Moby,
I'll try some different tube combo's and take it from there.

But have in mind that 4 6BA6 or similar wired in triode mode must have a "rich" at least 10K / 600 ohm . I would rather say 15K . Also, capability of SCAMP is max -17V CV so maybe tweaking the PM to another beast is not a good idea. I'm not desperate with PM sound at all , (with close matched tubes and transformers) but it has strong character and maybe is not best compressor for mastering purpose. Also, nobody mentioned that when you match tubes in CH1 you have to match another channel close to that if you want to use it in link (stereo) mode. So, order some more 6BC8's and match them in dynamic mode. It will sound great for vocals, guitar and some kind of drums smashing  ;D. For mastering wait a bit for some more sophisticated design 

 

[ChuckD]

I wanted to post here since this is where I got the wiring guide (first page).

Is there a ground loop on the wiring guide?
Does the Green/Yellow wire and Blue Wire on the Bypass PCBs both go to Ground through different paths?
Would this cause a loop?

Chuck

 

[lolo-m]

I wanted to post here since this is where I got the wiring guide (first page).

Is there a ground loop on the wiring guide?
Does the Green/Yellow wire and Blue Wire on the Bypass PCBs both go to Ground through different paths?
Would this cause a loop?

Chuck

If you want to be sure there's no ground loop you can star ground every board (I mean one point on the chassis leading ground to every single board). But hum in the PM670 comes mainly from an unregulated filament supply, bad matched tubes and magnetic field provided by a too hardly used power transformer. Check also a post of Moby about wiring the filament to cancel a lot of hum due to electrostatic field effect in the PM660/670 thread.

 

[ChuckD]

Hey Lolo

I have another issue :

When I try to calibrate one channel I cannot make both Your Timing circuit and Analag's circuit equal to -4.5.  When I tweak yours to -4.5 then flip switch Analag's reads -4.99. Same goes if I try the other way they are always -0.4V apart.
So when I flip the meter is now not at 0 GR

This is not true for the other channel which I can get them 0.04V apart. Which tracks very well on the meters in either mode.

What could cause this gap between the 2 circuits ?


Thanks Again

Chuck

 

[lolo-m]

Did you follow exactly the procedure in my first post ?
This is the only way to get the mod working properly.

The PM660 board should be set to the normal setting first. Then the SCAB can be attached and set as explained in first post.

I had problems too when I did the mod and finally find that procedure to have a good tracking in both modes.

Hope this helps.

 

[ChuckD]

Ok Sorry

I think I did this wrong then!
It still kinda works though ?  Here is what I did

1) setup RV3 for -2.5 V and RV6 for -4.5V at right side of R17 (middle pin of pot) .
2) Then set RV201 for -4.5  middle pin to GND (in both modes)
3) Set RV101 for -4.5 middle pin to GND  (in both modes)

So this is wrong then??

I think you said to :

1) setup RV3 for -2.5 V and RV6 for -4.5V at right side of R17 (middle pin of pot) .
2) set mode to Analag's , set RV201 and RV101 so that their corresponding "To Attack" connectors have 0VDC across them???

Is this correct?

Chuck

 

[lolo-m]

Ok Sorry

I think I did this wrong then!
It still kinda works though ?  Here is what I did

1) setup RV3 for -2.5 V and RV6 for -4.5V at right side of R17 (middle pin of pot) .
2) Then set RV201 for -4.5  middle pin to GND (in both modes)
3) Set RV101 for -4.5 middle pin to GND   (in both modes)

So this is wrong then??

I think you said to :

1) setup RV3 for -2.5 V and RV6 for -4.5V at right side of R17 (middle pin of pot) .
2) set mode to Analag's , set RV201 and RV101 so that their corresponding "To Attack" connectors have 0VDC across them???

Is this correct?

Chuck

Between the 2 pins of each "to attack" SCAB connector you must read 0V, in analag's mode with no compression and no signal.
It isn't easy to measure this but it is the only way.

 

[ChuckD]

Nope this still did not work for me.

I have done what you said but the one side does not match in 2 modes
I my way of doing it I had it much closer but it was a compromise between the two.

What is strange is it only happens on one channel. The other channel this method works perfect.
Any ideas what could cause the two modes to be so far from each other on one channel only??


Chuck

 

[lolo-m]

Nope this still did not work for me.

I have done what you said but the one side compresses way more then the other. In both modes.
In the Analag Mode the meters seem to match but in SCAMP mode they do not at all.

As the SCAMP is a basic voltage follower I can't understand what's happening.
If tracking is "OK" in analag mode and not in SCAB mode, maybe you have to check again if you didn't make an error in the resistors around the opamp. It must have a gain of 1. If everything is OK, you maybe have a problem of solder short somewhere in the SCAB. Have you tried to swap the channels of the SCAB ? Does the problem changes too ? If it does, the problem is on the SCAB, if it doesn't the problem is somewhere else.

 

[ChuckD]

Yes if I do this the problem does switch sides. So it is something on the right 20X side of my SCAMP board is screwy.
I tried swapping the opamps since they were in sockets and thought I'd give that a quick try. It did not make a difference.

Tomorrow I will have to unhook the SCAMP board and start looking at the bottom solder joints.

Chuck