[lewilson]

I think Im going to try dual ganged 25k pots in my poorman 670. It looks like the taper will be o.k when each pot is in parallel with a 40k resistor. Ill let you know how it goes

[Moby]

Why? You want to raise input resistance? Or something else?

[bluebird]

First off thanks to Volker and Analag for getting together such a great project !

Its been a while in the making but I finally finished my unit.

The first problem I had was a funky looking waveform even without any compression. I found out I had a meter with a high internal resistance so I ended up replacing R6 (22K) with 100 ohms. If your not using the recommended meter I suggest you check your output on a scope to make sure your not starving your 6BC8 plates.

The Main mod I wanted to suggest has to do with the time constants.

I just wasn't getting the flexibility out of the attack and release I wanted.

So I started off by adding a switch to change C4 (10uF) I used these values.
10uF, 6.8uF, 4.7uF, 1uF

I still felt like the attack knob wasn't changing the time constant enough.
I glanced over the original 670 schematic and realized it has a resistor in parallel with the time constant caps not in series that changes.

EDIT I'm sorry there are also resistors in series with the capacitors in addition to parallel. these would have an effect on attack time. EDIT

So basically I PUT (RV7) IN PARALLEL WITH (C4).

I put RV7 on a 5 position switch and used these values:


25K, 65K, 150K, 470K, and an open position (you could use 2 megs)

The higher the resistance the longer it takes the cap to discharge through it. longer release.
the smaller the cap the shorter time it takes to fill up. shorter attack.

switching up the combination of the two makes for a lot of flexibility.


I can't tell you cool this thing sounds....

If you already have a panel with only one hole for a time constant switch you can use a double pole six position switch. Play around with different caps and resistors in parallel. Heck just use the fairchild values!

Anyhow I hope someone is brave enough to try this. It really turns this thing into a magic squasher.

Peace,
Bluebird

[CJ]

Good Lord I though you were gone for good, how wonderful to see you!

Rock on.

What happened to the rest of the band?
I still play that CD,  love it.
Thanks again, cj.

[Moby]

I found out I had a meter with a high internal resistance so I ended up replacing R6 (22K) with 100 ohms. If your not using the recommended meter I suggest you check your output on a scope to make sure your not starving your 6BC8 plates.

Thanks god that somebody experienced that problem. I still don't have my unit finished but it was obvious that "VU" must be some low internal meter (ma meter or ua meter). BTW, I don't think that scoping the waveform is necessarily for checking unit. Just proper supply voltage after meter. I presume that you had a huge voltage drop with few kohm meter so tubes were working with low voltage  . Bluebird, can U tell us about your voltage after meter? It will be great to know is somebody experienced same problem as you.

I still felt like the attack knob wasn't changing the time constant enough.
I glanced over the original 670 schematic and realized it has a resistor in PARALLEL with the time constant caps not in series that changes.

Hey, that's great pimp! Do you have a time to draw a final combination (values) of your time constants?

[EmRR]

yes!  submit drawings if possible!  That way you won't have to explain it 38 times!   

[bluebird]

Still Alive C.J. !!

[Kid Squid]

Nice !

Steve

[Moby]

Heck just use the fairchild values!

Thanks for drawing  BTW, did you tried fairchild values?
Also, did you tried without capacitor in some position?

[bluebird]

No, I didn't try the exact fairchild values. Remember I'm using TWO seprate switches. The fairchild had a double pole switch.

The best way to find the values you like (if using a single switch) is to use a decade capacitor box and a decade resistor box in parallel and just mess around with the values until you find the combinations you like.

A setting without a capacitor would mean you are left with (C3) 0.22uF. I'd imagine you would get a distorted signal when turning the threshhold up at all. I'm not sure 0.22uF is enough to filter the DC control voltage. Hey it might sound cool...

[Moby]

Ok, I compiled this picture so we can see what's going on with the original 670 time constant. Fastest attack is 0.2msec and position 6 seems interesting since is automatic, depending from program material. I don't want to criticize the PM just to make it closer to fairchild.  I have a feeling that it has "too poor time constant" so it will be great if we can hear the author comment about. I know that 10uf with variable 10k in series works, but I'm afraid that it has much larger attack time than 0.2ms , not sure about release  . Bluebird's version is something good for a start, but I'm afraid that without two separate switches it will be too limited. Hmm, why Analag went with 10k and 10uF? I wonder

[khstudio]

Hmm, why Analag went with 10k and 10uF? I wonder

Interesting... Maybe because it works. How good though I've yet to hear for myself.
BUT if we're trying to copy a Fairchild... why not go all the way & use the original time constants & values?

The question is... will it work correctly on the PM version?

I don't know enough about it to dive into it without asking.


Also,
That's great info about the METER's!!!
I got some "old school" meters that are pretty heavy duty & cool looking but I think they MAY be too beefy for the PM670.
They were originally labeled - one for 100 VDC & the other 100 mA.
I tried removing the resistor inside & put the diodes on them JUST to see how they'd respond as "AC" VU's & the needle hardly moved at all... compared to ALL my other AC VU's like Modutec, etc...

What "resistance" should I be getting for the PM670 to work correctly???

May be time to look for other meters... but I'd like to get these to work.

[Moby]

I tried removing the resistor inside & put the diodes on them JUST to see how they'd respond as "AC" VU's & the needle hardly moved at all

If I understand correctly PM meter works like a DC ma meter since it's connected in series with DC supply. There is no AC voltage. Just DC voltage drop around meter resistance. I think that resistance have to be few Kohms but less can work even better since current is not static because of constant gain change of Valves. I asked few times for DC currents through meter but nobody gave me an answer  .
i would try 10ma meter and try to tweak with some resistance across and in series to make max movement for max GR.

BUT if we're trying to copy a Fairchild... why not go all the way & use the original time constants & values?

The question is... will it work correctly on the PM version?

I think that it should work, I don't see any reason for not... just, c3 have to be changed to 2uF....R18 is not important , I think.. 

[khstudio]

PM meter works like a DC ma meter since it's connected in series with DC supply. There is no AC voltage.

Sorry, I though I was clear when I said:

"JUST to see how they'd respond as "AC" VU's"

The resistance on the meters I have measures:
DC 100 Volts meter = 100k
DC 100 mA Meter = 1 ohm

They look like the same meter & most likely just have different resistors (or not ) in them.

Do you think these will work???

[khstudio]

What TYPE of capacitors "Should" be used for the Fairchild time constants???

Or should I ask..

What TYPE of capacitors did the REAL Fairchinld use?
or does it even mater?

I'd think it would... tants or electro's?

[[silent:arts]]

olives in oil are known to be the best

tantals are unethical these days (and I haven't seen one in a Fairchaild yet)
electros are not the best for "time cocnstant"

mhm, I wonder why there is a film cap in analags design.
must be just for the look and the costs I think

never mind
Volker

[khstudio]

mhm.. I wonder why people can't give a straight answer without being sarcastic?

[[silent:arts]]

sorry mate, nothing personal, will try to avoid it

back on topic:
what capacitors did fairchild use ?

[Moby]

Well I'm not sure about original Fairchild capacitors but since they were stated as 180V polarised (20 uf) I'm pretty sure they were ELCOS. Tant's at 150V will be better but it will cost  Some block bypassing with Elco can be nice. Other, smaller values can be anything "block" MKT, MKS, Or MKP....

[[silent:arts]]

there is no high voltage in this stage.
in my opinion you are fine with - let's say 63V.

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