[BUILD] NU FEDERAL AM-864 PCB OFFICIAL BUILD & HELP THREAD

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fiat_cc

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So, it works beautifully with the 12au7’s in it. When I plugged it up initially I only had output connected and there was some hum, and my heart sank. As soon as I connected a balanced input load, the hum disappeared. The unit is respectably quiet, and seems to compress as it should. Cranking the output gives a nice gritty overdrive too.
The client is very happy!

For anybody who is interested in running this unit with the 12AU7 tubes (as a closer match to the original Federals 6SN7GT) with correct heater voltages, the mod is very simple! Just put a jumper wire from each terminal of the 12au7/12ax7 heater terminal block (this ties pins 4 and 5 together), then solder one leg of the heater supply directly to the pcb pad for pin 5, and the other to pin 9. A 12ax7 will provide too much gain running this way I think, so try the 12au7.
 

fiat_cc

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Completed unit in last stages of bench test and calibration.
 

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fiat_cc

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Clients are very happy with the unit. They’ve spent the day tracking, and mixing using the unit, and have had many nice things to say about it!
 

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bernbrue

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Congratulations, Reuben. The unit looks great! Sometimes troubleshooting can be hard, but when everything is working in the end, it makes me happy as well.  Go on!
Bernd
 

fiat_cc

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bernbrue said:
Congratulations, Reuben. The unit looks great! Sometimes troubleshooting can be hard, but when everything is working in the end, it makes me happy as well.  Go on!
Bernd

Thanks Bernd. It’s been a process, no doubt, but a good outcome for all concerned.

It’s an interesting unit. The client described it as quite transparent at some settings, when tracking guitars etc, and said you more ‘feel’ it than hear it. Then at other settings, it can give that really driven , ‘slammed’  vintage sound. A cool unit for sure.
 

fiat_cc

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A quick question...
My client loves the unit. It’s quiet in operation, but he said it’s noisy when bypassed. Now, looking at the schematic, when bypassed, it is for all intents and purposes just shielded mic lead running from the back panel, to the board, then back again. Any ideas why this would be noisy?  I was considering connecting all pin 1 earth connections on the back of the xlrs to each other but not sure if that will help or worsen any ground loops.
 

hugo

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fiat_cc said:
A quick question...
My client loves the unit. It’s quiet in operation, but he said it’s noisy when bypassed. Now, looking at the schematic, when bypassed, it is for all intents and purposes just shielded mic lead running from the back panel, to the board, then back again. Any ideas why this would be noisy?  I was considering connecting all pin 1 earth connections on the back of the xlrs to each other but not sure if that will help or worsen any ground loops.

https://www.rane.com/png/n110fig1b.png
 

shot

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After a lot of time doing some other projects and having some family issues that kept me off the soldering, I'm trying to get back to this compressor.

I had a few issues that I found were wiring related and I fixed that. No more problems with input gain pot. It passes signal as it should. But still there's an issue with amount of compression that my unit is experiencing.

The problem is whatever level of signal it gets, it barely touches it. I can crank up the input gain to the point distortion is very obvious. If I push more signal into it, it actually severely distorts (in a nice way!). But compression is not happening. Just some slight compression that the meter is showing but it can't be heard. Yes, I see some of it on the recorded waveform, but when A/B-ing signal it's negligible.

Also, potentiometers are very non-responsive. Changing attack and release barely does anything. I can notice when release is set to longer value it makes GR needle respond slower but there's no change in sound. Treshold may make it enter into more gain reduction on the meter but that doesn't sound any more compressed. Ratio seems as if nothing happens wherever I set it.

I've tried replacing tubes. Got 8 different 6BA6 (two pairs plus four different singles) and I tried all of them. Couple of them were crappy and just made sound more distorted and lacking low end. I guess those were dead ones or fake ones. But other didn't make any significant change. Just a bit of color change (or maybe it was placebo...).
Then I tried changing 6AV6 to a couple of the same brand (NEC) and nothing happened.
I'm using 6N2P-EV instead of 12AX7 but I guess it's not an issue. I have used this tube twice before when I had no 12ax7 and it was okay. To me it even sounds better (or again another placebo...). It is supposed to be russian equivalent.

I assumed it could be that compressor is not biting enough because I used 1:1 input transformer so I replaced it with a 1:6 transformer I had around (OEP A262A3E). That didn't help neither. It just made the unit get more volume on the input and subsequently enter into distortion earlier.
I put back the 1:1 transformer and then tried bypassing input pad - the change was similar to having 1:6 input transformer. Higher level and more distortion...

Pots are wired correctly and have their resistor added where needed. Heater voltage is spot on.

Voltages from the PSU are a bit different than what is noted in first post, but not that much
B+ 220V
A 8.15V
B 217V
C 217V
D 186V
E 1,16V
F 1,16V

G 219V
H 219V

Looks as if D is 30v higher and E/F are double than they should.
Are those voltages too off?

What can I do to lower the treshold or in any other way make it compress more?
I've watched the video posted here of the compressor in action and it's compressing  way more than mine is.

:)

Luka
 

bernbrue

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Hi,
you need an input transformer with a secondary centertap, otherwise it won’t compress. Please check the wiring of the input transformer once again. Then use a ECC83 for output and test it once again. You should change the heater wiring so that both halves of the double triode get 6,3V as Reuben pointed out a few post above.
Cheers
Bernd
 

shot

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Hi Bernd,

I'm already using center tapped input transformers. Both transformers I tried are with dual primaries and secondaries wired to have center tap. But you gave me few ideas to try to measure what is going on from the secondary center tap to the grid of the tubes. I'll do some tests and also try to get a different transformer and get back with results.
thanks!

:)

Luka
 

fiat_cc

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So, I just seem to have an endless string of bad luck with this machine.  An input transformer died on me, and created all sorts of noise and havoc.  I now have a replacement input transformer, and channel 1 is beautifully quiet, channel 2 is slightly noisier, but seems to be usable (I think), but as soon as you engage the link switch, it gets horrendously noisy.  Bypassing channel 1 whilst linked makes the noise much worse, bypassing channel 2 makes the noise completely disappear.  Something is obviously up with channel 2 to create the extra noise when in mono mode linked mode. I would usually jump straight to grounding issues, but I can't find any (not to say there isn't one I haven't found...).  What would be possible causes for added noise on channel 2, and for the noisy interaction between channels when linked?
 

fiat_cc

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A little more info...
The basic him in channel 2 is there immediately on power up,  before tubes conducting, so must be in the output transformer area.

Link issue disappears with channel 2 v1 and v2 removed, so possibly two separate issues.
 

fiat_cc

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Input transformer was just hopelessly microphonic, and far more susceptible to RF interference than the other.  I swapped channels and the issue followed the transformer. Replaced transformer, and those issues are gone.

In terms of link, I just ran the two link nodes on the boards to a toggle switch on the front panel.
 

bernbrue

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Hi,
So my guess is, that something in the input tx and/or  input pot area is wrong. Probably a wiring error,  since  you already reported about microphonic input transformer. The channel link just connects the two center taps of the input transformer/sidechains. Check values/ voltage rating of C15 and C13 and the pot wiring in the sidechain as well.
Bernd
 

fiat_cc

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Thanks Bernd.
New input transformer got rid of microphonics etc, and had issues regardless of which board it was hooked up to.  This link issue seems to be related to channel two, so I'll re-re-check wiring on that board and it's associated pots. 
 

Tashi

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Hi Bernd,

What would happen with a stereo unit if I used one B+ supply for both sides? (with sufficient current and filtering - ie a 5H choke).
Would there be some interaction between the channels or would it be okay?

Thanks!
 

bernbrue

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Hi,
honestly, I’m not shure, what would happen. I don’t know of any dual channel tube compressor with a single B+ supply for both channel. Parts are cheap, so build a psu for each channel.
Bernd
 

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