MCI Tube Compressor(now with some gear porn)

[mjrippe]

The "other" comp has two Threshold pots where the Release pots are on the OP's unit,  No Release pots on the "other".  Also Lateral/Vertical limiters were used for disc cutting.  The Fairchild 670 being the most famous example...

 

[MagnetoSound]

Yeah, I think pucho meant to say 'no release pots'.

 

[pucho812]

Yeah, I think pucho meant to say 'no release pots'.

I did.

The "other" comp has two Threshold pots where the Release pots are on the OP's unit,  No Release pots on the "other".  Also Lateral/Vertical limiters were used for disc cutting.  The Fairchild 670 being the most famous example...

yes... does the lateral/vertical make the unit behave as a duel mono?

 

[pucho812]

Don't pull any tubes yet. Low, distorted signal could indicate a loss of power in one or more of the stages. It is not uncommon for B+ resistors to crap out. First check I would do would be to look for voltage on each of the suspect tube plates to verify whether B+ is getting there.

testing testing.  o.k. for starters I have an unmarked ceramic resistor that is falling apart. On one side of the resistor I get around 300VDC on the other side I get  around 0VDC and that ties into the 0b2 regulator.  Heaters are 6.3 Vac all around. As far as B+ measurements, I get around 300Vdc at the 6aq5 tubes, 150Vdc at the 6072 tubes and around 0Vdc for the 6386 tubes. hmmmmmmm. No schematic, Challange accepted.

Here is some 0b2 data

http://www.duncanamps.com/tdsl/show.php?des=0B2


Should be easy to work out the ceramic resistor value based on the fact that 300VDC goes into through the resistor and ties to pin 1 of the 0b2 regulator tube. As for the lower the normal b+ on the 6072 tubes and the no B+ on the 6386 tubes, Should be easy to work that back to  one of the old can caps, there is one here that looks original and is paper covered. Old skool.

 

[MagnetoSound]

Regulators can short, so that would be a good check once you have replaced the open resistor. It will be fairly obvious, of course, if that is the case.

They are one of the cheapest tubes to get though. 

 

[pucho812]

Regulators can short, so that would be a good check once you have replaced the open resistor. It will be fairly obvious, of course, if that is the case.

They are one of the cheapest tubes to get though. 

yes sir NOw to figure out that open resistor. It's ceramic and unlabled  due to falling apart.

 

[Jean Clochet]

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[pucho812]

the resistor all shares a cap in parallel right at the turret post for the wiring for the ob2. it shouldn't make much a difference should it?

 

[Jean Clochet]

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[pucho812]

O.k. let me try and explain better.  Coming from the bridge rectifier it feeds 300VDC into  the unknown resistor(now suspected a 13K as a good starting value) at a turret connection. The output of the resistor is attached to a second turret connection. That connection is where the wire going to the 0b2 is connected. At that same connection we have a capacitor .02/600V cap tied to ground.

 

[Jean Clochet]

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[pucho812]

thanks.  I'll keep you all posted

 

[jamesfei]

You can see some pictures of what looks like an older version of the compressor at http://www.mcirecording.com/. There is some beautiful wiring in those early MCI mixers.

Best of luck in the restoration, it's a rare find!

 

[pucho812]

funny my math got me 12K for  the resistor value ???  @ +/10% close enough.Those are some nice photos there. I put in a 12K 10 Watt resistor. 10 Watts is way over kill. It barely gets warm. It was hard to connect as it is large in size, I may scale back to a 5 watt or even smaller. Anyway so far looks like good. I now can switch it to GR mode and get the meter moving to 0Vu without audio or any signal present  Meter adjustment pots in front and I get exactly to 0Vu. B+ is around 250VDC  on the 6386 tubes now vs 0VDC before. now I just need to run audio and listen as everything else seems to measure with in spec from the data sheets.

 

[emrr]

I sure wish you could buy those Bud project chassis these days. 

 

[Jean Clochet]

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[pucho812]

yes. a complete rev eng is in the works.

 

[Jean Clochet]

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[emrr]

Here's the auction info and pictures for the one I saw.  A MIX magazine article about Jeep Harned from 1982 shows a similar looking unit with caption "MCI limiter, 1961".

Up for bid is a VERY rare vintage MCI tube 2 channel compressor /limiter used for stereo mastering. This unit was one of 4 that were designed and hand built by Jeep Harned back in 1964 at his original music center store/shop in Ft. Lauderdale, Fl. This is the same type comp/lim. that Mac Emmerman used in his disc mastering system at Criteria Recording Studios up until around 1978 when he sold his original Scully Westrex system. The MCI comp/lim. is a Jeep Harned version of the Fairchild 670 45 45/lat vert type triode comp/lim amp. The unit uses 6ES8, 12AT7 and a pair of 6AQ5 connected as triodes. This unit has an outboard regulated Westrex B+ supply and a 6.3 volt DC power supply for the filaments. The unit uses a triad HS-14 for the input, a Stancor WF-22 interstage and a Langevin 322B output for each channel. This MCI amp has adjustable release time for each channel and adjustable threshold for each channel mounted on a remote panel. The original metalwork had the threshold adjustments on the front of the unit but Jeep changed this to the release time as everyone wanted this adjustment. The threshold was fixed with 1/4" phone jack connectors on the back of the unit if threshold adjustment was wanted. I have included a panel with controls and 1/4" plugs for this. I purchased this unit from Jeep Harned himself in 1967. It is in working condition and sounds great! This is the perfect unit for that clean tube sound. Don't miss out on this very rare piece of recording history. This may be the only one of the four left in the US.

8/2003  $7100 ending price

 

[Jean Clochet]

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