MCI Tube Compressor(now with some gear porn)

[pucho812]

Does anyone have any information or a schematic about the extremely rare MCI tube compressor. There is something like only 5 world wide. I happen to have acquired one through horse trading for a couple of DIY pre's. I scored big time me thinks.  The unit looks like the photo of one I have seen but has different controls. It was said to  have originated at Bradley's barn in Nashville. I will definitely will be posting some good ol fashion gear porn on this beast. The unit was last worked on some time ago and was left off with, we can't test the tubes but PSU is o.k. So the problem should be a bad tube.

So here is what details I have so far upon first inspection

Unit is a stereo/duel mono compressor via stereo switch. Per each channel It has input and output volume control with the inpu and outputt being a balanced 600 Ω altec attenuator, threshold controls,release time controls and the vu meter can be switched between in, out and Gr.

UTC iron for the I/O

A-11's and HA 134's

Tube Compliment is 4 X 6aq5's, 2 X 6072(AKA 12ay7) 2 X 6386(now available through JJ and are not bad) and an ob2 in the PSU.

 

[emrr]

I remember one that was on ebay 6-8 years ago, I probably saved pictures.  Doubt there'll be any info outside of rev/eng. 

 

[pucho812]

I remember one that was on ebay 6-8 years ago, I probably saved pictures.  Doubt there'll be any info outside of rev/eng.

i agree, but worth asking

 

[Biasrocks]

Very cool find, hen's teeth come to mind.

Would love to see some pics and I think some rev-eng is in order.

Mark

 

[pucho812]

picture 1

 

[pucho812]

picture 2(so far)

Only 2 pics so far. Tested all the tubes except the 6386's. 3 out of the 4 6aq5 tubes tested in the replace catagory and questionable catagory. Good thing I got all those jan 6005's for la2a repair on hand.

 

[pucho812]

initial audio test proved a  failure even after replacing the 6aq5's. Level going in is healthy and is reflected  on the VU meter set to input. Audio Coming out is low level and distorted, VU meter set to output reflects the audible level. Meter set to GR does not move at all. Possible failure of the 6386 tubes? I wish I could test those on my tube testers.  Lucky for me I happen to have access to a known good set of 6386 tubes so I can swap them for some quick testing.

 

[pucho812]

Oh and check out this wiring job.

Sexy Sexy.

 

[nielsk]

Schwing!

 

[pucho812]

I remember one that was on ebay 6-8 years ago, I probably saved pictures.  Doubt there'll be any info outside of rev/eng.

I would be curious to see those photos if you got them, that way I can compare and contrast things.

 

[shabtek]

wow

 

[emrr]

Don't think it's the same one.  I'll be home in a few days and can look for pics.  It looks like it was rebuilt in the last 10-15 years already. 

 

[MagnetoSound]

initial audio test proved a  failure even after replacing the 6aq5's. Level going in is healthy and is reflected  on the VU meter set to input. Audio Coming out is low level and distorted, VU meter set to output reflects the audible level. Meter set to GR does not move at all. Possible failure of the 6386 tubes? I wish I could test those on my tube testers.  Lucky for me I happen to have access to a known good set of 6386 tubes so I can swap them for some quick testing.

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.

 

[Rob Flinn]

Agree, check the anode resistors !

 

[Viitalahde]

That thing looks awesome. It was definately rebuilt some years ago from the looks of it. The issues might not be big at all.

 

[pucho812]

t looks like it was rebuilt in the last 10-15 years already.

Yes. Some of it  looks far to new to be original. I did notice a couple of wires connected at one end only, going from a ground lug to to where.

initial audio test proved a  failure even after replacing the 6aq5's. Level going in is healthy and is reflected  on the VU meter set to input. Audio Coming out is low level and distorted, VU meter set to output reflects the audible level. Meter set to GR does not move at all. Possible failure of the 6386 tubes? I wish I could test those on my tube testers.  Lucky for me I happen to have access to a known good set of 6386 tubes so I can swap them for some quick testing.

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.

Correct and that is quite possible, That will be todays duties, getting in on the PSU and make sure it is working to some sense of reasonable results for the heaters and b+ on all tube sockets. I noticed the PSU has 2 mutican caps, one is obviously not original and one is. hmmmm. As for pulling tubes I already had 3 of the 6aq5 test as fail on my b&k tube tester. Yes far from precision testing and so forth but they were no bueno.

 

[pucho812]

Don't think it's the same one.  I'll be home in a few days and can look for pics.  It looks like it was rebuilt in the last 10-15 years already.

Yes. Some of it is obviously not original, especially in the wiring used and some p-touch labeling in the front. I did notice a couple of wires connected at one end only, going from a ground lug to to where(just clipped off the unconnected end but i doubt that is causing problems. As for being the same one, this one has obviously been modded in comparison to pics I have seen. For example this one has threshold pots and a stereo/mono switch.

initial audio test proved a  failure even after replacing the 6aq5's. Level going in is healthy and is reflected  on the VU meter set to input. Audio Coming out is low level and distorted, VU meter set to output reflects the audible level. Meter set to GR does not move at all. Possible failure of the 6386 tubes? I wish I could test those on my tube testers.  Lucky for me I happen to have access to a known good set of 6386 tubes so I can swap them for some quick testing.

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.

Correct and that is quite possible, That will be todays duties, getting in on the PSU and make sure it is working to some sense of reasonable results for the heaters and b+ on all tube sockets. I noticed the PSU has 2 mutican caps, one is obviously not original and one is. hmmmm. As for pulling tubes I already had 3 of the 6aq5 test as fail on my b&k tube tester. Yes far from precision testing and so forth but they were no bueno.


Below you will find a picture of an another MCI compressor I found Note how this one has a different label for stereo/mono operation as well as no threshold pots.  ???

 

[MagnetoSound]

I see two threshold pots, no?


(PS - yeah, I only meant not to pull the 6386s til you'd checked for voltage. )

 

[pucho812]

I see two threshold pots, no?


(PS - yeah, I only meant not to pull the 6386s til you'd checked for voltage. )

yes. one for each channel Yes of course just dumping the 6386 tubes for new jj versions would be too expensive just to try.  plus I would hate to dump vintage 6386's if they still have life left.

 

[lassoharp]

Cool find.    8)

plus I would hate to dump vintage 6386's if they still have life left.

They may test fantastic on a tester and even look "matched" but how they behave under GR could easily be a totally different result.  They could look less than ideal on a tester but be well matched for the circuits compression duties.  I would be inclined to favor the tubes the last owner had installed vs a new set of JJ or NOS random 5 star GE in terms of real circuit balance.

A quick method would be measuring DC voltage difference between each 6386 plate for static vs several amounts of GR.  There may also be a balance circuit of some type for fine tuning.