vintage 6386 Vari-Mu compressor help

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davidaparry

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Joined
Dec 28, 2021
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1
Location
victoria BC canada
howdy! i recently acquired a bit of an oddball compressor. it has similar tubes to the Gates STA. i cant find much about it on the internet other than a manual / schematic. the person i bought it off didnt know much about it - he said he got it from a well known recording artist who said it was modded but thats all he knows.

anyways, my dilemma is that its kind of working but misbehaves a lot and has some pretty unusable grit in its current state. there is a meter zero and meter sensitivity pot. the meter zero does what it should and the meter sensitivity seems to be a threshold and maybe attack and release all in one. the two scenarios that this pot gives me is:

1. meter shows compression, anytime it compresses it also adds some ugly distortion to the signal. the signal that i hear that isn't the grit sounds nice with some good sounding compression, albeit with pretty fast attack and release. maybe too fast. it also has occasionally thumps of way too much GR. its just super sensitive and random feeling.

2. turning the pot all the way CCW seems to lower the threshold, and slow the attack and release down. now there's more compression showing on the meter and it cleans the signal up with much less grit if any. the problem here is that now there are loud audible thumps with sudden excessive gain reduction that happen unpredictably but correlated to the audio signal. also the meter is completely unreliable here. if i kill the input signal the meter just oscillates on its own.

with both scenarios, the gain staging (input) has little impact if any on the tone. the input and output knobs work as they should and it would be a fine tube preamp if the compression circuit doesn't engage.
i do think it has the potential to sound quite nice and would love to hear anyones insights in to what to focus on! i will be around a retro STA next week and can swap the tubes around and see if that helps.

manual / schematic: https://steampoweredradio.com/pdf/harkins radio/auto level.pdf
 

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Bigtone had that thing 3 years back, went through his hands. He doesn't post or read here. Note it's had the living shit modified out of it, whatever it is.

I can't get that manual link to load.


It generally sounds like all the problems you experience with unbalanced and bad tubes.

 
Last edited:
Looking at the schematics I cannot see any balancing adjustments.

Without these the compressor needs tubes that are internally well balanced to function properly.

6386 is an expensive tube so it would make a lot of sense adding balance adjustments. Less tubes will be rejected

Look at the schematics for UA 175, RCA BA-6A and other vari mu compressors you will normally see a potentiometer between the two cathodes on one between the two plates of the input tube.
 

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