STA-Level: 6386 to 6BA6 T-Bar wiring

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Rob Flinn

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What is actually the exact process for the pushpull balance setup for STA? Injecting high level 1khz sinewave until the amp starts distorting then matching the AC voltages on cathode and anode?
It's to do with the sidechain. The sidechain puts a control voltage into the audio circuit. The audio circuit is balanced to cancel the sidechain signal out. The balancing circuit is just to fine tune the cancellation because both sides of the circuit are unlikely to be perfectly matched. The reality is the gain curves under compression will not be exactly the same, so although the balancing circuit works quite well it is still a bit of a compromise. You will only notice the thumping from the sidechain when the box is actually compressing & if the 2 halves are out of balance. One way to do this is to connect a signal generator between the earth & the point that the sidechain connects to (I think on the low side of the input pot, sorry I don't have diagram) then adjust the trim so that you can hear the smallest amount of the oscillator. It is worth being aware that at different levels of compression the null point is different unless the gain curves of both halves are perfectly matched.
 

emrr

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exactly like all other vari-mu - insert common mode source and tune it to minimum output. Look at RS124 which injects a neon osc thump, and the various obscure RCA Photophone units that use 6.3VAC filament tap to inject a tone.
 

MaxDM

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It was common to use separate multi-grid tubes. Look at all the compressors with 1612's, 6SK7's, and 6K7's.

Once there were dedicated tubes, no accountant would chose to pay for twice the sockets, twice the tubes, more wiring time.

Ok, but they were designed for variable gain.

6ba6 was not.
 

MaxDM

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It is indeed a remote-cutoff pentode, as is the 6K7 and 6SK7.

I responded to:

OOPS!!!

Right you are.

That's promising then.

For some reason I was convinced it wasn't. Maybe I was thinking of the 6AU6!
 
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mjrippe

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I don't recall where I got this schematic, but it is from a 1959 Japanese limiter using (2) 6BA6, so they were used back in the day.
 

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Winston OBoogie

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There's a *BA6 limiter described in Tremaine's Audio Cyclopedia too.
Can't remember if it uses 12BA6 or 6BA6 but, same difference.
 

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