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

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morls

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If you find a good match for the 6ba6 pair you can normally null any slight thumping with the balance trim on the 6V6's
Ok, thanks. In this case would it be best to just leave the trimmer in place, even though it's wired to different pins than intended?
 

Rob Flinn

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Ok, thanks. In this case would it be best to just leave the trimmer in place, even though it's wired to different pins than intended?
Not really understanding what your saying. I have built a few sta levels point to point & only ever needed the trim on the 6v6's
 

morls

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Not really understanding what your saying. I have built a few sta levels point to point & only ever needed the trim on the 6v6's

Sorry, I wasn't clear.

On the main PCB there is a trimmer installed across pins 2 and 8 of the 6386, labelled "balance". I'm not sure whether this will work with the 6BA6s, as they have cathode and grid#3 connected.

6386 adapter wiring with trimmer.png
 

MaxDM

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so, in my mind, the question arises:

If it's so easy to make a compressor circuit using normal pentodes, why wasn't this common practice in the tube days? Surely there must be an advantage to using dedicated tubes?

...or was it? I haven't found any articles.
 

MaxDM

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This page describes the approach taken by Manley Labs. I've gotta say, I was blown away by the response to my initial enquiry about getting some boards. I got a very quick reply from the company's president, EveAnna Manley, who put me in touch with the parts department and we went from there. Amazing support for a humble DIYer!

Manley Labs T-Bar
That's nice of them to collaborate with you.
I was concerned by their choice to make production units with the 5670, which only looks like a 6386, and is quite incorrect as a tube for this application, for some time.

Personally, I would have never dreamed of trying to pull something off like that, but they seem to be doing well regardless.

From their site:

...The 5670 equipped Variable Mu's were well received, and retained many of the characteristics that made the original version of the Variable Mu® popular. They did sound different than the original, especially when pushed past 6db or so of limiting..

?

does not compute
 

Rob Flinn

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Sorry, I wasn't clear.

On the main PCB there is a trimmer installed across pins 2 and 8 of the 6386, labelled "balance". I'm not sure whether this will work with the 6BA6s, as they have cathode and grid#3 connected.

View attachment 82104
Didn't realise you were using a pcb. It should work, try it out, see if it works, that's what I do. It can't be difficult to bypass if it doesn't.
 

Winston OBoogie

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make production units with the 5670, which only looks like a 6386, and is quite incorrect
I agree. it was a strange choice. The 5670 is certainly a good tube, and mostly overlooked for audio despite there still being decent quantities of GE NOS available, but it's not a variable mu tube. At least not to any extent that an E88CC or a 12AU7 could also be a variable mu tube.
 

Winston OBoogie

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If the value of the trimmer, along with any resistors in series with it, is about equal to what is shown on a STA-level between the cathodes, then it does no harm to have that DC balance control for the 6386/6BA6 tubes in place. Along with a trim in the 6V6 circuit for balancing the AC signal etc.

With a well matched pair of 6BA6's that cathode trim would be around the centre position anyway.
 

Rob Flinn

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I always figured that in the sta level there are 3 valves in the path before the signal gets tapped off to the double diode. This probably helps to average out the differences between the 2 halves of the circuit. By swapping your valves around a bit you can achieve quite a good balance in the first place. Therefore maybe one doesn't actually need a trimmer on every stage in the circuit. I certainly have never had an issue nulling the thumop with just the one trimmer. IMHO drip tends to take a circuit that isn't broken then over engineer some unecesessary fixes into it.
 

emrr

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so, in my mind, the question arises:

If it's so easy to make a compressor circuit using normal pentodes, why wasn't this common practice in the tube days? Surely there must be an advantage to using dedicated tubes?

...or was it? I haven't found any articles.

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.
 

TLRT

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If you find a good match for the 6ba6 pair you can normally null any slight thumping with the balance trim on the 6V6's
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?
 
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