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?
STA-Level: 6386 to 6BA6 T-Bar wiring
- Thread starter 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.
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.
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.
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:
?
does not compute
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.
scott2000
Well-known member
Is there a trim also on the 6V6?
There is only a trimmer on the 6V6 on a real sta level.
Winston OBoogie
Well-known member
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.
scott2000
Well-known member
Yeah. I thought Gregory's builds were more or less true to originals which is why I was curious...
Winston OBoogie
Well-known member
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.
With a well matched pair of 6BA6's that cathode trim would be around the centre position anyway.
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.
Winston OBoogie
Well-known member
The Hippocratic Oath - "First Do No Harm" is generally a good rule.
Jase_Sanderson
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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.
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?
