letterbeacon
Well-known member
A blast from the past!
I had loads of fun building this limiter from scratch and used it on some home recordings, but then one day I stopped getting signal from the output of it. Soon after that I got a new job and no longer worked as a sound engineer, and that meant my BA-6A was put on a shelf and gathered dust for a couple of years!
I decided to get it down the other day and fix it and it's been tricky trying to get my head around how I built it three years ago!
The first thing I did to get signal running through it again was to swap out the output attenuator. I originally used a Hairball 600r attenuator with a massive 40dB pad just before it, the idea being that most of the power would have dissipated before it got to the Hairball attenuator. I had a feeling that I built the pad wrong, or it didn't dissipate enough power, so I replaced the pad and the Hairball attenuator with a big 15W Mallory one, as recommended by Drip in his BA-6A build.
That seemed to do the trick, and I now had signal running through the limiter, and it appeared to be working again.
When familiarising myself with the schematic and my layout, I noticed that I'd connected pins 3 & 5 of the 6H6 to the non-transformer side of the input attenuator - essentially connecting 3 & 5 of the 6H6 to pin 4 (the grid) of the two 6SK7 tubes. I'd purposely run an extra wire to do that, for some reason.
Why did I do that? I must have made a mistake. I disconnected that.
When I turned the limiter back on, it started behaving very strangely. When I fed signal into it, it was compressing it hard, even when the input attenuator was fully attenuating. In fact the input attenuator didn't have any effect - it was as if it was being bypassed. The threshold control I had modded no longer seemed to have effect, and neither was there any difference between the 'Single' and 'Double' modes. When I turned GR off, the unit distorted somewhere internally.
I couldn't work out what was going on, and why so much signal was getting into the circuit when the input attenuator was fully attenuating. I desoldered the connection to the input attenuator from the output of the input TX but still the signal came through into the circuit full blast. Very strange.
I couldn't work out what was going on, and I've now put everything back (apart from the new output attenuator)- I've reconnected the input attenuator with the input TX and the connection between the 6H6 and the 6SK7 and it now seems to be behaving itself again.
So the question is - why did I solder that connection between the 6H6 and the 6SK7? Was it accidental? If it was a mistake, then why does my limiter behave so weirdly when it's removed?
With it removed and the connection from the input TX to the input attenuator removed, how is signal getting in to the circuit?
If anyone can shed any light on this conundrum, I would very much appreciate it!
I've attached the schematic for reference.
I had loads of fun building this limiter from scratch and used it on some home recordings, but then one day I stopped getting signal from the output of it. Soon after that I got a new job and no longer worked as a sound engineer, and that meant my BA-6A was put on a shelf and gathered dust for a couple of years!
I decided to get it down the other day and fix it and it's been tricky trying to get my head around how I built it three years ago!
The first thing I did to get signal running through it again was to swap out the output attenuator. I originally used a Hairball 600r attenuator with a massive 40dB pad just before it, the idea being that most of the power would have dissipated before it got to the Hairball attenuator. I had a feeling that I built the pad wrong, or it didn't dissipate enough power, so I replaced the pad and the Hairball attenuator with a big 15W Mallory one, as recommended by Drip in his BA-6A build.
That seemed to do the trick, and I now had signal running through the limiter, and it appeared to be working again.
When familiarising myself with the schematic and my layout, I noticed that I'd connected pins 3 & 5 of the 6H6 to the non-transformer side of the input attenuator - essentially connecting 3 & 5 of the 6H6 to pin 4 (the grid) of the two 6SK7 tubes. I'd purposely run an extra wire to do that, for some reason.
Why did I do that? I must have made a mistake. I disconnected that.
When I turned the limiter back on, it started behaving very strangely. When I fed signal into it, it was compressing it hard, even when the input attenuator was fully attenuating. In fact the input attenuator didn't have any effect - it was as if it was being bypassed. The threshold control I had modded no longer seemed to have effect, and neither was there any difference between the 'Single' and 'Double' modes. When I turned GR off, the unit distorted somewhere internally.
I couldn't work out what was going on, and why so much signal was getting into the circuit when the input attenuator was fully attenuating. I desoldered the connection to the input attenuator from the output of the input TX but still the signal came through into the circuit full blast. Very strange.
I couldn't work out what was going on, and I've now put everything back (apart from the new output attenuator)- I've reconnected the input attenuator with the input TX and the connection between the 6H6 and the 6SK7 and it now seems to be behaving itself again.
So the question is - why did I solder that connection between the 6H6 and the 6SK7? Was it accidental? If it was a mistake, then why does my limiter behave so weirdly when it's removed?
With it removed and the connection from the input TX to the input attenuator removed, how is signal getting in to the circuit?
If anyone can shed any light on this conundrum, I would very much appreciate it!
I've attached the schematic for reference.
