My PTP La2a Clone has way too much gain. Please help

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J

Janalex

Well-known member
Joined
Oct 8, 2014
Messages
166
Hi everyone,

I'm very frustrated with this build. We have followed the layout to the tee going over every connection. Did a nice clean job with the heater wiring. Used the original a10 and a24 transformers and allied power transformer.  All top notch complements. RCA valves. This beast will not allow for peak reduction above 60 without going into a  feedback loop and becoming unstable. We narrowed it down to something with v3 and v4. Voltages measure correct. Even just touching the meter to the unit stabilizes it.  The insertion of a ceramic cap also removes the oscillations. I would prefer to get to the heart of the problem first before trying modifications. I know others have experienced some similar issues however nothing I've read offers is pointing me in the right direction. Please help.
 
Greetings, folks.  I've been working on a two-channel LA-2A style opto limiter in my spare time.  The sidechain and metering circuits are adapted verbatim (with the exception of a 60V zener in place of the neon), and I'm using Kenetek T4s.  Everything works and sounds pretty good at this point, except for one thing:

Whether or not there is any signal at the input of the device, when I crank the "peak reduction" knob, the VU meter's needle buries itself to the left.  During this time, I'm measuring about 28Vrms at pins 3 and 5 of the T4, so something fairly significant is happening.  I suspected there was likely some oscillation that I wasn't hearing.  Sure enough, I scoped it and there seems to be a high frequency oscillation around 46kHz.  But it's only there when I crank the peak reduction knob.  There's a TINY bit of it at the output (2mV rms), but the VU meter doesn't see it.

I suspect it's coming from the 12AX7 in the sidechain, as turning the "stereo adj" pot down to ground makes the oscillation disappear.  There is no measurable AC going into the peak reduction control with no signal present (as expected).  I've tried a few different tubes in that position, but no difference.  Anyone else ever see this?  Is it kosher to put a low pass filter between the 12AX7 and the 6AQ5?  Say, maybe a cap across the stereo adj pot?

Incidentally, what's an LA-2As stereo adj pot default position?  I would assume full CCW, yes?

Thanks,
IZ
Click to expand...

I noticed that this exact issue was later remedied by another member in another post with a .1uf decoupling cap. Can anyone expand on this and recommend where to install it?
 
To all interested the issue was with the wiring of the dual t4bs. I had them wired through a 3pdt switch including the lead running to pin 3 by way of c11.  By running a lead from c11 to the switch and then to each t4b I was generating the oscillation.  When I changed it to wiring the t4bs in parallel each through its own c11 cap the issue was resolved. I see drip only uses a single c11 in their dual fast/slow build but I thought it would be more correct to wire each to a separate c11. The other two t4b connections run through the switch. I was attempting to separate the circuit as much as possible which I see isn't always necessary as drip uses a spdt switch.
 

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