Hello, I have a D&A PDX-400 amplifier with 6LQ6 tubes that have been checked and they are anywhere from 90-100% with no leaks, good grid however I have very low output say 100 dead key swinging to 250-300 with a 3-4 watt carrier. Is there anything that I should be looking for to rectify this situation? Caps have checked good with no shorts but 1 driver tube plate does turn orange and I have swapped it out and still does the same thing. Please elaborate if possible. Any help would be greatly appreciated . Thanks, 313
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D&A PDX-400 amp has a problem
- Thread starter 313inthegarden
- Start date
I also have a pdx 400 with low output,having tubes checked,if they are ok,what lese could be the problem
There are so many different configurations on D&A amps for the "low" side. Some have a relay that bypasses the output tubes and runs just the drivers on low. Others (even ones that don't have the "250" switch) run all the tubes but cut the plate voltage in half for low side. Many times over the years I have found either the "high-low" relay quits working (causing it to be stuck on low) or the high-low switch stays stuck in low.
A consistent problem is getting the driver Plate Tune and final Plate Tune both peaked to resonance.
D&A used a control with no end stop for these two controls. The plate cap of each tube, or pin 9 on a baldy driver acts like a capacitor to ground. Those four "parasitic" capacitors are wired in parallel with the Plate Tune control. Doesn't sound like a big deal, but the actual capacitance of each tube can change a lot from one brand of tube to the next. That part of the tube's design had no impact on how the tube performed in a color TV. As a result there was no incentive to control that parameter in any way.
If you measure that plate capacitance for one batch of tubes, it could be a lot more in a different batch. This has the effect of pushing the proper setting of that front-panel knob a half-turn or more past the control's end of travel.
The control will now be "peaked" (or seem to be) with the plates on that control either fully apart, the minimum setting. Or they'll be meshed fully together at max capacitance.
Looks like a "peak" as you turn the knob, but it's just the control running out of range.
A control that's resonant and truly "peaked" will have TWO peaks in one whole turn of the knob. They might be close together, or 180 apart, but if you can identify two peaks in 360 degrees, that circuit is good to go.
And if you see only one peak, the coil attached to that control needs to be adjusted, to bring the peak setting back within the travel of the control.
A control that has only that fake single peak can hold back the power, and cause the tubes to run hotter than is safe for them.
73
D&A used a control with no end stop for these two controls. The plate cap of each tube, or pin 9 on a baldy driver acts like a capacitor to ground. Those four "parasitic" capacitors are wired in parallel with the Plate Tune control. Doesn't sound like a big deal, but the actual capacitance of each tube can change a lot from one brand of tube to the next. That part of the tube's design had no impact on how the tube performed in a color TV. As a result there was no incentive to control that parameter in any way.
If you measure that plate capacitance for one batch of tubes, it could be a lot more in a different batch. This has the effect of pushing the proper setting of that front-panel knob a half-turn or more past the control's end of travel.
The control will now be "peaked" (or seem to be) with the plates on that control either fully apart, the minimum setting. Or they'll be meshed fully together at max capacitance.
Looks like a "peak" as you turn the knob, but it's just the control running out of range.
A control that's resonant and truly "peaked" will have TWO peaks in one whole turn of the knob. They might be close together, or 180 apart, but if you can identify two peaks in 360 degrees, that circuit is good to go.
And if you see only one peak, the coil attached to that control needs to be adjusted, to bring the peak setting back within the travel of the control.
A control that has only that fake single peak can hold back the power, and cause the tubes to run hotter than is safe for them.
73