Just finishing up a KT88 PP amp using dyna clone iron. It uses Pete Millet's compactron PP driver boards. Schematic here:
Push-pull driver board
The boards are built pretty much to this schematic.
My problem is that any feedback applied from the opt causes instant
oscillation. Both channels act the exactly the same leading me to believe I'm overlooking something simple. It plays well otherwise except the highs seem to rolled off a bit.
Thanks
Push-pull driver board
The boards are built pretty much to this schematic.
My problem is that any feedback applied from the opt causes instant
oscillation. Both channels act the exactly the same leading me to believe I'm overlooking something simple. It plays well otherwise except the highs seem to rolled off a bit.
Thanks
The wiring to the grids may be reversed, check their connections. Or possibly the primary wiring is wrong.
You have positive, not negative, feedback.
Use the same polarity of output transfrmer connections as the schematic in this Dyna manual.
The color coding will be the same as your transformers.
https://hafler.com/pdf/dynaco/Dyna-ST70.pdf
You have positive, not negative, feedback.
Use the same polarity of output transfrmer connections as the schematic in this Dyna manual.
The color coding will be the same as your transformers.
https://hafler.com/pdf/dynaco/Dyna-ST70.pdf
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All the transformer wiring should be good. I have that schematic. I however did not consider reversing the grids. I will try it.
Then that will fix it.
Bear in mind that the output tube biasing will change when you swap the grid wiring,
if you have separate biasing for each output tube (which you do).
At least turn down the highest set pot first, before applying the power.
Bear in mind that the output tube biasing will change when you swap the grid wiring,
if you have separate biasing for each output tube (which you do).
At least turn down the highest set pot first, before applying the power.
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Got it. That did the trick. Sounds great but needs a little more gain to achieve full output I think. It does good with my Aikido driving it. Also the B+ is a bit higher than I would like. Using solid state diodes atm. May put a 5AR4 in it. Main thing is it's functional with feedback applied. The rest is minor stuff.
Thanks rayma!
Thanks rayma!
If you decide to use a 5AR4, add a series 1N4007 with each tube diode.
This prevents arcing with some current 5AR4 types.
This prevents arcing with some current 5AR4 types.
Attachments
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I appreciate it very much. It's actually a GE branded Mullard but that mod is cheap insurance regardless.
If that's the case, you won't have any problems with that 5AR4.
I have Mullards in my amp also. The stock Dyna Stereo 70 tubes were Mullards (except for the 7199).
They last forever. And the stock Dyna PAS-3X tubes were real Telefunkens.
I have Mullards in my amp also. The stock Dyna Stereo 70 tubes were Mullards (except for the 7199).
They last forever. And the stock Dyna PAS-3X tubes were real Telefunkens.
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Hafler bought huge quantities of tubes at a time for the Dynaco kits, so they were cheap back then,
even in the 60s and 70s. Same for the transformers, chassis, and pcbs. The Stereo 70 sold for just $99
in kit form for most of its product life.
even in the 60s and 70s. Same for the transformers, chassis, and pcbs. The Stereo 70 sold for just $99
in kit form for most of its product life.
From what little bit I've read, he was a true visionary.
Demand for the vintage Dynaco stuff after all these years is proof.
Demand for the vintage Dynaco stuff after all these years is proof.