Roberto,
Thanks for sharing your observations. Are we talking about the zener D1 between the plate and screen grid? I’m not sure how you have the bleed resistor connected “to guarantee 1 mA across the zener to ground”.
http://rh-amps.blogspot.com/2013/02/rh84-amplifier-revision-2_26.html
Thanks for sharing your observations. Are we talking about the zener D1 between the plate and screen grid? I’m not sure how you have the bleed resistor connected “to guarantee 1 mA across the zener to ground”.
http://rh-amps.blogspot.com/2013/02/rh84-amplifier-revision-2_26.html
@Francois G
Yes, D1: I installed a resistor from g2 to ground with a value of
(315-21) V / 1 mA = 294 V / 1 mA = 294 kOhm -> anything from 270k to 330k is fine.
This way, even if the tube is not conducting, the zener works properly and screens see the right voltage, not full B+.
Another thing that I would try is to connect C2 not to ground but to the secondary of the output transformer.
This wat the DC path is to ground through the LM317, but the AC feedback is taken from secondary of the output transformer to the cathode of the output tube, like a cathode feedback.
I've built some amps with cathode-feedback taken from secondary (0-8 Ohm on SE, 0-4-16 Ohm on PP) and it works fine.
This is one of the latest guitar amps I've designed for a guitar amp forum, with switchabble negative-zero-positive voltage feedback from secondary.
http://www.diyitalia.eu/download/file.php?id=16420&mode=view
Yes, D1: I installed a resistor from g2 to ground with a value of
(315-21) V / 1 mA = 294 V / 1 mA = 294 kOhm -> anything from 270k to 330k is fine.
This way, even if the tube is not conducting, the zener works properly and screens see the right voltage, not full B+.
Another thing that I would try is to connect C2 not to ground but to the secondary of the output transformer.
This wat the DC path is to ground through the LM317, but the AC feedback is taken from secondary of the output transformer to the cathode of the output tube, like a cathode feedback.
I've built some amps with cathode-feedback taken from secondary (0-8 Ohm on SE, 0-4-16 Ohm on PP) and it works fine.
This is one of the latest guitar amps I've designed for a guitar amp forum, with switchabble negative-zero-positive voltage feedback from secondary.
http://www.diyitalia.eu/download/file.php?id=16420&mode=view
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I forgot to specify: the resistor will dissipate 330 kOhm x (1 mA)^2 = 330 mW so 1W resistor is fine.
Thanks. One will also have the watch the voltage rating of this resistor with close to 300 V across it.
Cool! How are they? I didnt had the currage to order the transformers right away. Im building high sensitive speakers, before building a 4w SET amp 🙂
Are planning a build with Them?
They are nice and petite. Just a simple RH-84 is what I'll use them for.
One can see that the builders have paid attention to the details. They are squeaky clean. Stainless steel nuts and bolts. Wires are very well terminated. But it will be a while before I use them.
One can see that the builders have paid attention to the details. They are squeaky clean. Stainless steel nuts and bolts. Wires are very well terminated. But it will be a while before I use them.
Sounds good. Do you make the pcb’s for your builds, or was it someone else i saw doing that…