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In Reply to: RE: first try at a 6072a/71a srrp amp... posted by vinnie2 on August 30, 2016 at 15:52:49
You need to connect the grid of the top triode to the plate of the bottom triode. Don't forget the grid stop resistor.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/30/16Follow Ups:
Opps, drafting error. That connection was made, forgot to draw it in. Where would you expect the grid stop resistor to be? There is a 1k resistor between the grid of the first section of the 6072a and the input, and there is another 1k resistor between the plate of the first section and the cathode of the second that I forgot to label. There is nothing else on Gordon's original schem that I see.
Dang found another error! There is a 10uf cap going from the plate of the second section of the 6072a to ground.
Edits: 08/30/16 08/30/16 08/30/16 08/30/16
Don't you need to reference 6072 heaters to something other than ground?
Not sure what you mean.
The cathode to heater potential in the upper tube of an SRPP circuit should be within spec.
Did you check the tube manual?
I use a typical resistor divider off the power supply to set the AC heaters on a DC bias above ground for SRPP.
I measured the voltage on both heaters and it is 6.3vac on each. Is that what you mean?
No, in the tube manual there is a voltage spec. for heater to cathode potential, this is for IDH tubes where the heater is a seperate element.
For 6072, the spec is 100 volts.
In SRPP, the upper tube cathode is at the same DC voltage or close to it as the lower tubes plate.
It is a good idea to raise up the AC heaters on a DC bias above ground to stay within the heater to cathode voltage spec.
I am a little confused on how to measure this. The 6072 has a common pin and then one other pin for each filament.
I will have to check on that. Thanks.
Connie, your coupling cap from SRPP to 71A is not right. .01 is too small. Find a calculator online and recalc.
Who's Connie? Actually that value of cap is what I generally use for couplers these days. I have found by trial and error that the smaller the cap I can use the better the sound.
Sorry, spellchecker decided to rename and regender you. Check that top triode cathode voltage.
No, the cathode pin of the upper triode measures what DC voltage.? If this is different by anywhere near 100volts, which is max rating of 6072, then you need to reference the center tap of the heaters transformer to a higher DC potential. You are just changing "sea level".
http://www.tubebooks.org/tubedata/HB-3/Receiving-Type_Industrial_Tubes/6072.PDF
See where it says peak heater-cathode voltage. The original Bugle must have done this.
Grid-stop resistor goes right ON the grid pin, with nothing else connected to the grid pin.
There is a good theoretical argument for dumping the Rk bypass capacitor in SRPP. You might want to try it. It equalizes current flow around the circuit. This is assuming that Rk1 = Rk2, where Rk1 is the cathode resistor on the lower tube section, and Rk2 is the resistor between the plate of the lower tube and the cathode of the upper.
They are the same, I just did not mark Rk2 like I should have. I will give that a try and see what happens.
What value should the grid stop resistor be? Would it tie to the grid on the grid side of the 1k resistor on lower section?
If 1k is good enough for the bottom triode it's good enough for the top triode.The grid of the top triode gets a grid stop resistor that connects to the plate of the bottom triode.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/30/16 08/30/16
So the 1k resistor to the grid of the bottom triode is a grid stop resistor? That explains what it is doing there. I thought the value was too low for a grid stop. I will give it a try, but Gordon's original schem does not show a grid stopper on the grid of the second triode.
Edits: 08/31/16
"So the 1k resistor to the grid of the bottom triode is a grid stop resistor?"
Yes.
"Gordon's original schem does not show a grid stopper on the grid of the second triode."
I see that but if the bottom triode "needs" a grid stop resistor then the top triode does also.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I can not tell you why, but it sounded worse with the grid stopper on the grid of the top triode.
Well, that's interesting.
It shouldn't have made any difference except stopping oscillations (if there were any).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Maybe I will try it again later when I get the rest of the wrinkles out.
Tre' did not comment on this, which makes me question myself in saying it, but 1K actually seems like a high-ish value for a grid stop resistor on a 6072, assuming the dual triode shown IS a 6072. If you can hear it, then you might consider going to a lower value or even starting with 100R, which would probably be sufficient. Carbon.
I thought 100k was the usual starting point for grid stops?
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