A question on filtering caps

[Wakculloch]

We are not talking about cathode biased amps, we're talking about single ended, class A amps, ie Champs.
Sag is a reduction in the HT voltage caused by increased current draw as an amp approaches, or exceeds full power. If you can hear a 5V drop in HT you really should have your ears insured for millions cos they're precision instruments of a subtle and hithertoo unknown ability.
You cannot turn it up till it sags, it's a physical impossibility.
I give up.

[MWJB]

"We are not talking about cathode biased amps," Yes we are..."we're talking about single ended, class A amps, ie Champs." See, I told you we were!

"You cannot turn it up till it sags," I can, trust me ...I'm very brave like that.

Your position is still not backed up by substantial details & figures, or listening tests.

"If you can hear a 5V drop in HT you really should have your ears insured for millions cos they're precision instruments of a subtle and hithertoo unknown ability." No, anyone should be able to hear it. My own Champ based amp (which has been subjected to every 2A 8pin & SS rectifier that will fit) is biased cool-ish, so voltage drop between GZ34 & SS is a couple of volts...the difference is clearly audible, not just to me.

Try it, don't rely on assumption.

[catnine]

"So instead of two dropping resistors and three filter caps I would have three dropping resistors and four filter caps and adding 1k ohm before the plate I would lower my B+ on the plate so I could use a lower than the 680 ohm cathode resistor to say 500 ohm bias the 6V6 closer to the mid point of saturation and clipping and at the same time have a safer 30 uf reservoir cap for the 5Y3 and the second filter cap of 47uf would make a stiffer power supply to retain the bottom end I have now?"

Firstly how much plate voltage do you have right now? Remember to deduct cathode voltage to get your corrected plate voltage.

All AA764 Champs use a 470ohm cathode resistor, to be biased halfway between cut-off & saturation you'd probably be looking at a plate voltage (measured to ground) of 425vdc+ (400v corrected). Of course, by then the 6V6 will be dissipating 20W, but thousands of them do and have lasted decades doing so.

There's no reason why you shouldn't go larger on the cathode resistor, to calm down plate dissipation but make sure that you are doing it because you like the sound of the amp like that. I have an 800ohm in my SF Champ & 440vdc from plate to ground, for that reason.

Here is what I have . Aweber champ PT rated at 330-0-330 VAC @ 100mA, 4 amp 6.3 VAC heaters and 3 amp 5 volt .

Right now since my line VAC varies from 118VAC to 119.5 VAC I of course get different reading but on average the line VAC is 118.5 VAC at that voltage my B+ is 372 VDC and plate 361- 25.2 cathode is 335.8 this is across a 680 ohm cathode resistor so my IPD can vary from 12.44 to 12.75 (37 mA )depending what the line is at . As you know the more you raise the cathode resistor value the B+ and plate rise so when I had a 750 ohm cathode res the B+ was 380 VAC .

The amp sound fine to me at these voltages , there are odd times where the line will be at 120 VAC and then I reach near 14 watts IPD so I let the 680 stay not that 14 watts will kill the tube but I don't like the tone biased that high.

MY main concern is really in the reservoir cap of 47uf stressing the 5Y3 . I did replace it with a 30 uf but what this did was cut bottom and bring on icepick highs . By the advice of Bruce of Mission amps in another post here , he advised me to place a 47 ohm resistor between pin 8 of the 5Y3 and the B+ junction . I did this because the resistor will dampen the current surge drawn on the 5Y3 and there fore not stress the 5Y3 . I now have the bottom back and the highs cut down . I did have a 47uf reservoir cap right off the 5Y3 for a year with no ill affects until I read about the allowed rating of the reservoir cap fora 5Y3 is 20uf .

My 73 SF champ that was all original and needed a cap job came with a 40/20/20 uf cap can and I replaced it with that same value CE cap can and had an old 5Y3 and never had an issue and not knowing never gave this a thought . That 73 champ had a much higher B+ and plate but then I was using a 1k ohm cathode resistor to get it below the 17.5 IPD.

This is why I brought this subject up because I am trying to understand it all. I know from little I can find on any tube charts a GZ-34 is rated at a max of 60uf for the reservoir and the GZ-34 draws about the same amp as a 5Y3 however if I use a gz-34 my plate will rise far above what I need , less than a SS rect and both would require a good resister to lower the voltage so this is not where I want to go.

As the AA764 champ build I have now I see no arcing in the dark powering up this amp . This 100 mA PT after an hour of hard playing just barely gets warm to the touch at levels above 6 on the vol. The 12ax7 and 6V6 might draw 45mA tops . My voltages are not high really . My main idea above was to bring the 6V6 in the center of saturation and cutoff , use a lower 500ohm cathode resistor to do so and to do that hence my question of adding an extra cap and resister to the rail before the plate 1) to lower the plate and 2) use a lower 30uf reservoir cap to put less stress on the 5Y3 .

However I trust the advice Bruce offered and it does seem to be fine .

[MWJB]

With the voltages you have a 470ohm cathode resistor would be typical. If you like the sound of the 680ohm, then by all means stick with it, but your plate dissipation is relatively modest for a champ.

"I did have a 47uf reservoir cap right off the 5Y3 for a year with no ill affects until I read about the allowed rating of the reservoir cap fora 5Y3 is 20uf ." 47uf will be fine with any of the commonly used 5V rectifiers (5Y3/5V4/GZ34). The data sheets are cautious.

"My main idea above was to bring the 6V6 in the center of saturation and cutoff , use a lower 500ohm cathode resistor to do so and to do that hence my question of adding an extra cap and resister to the rail before the plate 1) to lower the plate and 2) use a lower 30uf reservoir cap to put less stress on the 5Y3 ." You would need a resistor much less than 500ohms to get halfway between saturation and cut-off (or a lot more plate voltage). Don't worry about doing this, you have already said that you prefer the tone of the 680ohm cathode resistor...that's the acid test.

1) Not necesary. As previously stated IF you were tryin to drop the B+, using zener diodes would be preferable to stringing along more filtration nodes & dropping resistors.

2) Not necesary.

"However I trust the advice Bruce offered and it does seem to be fine ." Undoubtedly.