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5AR4 questions

Home and my pin 3 voltage on all 6bq5 tubes was 11 volts. Up from 9 volts with dropping resistor. I have 180 ohm cathode resistors and 315 average plate voltage 30.5 ma per tube outputting 9.3 watts. Heater voltages improved a little after removing the dropping resistor. I’m OK with the 9.3 watts on each tube. Running at less than maximum.
Higher plate or cathode voltage will increase output is what my math showed. Is this the reasoning for these resistor values.
 
Which goes back to low primary voltage. Losing my dropping resistor changed my output from 7.5 watts to 9.3 watts.
Adding in the yellow sheet mod tonight.
 
Both can increase output but for different reasons More plate voltage allows for more peak voltage swing. That one is pretty clear cut.

Increasing the idle current in a AB1 cathode bias amp can increase power output, but only if its currently getting into crossover distortion before it starts clipping on the peaks. Cathode bias shifts as power output increases, and if it shifts too much you get each tube conducting less than 180 degrees which is where the crossover distortion comes in. If the peaks are flattening before it starts forming a step at the zero cross though, bumping the current won't do anything for you.

With a fixed bias amp, the idle current has very little to do with max output power, so long as its high enough to not have crossover distortion. It can have a fair effect on overall distortion though. Generally fixed bias amps will idle at lower plate dissipation though since they don't have that bias shift thing to deal with.
 
That’s a lot to take in for my level. What an educational experience for me. So is it the greater value between plate and cathode voltage that can control your output wattage as long as you’re not distorting the phases at the 180 crossover point? The distortion thing will have me hitting the books to put together a visual that I can grasp. Is it safe to say that my 9.3 watts is ok. Maybe a little cooler in temp and less abuse?.
 
9.3 watts is easy on the tubes, they are rated for 12 watts plate + 1.2 watts of screen so you're not stressing anything.

but yea voltage is the big limiter on power output. If its 300 volts plate to cathode, in theory you could swing 600 volts plate to plate. It won't ever quite get that far but thats a theoretical maximum. If you bump the voltage to say 320 volts plate to cathode, now you can swing 640 volts plate to plate. More volts across the same load = more watts.
 
Addendum..remembering back to the restore. I could not get a Pilot 240 schematic WITH the simpli-magic test panel used to set bias by sound and/or voltage. My schematic says 180 ohm cathode resistor. This amp with the test panel has 330 ohm cathode resistors. I’m getting 11 volts on the cathode. 11 volts on cathode and 307 volts on plates. 5.1 watts??? Should I leave it alone.
 
chetgallery said:
Addendum..remembering back to the restore. I could not get a Pilot 240 schematic WITH the simpli-magic test panel used to set bias by sound and/or voltage. My schematic says 180 ohm cathode resistor. This amp with the test panel has 330 ohm cathode resistors. I’m getting 11 volts on the cathode. 11 volts on cathode and 307 volts on plates. 5.1 watts??? Should I leave it alone.
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Not sure what exactly you're calculating. Sounds like the discrepancy in cathode resistor values likely a matter of whether there is one resistor per pair of output tubes or one resistor on each tube. When a shared cathode resistor is used, it handles the current for two tubes, so for the same bias point half the resistance for two tubes that you need for individual resistors on each cathode. 11 volts across 180 ohms is 61mA for two tubes, half that per tube, with 296 volts across the two that's 9 watts dissipation. If you're getting 11V across individual 330 ohm cathode resistors, that's 33 mA or just under 10W dissipation with the same plate voltage. I may be misunderstanding what you're asking.
 
chetgallery said:
You could bump that voltage up by lowering the cathode bias resistor? And vice versa?
Click to expand...
yes, though its not a linear relationship. Meaning if you make the resistor 10% smaller, it doesn't necessarily mean that the current increases by 10%.

but yeah smaller resistor = higher current. The grid becomes less negative relative to the cathode which winds up the current. Grid is at 0v, cathode at 11 relative to ground, or if you put the negative lead on the cathode and measure to the grid, it will be -11. The more negative it is, the less current flows, and the less negative it is the more current flows. 0v G-K is the theoretical maximum current, and there is some point where current hits zero but that depends on plate and screen voltage.
 
It was a math problem on my end. I divided my current by 2 when there was clearly 1 cathode resistor per tube. 33 ma and 10 watts PER TUBE. Having the wrong schematic led me down the wrong path. I will bring it up on a variac tonight. installed the yellow sheet diodes last night. Then I will get all voltage readings again.
 
looks like a number of variants, the schematic I found was a 180 per-pair, and I see somewhere a 150 per-pair referenced. Often you'll get it per-pair since that helps stabilize the voltage somewhat within the channel.
 
Maybe I should post a pic of the test panel on this amp. It has 2 pots to balance the bias and test points to hook up a meter. During the original restore I had no luck finding this specific schematic. It had been on AK maybe in the early 2000's but is long gone. Explains each cathode having a dropping resistor. I think I'm good with the voltages I have.
 
Yellow sheet mod installed. Thanks to whoever posted that. Replaced the cathode resistor’s. Originals were not anywhere close to each other. Maybe these pics of the test panel and bias pots will refresh someone’s memory. New voltages are 10.7-11.0 volts on cathode resistors. A little under on wattage. But I want to make the 6bq5’s last. Many thanks to all who helped me along the way. I learned a lot. Feeling like a lesser of a hack after the schooling I got on this post. If any other issues pop up I will post. Again thanks for the knowledgeable replies and tribal knowledge.IMG_0767.jpegIMG_0766.jpegIMG_0767.jpegIMG_0766.jpegIMG_0766.jpegIMG_0767.jpegIMG_0766.jpeg
 
It's probably the photo angle but just making sure that the bottom left EL84 isn't redplating, in the photos it looks different than the other three. If it's a photo artifact, no worries. If it is looking a little red on the plates I'd shut it down and reassess.
 
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