Question in RDH4

[Alan0354]

check these curves out: http://www.mif.pg.gda.pl/homepages/f...137/6/6888.pdf

This one will really mess with you guys, it shows a pentode being driven from all three grids.

If round curve is good for blues, I want a pair of this!!!!

[Austin]

Alternative Pentode Connections. (Page 1)

[Alan0354]

Ha ha, I went out and bought a few 1.2K 2W, 680 2W resistors. Going to experiment with the screen resistor!!! Put them in series!!! Round it up!!! I don't even care whether they are carbon film, wire wound or what, 10 cents each!!! I am so lucky to have a surplus store where I live, I just bought 100 of 10uF 35V tantalum cap for 20 cents each!!!!

This place is very helpful, thanks to all of you guys.

Alan

[J M Fahey]

Going to experiment with the screen resistor!!! Put them in series!!! Round it up!!! I don't even care whether they are carbon film, wire wound or what, 10 cents each!!!

Well, that's the mentality !! Experiment and post !!!

[Alan0354]

Well, that's the mentality !! Experiment and post !!!

I'll do this in a day or too, I am just busy right not on some pcbs. I am going to run it through the THD Hot Plate to really push it all the way.

[paggerman]

I don't know if everyone else has the Addenda (I guess not, as many copies would have been printed and distributed before the Addenda was produced in February 1953), but there is no addendum for pp549.

Is there demand for the Addenda? If anybody wants me to scan it, let me know.

[Alan0354]

I did the screen first the original 470, then about 1.8K, then 4K. I use the THD and attenuated the output so I can crank full tilt. The sound was not good with such high attenuation. But I can compare the sound. There is no significant difference in the sound, maybe the quality of the high attenuation drown the little difference. There is no change I can crank the amp up without the attenuator so that's that.

One thing surprising, it is louder with the 4K resistor which is opposite from what I expected. But the sound is the same in all do respect. I am not going to split hair on little detail, it's not worth the mod to get whatever a little difference that I can't notice.

So that's pretty much the end of this idea.

[Enzo]

I have no idea what really happened, but with the larger screen resistance, did your tube current diminish, and with it the B+ rise? And that perhaps causing a overall signal level increase because all the preamp stages would have a few extra B+ volts?

[jazbo8]

That is strange, aren't there some attenuators based on lowering the screen voltage (higher screen resistance), how would they work then? Gotta be the PS...

Jaz

[Austin]

I did the screen first the original 470, then about 1.8K, then 4K. I use the THD and attenuated the output so I can crank full tilt. The sound was not good with such high attenuation. But I can compare the sound. There is no significant difference in the sound, maybe the quality of the high attenuation drown the little difference. There is no change I can crank the amp up without the attenuator so that's that.

One thing surprising, it is louder with the 4K resistor which is opposite from what I expected. But the sound is the same in all do respect. I am not going to split hair on little detail, it's not worth the mod to get whatever a little difference that I can't notice.


So that's pretty much the end of this idea.

maybe it seemed as loud but lacked the punch due to what I call "the swell effect"? Maybe try a 16 ohm load instead of an 8 ohm load to really test the screens.. The increased voltage swing of the plate should really exaggerate the effect of the screen resistor.. I have seen my 6bq5 screens light up when experimenting with higher loads with the simple crackle of plugging and unpluging the input while under full volume. Not recommeded, I need higher screen resistors or just triode mode.. I may have damaged my tubes already and am waiting to get some more el84/6bq5 to have a comparison.

[Alan0354]

I did not measure the voltage or anything while I was playing for obvious reason........my hands were busy!!! When I measure static, it did not tell me anything as there is not much voltage drop across the screen resistor. Maybe I need a higher value resistor, but again, I need an even bigger resistor to take the power. It's just not worthy, the result is disappointing and I tore it down already. The connection is correct, that I double check. The increase in volume is quite noticeable. It actually get a little bunchier. But anyway, it's not worth doing it. I have to hear obvious improvement before I consider valuable. I was so excited about it that I actually dropped one project in the middle, went out and bought the parts.

BTW, this is with THD attenuator and the amp at full blast and the amp is definitely clipping. I did compare when it is clean too. No noticeable difference at all.

Hearing speaks a thousand words.

[Alan0354]

I have been thinking long and hard why, I might be confused on the switching. I had a three position switch to get 470, 2K and 4K instantaneously. I am sure the circuit is correct, but I might be confused the position as I cannot picture back the position that has the loudest sound. But I cannot verify anymore. Anyway, they don't sound different, it's not worth the effort.

[Austin]

I have been thinking long and hard why, I might be confused on the switching. I had a three position switch to get 470, 2K and 4K instantaneously. I am sure the circuit is correct, but I might be confused the position as I cannot picture back the position that has the loudest sound. But I cannot verify anymore. Anyway, they don't sound different, it's not worth the effort.

I bet the values of those are not high enough to make a difference. I would try a 1k a 10k and a 100k to find the right ballpark, as maybe your screen isn't drawing enough current with the tube you are using and the voltage and load variables to cause any sag through the resistors you tried already. By looking at the curves of the tube on the data sheets you can see how any variation in the screen voltage should have a profound effect on the power and response.

[Alan0354]

I bet the values of those are not high enough to make a difference. I would try a 1k a 10k and a 100k to find the right ballpark, as maybe your screen isn't drawing enough current with the tube you are using and the voltage and load variables to cause any sag through the resistors you tried already. By looking at the curves of the tube on the data sheets you can see how any variation in the screen voltage should have a profound effect on the power and response.

I'll try this the next go around, right now, I moved onto something else. I do put a switch to instantaneously switch between resistors for better comparison, so it's not like changing the resistor. I yanked everything out already.

[Alan0354]

I did tried 51K, it really attenuated the output and more distorted. BUT, it does not sound that good, it lost the punch and became wimpy. I only ran it through the THD Hotplate to lower the volume, but the THD is pretty good, so I won't blame on the THD. It does not hold candle to the THD for attenuation and distortion. I tried 5K and 10K, not much attenuation, it just get "softer". Not exactly a good thing. All in all, you have to try it to see, I am not impressed with it. Maybe just 5K to brown it out just a little.

Anyone try the THD HotPlate? It is very good. I used big rheostat ( wire wound resistor with adjustable tap) to adjust attenuation. It does not sound as good. They really compensate it well. The -4dB, -8dB are very true to the sound. -12dB is still very good. It gets degraded when you put onto -16dB with adjustment. It's expensive, but worth every bit of it.

BTW, it's not as if I don't have question in RDH4, I just don't have time to study, my study has been very slow.