RCA SE amp - suppressor grid quandary.

[DummyLoad]

found this in the RCA RC-19 RTM. as per subject it's another single tube A1 amp. please review attached .SCH or .GIF. 

points of interest are the tone stack connected to G3 (suppressor) of V2. it seems to me as though this is biased up at around 100:1 of 6L6 plate and a tone network makes up the lower half of the voltage divider. another is the apparent (to me) NFB fed to the cathode of V2 via C2 - an 820pF cap.

why use G3 at all? gain reduction?

much thanks in advance for any enlightenment.

peace.

--DL
   

[jojokeo]

My first thought of G3 was being used as a type of presence control and/or maybe a shelving parameter-type of set-up? Then the pF cap being a smoothing-type of high end parasitic oscillation reducer? Enquiring minds want to know...

[Willabe]

I don't know much, but I have 3 thoughts/ questions......

1. To isolate the vol from the tone?

2. To change the Z of G3/ tone pot?

3. That G3 is more sensitive at controlling the plate than G1?

Looks interesting, wonder how it sounds/responds, to picking.       


                 Brad         
           

[HotBluePlates]

3. That G3 is more sensitive at controlling the plate than G1?

G3 can't possibly be higher mu that G1, because it's farther from the cathode.

If this were on G2, it would make some sense. If the 0.1uF cap reactance was large compared to the impedance looking into G2, you'd lose gain at/below that frequency. So a too-small G2 cap to ground was shave off bass.

But almost no one dinks with G3.

It looks too me like a feedback-based tone control, adjusting feedback from the output tube plate to the cathode.

I have a pdf copy of RC-19; what looks like a fly-speck on the line running into G3 is actually a connection of G3 to the cathode. So C2 does not cross over the line running to G3; there should be a dot at that intersection indicating a connection.

This makes sense if you consider the feedback would almost have to be positive going from the output tube plate to G3, but is negative feedback if it goes from the plate to the 6J7 cathode.

Given that it's caps across the cathode resistor, it raises bass by reducing feedback at lows. As the pot is rotated to the "top" the caps reduce local negative feedback across the cathode resistor, and simultaneously shunt the HF feedback signal to ground. So at that point, it acts to boost treble.

[DummyLoad]

I have a pdf copy of RC-19; what looks like a fly-speck on the line running into G3 is actually a connection of G3 to the cathode.

it is... i have a hard copy and had to put it under a mag-glass to see it.

thanks for pointing that out - it's pretty easy to figure out now knowing that.

guitar amp potential is there - two pentodes cascaded driving a single tube class A. yummi! imma' pencil up something that i think will work. :-) 

i'll fix the schematic and post.

peace.

--DL 

[DummyLoad]

what do y'all think? 

hack it up - i'll bread-board it - although i must admit it looks interesting enough stock.

peace.

--DL

[PRR]

I assumed 6J7 gain of 100 and 6L6 gain of 10 to plate, used an ideal G=1000 amp.

The pot should be reverse-audio. The only nearly-flat setting will be <10% down from the top resistance-wise.

It is a scratch-filter, with an extreme setting that gives some high-boost, probably more than you'd want on AM or 78. The hi-cut also reduces THD in the amp so scratch harmonics are damped.

It is of course NFB and will damp speaker bass resonance (at all settings).

[DummyLoad]

PRR, thank you!

the frequency plot is much more than i expected.

The pot should be reverse-audio.

book just states 5000 ohm potentiometer - no mention of taper. i guess they assumed we'd know that. i certainly didn't. you've mentioned it in past threads about use of rev-log pots in NFB tone networks. now it sinks in. somewhat.

thanks, again.

--DL