RCA 6V6PP amp schematic (Schade fb)

mctavish · 25th February 2012, 10:00 PM

rca6v6pp.jpg

I have a few questions about this schematic, first off has anyone seen it or built it?

The resistors in the concertina seem extremely large, is there a reason for this?

Also, the screens for the 6AU6's share the same 1 meg dropping resistor and have no decoupling, I assume this is part of the design?

Thanks for the help, I am trying to better understand this circuit and determine what could be done to modernize it.

Paul Scearce · 25th February 2012, 11:29 PM

I have a downloaded copy of the 20W version of this schematic, and it shows 39k resistors in the concertina instead of the 390k shown here.

I may be confused about the terminology, but I think of Schade feedback as being connected to the plate of the driver tube. The feedback in these amps is connected to the cathodes of the driver tubes. This type of feedback keeps the driver tube in a very narrow range of operation, especially considering the 6au6 only has to deliver ~ 12v peak to the 6v6. The designers at RCA may have figured any changes in the screen current would remain balanced. I have heard the 20w version of this amp, and felt it sounded very good. I don't know if adding a capacitor to the screens would change the sound.

Eli Duttman · 26th February 2012, 12:18 AM

I have bones of contention with that design.

The value of 1st filter cap. in the PSU badly exceeds the published limits for 5Y3s. Back in the day, rectifiers were comparatively inexpensive and RCA was the last to avoid selling more tubes, due to premature failure. The unbypassed voltage amplifier cathode resistor provides some linearization, but the 'U7 triode is not inherently linear.

IMO, "Cheap Charlie" was at it again.

Cassiel · 26th February 2012, 06:41 AM

6V6 P-P Amp From RCA Manual RC-19

revintage · 26th February 2012, 09:11 AM

First this has no Schade fb. Secondly this one is with to many oddities to modernize.

mctavish · 26th February 2012, 11:05 AM

Thanks guys, I was incorrect calling it "schade" feedback, should have said without global FB. The other thread linked goes through in detail the flaws.
At first glance I just thought it was an interesting circuit that I had not seen before.

revintage · 26th February 2012, 11:47 AM

Schade or not, the lack of global feedback around the OPT is actullaty the positive feature on this topology.

mctavish · 26th February 2012, 12:13 PM

That is what had first caught my eye. I dont see many (vintage) PP amps with this style of feedback. Know of any others with less shortcomings? I have the breadboard ready for something new to experiment with.

revintage · 26th February 2012, 03:05 PM

The AU7 is totally unlinear and should be substituted. 6H30?

Also, without calculating, I would try a triode instead of pentode driver to lower massive gain before feedback applied. But just guessing here.

25th February 2012, 10:00 PM	#1
mctavish diyAudio Member Join Date: Feb 2008 Location: Niederösterreich	RCA 6V6PP amp schematic (Schade fb) rca6v6pp.jpg I have a few questions about this schematic, first off has anyone seen it or built it? The resistors in the concertina seem extremely large, is there a reason for this? Also, the screens for the 6AU6's share the same 1 meg dropping resistor and have no decoupling, I assume this is part of the design? Thanks for the help, I am trying to better understand this circuit and determine what could be done to modernize it.

26th February 2012, 12:18 AM	#3
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	I have bones of contention with that design. The value of 1st filter cap. in the PSU badly exceeds the published limits for 5Y3s. Back in the day, rectifiers were comparatively inexpensive and RCA was the last to avoid selling more tubes, due to premature failure. The unbypassed voltage amplifier cathode resistor provides some linearization, but the 'U7 triode is not inherently linear. IMO, "Cheap Charlie" was at it again. __________________ Eli D.

26th February 2012, 06:41 AM	#4
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	6V6 P-P Amp From RCA Manual RC-19 __________________ Hello

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25th February 2012, 11:29 PM	#2
Paul Scearce diyAudio Member Join Date: Feb 2012	I have a downloaded copy of the 20W version of this schematic, and it shows 39k resistors in the concertina instead of the 390k shown here. I may be confused about the terminology, but I think of Schade feedback as being connected to the plate of the driver tube. The feedback in these amps is connected to the cathodes of the driver tubes. This type of feedback keeps the driver tube in a very narrow range of operation, especially considering the 6au6 only has to deliver ~ 12v peak to the 6v6. The designers at RCA may have figured any changes in the screen current would remain balanced. I have heard the 20w version of this amp, and felt it sounded very good. I don't know if adding a capacitor to the screens would change the sound.

26th February 2012, 09:11 AM	#5
revintage diyAudio Member Join Date: May 2007	First this has no Schade fb. Secondly this one is with to many oddities to modernize.

26th February 2012, 11:05 AM	#6
mctavish diyAudio Member Join Date: Feb 2008 Location: Niederösterreich	Thanks guys, I was incorrect calling it "schade" feedback, should have said without global FB. The other thread linked goes through in detail the flaws. At first glance I just thought it was an interesting circuit that I had not seen before.

26th February 2012, 11:47 AM	#7
revintage diyAudio Member Join Date: May 2007	Schade or not, the lack of global feedback around the OPT is actullaty the positive feature on this topology.

26th February 2012, 12:13 PM	#8
mctavish diyAudio Member Join Date: Feb 2008 Location: Niederösterreich	That is what had first caught my eye. I dont see many (vintage) PP amps with this style of feedback. Know of any others with less shortcomings? I have the breadboard ready for something new to experiment with.

26th February 2012, 03:05 PM	#9
revintage diyAudio Member Join Date: May 2007	The AU7 is totally unlinear and should be substituted. 6H30? Also, without calculating, I would try a triode instead of pentode driver to lower massive gain before feedback applied. But just guessing here.

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