Out of the Box: Part 6, a 25W PP UL 6L6GC Monoblock

[jhstewart9]

That is the paper with the results & test circuits.
I was surprised, the DC NFB from the following cathode did not do much.🙂

 

[PRR]

....The circuit from HB3 is frequency selective to achieve a roll off in the audio range....

I am not sure what you mean. I do not see audio frequency selectivity in these plans. 20cps-11cps is far below the state of common loudspeakers of the 1930s and into the 1940s.

 

[Kay Pirinha]

Here is a trick amp where internal NFB goes from the output plates to the first stage cascode diff amp. I'd seen something like this with NFB to the first stage PP pentodes. But pentode grids need current, this one doesn't. CFs drive the OP 6V6's so as an AB2 amp it can do 26 watts on a lab supply.

Are both the 150 Vdc supplies for the cascode's upper grids really needed? Wouldn't appropriately designed voltage dividers from the output plates be an alternative? FB needs to be adjusted by RC networks in parallel with the lower divider resistors, of course.
Best regards!

 

[jhstewart9]

I used all those extra power sources in the schematic in order to simplify the result. Otherwise the schema got too crowded, difficult to see what was real or not. The on board PS as used is attached here.
One of the objectives of the amp was to test its response on a limited PS. The Hammond 271X is on the small side but seems OK at steady state. The PS response to a pulsed 10 watt load is also attached. The B+ drops ~3.5V (red trace) & then recovers.🙂