PP 300B questions

[Grover Gardner]

As it happens, Patrick Turner has some excellent information about choke-loaded driver stages for AB1 amplifiers:

http://www.turneraudio.com.au/audiofilt ... page3.html

http://www.turneraudio.com.au/300w-1+2- ... jan06.html

[J-ROB]

Well I actually want to do a common-cathode PP driver. Assuming the inductance would be sufficient (say a small 8K p-p tranny with a pair of triode-wired 6V6's) and I include a balancing pot on the cathodes to ensure equal current, then I could cap-couple the plates to the output tubes, yes?

Take a look at the Peerless A100A circuit. The center tapped choke in that design, used as cathode load for PP 6SN7 driver, was a replacement grade Peerless output trans (10k I think).
There you have a low impedance driver with low DCR to ground if you really want to drive the snot out of a triode grid.

Peerless A100 was Class AB2 6A5g (indirectly heated 6A3 variant). I wrote this amp up in one of the last SP issues. I still have the pair I built 20+ years ago.

Check the Altec 1570B circuit for a beefier version of this topology.

[Grover Gardner]

Interesting Joe, thanks for the links. I'll take a look.

[FerdinandII]

Love that Altec circuit........ starts out like a Williamson and then KICKS IT to another level, with triode-strapped 6W6GT choke loaded cathode followers driving 811A outputs!

[dberning]

David, your designs never cease to amaze. Now, given I'll be using transformer output, is it really practical to bias the 300B's that low? Won't the plate impedance go up considerably? Or does it not make a difference?

The plate impedance does go up with the lower plate current. In the 300B P-P amp I make, the output impedance at the speaker terminals is 3 ohms (this is a no feedback design) when the bias current (for two tubes) is 19 mA and 2 ohms when the bias current is 37 mA. The ZOTL works just like an output transformer in terms of reflected impedance. The p-p effective load on the tubes is 96oo ohms when a 6-ohm load is connected. The effective turns ratio is 40 plate-to-plate. The 9600 ohms is quite a bit higher than most folks would use for a P-P 300B, but I like to light-load the tubes to get the lower output impedance with low bias currents. I didn't check it, but given these output impedances, the plate resistance of each 300B is probably about 1600 ohms at the higher bias current of 1/2 X 37 mA. This seems reasonable.

David

[Grover Gardner]

David, your designs never cease to amaze. Now, given I'll be using transformer output, is it really practical to bias the 300B's that low? Won't the plate impedance go up considerably? Or does it not make a difference?

The plate impedance does go up with the lower plate current. In the 300B P-P amp I make, the output impedance at the speaker terminals is 3 ohms (this is a no feedback design) when the bias current (for two tubes) is 19 mA and 2 ohms when the bias current is 37 mA. The ZOTL works just like an output transformer in terms of reflected impedance. The p-p effective load on the tubes is 96oo ohms when a 6-ohm load is connected. The effective turns ratio is 40 plate-to-plate. The 9600 ohms is quite a bit higher than most folks would use for a P-P 300B, but I like to light-load the tubes to get the lower output impedance with low bias currents. I didn't check it, but given these output impedances, the plate resistance of each 300B is probably about 1600 ohms at the higher bias current of 1/2 X 37 mA. This seems reasonable.

David

That's very interesting, thanks for the reply!

[SoundMods]

David, your designs never cease to amaze. Now, given I'll be using transformer output, is it really practical to bias the 300B's that low? Won't the plate impedance go up considerably? Or does it not make a difference?

The plate impedance does go up with the lower plate current. In the 300B P-P amp I make, the output impedance at the speaker terminals is 3 ohms (this is a no feedback design) when the bias current (for two tubes) is 19 mA and 2 ohms when the bias current is 37 mA. The ZOTL works just like an output transformer in terms of reflected impedance. The p-p effective load on the tubes is 96oo ohms when a 6-ohm load is connected. The effective turns ratio is 40 plate-to-plate. The 9600 ohms is quite a bit higher than most folks would use for a P-P 300B, but I like to light-load the tubes to get the lower output impedance with low bias currents. I didn't check it, but given these output impedances, the plate resistance of each 300B is probably about 1600 ohms at the higher bias current of 1/2 X 37 mA. This seems reasonable.

David

That information is awesome!! Thank You! Thank You! Thank You!

[Grover Gardner]

David, your designs never cease to amaze. Now, given I'll be using transformer output, is it really practical to bias the 300B's that low? Won't the plate impedance go up considerably? Or does it not make a difference?

The plate impedance does go up with the lower plate current. In the 300B P-P amp I make, the output impedance at the speaker terminals is 3 ohms (this is a no feedback design) when the bias current (for two tubes) is 19 mA and 2 ohms when the bias current is 37 mA. The ZOTL works just like an output transformer in terms of reflected impedance. The p-p effective load on the tubes is 96oo ohms when a 6-ohm load is connected. The effective turns ratio is 40 plate-to-plate. The 9600 ohms is quite a bit higher than most folks would use for a P-P 300B, but I like to light-load the tubes to get the lower output impedance with low bias currents. I didn't check it, but given these output impedances, the plate resistance of each 300B is probably about 1600 ohms at the higher bias current of 1/2 X 37 mA. This seems reasonable.

David

That information is awesome!! Thank You! Thank You! Thank You!

Yeah, I'm still reeling over the idea. In relation to reflected loads, many years ago there was a debate between Mike Lafevre of Magnequest and some other folks about mismatching speaker outputs to reflect a higher load back to the tubes. I seem to recall that Mike objected on the grounds that this messed around with...peak voltages, or currents, or something else that lead to less than ideal transformer performance. Anyone else remember, or have an idea what he was talking about?