Output Driver Info

[Aaron]

I recently bought an old Fender 160 PS head. My plan is to use it for parts, (especially the GEC KT88's!), but have a few questions.

I have no idea on how the driver transformer works? How does this amp put out 150w from just a pair of KT88's?
If I didn't use the driver transformer what would the out drop down too?

Thanks,
Aaron

[JazzGuitarGimp]

Just like the output transformer, the driver transformer converts the high impedance output of the PI to a low impedance source. This allows enough current to flow into the grids of the KT88's to push them well above 0V, thus causing the KT88's to conduct more, pulling the anodes closer to 0V. This allows for more voltage swing on the OT primary, yielding mire output power. This is known as Class AB2 operation.

[VacuumVoodoo]

The "driver" transformer is the phase inverter driven by the 6V6 SE stage.
The PP output with 2xKT88 (originally this amp used GE 6550 ruggedized version made for Fender) operates in class B2 courtesy of the PI transformer supplying enough current to the control grids. This together with 700V on plates and OT Raa around 2.8k is enough to deliver 150W output.
I'm gearing up to rebuild my PS160 into a 4 channel monster.

[Stevem]

nice find! I hope your build goes well for ya!
If your retain its driver / output stage set up in class AB2 be sure to follow the output stage set up notes on the schematic!
You might also want to go to the Time Electronics site and contact the owner R Koerner, as he is a expert on the Fender 400 ps amp which is a 435 watt class AB2 amp, and may be he can give you some info on setting up and or any qurcks that your amp may have!

[JMFahey]

You want to build a WW Beetle with Ford 350 truck parts?

I'd leave the power stage as is , strip the Bass oriented preamp and build some firebreathing monster preamp to drive it.

IF you discard the driver transformer and want to build a conventional PI driven amp, available power will drop to some 80W but much worse, since tubes will be able to supply roughly half of expected current, your OT now will be the wrong impedance and you'll need a new one ($$$$$$$)

jm2c

[Aaron]

So what would the difference be between running the 6V6 Driver or PI-DC Cathode Follower circuit.
From various info on here they both seem to be AB2.

Thanks,
Aaron

[JazzGuitarGimp]

It depends on what tube you choose for the DC coupled driver. You'd likely be better off using IRF820's in a DC coupled source follower than tube drivers.

[tictac]

You'd likely be better off using IRF820's in a DC coupled source follower

would you have a schematic diagram of that circuit that you could post?

Only ones I've seen have been bits & pieces; no practical circuits with components values and that actually have been proven to work well...

Thanks

TT

[Aaron]

It depends on what tube you choose for the DC coupled driver. You'd likely be better off using IRF820's in a DC coupled source follower than tube drivers.

I was planning on using a 12AU7 PI into a 12BH7 CF, and from reading other threads maybe a 12BZ7.

Thanks,
Aaron

[es345]

would you have a schematic diagram of that circuit that you could post?

Only ones I've seen have been bits & pieces; no practical circuits with components values and that actually have been proven to work well...

Thanks

TT

Have a look here http://music-electronics-forum.com/t286 ... post360510
Different tubes but it has a PI with a MOSFET follower circuit. I use this bassamp now for more than one year without any issues.

Hans- Georg