Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Foreplay III => Topic started by: xcortes on March 19, 2014, 12:43:06 PM

Title: FP3 goes to the movies
Post by: xcortes on March 19, 2014, 12:43:06 PM

I want to convert a FP3 with the extended mods to six channels buffer. I know I could do some fancy things including compactrons to keep the SR, etc. but I want to keep it simple. My idea is to remove the regulator and use each section of the three 12au7s as a cathode follower. I will use C4S on each of the six CF. I will reduce the plate voltage a little bit by replacing each 270 ohm with a MQ exo 010 chokes on hand (1000 ohms) so I should be getting a plate voltage very close to 150V. What voltage do I need on the grids? I'm guessing something like 70V. And how to I set them? Do I make a voltage divider from the B+? Do I need I bypass cap?

Thanks,

Title: Re: FP3 goes to the movies
Post by: Paul Joppa on March 19, 2014, 01:09:41 PM

You need a negative supply for the C4S's - at least 10 volts, I'd go for 15. You can implement this with a 431 chip; use a bypass cap of at least 10uF for stability.

Title: Re: FP3 goes to the movies
Post by: Paul Birkeland on March 19, 2014, 01:10:04 PM

Yeah, there's enough B+ and heater current to do this with the PT-3, so you're OK there.

You'll want to filter the living hell out of the high voltage supply; the extra current will demand it.  The EXO-010 is 10 or 15mA.  15mA is enough, 10mA probably is not.

For the buffer, run your inputs up to the front of the chassis.  Use a voltage divider from the B+ to ground that is two 249K resistors in series.  Tie that junction to the grid of each cathode follower.  For the incoming signal, use a 0.1uF cap between hot and that junction to keep the DC off your jacks.  A 1M resistor from hot to shell on each RCA input jack will ensure that both sides of that 0.1uF coupling cap have a path to ground.

There's enough current in the PT-3 to put a truckload of 0D3's in there if you don't want to give up on shunt regulation. 

-PB

Title: Re: FP3 goes to the movies
Post by: xcortes on March 19, 2014, 01:43:31 PM

Sorry. It's 001 not 010. 20mA so I'm Ok there.

Do I need the 1M if I install a 100K attenuator at the input?

And yes, I may do the 0D3s one day but this one has to go off quickly. Even withour SR it'll be awesome for a HT.

I'll do the strings of 249k to set the grid voltage at 75v. Thanks.

And another question. Why is it recommended to run the inputs to the front?

Thanks

Title: Re: FP3 goes to the movies
Post by: Paul Birkeland on March 19, 2014, 02:46:45 PM

I was speaking generally about wiring the inputs to the front of the chassis, as you'll need some space, and room for your caps.

You don't need 1M resistors if you plan to use trim pots.

Also, bias the heater up a bit, maybe to 100V.

I do not believe DC heaters will be advantageous here. If you're in a hurry, it will save some time and cost.