Foreplay III Redux

[Thermioniclife]

Hi Folks,
I decided to build a FP3 type preamp from scratch. I had a legit unit years ago that I do not have anymore and I am quite Sentimental about it. So this is the start of the process. I would like to keep some of the elements of the original but would also make it a little different at the same time. I have the original schematic and manual and a new PT-10 transformer thanks to Queen, luv ya kiddo.
I decided on 7193 tubes in lieu of 12au7's this  requires 4 tubes as they are a single triode each and have a top connecting plate and grid connection. they remind me of Mickey Mouse ears. I forgot to mention that this is probably going to be a lengthy post. I wanted to design an unusual chassis design ala Don Garber in his line of equipment called Fi audio made in N.Y. quite some time ago.
When I was still working I had access to a press brake and a very experienced operator but alas no more, so that being said and the fact that the build itself would be problematic at best I decided to take a more conventional approach. Do a search on Don Garber and Fi audio to check out his cool designs.
I told you this may be long. Ok so I will have 3 inputs and 2 outputs as the original but I will use a motorized volume pot controlled via Ir, manual input select and would like to incorporate a balance control. Studying the schematic I realized the orig fp3 has a different volume control wiring scheme. As it appears that Paul Joppa(the man) wired the stepped attenuators so the grid of the voltage amp section will always have a constant load regardless of attenuator position, (unless I am wrong which I frequently am)
I think this is pretty cool and I think I have a plan to use a balance control and the original plan of individual input padding resistors.
Feel free to chime in Paul if you care to.
Did I mention this may be long? So I think I have all the parts amassed and have designed a preliminary chassis plate using Front panel express. I have some nice curly maple lumber on hand for the base, it has a really nice grain. Attached is a screenshot of the chassis as it stands now. It is 10x10 inches to accommodate the extra tubes and a couple of pics of Don Garber designs.
I will try to keep you informed regarding my progress. Having said that this project may be put aside for a while because I have a Mainline arriving in two days but maybe I will shelf that and continue with this project instead. Choices. Difficult choices indeed.
Ok i'm out for now, stay tuned to this channel if your interested.

[Paul Birkeland]

In front panel express, if you have something like a 2.5mm thick panel, you can run a cavity down the top of the panel that is 1.25mm deep and 1.25mm wide, then when you get the panel it's very easy to bend by hand where that cavity is.

On the downside, the anodizing will crack on the outside radius of the bend, but that would otherwise be buried in your chassis anyway.

[Thermioniclife]

Thanks PB,
This panel is 2.5 mm and I understand your point but as much as I would like to do something like that, I feel like the build will be to problematic for me at this time but I may reconsider. FPE uses a 2000 series aluminum and I need to research  the properties of this grade of aluminum   I have used their products in the past on many occasions and I have been happy with their manufacture. I would love to do a Garber inspired design but the cost of FPE is quite costly and I would hate to throw those bendments into the scrap bin. Having said that as I stated before I may reconsider. I am pretty confident that I can build the circuitry and the mods I would like but I will probably need some input in the future, so if you and PJ can keep an eye on this post I would appreciate it.
As I stated earlier I am expecting a big brown truck in two days with a Mainline and I am not sure what path I will take next.
Regards,
Lee

[Paul Joppa]

"... the orig fp3 has a different volume control wiring scheme. As it appears that Paul Joppa(the man) wired the stepped attenuators so the grid of the voltage amp section will always have a constant load regardless of attenuator position

The idea was to arrange the switch so that if a contact failed, the grid would still have a path to ground and would not make a pop. It was a way to make an affordable switch work without generating switching noises. It also implemented about 10dB level reduction, since the preamp otherwise has too much gain for most systems.

The switch could be re-purposed as a normal  potentiometer at 15K input impedance, which would work as a passive preamp. It was rarely done, though.

[Thermioniclife]

Thanks Paul J,
I am going to use a stereo pot in lieu of stepped attenuator.
I have a 20k alps, I plan on having ir remote control of volume. the 20k pot is not motorized but I have several motorized alps pots
and plan to disassemble one and swap the parts. one of the motor pots that I have is a 10k perhaps if I add 5k to the padding resistors it might keep the original characteristics of 33k to 48k. I would also like to employ a balance control, I will make a drawing and post perhaps you or PB could check it out and give me your thoughts.
I'm expecting ups today with a Mainline!!

[Paul Birkeland]

I will also shamelessly plug that I managed to find an unbuilt Extended FP-III kit that I popped up on eBay.  You can help fund the 833 amps that I'll use to keep Doc B. warm at the office this summer

[Thermioniclife]

Hey PB,
I have amassed all the tubes and parts to build a FP3 from scratch using 7193 tubes. I am going to try to post the build along the way so a sorry can't buy an orig kit. if you read the above post I have some questions that you, PJ or Doc may care to respond to.
I would like to use a stereo pot in lieu of a stepped atten. I have a 20 k and a 10 alps pot and want to use a balance pot.
see attached drawing showing what I propose, I would like to keep individual padding resistors.
keep in mind the drawing leaves out the input switch and only shows one channel but shows how the bal pot is wired.
Hope I did not confuse you to much. any thoughts will be greatly appreciated.
Just received a Mainline kit not 30 minuets ago.

[Paul Birkeland]

The exit of your balance pot needs to go to the input of your volume pot, not the wiper.  The wiper would go to the tube grid.

[Thermioniclife]

On the Fp3 the input to the stepped attenuator is the wiper. this is not a conventional method but if you look at the FP3 sch. the input to the volume stepped atten is the wiper and the out to the grid is one leg of the resistor series and the other leg is ground. I suppose I should wire the vol pot with wiper to grid as is normal and that would work  as I am not going to use a stepped atten  but a volume pot Thanks for the reply.

[Paul Birkeland]

The FP-3 didn't have a balance control, so that kind of arrangement wasn't problematic.  If you wire up what you have drawn, the left to right balance will change substantially as you move your volume pot, which you don't want.

[Thermioniclife]

I think I see your point, if I do, than I will run the balance pot as I described and feed the wiper of the bal pot to the end of the resistance track of the vol pot and use the wiper of the vol pot to feed the grid as is in a normal config. Any suggestions as to the value of the bal pot? I think I have a 50k linear somewhere unfortunately I need a shovel and a rake to sort thru my tightly controlled inventory system. and this is stuff that I am currently engaged in. I have several other piles in different rooms. I am ashamed actually. see attached. lol.

[Paul Birkeland]

Sure, 50K is a value that could do the job.

[Thermioniclife]

Ok, the ball is rolling.
FPE arrived this morning, now all I have to do is figure what all this stuff is! I installed the chassis parts. I am not sure what
I will use for a VC, I have a few options. Stay tuned for the next exciting episode of this adventure. Same Bat channel

[Thermioniclife]

Tube compliment, 4x 7193 and 0D3 for now.

[Thermioniclife]

Ok todays progress.
Wired the PEM to the PT-10 and completed the HV supply. Inputs are done, I decided to place the padding resistors at the rca's on terminal strips. Outputs partially done. Wired the tube sockets for the DC heater supply except the ground reference. That's about it for today. Peace out, stay well, stay tuned.