Foreplay II rebuild and C4S questions

[TheMilford]

I have two old FP IIs. My main one was upgraded to a 6k1vf transformer and DC filaments years ago and remains my main preamp for over a decade... the other was a kit that was overmodded that I stripped and planned to rebuild with 6SN7s... Also, with the 6k1vf PT. Years ago I was told it should operate fine with the slightly larger PT, probably on the old forum. I was wondering which parts values, if any should be changed on this rebuild?

Also, for my main FP II, I've always wanted to do the C4S upgrade but never got around to it before the FP III was introduced, I just never had the $$... I was hoping that since this product is so old now that the C4S circuit is somehow available for free or honestly if anybody has some board I would buy them. Can I still buy the manual and get the circuit that way? I'm not ready to let these guys go just yet... but I plan to upgrade to the current line eventually.

 

[Paul Birkeland]

The C4S circuit is not patented and is available to use.

The Foreplay III was the Foreplay that was most suitable for conversion to the 6SN7.  If you want to build a Foreplay II with 6SN7s, then you'll need a bigger chassis plate, a bigger wood base, a bigger power transformer, a different layout...  It's certainly possible, but it's miles away from what I would consider supporting a legacy product.

As far as boards go, we do have C4S boards in current production, but they are quite a bit larger than what would fit in the old Foreplay.  The exception to that might be buying two S3X C4S kits and modifying those to work as a current source and current sink. 

-PB

 

[Doc B]

I'll suggest selling them on ebay and building a Quickie with the PJCCS. It sounds better.

 

[TheMilford]

Thanks guys.

Can you point me to a schematic for the C4S w/ foreplay values?

Also, the FP II I have is already drilled with octal sockets installed. The Larger PT is borderline but willing to push things. FWIW: I remember calling Hammond at one point and them telling me they thought the current draw of the 6SN7 should be fine and that these PTs were underrated . Is there a more suitable octal tube for the older FPII circuit? I'd like to keep this chassis and base.

Looking into the Quickie either way.

 

[Paul Birkeland]

Can you point me to a schematic for the C4S w/ foreplay values?

With a different power transformer, the values will be different, especially if more high voltage current is available.

FWIW: I remember calling Hammond at one point and them telling me they thought the current draw of the 6SN7 should be fine and that these PTs were underrated.

That would be the first Hammond power transformer ever wound to have that attribute.  Generally they are exactly the opposite, which is a big part of why we stopped using their products long ago.

With a different power transformer, the values will be different, especially if more high voltage current is available.

Is there a more suitable octal tube for the older FPII circuit? I'd like to keep this chassis and base.

No, there is no suitable octal tube for use in the FPII circuit.  What you're talking about is making a completely different preamp.

 

[TheMilford]

Weird that you are not familiar with this transformer which used to be recommended on the forums as an upgrade for the Foreplay 6K27VF transformer...

https://www.alliedelec.com/m/d/c775b06be968dd7e24ffae3012c33577.pdf
I'm using this very PT in my mostly stock Foreplay II and all the voltages check with the manual. I Just  swapped the diode bridge with a full-wave, center-tap scheme.

FWIW: When I called Hammond back in 2014 they said the filament current could handle 1.5a

The only place I would have gotten the idea for a 6SN7 foreplay would have been on one of the Bottlehead forums either here or over on the old AA space... Not sure those older posts still exist. This conversation is making me feel like I'm crazy or something.

In any case what would happen if I were to simply wire this up as I have described, with 6SN7s not changing any of the stock values?

Also,I  would REALLY like to find the stock Foreplay C4S schematic and values. As I said I'm happy to pay for the manual as you guys used to do it in the past.

Cheers,
D

 

[TheMilford]

So, just to clarify:

1. I have a stock FPII for which I would like to add C4S. I would like to never get rid of this. It has served me very well and I love it. BAM!

2. I have a partially built FPII drilled for octal tubes (not my idea, got it somewhere in the archives maybe 10-12 years ago) I would like to suss out which values to go with... I wish I could find the old thread detailing this but alas no dice. HELP!

 

[Paul Joppa]

Dang, I made a post but it disappeared before it got posted!

Basically, I noted that the FP-III replaced the earlier versions in 2005. I did the modification, which included adding a voltage regulator and changing the operating voltages and currents of the tubes, among a few other things, but the original design was by the late George Wright. I also re-designed the C4S circuit board at that time. I have no records of the previous designs, beyond a battered copy of the original Foreplay manual. Specifically, I have no record of the C4S component values used.

Back in the day, there were an enormous variety of modifications discussed for the Foreplay and S.E.X. amps.  Many of them would not be supported today - we've learned a few things over the years!

FWIW, I would not recommend or support using any component beyond its specs.

Also FWIW, the old Forum is archived at https://www.audioasylum.com/forums/bottlehead/bbs.html

 

[TheMilford]

Thanks Paul! Very helpful.

Seems a shame the documentation for the original Anticipation is lost to the ravages of time. Maybe somebody here can chime in with schematic and values...

 

[Natural Sound]

I might have a couple of c4s boards that were originally purposed as a constant current load for the voltage amplifier section of a 6DN7 (S.E.X. 2.0). You would need to get some advice from PJ or PB on what resistor changes would be necessary to make them suitable for your FPII. If I can find them you can have them, no charge.

 

[bernieclub]

Just dawned on me that I have an old FP11 with c4s boards that I cannibalized long ago.    You're welcome to the boards.  PM me.
Bernie 

 

[Paul Birkeland]

I may have an Anticipation manual laying around somewhere in a dusty file cabinet.

FWIW, the stock Foreplay I and Foreplay II power transformers didn't have a whole ton of high voltage current available, so the currents run in the C4S upgrade were pretty minimal. PJ solved this with a 15 lb. hammer when be brought out the FP-III and the PT-3 power transformer that accompanied it.

Since you have a different power transformer, there is little value IMO to using what was in the original FP-II anticipation.  The 6SN7 can run with a myriad of bias voltages and currents, I'd be happy to suggest some based on the power transformer you have available.