Adding a B7 Trans to and FPII ........is this correct? (long)

[Frank Mena]

This post may be of interest to others who want to do this to their FP3.  I think the principles would be the same.  I want to check first with the board, that what I am going to do is correct, so please correct me if I am wrong.  I’ve searched the archives and this is what I’ve come up with... here goes.......

I wish to add a B7 (15k:500) trans  to my Foreplay II preamp and have the output go through the B7 to decrease the  gain  and output impedance.  I have the old CS4 version with separate VA and CF boards for each 12AU7.  My Foreplay also has the walwart  for DC filament heaters.   I also wish to eliminate the CF stage.

This is what I want to do......
 ...............take out the CF board entirely (disconnect the B+ (pin6), disconnect the cathode (pin8) and disconnect the ground).    I can then take output cap and instead of having it go to the RCA out (from pin 8 or cathode) I can connect it to the B7 primary (side with two terminals ......dot is hot).  The non- dotted lead on the primary is then connected to the cathode (does it matter if it is pin 3 (cathode section 2) or pin 8 (cathode section1)? ....pin 3 has the 1800 ohm bias resistor).       

The B7 secondary (three terminals) is connected to the RCA out while the middle terminal is not connected and the last terminal is connected to ground.   I’ll assume that the output cap value should be increased in value to something  like  a 15 µFmotor run?  Can I also double the current in the VA stage by using a 499 ohm resistor instead of the 1K for R1?
I’m running the FPII into a Paramour  (with iron upgrade) with an 12AT7 driver .

Does all this look correct?
Thanks in advance
Frank M

[JC]

I can only speak about this in a very general sense, since my FP2 does not have the CCS's, but it sure seems to me that you could just put a coupling cap after the VA section to feed the primary of the transformer, and do away with the CF section altogether.  With the C4S as the plate "load" for the VA and a coupling cap to the transformer, it would be a sort of "parafeed" with the C4S taking the place of the more usual plate choke.

Just a thought; I'm not really sure of the output impedance of the VA section relative to the input impedance of the transformer.

[Frank Mena]

What I want to do is essentially this......(copied from an old post from Paul Joppa)

Paul Joppa wrote
 ....You could also take the output from the VA stage plate through the output cap to the transformer dotted terminal instead of the CF cathode. This basically bypasses the CF stage entirely. The unbypassed bias resistor at 1800 ohms would increase the impedance too much, so in that case return the non-dotted lead to the VA cathode instead of ground. If you do this, you can carry double the current in the VA stage C4S, ......

Frank M

[Caucasian Blackplate]

...take out the CF board entirely (disconnect the B+ (pin6), disconnect the cathode (pin8) and disconnect the ground).    I can then take output cap and instead of having it go to the RCA out (from pin 8 or cathode) I can connect it to the B7 primary (side with two terminals ......dot is hot).  The non- dotted lead on the primary is then connected to the cathode (does it matter if it is pin 3 (cathode section 2) or pin 8 (cathode section1)? ....pin 3 has the 1800 ohm bias resistor).       

The B7 secondary (three terminals) is connected to the RCA out while the middle terminal is not connected and the last terminal is connected to ground.   I’ll assume that the output cap value should be increased in value to something  like  a 15 µFmotor run?  Can I also double the current in the VA stage by using a 499 ohm resistor instead of the 1K for R1?
I’m running the FPII into a Paramour  (with iron upgrade) with an 12AT7 driver . 

The output impedance will be roughly the same.  The 2.2uf cap is fine for a 15k B7, connect it to the plate of each triode that you  are left using (the wire going from each VA board adjacent to the MJE-350).  Tie the non-dotted lead to pin 3 of each tube.  Use an HLMP-6000 in place of the 1.8k resistor, or put a 100uf @ 16v bypass capacitor across it.  The 15uf motor run would be overkill, you can go down to 1uf if you like.  Increasing the current will be fine since the CF is gone, along with its current draw, though I think bumping the cathode resistor down to 1k might be a good idea.

[Frank Mena]

Thanks Paul.  I think I got it, ....but just in case I want to verify a few things in your post.

If replace the 1.8K bias resisitor with the HMLP-6000 does the cathode side of the HLMP-6000 face the tube (pin3)?

Is the 1.8K bias resisitor I mentioned, the same resisitor as the cathode resistor you mentioned in your post, and if I plan to replace it with the HLMP I assume it does not need too be bumped down to 1K? or are we talking about some other resisitor, since I don't see any other resisitor on the cathode pin.

Thanks again for all of your help. It really is appreciated
Frank M

[Paul Joppa]

...
If replace the 1.8K bias resisitor with the HMLP-6000 does the cathode side of the HLMP-6000 face the tube (pin3)?

Is the 1.8K bias resisitor I mentioned, the same resisitor as the cathode resistor you mentioned in your post,...

No, and Yes.

The 1.8K can be replaced with a smaller value if the current is increased to maintain at least 1.5v at the cathode, or it can be replaced with a HLMP-6000 LED which wil force 1.57v bias. You can raise the current to 3.8mA as in Foreplay III, or even higher - as long as the plate voltage stays low enough to leave some headroom. I'd use the 3.8mA, which is an R1 value of 237 ohms.

Be sure to keep the B7's well away from the power transformer so they don't pick up any hum!

Incidentally, the next stage would be to use the former CF triode as a shunt regulator. But try your planned experiment first to be sure this is the sound you want.

[Frank Mena]

Thank you Paul.  Parts have been ordered and I'll post back with my impressions.  You really piqued my interest with your statement of using the former CF triode as a shunt regulator as  the next stage.    I most likely will give it a try it but first one step at a time.

Thanks again.

Cheers
Frank M

[Tube Newbie]

Hi Frank,

Have you moved forward on this yet?  I was just wondering how you faired.

This is all very interesting to me.

Thanks,

From the new guy,

Ron

[Frank Mena]

Hi Ron...Yes I did do it  (adding the two B7's .....one for each channel ) and have posted my impressions and changes in a post called In Heaven with B7..or something to that effect.   

In summary I really liked the effect it gave especially with my vinyl set up.

Cheers
Frank M