Seduction Questions

[xcortes]

Question A: There's only two led bias resistors for four C4S circuits. I assume each one is shared for two C4Ss?

Question B: I want to convert a Seduction to a single gain stage sans RIAA eq. So I have one extra section per channel and plenty of compliance. I'm thinking of using one half of the 6922 to regulate the other. Each channel would have a C4S (9mA) followed by one half of a 6922 as a regulator with plate voltage 100v and set up for 4mA, the other half of the 6922 being the gain stage with a C4S at 70v and 4mA.

How do I set up the regulator half of the 6922? If I get this right from the curves I need around 3.1v on the cathode for 110v at 4mA. Maybe I can use two LEDs in series to get 3.2v good for around 112v on the plate?

This is my first attempt at the Strain Gauge preamp (with batteries to bias the SG).

Thanks

 

[Grainger49]

Xavier,

I have the first run Seduction with C4S.  It has 4 LEDs in the C4S board, all HLMP 6000s.  

Has this changed?

BTW, I just bought a hand full of HLPM 6000s.  If you need some let me know.  It was more than a lifetime supply.

 

[xcortes]

Ha, only four LEDs, didn't catch that.

I think the C4S hasn't changed. I also have four LEDs, four current setting resistors and two LED bias resistors. The thing is, a C4S circuit uses two LEDs and one bias resistor.

Here we're talking four circuits but only four LEDs and two biasing resistors. So it seems like the biasing "side" of the C4S circuit is shared.

And thanks, I may indeed need some LEDs. Will let you know.

 

[xcortes]

I found the answer to question A in an older post by PJ

"the existing board has a single input and only two bias chains".

Looks like the connections for Question B may be complicated.

 

[Grainger49]

Yup, I had to look.  I have some Anticipation (FP 1 & 2) C4S boards and they are double LEDs per board.  I did see in the book that the two LEDs are in series.  Maybe the intermediate voltage could be used on two sets of transistors?

I tore into the Seduction board about a year and a half ago and sketched a schematic.  I'll be darned if I can find it today.  

 

[xcortes]

I think that what would make more sense is, instead of using the two extra sections to SR, use them to bias the SG. If the experiment sounds well I can then either rebuild or build something new. I'll post a schematic later.

 

[Paul Joppa]

Question A: There's only two led bias resistors for four C4S circuits. I assume each one is shared for two C4Ss?

Question B: I want to convert a Seduction to a single gain stage sans RIAA eq. So I have one extra section per channel and plenty of compliance. I'm thinking of using one half of the 6922 to regulate the other. Each channel would have a C4S (9mA) followed by one half of a 6922 as a regulator with plate voltage 100v and set up for 4mA, the other half of the 6922 being the gain stage with a C4S at 70v and 4mA.

How do I set up the regulator half of the 6922? If I get this right from the curves I need around 3.1v on the cathode for 110v at 4mA. Maybe I can use two LEDs in series to get 3.2v good for around 112v on the plate?

This is my first attempt at the Strain Gauge preamp (with batteries to bias the SG).

Thanks

A: Yes, the C4S's for the first stage share a bias chain and those for the second stage share another bias chain.

B: Our hybrid shunt reg uses the 431 chip in the cathode - the Paramount manual shows the circuit. The chip adjusts the bias of the tube to maintain the regulated voltage.

 

[xcortes]

Our hybrid shunt reg uses the 431 chip in the cathode

You're right! The 431 regulates and the tube just "amplifies" the regulated voltage. Forgive me, I'm still learning C4S and SR concepts. It's funny how, after years of nonsense, everything starts making sense.

 

[xcortes]

At some time I thought of getting advantage of the current at the tube and the SG being the same to use one to regulate the other. But I couldn't figure out how to do it. Well, these guys have come up with an idea (and tested it) for doing it that seems great:

http://www.vinylengine.com/turntable_forum/viewtopic.php?f=19&t=28750&start=240

I assume they use the CF on top because at 2.2mA and 90v r-p plate resistance is highish at around 8k. The interesting concept is that, since the bias setting "resistor" is fixed at 1k (the SG resistance) the only way to increase current is by increasing plate voltage. But I can live with low gain and with a 8k output impedance. So how about I use the second half of each 6922 to shunt regulate instead of a CF?

 

[Paul Joppa]

That sounds like an excellent approach to me.