More "how to add transformers" questions

[xcortes]

Sorry for more questions along the same tune. As I've said before, contrary to my main system where less i more, in the office system I want to add transformers because I expect them to tame the digital beast (if I'm not right is another story).

Since I'll have an extended FP idle once the BP is finished i think it would make sense to use it as follows (have all parts):

Mount two pairs of B7s at the null point besides the pt a-la Sex amp. Rewire one input through the B7 10k transformers and from those to the goldpoint 10k attenuators already in place. Rewire the attenuators as normal attenuators instead of shunts. Eliminate the selector switch.

Take the signal from the VA tubes to the other pair of B7s through a parafeed cap and from there to the output effectively bypassing the CF.

Now the questions:

The output B7s have 8k primaries. Do I maintain the 12AU7 operating points or should I change them?

When getting rid of the CF stage, do I just ground inputs and outputs but leave the C4S and the the tube current in place to avoid messing the current balances of the whole amp?

What would be a goof parafeed cap size for starters?

Thanks again

[Paul Joppa]

Even at the "null" I worry (about the input transformer especially) picking up hum. The point is suitable for the output stage of a power amp but preamp signals - especially inputs - are much lower levels. So give it a try but be aware of the possibility.

The CF stage does no harm, just take the parafeed cap from the VA plate instead of the CF cathode.

The FP-III already runs as much current as is practical without dipping into grid current, so don't adjust the operating point.

I have no reliable information on parafeed cap value, so start with the stock, and listen to the bass.

[xcortes]

Thanks Paul,

Very simple. I'll try it and report back. This digital system is getting shape!

[xcortes]

In another thread Doc suggested "if you are going to put 8K B7s in your FP3, why not convert it to a tube that is a better match for them? You don't need the extra triode for the CF, and you really want a tube with a lower plate impedance than a 12AU7. Maybe try 417As?"

Not bad since I can use the chassis, switches, jacks, power supply, etc, etc vs building a new Soul Sister.

I see two ways. One would be just to dump the regulation and use the already existing C4S (16mA) to feed the 417As. 16mA and an HLMP to have a 1.6v bias would get me to 125v on the plate and 2,000 ohm plate resistance.

Another way would be to maintain the regulation and then use 8mA C4S. The same bias gets to something like 110v which give me approx 3,000 ohm plate resistance.

The curves look more linear at 16 than at 8 but then there's the regulation vs none.

Thoughts?

Thanks

[xcortes]

A third way would be to do a low current soul sister using instead a 5670/2C51/WE396A. 8mA at 1.6v bias is 150 volts and maybe around 4,000 ohm plate resistance.

Would it be better?

[Paul Birkeland]

One would be just to dump the regulation and use the already existing C4S (16mA) to feed the 417As. 16mA and an HLMP to have a 1.6v bias would get me to 125v on the plate and 2,000 ohm plate resistance.

The PT-3 has a fair amount of high voltage current.  Go back to the single voltage doubler supply of the standard FP-III.  That will get you a hair under 400V, which you can regulate down to 300V (shove 30mA through), then you can feed the tube plates 16mA, the C4S LED's 4mA, and have 10mA left for regulation.  You'll have to look for a dual triode that will be happy seeing 10mA at 300V, and will have a cathode voltage at this operating point between 4 and 20V. 

-PB

[xcortes]

With those 400V I could buy a soft start retrofit kit and basically build me a Smack right?

[Paul Birkeland]

The Smack didn't need that much voltage, and actually you don't really need the 400V supply in your case either, you can always try regulating to just under 200V, and with a plate voltage under 150V, compliance should be OK.