FC1 questions

[Jim R.]

How many switches would it take to make an arrangement to switch the FC-1 from 2.5 to 5 volts?

Considering building the paramount 1.1s as a cap coupled 300b amp but with the option of rrunning something like the JJ 2a3-40 or any of the sophia 2.5v 300bs.

The switches would be located underneath and not easily accessible so there could be no accidental shanges in the filament voltage.

Any other downsides to this?

Thanks,

Jim

[Paul Birkeland]

You might just be able to wire it (the FC-1) for 2A3 operation and use that with the 300B, just keep an eye on what the filament voltages actually are. 

Otherwise, I think you could do it with a 5P2T switch, but it would need to be rated for a lot of current. 

[Jim R.]

PB,

Really? So if I put an eml 300b in that socket wired for 2.5v, Jac is not going to complain? ;-)
Or is there some other interaction between the impedance of the cathode of the 5v tubes and the 2.5v winding that I'm not aware of?

Think I'll look for a good heavy duty switch.

-- Jim

[Paul Birkeland]

(See my edit)

[Paul Joppa]

Jac will object if you deviate by 3% from the specified heater voltage. Paramount does not offer that much precision, and neither does your power company  :^)

Today and tomorrow I am drafting a writeup on how to adjust the voltages in a Paramount. But any such adjustment must depend on the power line voltage remaining stable.

When and if the Paramount is ever revised, it will most likely use a chip regulator for the filament supply. Prior to the BeePre we have stayed away from regulators and even capacitors for filament power because they all have poor reputations for sonic refinement. However, our experience with the BeePre (which is regulated) has been very positive. We will continue to evaluate such possibilities.

[johnsonad]

Paul, I look forward to reading your paper.

[Jim R.]

Paul,

Hate to say it, but Jac also says no-no to shunt regulated filament supplies -- it's another one of those deal breakers on the warranty. Last time I checked anyway.

Why do I have to make things so complicated? I don't want complicated -- think I'll just stick with the 2a3 amp and be done with it.

-- Jim

[Paul Birkeland]

It looks like he specifies not using a current source to power filaments.  He has a lot of cautionary warnings regarding regulated DC, most of which we adhere to.  It's somewhat interesting, as the technology in the Smash filament supply ticks all his required boxes.  (No, you can't put that supply into the BeePre/Paramounts for many, many reasons)

[RPMac]

PB, thanks for answering my dumb filament question before I ask it!

I have another question on switch ratings...
Are the amp ratings for switching under load?
If the switch is only used when there is no power present (used as a means to easily change connections), is the same rating required?

I have wired my Stereomour with 4P2T switches to change the B+ setting from 2A3 to 45.
I think the mini-switches are rated 250vac at 1 amp.

[Paul Birkeland]

You'll want to get a switch rated for the current being used, likely with some extra margin. 

[Paul Joppa]

There are two concerns about switches. One is the resistance of the contacts under normal load - especially important with filament/heater current which can be quite large. A tenth of an ohm, feeding a 2A3, drops the voltage by 10%.

The other concern is the current being switched. This is the inrush current, which in a tube filament/heater may be five times the operating current (12.5 amps for a 2A3). For a high voltage power supply it's the peak current into the initially-discharged filter capacitor - eight amps is what I get for switching on the high voltage in a typical small power transformer (PT-7).

There are more concerns for various circumstances - you can fill a book with them and not run out - but these seem most relevant to the current [sic] discussion.