Quickie on order, mulling over ideas

[TadS]

Hi Learned Folks--

Curiosity got the best of me and I ordered a Quickie a few weeks ago, even though I did not have a preconceived purpose for it.  Now, I have a friend who is trying to put together a system on the cheap, and in my parts box, I have materials to put together a nice little LM3886 chipamp.  I am considering a build with the quickie in the same chassis.  I am not so sure my friend would like to fool with batteries and I am considering a power supply for it. 

I know that a big advantage of the quickie is its no-ripple supply.  However, just thinking here, I found an old transformer from a piece of lab equipment that I salvaged about 15 years ago; it has two secondary windings, one with a center tap; 27-0-27, and 13 V on the other.  I slapped together a PS with the CT'ed winding and am getting +/-37 VDC on each rail (74V total).  Is this too much for the quickie tubes?  How about the 13VAC winding for the filaments?  Would I need to step that down? Any good options for that?

Maybe I am just thinking too much while I am waiting on the mailman, and maybe I should just build it stock first (well, I did get the PJCCS).  But I thought I would see what the knowledgeable collective has to say while I am waiting.

Thanks!
--Tad

[Paul Birkeland]

There is potentially an interesting project here, but I'd certainly build the kit in stock form first.  To make a Quickie with AC power, you'd want three independent power supply windings (I'd recommend two 3V windings for the filaments).

I built such a beast some time ago - I could nearly have bought a Foreplay III (they were available at the time) for what went in to all the parts.

-PB

[TadS]

Thanks, PB.  I had forgotten that the filaments need separate floating voltages.  As you suggest, I will put it together stock before playing with any alternate power ideas. 

Considering the transformer I mentioned in the original post, would 74V be too much for these tubes?  Also, if I run the plates at higher voltages, should the filament voltages be increased some as well.  I thought I saw a post in the archives that suggested that the voltage from a single rechargeable (1.2V) was okay since the voltage on the plates was only 36V. I would have to comb through the past posts to see where I read that, but I took it to mean that higher filament voltages may be needed with a bigger rail supply.

--Tad

[Natural Sound]

would 74V be too much for these tubes? 

According to Paul Joppa, 67 volts is the maximum rated screen voltage for the 3S4.

Hope this helps.

[Paul Birkeland]

Considering the transformer I mentioned in the original post, would 74V be too much for these tubes? 

You can always raise the 4K resistor to compensate for more raw B+, just use Ohm's law.

Also, if I run the plates at higher voltages, should the filament voltages be increased some as well. 

No, the filament voltage is invariant, unless you rewire the socket to run the tube at 3V.

[TadS]

Thank you both!

I did finally turn up the datasheet on the 3S4 and it looks like my junk box transformer would be too much with out shunting some of that voltage.  Since I don't have the circuit yet, I am guessing the purpose of the 4k resistor is for that.  I know I am putting the cart before the horse since my kit is not delivered yet.  However, I did get the shipping notice, got excited, and started postin' questions. I will build as stock first and then playing mods.  Thanks again,

--Tad

[Paul Joppa]

I think I've said this before, but here I go again -

If you have a good power supply with stable voltage, then you can use fixed bias for the tubes - such as an LED or two depending on the chosen operating point. If you have 200mA or more available from the filament power, you can do like the BeePre and use a resistor in series with the filament for bias.

I know that's not enough specific details to go build; I haven't worked out the details either. Perhaps I should do that, but it's busy time here so that won't happen before January...