Quickie as cathode follower help needed

[Paul Joppa]

The 3S4 and related tubes were designed for carbon-zinc batteries - alkalines have a much greater lifetime but are otherwise similar. So they were intended to work well with a much wider range of filament voltages, and to not be damaged by low filament voltage.

In Quickie, the plate voltage is low, 36v compared to rated 90v. Cathode poisoning happens when the plate voltage drives positive ions into the filament, so the low voltage means less energetic ions - and less damage. In addition, Quickie operates at about 2mA plate current, compared to rated maximum 9mA. That means the space charge remains adequate to protect from poisoning down to a lower filament voltage than normal - I estimate another 10% beyond the usual range for these tubes.

Grid current is not usually a problem since the alloy used for the grid in these battery tubes is chosen to have a contact potential that delays grid current until the grid is more positive than the usual AC-heated tube - perhaps +0.5 to +1.0 volts IIRC.

[cpaul]

Wow, thanks Paul.  That's a very helpful answer, and things I definitely did not know.  A group of audio engineering friends were discussing the possibility of A2 operation because of the rough numbers I'd thrown out when talking about the amp.  And when I try to measure voltages to figure out exactly where the amp is operating, I'm getting odd results.  I'll assume there's no harm if it did happen, except perhaps to sound quality...

As for the filament, I have a pile of alkaline batteries from another device that have 1.2-1.3v left and I was hoping to use them up.  The first pair I used lasted something like 15-20 hours (maybe more) which I was happy with.  But knowing that meant dropping to 1v or possibly below, I started wondering about potential harm to the filaments.  I'll assume there isn't a problem.  Thankfully we're not talking about replacing a pair of Western Electric 300Bs in any case.

I do need to try this CF version with a full set of new/charged filament and B+ batteries to see how it sounds at much better voltages.

[Paul Birkeland]

Very low battery voltage indeed isn't great for the tube, but it sounds terrible so it's not like you're going to spend hours and hours operating the preamp in that region.

The datasheet for the 3S4 makes it look pretty resistant to drawing grid current.

[cpaul]

Thanks PB.  If you're willing and able, I'd love to know how the data sheet indicates resistance to drawing grid current.  It's not something I've ever worried about before so I don't know how to assess that.  Either way, thanks for the input.

[Paul Birkeland]

The presence of any grid current information, especially on triode curves for a pentode, is pretty rare outside of powergrid transmitting tubes. 

Still, you might try clip leading in one extra 9V battery to see what happens.  An extra 9V on the plate certainly won't hurt anything.

[cpaul]

Thanks.  Interestingly that is on my to do list, but not specifically to address possible A2 operation.  I'll report back if there's anything to report.

[cpaul]

More battery experience with this amp, though I seem to recall posting something like this elsewhere for the Quickie too.  I started hearing some distortion yesterday and thought it might be because my filament batteries had drifted below 1v.  The last I'd checked a few days before the 9v B+ cells were fairly strong at over 8v.  I checked the D cells and one was about 0.9v and one just under that.  But after replacing them I was still hearing distortion.  Ultimately I checked the B+ batteries again and found 3 at essentially 0v and one at just over 8v.  I was still getting reasonable music out but definitely distorted.  Hard for me to fathom that the amp could run that low but it did.  I replaced them and it's working fine now.  I kept the newer D cells in but may try the old ones just to see what happens at 0.9v or less.

[cpaul]

I was recently given a pair of Paramour amps to try out.  They are on loan with a pair of Fostex speakers in reflex boxes because I'm thinking about new amps and speakers.  So far I've only tried the CF version with  the Paramours but I'll be trying out the standard Quickie soon.  I had to pillage the stepped attenuator of Quickie because the conductive plastic pot/attenuator I had in the CF version was suddenly making all kinds of static noise when adjusting it.  So it awaits a new volume pot.

Anyway, it all sounds terrific.  Even the tiny line transformer output of the Paramour sounds great with robust bass (probably because it's parafeed?).

Cheers,
Carl

[Paul Birkeland]

Yes, parallel feed output splits the duties handled by a typical series feed capacitor into two individual components (the plate choke and the parallel feed output transformer).  In testing the Kaiju plate choke and the Stereomour output transformer together, I am down about 2dB at 6Hz.

[cpaul]

   Yikes!

[cpaul]

I had to rob Quickie of the 100k stepped attenuator for a volume control for the CF version, so Quickie was unavailable for a while.  But I finally got a "Blue Velvet" attenuator (unfortunately quite possibly a Chinese knock off as I've learned) and installed it last night.  So now I'm listening to Quickie with the Paramours.  I've even done a little A/B testing, definitely not even blind much less double blind!

After some listening I now think Quickie with the PJCCS outpaces my CF version.  It is somewhat more pronounced and "tight" on the low end, and may have a touch more sparkle on the top end.  Not drastic but noticeable.  Mids are pretty similar I think.  In any case, it's quite nice!  And given the gain situation, the microphonics aren't bad at all when driving the Paramours (and other tube gear).  Glad to have both of them back. 

The real benefit of the CF version is lower microphonics and other noises, which are terrible in the Quickie when driving solid state gear.  With that in mind I look forward to hearing the CF version drive my Marsh A200 and the Maggie 3.3r speakers in the barn man cave sometime next year when it warms up again!

[Bonzo]

Built one myself! But with chokes.
Quickie with PJCCS is better, but as cpaul wrote CF is the way to go with high sensitivity power amps (if you don't want to pad the standard Quickie output, which is the cheapest option).
Some shots attached.
Thank you all for the super fun project!

[cpaul]

Nice looking amp, Bonzo, and thanks for the post.  I see what looks like Hammond 156C, which looking at the specs seems like the choke of choice (including the price!).  150Hy and 3700R with 8mA.  I will definitely look into using that.  I suspect it will be better.

Finally got a chance to hook the CF version to my Marsh A-200 and it sounds terrific with almost no microphonics.  The gain (or loss really) isn't ideal as I can't get full output from the Marsh (then again, no distortion either).  But at 100wpc, even my Maggie 3.3r are quite loud with the CF version, too much for me.  The other solid state amp I've tried it with is a DIY LM3886 chip amp, which also sounds quite good with no microphonics.  In short, this was definitely a successful experiment.

BTW, I didn't try padding the output of Quickie but padding the input did little or nothing for microphonics in my case.  Hence my looking for another approach.

[Bonzo]

I personally use a pair of Kimber cables with 16 db attenuation (more than quickie's gain!).
Sound good, and made me turn the volume up a lot (exciting experience, even if I was cheating :-) )