Cavalli Compact Tube Hybrid (CTH) Tube & Tweak thread. - Page 29

Guess it's just an amazing little amp

Think smeggy might have been 1st to use 6n6ps but others e.g.barleyguy and I have as well.

Maybe tubes unexpected to sound well do so because of their light loading / high impedance typical of Alex's designs. And decent plate voltage.

You have one of these around yet?

Funny story - During my 1st PCB build my bench must have been too messy because I had to take a few moments to find the partially populated PCB on my bench... I'd actually "lost" it due to its size

6F8G -> 12au7/9-pin mini adapter heads-up

Couldn't resist trying this and ordered one of those $25 6F8G -> 12AU7/mini adapters off the bay. They are no good for the CTH. Edit: dpmiller found the 6F8G to 6CG7 one is the one to get.

CTH uses pins 4 & 5 for (serial) heater supply. Center-tap pin #9 is unused in CTH and it looks like that adapter relies on is use... Note circled mini heater pins 4 & 5 are shorted in this adapter:



Just before powering up a CTH of mine using this adapter I thought "Hey, I better check this thing out w/an ohmmeter." Glad I did. CTH's heater circuit would not have been happy.
So a bit of re-wiring of the adapter & she'll be good to go.

Think it depends on your tube's (series) heater draw... Most target tubes draw up to 350ma or so & IIRC the full draw of the amp with that was about 550ma (?). So going > 750ma WW should not yield a benefit then (.833ma being fine in most cases - it's what I use).

If using tubes w/heater draw > 500ma or so you could move to a 1A wallwart, but doubt you'd hear a difference. Wallwart would just run cooler. We like to have at least 25% headroom in transformer/wallwart capacity so that they don't get too hot/stressed.

Ack! Yes I should have picked up that one Thanks for clearing this up.

BTW, quick re-wire (split 4/5 pins & moved pin #9 wire to then empty pin #5) and its working but I'm getting some hum w/the 6F8Gs - do you?.
ANY other tubes in this CTH have dead silent/black background.
This is a new, bigger, better CTH build of mine BTW (more room, all runs > 3in are shielded, etc.).

A good way to build a CTH, 6F8G update

My latest CTH is also my quietest. Perhaps because of the space, shielded runs, 24V reg clearance - who knows but I'm happy.



And no hum issues anymore w/RCA 6F8G. These must have helped: 1) let tube warm up a bit, 2) made cap/grid wire a shielded one (pin #5 of CTH's mini = ground) & 3) moving CTH around or off of Rotel CDP top.



I've tried a few tubes w/my new HF-2s (8416, 6922 PQ, etc) but none really hit me like this RCA 6F8G...

Smeggy & dpmiller know what they are talking about, these 6F8Gs are something special. Thanks for bringing them up

Not crazy about either of my 6f8Gs w/DT880s & haven't tried w/my D2000s (or HD650s on their way to me from FallenAngel). Liking the RCA in the pic better than Sylvania. Have a NOS RAYTHEON 6F8G VT-99 on the way & they are supposed to be nice.
Stayed up way too late listening to the setup in the pic... 1st time I've done that w/HF-2s. But it's likely other tubes in my stable mate well w/HF-2s - just have to crawl through them.

You might. Or perhaps something in your build, but I know you know what you are doing
I'm a big fan of shielded runs. With vol @ 3/4s I could hear the very faintest, hard-to-detect hum w/my most efficient cans between tracks on my other builds. But can't hear any hum or hiss on this one with any of the other tubes I've tried across all my cans.
A very, very tiny bit of hum is detectable in right channel between tracks w/my 6F8Gs that goes away when I grasp the cap/grid lead. Didn't notice it at all after tube was warmed up 1/2hr or so & moved amp away from CDP. Perhaps the nature of having a long grid line to top of tube around. But tube's fun value far outweighs any of this for me.

Thanks... Found CTH shines it's very best w/spread out/conventional build like this one. Have a few amps under my belt now & have heard several others @ meets & still come back to CTH goodness

Re: HB-360 - Yep, that's the one. Can't beat the price esp if you combine orders from that guy.