audio-talk

This looks a nice transmitter tube.



I might just have a go with this in my existing amp.
If it sounds good I might design another chassis, with 800V capability
and use my existing inputs and drivers, or maybe the 801A to drive it.

800V @ 60mA and -40V bias looks a nice operating point.

Has anyone here used it? or ever thought about using it?


Steve

Nope, but looks great in theory. Can you actually get them anywhere though?

 

Well I've got a pair coming this week so I'm quite excited.

After looking at the curves of all these transmitter types and listening to various conversations at Simon's do concerning class A2 operation, I have decided to go for zero bias class A2 with these valves.

I very very briefly tried zero bias A2 operation with my 801A amp last night.
I has to slug down the HT to try this and bypass the voltage droppers to the inputs and drivers. Even then, I was still exceeding the max anode dissipation of the 801A.

Terrified as I was of frying my lovely 801As, the experiment was necessarily very brief indeed, maybe around 45 seconds of A2 operation. The sound that came out was big, powerful and just as detailed as with my normal mode of operation with the 801As.

In one way this was quite a surprise to me as I had not been expecting much. After all A2 needs power driving circuitry and all I was doing was driving with cap-coupled AC/P1 small power tubes. These, although having a lowish Ra, were certainly not supposed to be anywhere near ideal for driving an A2 power stage. Yet drive it they did, very loud, very clean and nary a hint of distortion.

What's that all about then? I thought you either needed DC coupled cathode followers, or step-down interstage transformers for successful A2 work, yet these 801As seemed very easy to drive with the AC/P1s.

The 4304CBs have a gain of 11 against the 801A's 8, so should be even easier to drive

Googling for the 4304CB tube turned up this WAVAC amplifier from Japan that uses them. Must say they look frightfully cool when lit up.




I'm looking forward to trying them out.

Top-cap connectors might be a bit of a problem to get hold of though.

Steve

Must say they look frightfully cool when lit up.

They look frighteningly hot to me.

al newall wrote: They look frighteningly hot to me.

Easy tiger! I like 'em too but....

DTB

SteveTheShadow wrote:What's that all about then? I thought you either needed DC coupled cathode followers, or step-down interstage transformers for successful A2 work, yet these 801As seemed very easy to drive with the AC/P1s.

I suspect it's possible the caps may be charging up so the grids are actually going negative again. On average. Except on the tips of the positive peaks. I'm just guessing but that's typically what would happen in that sort of scenario. Which is why have to use something else instead of caps

Mike H wrote:

SteveTheShadow wrote:What's that all about then? I thought you either needed DC coupled cathode followers, or step-down interstage transformers for successful A2 work, yet these 801As seemed very easy to drive with the AC/P1s.

I suspect it's possible the caps may be charging up so the grids are actually going negative again. On average. Except on the tips of the positive peaks. I'm just guessing but that's typically what would happen in that sort of scenario. Which is why have to use something else instead of caps

Hmm that could be it.

When I do go A2 with the amp, the intention was to use DC coupled cathode followers to buffer the drivers to the power valves, maybe at +5V stood on their cathodes to bias the 4304 grids slightly positive at idle.

Cos of the highish gain of the 4304s I thought maybe a single 6AS7 could be configured as a cathode follower to drive their grids directly, one half per channel. That would have a very very low Zout.

I just thought it really interesting that an amp with a seemingly inadequate driver for A2 performed without horrible distortion every half cycle.

Steve

Mike H wrote:

SteveTheShadow wrote:What's that all about then? I thought you either needed DC coupled cathode followers, or step-down interstage transformers for successful A2 work, yet these 801As seemed very easy to drive with the AC/P1s.

I suspect it's possible the caps may be charging up so the grids are actually going negative again. On average. Except on the tips of the positive peaks. I'm just guessing but that's typically what would happen in that sort of scenario. Which is why have to use something else instead of caps

Good thought Mike, 'grid-leak bias' could be happening.

SteveTheShadow wrote:I just thought it really interesting that an amp with a seemingly inadequate driver for A2 performed without horrible distortion every half cycle.

Yes Steve indeed I'm not arguing with that bit, just me putting my analytical hat on and going "aha! Bet know what's happening there" (as a 'secondary effect' kinda thing)

Still doesn't alter that it worked!

Yep I like the DC coupled CF idea, there's another topic somewhere about exactly that. Think it was one of Phil's [many?] amps, you could DC bias the outputs by the grid bias of the CF's. Which sounds much more complicated than it really is.

The tubes arrived today.

To the lab Ygor BWHAHAHAHAHAH!

LOL

 

With a smile like that on your face Steve one wonders where the other valve might be ?

Blimey!

What are the chances of that eh?

That valve is the same shape as yer 'ead

DTB

Yeah just need a pair of deely boppers and you could connect 800V.