CV428 or 5B254M useful?

[rotaspec]

A bunch of CV428 tubes have come up for sale here at fairly good prices.
As far as I can tell these are 5B/254M made by ITT, and are some relative of a 807 with a loctal base.

I haven't seen much discussion on them (plenty of 807 articles though), so I thought I would ask what people thought of them in a SE or PP amp configuration with a B+ of around 400v.
One thing I noted from the data sheet for the 5B series is that cathode bias is not recommended. Anyone care to enlighten me why that would be? I'm such a noob that I have only built small cathode biased stuff so far.
I could do fixed bias with a separate transformer only, as I have no bias winding on the PT I have here.
I have only skimmed the datasheet, but it looks like a PP OPT of 6k primary is needed. How far could I deviate from that (8k)? Trying to keep the budget in line with the price of the tubes

Gary

[Eli Duttman]

The minimal info. TDSL provides suggests that, except for basing, the type is a 19 W. 6L6 equivalent. Do you have a pin out? Use the 8 K "iron" and triode mode. Genuine 6L6 family tubes are rated for a positive grid current regime. So, consider an AB2 or even B2 set of operating conditions.

[rotaspec]

Comprehensive data sheet is here:
http://www.r-type.org/pdfs/5b254g.pdf

Max anode plus screen dissipation seems to be 25W

Also seem to be popping up on eBay UK:
http://cgi.ebay.co.uk/ITT-CV428-5B25...52310002r28101

Nice photos there

[Eli Duttman]

The socket looks like some sort of rim lock. Perhaps an ordinary Locktal socket will do.

Compare to the 6L6G. Those tubes are a tad tougher, but not the equal of the 6L6GC.

[rotaspec]

Thanks for the info Eli.
For the price being asked for these in lots of 10 (1/4 of what is being asked on eBay UK), I might just give them a shot. I think something simple like the RH807 SE circuit would be a good way to get a feel for them for a start, then push on from there.
The base is called a B8B I think, and I've been told by one of the tube guys over here that the CV428 fits a Loctal socket OK.

[radiotron]

Yes, have seen these in Surplustronics, looks like a 6L6 stuffed into straight-sided envelope on loctal base with small top cap, guess similiar rating to 6L6G/GA, would not go past 15-18 watts total disspation, but could be useful sub for 807 at lower plate voltage, they remind me a bit of 6L6 badged Russian 6P3s relabelled as GCs (they're not a GC substitute - red plates at 20W/400V). My p-p amps have (still do just now) happily run 6P3s and 6L6G/GA/5932 with 350V across tube, strapped as triode with about 30V of zener diode in series with screen, the tubes you mention would probably be happy in similiar setup.

[aardvarkash10]

locktal 807.

[Svein_B]

The twin sister tube 5B/255M (CV391) is used in a successful SE kit amp sold in Norway.

It uses plate-to-grid feedback, Hammond 125ESE OPT, and is driven by E83F pentode.

Sound is said to be excellent, within the limitations of its power rating (6.5W).

Svein.

[Rod Coleman]

I wouldn't hesitate. The examples I have seen were all made by STC Footscray (London), and are absolutely best quality. Low price is on account of funny base and topcap.

Run the screens from their own supply of 300V. As the spec says, you can run 600V on the anodes, which will give 80W PP pentode mode AB2.

Low screen rating means triode mode is relatively poor, but who cares, just use pentode mode with a Schade network and away you go.

These are much tougher the the old kind of 6P3S, much less risk of gassy samples too.

[Eli Duttman]

Rod,

It seems that those tubes are excellent and well worth taking full advantage of. Once a positive control grid current regime is in play, hefty/low impedance, driver circuitry is a must. IMO, there's no reason the driver can't source both A. and V. With that in mind, buy O/P trafos with 10% cathode feedback tertiary windings. Schade stuff is unnecessary.

Tubelab's "PowerDrive" will do quite nicely. Employ the IRFBC20 as the MOSFET, as its C_{Reverse Transfer} is both low and stable.