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
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
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-CV428-5B2...emQQimsxq20100205?IMSfp=TL1002052310002r28101
Nice photos there
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-CV428-5B2...emQQimsxq20100205?IMSfp=TL1002052310002r28101
Nice photos there
Last edited:
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.
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.
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.
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.
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.
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 CReverse Transfer is both low and stable.
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 CReverse Transfer is both low and stable.
Eli, I am 100% with you on source follower drive circuits, especially where AB2 may be contemplated. But these also make the Schade network easy to drive. Is there something about the Schade sound you don't like? I would have thought that it offers more flexibility that a fixed feedback winding.
I know nothing about Schade. I KNOW that CFB in combination with regulated g2 B+ works very well. Add a few dB. of GNFB and the music will flow.
How about a variation on the Mullard theme? Use a 12AT7/ECC81 section as the voltage amplifier and a 5965 as the LTP. The FET buffers driving the "finals", via DC coupling, are a given. LED bias in the voltage amplifier will make the chance of instability extremely remote.
How about a variation on the Mullard theme? Use a 12AT7/ECC81 section as the voltage amplifier and a 5965 as the LTP. The FET buffers driving the "finals", via DC coupling, are a given. LED bias in the voltage amplifier will make the chance of instability extremely remote.
Eli, I'd say it would be good & simple to build, and will work (or can be tweaked to work) a treat.
Either way, with a pair of FETs and these cheap but excellent pentodes, one should be able to make a very fine amp for low cost. With regulated VG2 & dc source-follower grid-drive as a starting point, we leave a lot of design options open that are unavailable if triode mode is chosen.
Makes me wonder if a toroidal trafo could be pushed into service at this power level. That way, we could experiment with CFB by adding a hand-wound tertiary.
Either way, with a pair of FETs and these cheap but excellent pentodes, one should be able to make a very fine amp for low cost. With regulated VG2 & dc source-follower grid-drive as a starting point, we leave a lot of design options open that are unavailable if triode mode is chosen.
Makes me wonder if a toroidal trafo could be pushed into service at this power level. That way, we could experiment with CFB by adding a hand-wound tertiary.
It's an 807 repacked in a Loctal case, original use in transmitters. The top cap is there to ensure there is no socket flash-over at high plate voltages. The 255M has it's anode on the socket and no cap. These are prime quality tubes, but keep in mind the smaller envelope reduces maximum dissipation limits.
Spread the tubes out on the chassis and use forced air cooling. The right fan (large, slowly turning, blades) will move plenty of air, without making excessive noise. Those of us living on the North American continent have the simple option to use a "240" VAC fan in combination with the available "120" VAC mains power.
They can be considered a high quality 807. There are plenty of Loktal sockets about either the McMurdo ones or the really good NOS PTFE ones. Just remember to bend the locking tags in the center out a bit or they wil be hard to get out again.
I lashed a pair up in UL Williamson style using 7N7s, They dont seem to mind abusing the screen voltage rating too much. They were happy with 450V HT.
I agree parallel operation is the way to go, with fixed screens, cathode feedback and maybe even AB2.
Just like the 807 they like to oscillate. So grid stoppers at the socket and maybe some RF chokes in the anodes will be needed.
The 8 5B-255M that I have were made by ITT in 1982, so there must be plenty about. The price on these is going up steadily:-(. I think the Japanese and Taiwanese are very fond of them. The do glow a rather nice blue on the envelope.
Heres some links of people using them:
AudioJumble - Next Event
5B255M PushPullƒAƒ“ƒv
OB7
http://www.diyaudio.com/forums/tube...harmonic-distortion-curves-5b-255m-807-a.html
So the whole 5B-25X series are really nice valves
Cheers Matt.
.I lashed a pair up in UL Williamson style using 7N7s, They dont seem to mind abusing the screen voltage rating too much. They were happy with 450V HT.
I agree parallel operation is the way to go, with fixed screens, cathode feedback and maybe even AB2.
Just like the 807 they like to oscillate. So grid stoppers at the socket and maybe some RF chokes in the anodes will be needed.
The 8 5B-255M that I have were made by ITT in 1982, so there must be plenty about. The price on these is going up steadily:-(. I think the Japanese and Taiwanese are very fond of them. The do glow a rather nice blue on the envelope.
Heres some links of people using them:
AudioJumble - Next Event
5B255M PushPullƒAƒ“ƒv
OB7
http://www.diyaudio.com/forums/tube...harmonic-distortion-curves-5b-255m-807-a.html
So the whole 5B-25X series are really nice valves
Cheers Matt.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.