CV428 or 5B254M useful? - Page 2

Rod Coleman · 14th February 2010, 05:45 PM

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.

Eli Duttman · 14th February 2010, 06:23 PM

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.

Rod Coleman · 14th February 2010, 07:07 PM

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.

radiostar · 15th February 2010, 03:57 PM

Great tube!!! I have made a lot of pirate FM transmitters with them 20 years ago, 807 for high frequencies ...

ilimzn · 15th February 2010, 04:07 PM

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.

Eli Duttman · 15th February 2010, 07:15 PM

Quote:

Originally Posted by ilimzn

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.

jlsem · 15th February 2010, 09:13 PM

For NZ$50 per ten, you can afford to run them parallel.

John

Matt.B.H. · 15th February 2010, 10:11 PM

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

2nd & 3rd Harmonic Distortion curves for 5B/255M (807)

So the whole 5B-25X series are really nice valves:-)

Cheers Matt.

rotaspec · 16th February 2010, 12:34 AM

Well, certainly some interesting ideas and links, thanks guys.
I just put in an order for 10 of the 5B/254M.

The data sheet doesn't seem to show the top cap diameter. Does anyone know? I am guessing it's 6mm (1/4), if it's in keeping with the slim tube diameter of 30mm.

Gary

radiotron · 16th February 2010, 07:30 AM

Yes, 1/4" top cap, will be interested to find out how they work for you, cheers, Jon

14th February 2010, 06:23 PM	#12
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	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. __________________ Eli D.

15th February 2010, 03:57 PM	#14
radiostar diyAudio Member Join Date: Apr 2009	Great tube!!! I have made a lot of pirate FM transmitters with them 20 years ago, 807 for high frequencies ... __________________ tubes.rs facebook.com/tubes.rs

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14th February 2010, 05:45 PM	#11
Rod Coleman работник diyAudio Member Join Date: Jul 2004 Location: Warwickshire UK	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.

14th February 2010, 07:07 PM	#13
Rod Coleman работник diyAudio Member Join Date: Jul 2004 Location: Warwickshire UK	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.

15th February 2010, 04:07 PM	#15
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	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.

15th February 2010, 09:13 PM	#17
jlsem diyAudio Member Join Date: Jun 2003 Location: Dallas,TX	For NZ$50 per ten, you can afford to run them parallel. John

15th February 2010, 10:11 PM	#18
Matt.B.H. diyAudio Member Join Date: Jul 2009 Location: Cheltenham	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 2nd & 3rd Harmonic Distortion curves for 5B/255M (807) So the whole 5B-25X series are really nice valves:-) Cheers Matt.

16th February 2010, 12:34 AM	#19
rotaspec diyAudio Member Join Date: Nov 2007	Well, certainly some interesting ideas and links, thanks guys. I just put in an order for 10 of the 5B/254M. The data sheet doesn't seem to show the top cap diameter. Does anyone know? I am guessing it's 6mm (1/4), if it's in keeping with the slim tube diameter of 30mm. Gary

16th February 2010, 07:30 AM	#20
radiotron diyAudio Member Join Date: May 2007	Yes, 1/4" top cap, will be interested to find out how they work for you, cheers, Jon

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