CV428 or 5B254M useful?

[Svein_B]

Originally Posted by Matt.B.H.

. . .
They do glow a rather nice blue on the envelope.
. . .
So the whole 5B-25X series are really nice valves:-)

Svein

[rotaspec]

Originally Posted by Svein_B

Svein

Very cute

Is that your amp? What are the driver tubes?

I'm still waiting on acknowledgement of my order for the tubes

Gary

[Matt.B.H.]

Did you have any joy?

Cheers Matt.

[rotaspec]

I got the 5B254M in the end, but have been too busy with other projects to have done any more than running one valve up with about 250v anode voltage to watch the blue glow
I'm in the middle of a little ECL86/EL84/6P1 type PP amp at the moment. Maybe when that is finally running properly I will have time to sort out the iron needed to suit the 807/5B254M amp.
I have a power transformer that should suffice, but no OPTs yet (well, one from a EL34 PP PA amp that will do as a trial).

Gary

[Matt.B.H.]

Same here with my 5B255Ms. I have two nice Antec mains torroids but no output transformers yet. I have thought about winding my own, even worked out all the details but I keep getting sidetracked. Mostly fixing other peoples stuff, helping the sister-in-law move house, etc. Plus in a moment of madness I decided to build a TIG welder
The lash up I built to see what they were like used a crappy little guitar output tranny. It proved a point though, the screen voltage rating can be abused a bit as long as the screen dissipation is kept well below max. Blue glow is very nice
Also another thing is the 6L6 AB2 thread, I keep changing my mind......

Cheers Matt.

[Matt.B.H.]

Hello,
I guess this is the most appropriate place to post.

Im itching to get this build done whilst im waiting for some 300A rectifiers for my TIG (its on the back burner anyway, I cant really justify the cost of argon).

I have finally settled on a topology. 5B255M x 4, PPP, AB2, fixed screen with 10-20% cathode feedback. 450V HT regulated with 300V screen also regulated, all interlocked so any bias/screen/heater supply failure will shut it down. Also a time delay in the HT to allow warm up. All this will be done using simple discrete logic. No PICs or other digital nonsense in this one.

Im planning on using the MOSFET followers as in the 6L6 AB2 thread. This will allow DC coupling throughout. 7N7 splitter and possibly a 3A167M or C3G/M at the front. Loctal all the way That was the initial design goal anyway.

The question is now after much searching and diliberating (gibering about) What are the Edcor OPTs like? I see they can do some custom winds and after being let down by a UK winder Im really considering going with them. I will be asking them to break the primary at 10 and 20% impedence from CT and bring them out for CFB and still leave the 40% UL connections for experiment. 3K3, 100W tranny seems about right.

I know their build quality is good, at least better than Hammond of late and have heard of trouble with the Hammonds. Plus Hammond in their wisdom have chosen to use one tapped secondary on the latest 1650R, the 1650RA, which rules this one out for me(appart from no CFB) because I cant mis-wire it to get the lower load, Hammond dont reccomend this anyway. I can not get the standard 1650R here only the later 1650RA so it goes to Edcor. I cant afford some insanely priced trannys out there although I would love to have some, if only to measure how good or not they really are

Any advise will be much appreciated.

Thanks Matt.

[rotaspec]

Sounds like an interesting design. I looked at 7N7's for my 5B254M PP but the rarity and therefore price of them put me off. I do have a bunch of 7C6's (single triode + diode) that I might check out as a splitter though. I even thought of continuing the loctal theme with 7C5's as a cathode follower driver, but I will probably look at MOSFET's instead. The sheer size of the chassis will probably mean monoblock layout, especially if you are going PPP with 100W tranny's.

Edcor seems to be the one to beat in low cost iron, and will be my first choice (for budget reasons). There seems to be some report of long delivery times at the moment though.

For a good heads up on MOSFET drive (and Edcor OPTs) check out tubelab.com. George there has put down a good page on "Superdrive" MOSFET followers. He did a good comparison between OPTs on his SE page.

I really should get around to ordering some OPT's and do something with the 5B254M's that I have. The EL84 PP is all but done (just needs a wooden frame) so I might have some bench space to get on with the next one.

[Matt.B.H.]

Hi Rotaspec,
Sorry to jump your thread but I guessed it would be the best place for it.

Yes, Im going monoblock with seperate PS each. I got some of the Cannon copy Chinese connectors on Ebay for the umbilicals. Just search Millitary Connector. I tested them at 3KV for an hour no probs, so they do meet the original ITT/Cannon specs, nice. Be sure to use two pins for interlocking etc for safety.

I have a pair of Antek AN-4T360 which gives me a nice 500V or so either full bridge (both secondarys in parallel) or 506V with The usual CT type arangement and two diodes, both loaded at 600mA. This is using 2 x 1000uF Evox Rifa PEH169s in series with droppers accross em. So I reckon a Maida reg would dissipate 30W dropping it to 450V. A bit of burden but hey I did try the 5Bs at 500V and they didnt show any signs of distress. Mind you that was in a Williamson lame *** set up. Maybe I can live with 500V HT and 300V screen. I do dought that though in AB2, it is really pushing them.

Anyway, 7N7s are not rare. Ask about at your usual valve vendors. Here they are certainly cheaper than 6SN7. I got 8 for 3 quid a pair. They were in pairs because they were supposed to be matched. How the hell you can match dual triodes is beyond me but hey who cares for that price. I think this may be the Morgan Jones factor in action.

Cheers Matt.

[Matt.B.H.]

Hey rotaspec,

How you getting on with you 5B's?

I just completed the output stage prototype for my all Loktal amps getting nearly 100W RMS with four 5B255M.

They are good way better than 807's and the data is a lie they easily do 30W anode dis.

Also no worries on the screen, been running them at 498V UL, they love it.

Here are some bad pics:

[Matt.B.H.]

Hope they are here.