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Gday I am perusing Eimac data. I note apart from the 4e27 and 4e27a Eimac makes a 4-125a that has very similar specs. Has anyone tested and compared in triode the 4e27 and 4e27a pentodes which are half the price and even cheaper the 4-125a tetrode.
What are the differences between pentode and tetrode in triode operation.
Are Eimac any different to normal tubes. I have read somewhere that perhaps the tetrode screen is closer to the cathode then most tetrodes. Have I got that correct?
Follow Ups:
You can more or less ignore the G2 ratings in triode mode , have run 4-65A at 750V no problem , just check the current G2 is drawing . With these types with enclosed tubular anode and bright fil , you just can't tell what g2 is doing visually . Those old Eimacs need some colour on the anode to function correctly and away you go . I think I have some 4E27 , these are the ones with the 813 type base ? If so Ra as a triode is around 2k from memory . Try a 4-65A , these measure like an 845 as triode and you should be able to use these with a > 5k optx in SE .
Al
Read the manufacturer's maximum ratings carefully on "modern" RF power tetrodes. To improve efficiency in normal RF applications, the screen voltage was often much lower than for operation of audio tubes, which presumably means closer spacing from screen to cathode and correspondingly low maximum screen voltage rating. This allows larger plate voltage swing and better power efficiency.
The 1967 Eimac data sheet for the 4-125A/4D21 (see Frank's pages) lists maximum plate voltage for RF and class AB2 audio as 3000 V, but only 400 V on the screen. In class AB1 audio, the screen rating rises to 600 V.
With these low screen ratings, it is not surprising that Eimac did not list triode-connected parameters.
The 1968 Eimac data sheet for the 4E27A/5-125B does show a higher screen voltage rating: max. plate voltage 4000 V, max. screen voltage 700 V, but still no triode operation with such a difference between plate and screen ratings.
Edits: 04/10/14 04/10/14 04/10/14
Gday yes this is my concern that perhaps these sorts of tubes wont be optimum being run as triodes. Probably no risk of destruction but risk of sub par performance. I am considering pentode operation or tring to find real triodes instead. 75tl 211 still almost affordable and copper gm70 which is my tube to fall back to if these don't work out as that is what my new trans are specced for 7k. Well I am not sure if they are even built yet. I am trying to be patient. I don't need them yet after all.
I have a pair of higher load for the 211 allready(softones). As I though so indeed the plates must get hot. In the add for custom machined plate caps they boast plates never get red when using them ;\ I really need to tailor my cooling caps for ideal temps at my final op points. Perhaps a small hr2 grid cap drilled out may be all that's required at such reduced operation. Since the 4-165 is cheap I will; buy a pair and try them I guess if there is no definitive answer to which makes the best triode.
Then I will try the triodes! all good fun if it wasn't for the heater supply builds. I have new 30amp 5 volt switch modes to try the easiest way first with some hand wound toroidal cmc;s and a finished proper pair of supply's for the ten volts tubes. The negativeness for the eimac multigrids are adding up. Thank you Al and Tim
I don't know what switchmode supplies will sound like but filament supplies are what makes all the difference with these big spiral filament , spiral grid thoriated types . AC is definitely out . I used DC LCLCL supplies which is probably the main reason why I never managed to get past the breadboard stage . I think you are making the right move with 4-65A , filament requirements are less , Ra as triode is lower (1.6k) and sockets are no issue . You may need to find some heat dissipator caps though . Look up Ale Moglia's blog (Bartola Valves) , he has a 4-65A amp on the go at the moment
Al
Thanks Al I just bought a new pair to try. Much easier to find then 4e27
Things worth mentioning for anyone like me that might find this thread. I hope I am correct in saying.
Buying used tt filament on eBay is risky because the filaments go brittle when used.
The tubes with yellow like paint/epoxy) on the bottom are possibly risky if not all likely to loose there vacuum. And should be avoided.
The plates need to get red to operate correctly long term so plate cooling needs to be well considered.
As for switch mode power supples... have seen it done with 808 on valves world Japanese website which has inspired me for years to try build with weird funky but cheap to buy tubes. But those guys are supa cool for building to order from customers for all sorts of funky tubes but I suspect novelty and price constraints may effect sound quality standards.
hmm sofar that's it. I will keep trying.
What about Retrovox /David Crittle . It's the sort of stuff he may have in stock and at least on the same land-mass as you :)
Al
I didn't know he was still at it. I will look him up.:)
I have been running 5-125B, and with 10V rigging to the filament Iron, 813's on AC with no audible hum on efficient speakers. One winding and one hum pot for each tube. They both run U-L at 20 or 30% on a 10k a-a OPT. I like the 5-125B slightly better, and they make me 60W in Class A from just under 600V.
They will shortly get their first modification; trading a 6H6/FQP1N60 cascode LTP for a pair of EL84...also rigged as LTP.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
PP is a bit extreme for me. I have thought about it if you can get away with ac filaments it does not seem so extreme after all. For the record I am going to try 4e27 and a and the tetrodes and any other tube that I can run and listen for a while.
Tonight I have to think of a budget design for a amp.I have always resisted engineering amps. It occurs to me if I was comfortable with the technology it would be fun and rewarding to layback in bed on the lap top spicing your amp design trying tonnes of different ideas. More interesting then TV. If only? Lots of google spice for me to see if its tasty.
Gday Douglas
have you tried the cheaper (currently) 4d21 tube to compare?
Nah, not tried the 4D21 yet. I have not even tried boosting the g3 voltage( would probably try 9V with a battery for starters ). That one looks like it has potential, especially to firm up the knee at the upper left of the curves...which are dominated by the U-L percentage I happen to run.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Long time... You have the advantage of cancellation to keep your fils quiet but have you tried DC at all ? With SE I find DC is the only way.
Al
Never tried DC; never saw the need to go to all the trouble of rectifying and smoothing it...especially considering there needs to be 15A of it for two. AC did just fine, thanks to the cancellation you noted.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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