Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SnickSound on June 18, 2020, 10:48:03 am
-
This is another case of "using what I got". After a bunch of mods and rebuilds I have a pair of EHX 6973s and a Hammond 272DX power transformer. I'd like to use them.
My experience says the PT will net me just shy of 400V with diode rectification.
The 6973 datasheet (https://shop.ehx.com/catalog/addimages/6973eh.pdf) says 440V plates and 330V screens. Meaning I'm good for plates. But screens are a different story...
Can't drop the screens with a resistor, that would require something like a 33k and the sagging under load would be way too pronounced. Tube rectifier not an option (space constraint, doesn't fit the personality of the amp, and I'm using the 5VAC winding for something else)
But I notice the ratings when wired for ultralinear is 410V for both plates and screens. I'm intrigued as to why the screens can take more when wired this way? I'm guessing because the voltage actually goes down under load.
So I'm thinking... first I'll help myself by going with cathode bias which willincrease cathode voltage (thus reducing "screen to cathode" voltage).
And I'm gonna put fairly big screen resistors (at least 470R... maybe 1K per) so voltage sags a little under load.
Truth be told, I ran those 6973 in fixed bias with 365VDC on them for a little while, they didn't seem to care.
Unless anyone has a suggestion, or some anecdotal evidence to share.
These are just new production EHX 6973s which are cheap and plentiful.
-
If you run the tubes with cathode bias and use a 5Y3 you will have a much more friendly B+. Look at my voltages and note my PT is 640V versus 600V with your 272DX...
http://sluckeyamps.com/supro/supro.pdf
-
If you run the tubes with cathode bias and use a 5Y3 you will have a much more friendly B+. Look at my voltages and note my PT is 640V versus 600V with your 272DX...
http://sluckeyamps.com/supro/supro.pdf
Like I said though, tube rectifier is not an option this time.
-
> cheap and plentiful.
So throw 400V at them and see how they last.
This tube is a 6V6 with funny pins and more honest ratings. 1939's 6V6 "could" have been built on cheap 315V plate-stuff, so that's on the sheet. But by 1959 the craze for TV meant that all power tubes were using 500V plate-stuff, and G2 could probably be run over 400V for decades. In an AUDIO amp there is no good sonic reason to run the screens hard; you can get your 24 Watts with 300Vg2. But if you are stuck with parts on hand, just do it.
-
A Hammond 272DX is perfect for a tweed tremolux 5G9 https://www.hammfg.com/files/parts/pdf/272DX.pdf (https://www.hammfg.com/files/parts/pdf/272DX.pdf)
Seeing as how a 6973 is a noval 'equivalent' of a 6V6, you probably would get 5G9 B+ if you ran a 5U4G rectifier. (And you could use a smallish 40mA HT choke to supply the screen node, like a 5G9 does)
-
1960 datasheets state 410V max g2 volts in UL operation so 400V to the screens really shouldn't be an issue in pentode mode. you should be more concerned with the g2 dissipation. maybe start with 1K 3W g2 resistors, one for each tube. monitor screen power. you shouldn't exceed 2W average. measure DCV across 1K g2 R and calculate g2 dissipation. if you need more than 1K for g2 limit to keep g2 dissipation in check, you're probably going to experience some sag.
second option would be to use SS fixed power supply with a VVR for just g2 supply.
--pete