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EZ81 rectifier current rating?

1/21/1998 5:45 PM
Michael Tousek
EZ81 rectifier current rating?
I've managed to get hold of a late 50's Bogen tube hifi, and I'm thinking of brewing it into an 18-watt Marshall clone. The Bogen uses a 5Y3 rectifier; how would the current rating of an EZ81 (like the little Marshall uses) compare? I don't have an EZ81 listed in any of my tube books.  
Much thanks in advance.  
Michael Tousek
1/21/1998 6:31 PM
John Kos

You might try looking for a 6CA4 in your tube books. They are the substitutes for each other.  
John Kos
1/21/1998 6:48 PM
The EZ81/6CA4 rectifier is rated at 150 mA maximum DC current while the 5Y3 is listed as 125 mA. The 5Y3 is commonly used to run a pair of EL84's or 6V6's, so don't let the missing 25 mA bother you.  
I modified a Heath EA-3 (EZ81 rectifier and two EL84's) for guitar use, using the phase splitter of the small Marshall, Vox Top Boost preamp and tone circuit, and a Bassman-style presence control. It came with an ultralinear output transformer, but it sounds great for guitar nevertheless.  
What other tubes are in your Bogen amp ? Does it also have an ultralinear output transformer ? I'm sure others are curious as well.
1/21/1998 9:05 PM
Michael Tousek

Thanks for the EZ81 current spec.  
This Bogen is a model DB10-1. In addition to the 5Y3, it uses a pair of 6V6's, one 6SL7, one 6SF5, and a 6SC7. The 6SL7 has a metal bottle. These are octal tubes, of course, and I'm planning on replacing the big octal sockets with miniatures so I can use 12AX7's (as the 18-watt Marshall uses). I've thought of a couple ways I might do this, but I'm still not quite sure. I'm planning on trying it out with the 6V6's before switching to EL-84's.  
I really like the way this unit looks -- it's dark forest green with yellow silkscreened lettering, and gracing the front are four bronze-colored knobs which are so cool-looking that they must represent some sort of high point in knob design.  
As for the transformer, I don't know if it's ultralinear or not -- I hadn't even thought about that until you mentioned it. It's got two leads which run to the plates of the output tubes, and that's it. Don't ultralinear trannys have a set of leads which run to the screens as well? How can I tell if it's ultralinear, and what differences will it make if it is?  
Michael Tousek
1/22/1998 8:25 AM

Yes, ultralinear trannies have an additional  
set of leads running to the screen. They are  
generally taps at %40 of the winding length  
between the outside primaries and the CT.  
So you already know how to tell, it sounds  
As for audible differences, ultralinear  
adds linearity to the frequency response,  
essentially resulting in improvements of  
the "flatness" of the response curve of  
the amplifier. More usefull for hi-fi,  
since we guitar folks want "character"  
not precision.  
So, if you're reworking this amp  
for guitar use, thank the original designers  
for not using an ultralinear tapped OT,  
although you could always just not use  
the taps if they were there.  
Or experiment, maybe you'd like the UL  
... Whit
1/22/1998 3:35 PM
Michael Tousek

Is the tap for the screens the only significant difference in a UL OT and a "normal" OT? I suppose what I'm asking is this: if I don't use the screen taps, will this OT behave like a normal OT?  
Michael Tousek
1/23/1998 7:46 AM

As far as I understand it (and I'm always  
happy to be educated) that is the case.  
The UL taps just provide a source of signal  
around %40 of the distance through the primary  
winding, which can be fed to the screen of the  
output tube(s). It allows the screens to  
"ride up and down" with the signal at the  
plates, essentially a shadowing of the  
activity at the plates.  
The %40 figure was arrived at through  
That being said, I'm reminded of the  
Mark Twain line: "It's better to remain  
silent and be thought a fool, than to open  
one's mouth and remove all doubt!"...  
... but I think I'm tellin' the truth  
... Whit

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