[Amps] 3-400Z vs. 4-400A

[Tom Rauch]

> Because other options are open season... One 
> might judge the value of a Dahl retrofit xfmr. Some 
> vintage Henry amps (with excessive filament and 
> Anode values) arrived with 3-400z's on board, an 
> SB-220 might be considered a candidate should 
> the owner have a supply of cheaper 4-400's when 
> the original tubes go away.  The original post 
> asked for information to compare the two tube 
> types... the posted reply was very useful, on topic 
> information. 

Better get a new plate tuning capacitor and a new bandswitch while 
you are getting the new transformer...because the old components 
won't take an increase in voltage.

Watch the little fixed caps and electrolytics caps also.
. 
> I've found the 4-400 a bit "stiff to drive" compared 
> to the 3-500z, but it's a nice tube to play with.  One 
> could easily apply a bit of screen voltage for lower 
> drive (and higher gain) applications. The control 

In grounded grid there is hardly any significant change when you 
apply bias, except it evens out grid current more.

> The 4-400 is a tetrode and some very 
> innovative things can be done with that 
> old swan amp.  Just ask.... right here!

The best way to use the 4-400 is to plug it in and play, and live 
with the slight gain reduction. Otherwise, you might as well build a 
whole new amp. To make a Swan work like it should with a tetrode, 
you'd either do that or have external parts in other cabinets just to 
support the amplifier.

Of course I guess you could saw it in half and stretch the chassis so 
all the extra parts would fit, but I seriously think using it as-is 
would be best unless you want to build a new amp around the sockets 
and a few other parts.


73, Tom W8JI
W8JI@contesting.com