3-400Z

[Multi-Volti Devices]

Rich Measures AG6K told me about 10 years ago the reason for the lower mu
(160)on the 3-500Z was an Eimac design change coordinated with US amateur
amplifier manufacturers to alleviate parasitic oscillations. At some pointin
history he says the 3-500Z had a mu of 200 like it's baby bro' the 3-400Z.

Preventing parasitic oscillations became more difficult when other sources
for 3-500Z's appeared on the market - the Chinese ones were supposedly
dimensional copies of the Amperex design, which retained the higher mu of
200.

The Heath SB-1000 ran significantly higher plate voltage than the
SB-220...ever wonder why a one tube amp put out almost the same power as the
2-tube amp RATED power? I haven't verified, but I was told that the mu vs.
Ep curve will show that mu is not a constant and tends to be higher (with
this tube anyway -  I don't want to generalize for all tubes) at higher
voltage.

So, a higher mu tube at higher than usual voltage has a slightly higher risk
of parasitic oscillation because the tube gain is higher.

That might be a concern also with replacing a 3-500Z with a 3-400Z, and
because the HV is so high in an SB-1000, I would not put a 3-400Z there,
unless I had so many I didn't care about the tube's life.

When 4-400A's were readily found and given away as pulls from broadcast
rigs, I used to run my homebrew amp as high as the variac would go...a late
Silent Key (old age, not running theVariac up too high) who designed
broadcast tubes  for RCA told me the tubes will tolerate much higher voltage
than rated - they're 'flashed' at significantly higher voltage to 'burn off'
field points (basically burrs & other sharp edges).

VHF is more of a concern for reducing voltage rating, and I don't know if I
ever knew or forgot whether it's a dissipation or arcing issue...with rf
switches, voltage rating is reduced at higher frequency, so who knows.

Murray

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.asp?list=HEATH
To post - HEATH at LISTSERV.TEMPE.GOV
Archives - http://interactive.tempe.gov/archives/HEATH.html