[AMPS] For who well know 8877

[Maurizio Panicara]

----- Original Message -----
From: "Alfredo Rosati I5UXJ" <alfredo@cln.it>
To: <amps@contesting.com>
Sent: Saturday, December 29, 2001 6:24 PM
Subject: [AMPS] For who well know 8877



> my result are :
>
> 3500 volt , 1Amp , 70 watt input , 2025 watt output ,120 ma grid.

3500V is not enough (proper) for 2KW.
With 4KV and an optimized output circuit the IP should be around 0.8 A or
even less.

> i had tried also as data sheet of eimac told for 88-108 operation
> at 2000 ,2500, 3000 , 4000 volt.
>
> the is the same more grid current than the data sheet.
>
> i had tried some different output circuit but little difference.
>
> the tube is capable with 100 watt input , to draw 1,5 amp. and an output
> of 3700 watt (at 4000 volt).

Impossible!
Only with 2 tubes or using a wrong wattmeter and a wrong ammeter.
...or eventually a 3CX3000A7.

> the question is :
> is my tube exausted ?

May be.

> is my tube defective ?

I don't think so, but let the PA on with power and see what happens to its
parameters.

> people that working in brodcast fm , has told me , that they use
> amplifier at 2500 watt whit 80~100 ma grid
> 2000 watt 40~60 ma grid

A typical grid for an FM broadcast PA using a brand new 8877 (3cx1500a7,
yu236, etc) 4KV (3 phase PS) and 2KW is 15-30 mA, the drive power is around
40/45 W.
As the tube gets old, the drive has to be increased a little to try keep the
output power, and IG can go up to 60 mA or more.
Although no damage occurs for short term high grid since 8877 can manage a
certain
power, beyond 80mA it's a simptom that something is wrong and a good
broadcast
amplifier should trip off.
A wise service technician shouldn't overcome that protection....
An important fact known by broadcaster was some years ago the tendency for
some new tubes to have an higher grid level during the very first hours (up
to some hundred) of operations and then a decrease final stabilizing at a
lower value.
Hardly in amateurs application this can be noted as well as the with tubes
suffering abnormal fast aging.


> If I disalign the load coming down from 2000 watt to 1800 the grid
> current go down to around 40 ma.

This alone doesn't say much unless you say what happens to plate current.
Not necessarily a stated output power has not to correspond to a tuning that
involves a lower grid and still good efficiency.
In my opinion, all what You probably need is to optimize your output
circuit.
Another common situation for high grid tunings is a low plate voltage and an
insufficient power drive, but it doesn't look your case.
To finally clear with your doubts you could ask the broadcast friends to
plug your tube inside any of their known FM PAs and see what happens.

73,
Mauri I4JMY

> thank in advance of eventual answers
> hello i5uxj




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