[Amps] 8877 - HV with heater, or later?

[John Lyles]

Amp people,

Its an interesting discussion with lots of answers. I'll ask Reid 
Brandon, retired from Eimac/CPI, what he knows. I have a Henry 3K 
Classic that has not been fired up in years, that is a project. It has 
the 8877 with a 90 sec holdoff timer for the HV button to become active. 
I was talking to a friend today with a Henry 2K-4 that has dual 3-500Z. 
Someone had put a homemade step start on the power transformer, which is 
a combined plate/fil in that model. In mine they are separate. Brian 
Tatum says he has always put a step start on fil/HV both in his work. 
Others say 90 seconds is inadequate.

We know that direct heated thoriated tungsten filaments are low 
resistance when cold, so the inrush can destroy a filament if snapped 
on. One solution in commercial practice is to use a magnetic gapped 
transformer that will only supply 2 X the normal current into a short 
circuit. Thats what we did with broadcast transmitters up to 30 kW using 
Eimac tubes. For larger tubes of 100 kW or more, we use a ramping 
circuit that lasts 10-20 seconds and up to 8 minutes for $300K tubes, on 
the filament AC primary circuit. For the smaller tubes, HV is typically 
applied along with filament power. Meaning tubes like 3-400Z, 3-500Z, 
4-400A etc.

For indirect cathodes such as the oxide coated in 3CX1500A7/8877, 
3CX800A7, 4CX1000A, 4CX1500A etc, there is the requirement to allow the 
cathode to get hot enough before HV is applied. I believe, as some have 
said, that the real concern is pulling cathode current too soon, and if 
that is zero due to biasing or drive power control, HV should be OK 
sooner. It is always good to delay HV in any tube just to get the 
filament time to mechanically stabilize too. Also for any residual gas, 
it is bad thing to bring HV on a tube that has not been operated for a 
long time, without letting the filament heat up (if it has getter 
material under the filament). I don't know how important this is for 
oxide cathodes.

I'll ask Reid now.

73

John

K5PRO



> I'm overhauling a vintage 8877 amp, and it occurs to me that I've never
> seen a definitive answer to this question:
>
> Should HV be held off on a 8877 until the heater warm-up timer expires?
>
> Not that I claim any particularly deep or insightful knowledge of such
> things, but it seems to me that if the cathode has cutoff bias that it
> wouldn't make any difference when B+ is applied.
>
> I did find a reference to this question in the amps archives, but every
> design I've looked at recently holds the HV off until the cathode is ready
> to go.
>
> Maybe there's more to it than I'm seeing? I may use a separate HV switch on
> the front panel anyway just to allow for gettering, but I'd still like a
> consensus on the question.
>
> Thanks!
> Jim N7CXI